Sequence for Oral Examination

An oral examination should be accomplished in the following sequence: visual examination, pain relief and temporary restorations, radiographs, oral prophylaxis, evaluation of teeth and periodontium, vitality tests of individual teeth, determination of the floor of the mouth position, and impressions of each arch.

Figure 12-1 Complete intraoral radiographic survey of remaining teeth and adjacent edentulous areas reveals much information vital to effective diagnosis and treatment planning. The response of bone to previous stress is of particular value in establishing the prognosis of teeth that are to be used as abutments.

Figure 12-2 A, Diagnosis record for recording pertinent data.B, Treatment record chart for recording treatment plan and treatment progress.

Figure 12-3 Simple working chart. Restorations for individual teeth, crowns, and fixed partial dentures to be made may be marked on the chart and checked off as completed during mouth preparations.

Visual examination will reveal many of the signs of dental disease. Consideration of caries susceptibility is of primary importance. The number of restored teeth present, signs of recurrent caries, and evidence of decalcification should be noted. Only those patients with demonstrated good oral hygiene habits and low caries susceptibility should be considered good risks without resorting to prophylactic measures such as the restoration of abutment teeth. At the time of the initial examination, periodontal disease, gingival inflammation, the degree of gingival recession, and mucogingival relationships should be observed. Such an examination will not provide sufficient information to allow a definitive diagnosis and treatment plan. For this purpose, complete periodontal charting that includes pocket depths, assessment of attachment levels, furcation involvement, mucogingival problems, and tooth mobility should be performed. The extent of periodontal destruction must be determined with appropriate radiographs and use of the periodontal probe.

The number of teeth remaining, the location of the edentulous areas, and the quality of the residual ridge will have a definite bearing on the proportionate amount of support that the removable partial denture will receive from the teeth and edentulous ridges. Tissue contours may appear to present a well-formed edentulous residual ridge; however, palpation often indicates that supporting bone has been resorbed and has been replaced by displaceable, fibrous connective tissue. Such a situation is common in maxillary tuberosity regions. The removable partial denture cannot be supported adequately by tissues that are easily displaced. When the mouth is prepared, this tissue should be recontoured or removed surgically, unless otherwise contraindicated. A small but stable residual ridge is preferable to a larger unstable ridge for providing support for the denture. The presence of tori or other bony exostoses must be detected and their presence in relation to framework design must be evaluated. Failure to palpate the tissue over the median palatal raphe to ascertain the difference in its displaceability as compared with the displaceability of the soft tissues covering the residual ridges can lead to a rocking, unstable, uncomfortable denture and to a dissatisfied patient. Adequate relief of the palatal major connectors must be planned, and the amount of relief required is directly proportionate to the difference in displaceability of the tissues over the midline of the palate and the tissues covering the residual ridges.

During the examination, not only each arch but also its occlusal relationship with the opposing arch must be considered separately. A situation that looks simple when the teeth are apart may be complicated when the teeth are in occlusion. For example, an extreme vertical overlap may complicate the attachment of anterior teeth to a maxillary denture. Extrusion of a tooth or teeth into an opposing edentulous area may complicate the replacement of teeth in the edentulous area or may create occlusal interference, which will complicate the location and design of clasp retainers and occlusal rests. Such findings subsequently will be evaluated further by careful analysis of mounted diagnostic casts.

A breakdown of the fee may be recorded on the back of this chart for easy reference if adjustments or substitutions become necessary because of changes in diagnosis as the work progresses.

The fee for examination, which should include the cost of the radiographic survey and the examination of articulated diagnostic casts, should be established before the examination is performed and should not be related to the cost of treatment. It should be understood that the fee for examination is based on the time involved and the service rendered, and that the material value of the radiograph and diagnostic casts is incidental to the effectiveness of the examination.

The examination record should always be available in the office for future consultation. If consultation with another dentist is requested, respect for the hazards of unnecessary radiation justifies loaning the dentist the radiograph for this purpose. However, duplicate films should be retained in the dentist’s files.

Purposes of Diagnostic Casts

Diagnostic casts serve several purposes as an aid to diagnosis and treatment planning. Some of these are as follows:

Figure 12-4 A, Following mounting of the diagnostic casts, tooth arrangement for the mandibular occlusal plane requirements can be accomplished. B, Following placement of the maxillary anterior teeth in an ideal position, diagnostic arrangement of occlusion results in a space posterior to surveyed crown #27. If such a finding were objectionable, alternative arrangements could be investigated. This is not possible unless a diagnostic workup is completed. C, Occlusion of the mandibular removable partial denture will be enhanced by improving the maxillary posterior occlusal plane of the super-erupted molars.

Figure 12-5 Proposed mouth changes and design of the removable partial denture framework are indicated in pencil on the diagnostic cast in relation to the previously determined path of placement. This serves as a means of communicating with the patient and as a chair-side guide to tooth modification.

Mounting Diagnostic Casts

For diagnostic purposes, casts should be related on an anatomically appropriate articulator to best understand the role occlusion may have in the design and functional stability of the removable partial denture. This becomes increasingly important as the prosthesis replaces more teeth. If the patient presents with a harmonious occlusion and the edentulous span is a tooth-bound space, simple hand articulation is generally all that is required. However, when the natural dentition is not harmonious and/or when the replacement teeth must be positioned within the normal movement patterns of the jaws, the diagnostic casts must be related in an anatomically appropriate manner for diagnosis. This means placement of the maxillary cast in a position relative to the opening axis on the articulator, which is similar to the position of the maxilla in relation to the temporomandibular joint of the patient (Figure 12-6). The mandibular cast is then placed beneath the maxillary cast in a horizontal position dictated by mandibular rotation without tooth contact, at a minimal vertical opening.

Figure 12-6 Use of the facebow makes possible the recording of the spatial relationship of the maxillae to some anatomic reference points and transference of this relationship to an articulator.

The Glossary of Prosthodontic Terms* describes an articulator as a mechanical device that represents the temporomandibular joints and jaw members, to which maxillary and mandibular casts may be attached. Because the dominant influence on mandibular movement in a partially edentulous mouth is the occlusal plane and the cusps of the remaining teeth, an anatomic reproduction of condylar paths is probably not necessary. Still, movement of the casts in relation to one another as influenced by the occlusal plane and the cusps of the remaining teeth, when mounted at a reasonably accurate distance from the axis of condylar rotation, permits a relatively valid analysis of occlusal relations. This is more anatomically accurate than a simple hinge mounting.

It is better that the casts be mounted in relation to the axis-orbital plane to permit better interpretation of the plane of occlusion in relation to the horizontal plane. Although it is true that an axis-orbital mounting has no functional value on a nonarcon instrument because that plane ceases to exist when opposing casts are separated, the value of such a mounting lies in the orientation of the casts in occlusion. (An arcon articulator is one in which the condyles are attached to the lower member as they are in nature, the term being a derivation coined by Bergström from the words articulation and condyle. Many of the more widely used articulators such as the Hanau H series, Dentatus, and improved Gysi have the condyles attached to the upper member and are therefore nonarcon instruments.)

Sequence for Mounting Maxillary Cast to Axis-Orbital Plane

The initial steps allow recording of the maxilla–temporomandibular joint (TMJ) relationship:

The next steps allow transfer of the recorded relationship to the articulator:

The facebow is a relatively simple device used to obtain a transfer record for orienting a maxillary cast on an articulating instrument. Originally, the facebow was used only to transfer a radius from condyle reference points, so that a given point on the cast would be the same distance from the condyle as it is on the patient. The addition of an adjustable infraorbital pointer to the facebow and the addition of an orbital plane indicator to the articulator make possible the transfer of the elevation of the cast in relation to the axis-orbital plane. This permits the maxillary cast to be correctly oriented in the articulator space comparable with the relationship of the maxilla to the axis-orbital plane on the patient. To accommodate this orientation of the maxillary cast and still have room for the mandibular cast, the posts of the conventional articulator must be lengthened. The older Hanau model H articulator usually will not permit a facebow transfer with an infraorbital pointer.

A facebow may be used to transfer a comparable radius from arbitrary reference points, or it may be designed so that the transfer can be made from hinge axis points. The latter type of transfer requires that a hinge-bow attached to the mandible should be used initially to determine the hinge axis points, to which the facebow is then adjusted for making the hinge axis transfer.

A facebow transfer of the maxillary cast, which is oriented to the axis-orbital plane in a suitable articulator, is an uncomplicated procedure. The Hanau series Wide-Vue 183-2, all 96H2-0 models, the Whip-Mix articulator (Whip-Mix Corp, Louisville, KY), and the Dentatus model ARH (Dentatus USA, New York, NY) will accept this transfer. The Hanau earpiece facebow models 153 and 158, the Hanau fascia facebow 132-2SM, and the Dentatus facebow type AEB incorporate the infraorbital plane to the articulator. None of these are hinge axis bows; they are used instead at an arbitrary point.

The location of the arbitrary point or axis has long been the subject of controversy. Gysi and others have placed it 11 to 13 mm anterior to the upper third of the tragus of the ear on a line extending from the upper margin of the external auditory meatus to the outer canthus of the eye. Others have placed it 13 mm anterior to the posterior margin of the center of the tragus of the ear on a line extending to the corner of the eye. Bergström has located the arbitrary axis 10 mm anterior to the center of a spherical insert for the external auditory meatus and 7 mm below the Frankfort horizontal plane.

In a series of experiments reported by Beck, it was shown that the arbitrary axis suggested by Bergström falls consistently closer to the kinematic axis than do the other two. It is desirable that an arbitrary axis is placed as close as possible to the kinematic axis. Although most authorities agree that any of the three axes will permit transfer of the maxillary cast with reasonable accuracy, it would seem that the Bergström point compares most favorably with the kinematic axis.

The lowest point on the inferior orbital margin is taken as the third point of reference for establishing the axis-orbital plane. Some authorities use the point on the lower margin of the bony orbit in line with the center of the pupil of the eye. For the sake of consistency, the right infraorbital point is generally used and the facebow assembled in this relationship. All three points (right and left axes and infraorbital point) are marked on the face with an ink dot before the transfer is made.

Casts are prepared for mounting on an articulator by placing three index grooves in the base of the casts. Two V-shaped grooves are placed in the posterior section of the cast and one groove in the anterior portion (Figure 12-7).

Figure 12-7 The base of the cast has been prepared for mounting by placing three triangular grooves to allow indexing when mounted. The grooves are prepared with a 3-inch stone mounted in a laboratory lathe.

An occlusion rim properly oriented on a well-fitting record base should be used in facebow procedures involving the transfer of casts representative of the Class I and II partially edentulous situations. Without occlusion rims, such casts cannot be located accurately in the imprints of the wax covering the facebow fork. Tissues covering the residual ridges may be displaced grossly when the patient closes into the wax on the facebow fork. Therefore the wax imprints of the soft tissues will not be true negatives of the edentulous regions of the diagnostic casts.

For purposes of illustration, a facebow using the external auditory meatus as the posterior reference point, the Whip-Mix Facebow technique (DB 2000, Whip-Mix Corp, Louisville, KY), will be shown. The facebow fork is covered with a polyether, polyvinyl siloxane or a roll of softened baseplate wax with the material distributed equally on the top and on the underneath side of the facebow fork. Then the fork should be pressed lightly on the diagnostic casts with the midline of the facebow fork corresponding to the midline of the central incisors (Figure 12-8). This will leave imprints of the occlusal and incisal surfaces of the maxillary casts and occlusion rim on the softened baseplate wax and is an aid in correctly orienting the facebow fork in the patient’s mouth. The facebow fork is placed in position in the mouth, and the patient is asked to close the lower teeth into the wax to stabilize it in position. It is removed from the mouth and chilled in cold water and then replaced in position in the patient’s mouth. An alternative method of stabilizing the facebow fork and recording bases is to enlist the assistance of the patient.

Figure 12-8 Orienting the facebow fork to the maxillary cast and occlusion rims will avoid displacing the occlusion rim in the mouth through patient closure or another uneven force. Polyvinyl siloxane material has been evenly distributed around the facebow fork, and care is exercised to position the fork to be centered at the mid-incisal position without any fork extension posterior to the record base, which could cause discomfort.

If an earpiece facebow is to be used, the patient should be reminded that the plastic earpieces in the auditory canals will greatly amplify noise. With the facebow fork in position, the facebow toggle is slipped over the anterior projection of the facebow fork (Figure 12-9). The patient can assist in guiding the plastic earpieces into the external auditory meatus. The patient can then hold the arms of the facebow in place with firm pressure while the operator secures the bite fork to the facebow. This accomplishes the radius aspect of the facebow transfer.

Figure 12-9 The horizontal toggle clamp of the Whip-Mix earpiece facebow (1) is slid onto the shaft of the facebow fork protruding from the patient’s mouth. The patient then helps guide plastic earpieces into the external auditory meatus and holds them in place while the operator tightens three thumb screws (2) and centers the plastic nosepiece (3) securely on the nasion. The horizontal toggle clamp is positioned and secured near (but not touching) the lip. The T screw (4) on the vertical bar is tightened. note: Extreme care should be exercised not to tilt the facebow out of position when tightening.

If an infraorbital pointer is used, it is placed on the extreme right side of the facebow and angled toward the infraorbital point previously identified with an ink dot. It is then locked into position with its tip lightly touching the skin at the dot. This establishes the elevation of the facebow in relation to the axis-orbital plane. Extreme care must be taken to avoid any slip that might injure the patient’s eye.

With all elements tightened securely, the patient is asked to open, and the entire assembly is removed intact, rinsed with cold water, and set aside. The facebow records not only the radius from the condyles to the incisal contacts of the upper central incisors, but also the angular relationship of the occlusal plane to the axis-orbital plane.

The facebow must be positioned on the articulator in the same axis-orbital relation as on the patient. If an arbitrary-type facebow is used, the calibrated condyle rods of the facebow ordinarily will not fit the condyle shafts of the articulator unless the width between the condyles just happens to be the same. With a Hanau model 132-25M facebow, the calibrations must be reequalized when in position on the articulator. For example, they have read 74 (mm) on each side of the patient but must be adjusted to read 69 (mm) on each side of the articulator. Some later model articulators have adjustable condyle rods and may be adjusted to fit the facebow. It is necessary that the facebow be centered in either case. Some facebows are self-centering, as is the Hanau Spring-Bow (Whip-Mix Corp, Louisville, KY).

The third point of reference is the orbital plane indicator, which must be swung to the right so that it will be above the tip of the infraorbital pointer. The entire facebow with maxillary cast in place must be raised until the tip of the pointer contacts the orbital plane indicated. The elevation having thus been established, for all practical purposes the orbital plane indicator and the pointer may now be removed because they may interfere with placing the mounting stone.

An auxiliary device called a cast support is available; it is used to support the facebow fork and the maxillary cast during the mounting operation (Figure 12-10). With this device, the weight of the cast and the mounting stone are supported separately from the facebow, thus preventing possible downward movement resulting from their combined weight. The cast support is raised to supporting contact with the facebow fork after the facebow height has been adjusted to the level of the orbital plane. Use of some type of cast support is highly recommended as an adjunct to facebow mounting.

Figure 12-10 Facebow fork support used to maintain the fork and cast in position while mounting.

The keyed and lubricated maxillary cast is now attached to the upper arm of the articulator with the mounting stone, thus completing the facebow transfer (Figure 12-11). Not only will the facebow have permitted the upper cast to be mounted with reasonable accuracy, it also will have served as a convenient means of supporting the cast during mounting. Once mastered, its use becomes a great convenience rather than a time-consuming nuisance.

Figure 12-11 Facebow mounting is complete. The relationship of the maxillary cast to the articulator condylar components is anatomically similar to that between the patient’s maxilla and the bilateral temporomandibular joint (TMJ) complex. Any subsequent tooth arrangement and occlusal contact development will represent the mouth more accurately than more arbitrary mountings. The benefits of the anatomic similarity are seen in more accurate occlusion for the finalized prosthesis (i.e., less intraoral adjustment required).

It is preferable that the maxillary cast be mounted while the patient is still present, thus eliminating a possible reappointment if the facebow record is unacceptable for some reason. Not too infrequently, the facebow record has to be redone with the offset-type facebow fork repositioned to avoid interference with some part of the articulator.

Jaw Relationship Records for Diagnostic Casts

One of the first critical decisions that must be made in a removable partial denture service involves selection of the horizontal jaw relationship to which the removable partial denture will be fabricated (centric relation or the maximum intercuspal position). All mouth-preparation procedures depend on this analysis. Failure to make this decision correctly may result in poor prosthesis stability, discomfort, and deterioration of the residual ridges and supporting teeth.

It is recommended that deflective occlusal contacts in the maximum intercuspal and eccentric positions be corrected as a preventive measure. Not all dentists agree that centric relation and the maximum intercuspal position must be harmonious in the natural dentition. Many dentitions function satisfactorily with the opposing teeth maximally intercusped in an eccentric position without either diagnosable or subjective indications of temporomandibular joint dysfunction, muscle dysfunction, or disease of the supporting structures of the teeth. In many such situations, no attempt should be made to alter the occlusion. It is not a requirement to interfere with an occlusion simply because it does not completely conform to a relationship that is considered ideal.

If most natural posterior teeth remain and if no evidence of temporomandibular joint disturbances, neuromuscular dysfunction, or periodontal disturbances related to occlusal factors exists, the proposed restorations may be fabricated safely with maximum intercuspation of remaining teeth. However, when most natural centric stops are missing, the proposed prosthesis should be fabricated so that the maximum intercuspal position is in harmony with centric relation. By far the greater majority of removable partial dentures should be fabricated in the horizontal jaw relationship of centric relation. In most instances in which edentulous spaces have not been restored, the remaining posterior teeth will have assumed malaligned positions through drifting, tipping, or extrusion. Correction of the remaining natural occlusion to create a coincidence of centric relation and the maximum intercuspal position is indicated in such situations.

Regardless of the method used in creating a harmonious functional occlusion, an evaluation of the existing relationships of the opposing natural teeth must be made and is accomplished with a diagnostic mounting. This evaluation is performed in addition to, and in conjunction with, other diagnostic procedures that contribute to an adequate diagnosis and treatment plan.

Diagnostic casts provide an opportunity to evaluate the relationships of remaining oral structures when correctly mounted on a semiadjustable articulator with use of a facebow transfer and interocclusal records. Diagnostic casts are mounted in centric relation (most retruded relation of the mandible to the maxillae) so that deflective occlusal contacts can be correlated with those observed in the mouth. Deflective contacts of opposing teeth are usually destructive to the supporting structures involved and should be eliminated. Diagnostic casts demonstrate the presence and location of such interfering tooth contacts and permit visualization of the treatment that would be necessary for their correction. Necessary alteration of teeth to harmonize the occlusion can be performed initially on duplicates of the mounted diagnostic casts to act as guides for similar necessary corrections in the mouth. In many instances the degree of alteration required will indicate the need for crowns or onlays to be fabricated, or for recontouring, repositioning, or elimination of extruded teeth.

As was previously mentioned, the maxillary cast is correctly oriented to the opening axis of the articulator by means of the facebow transfer and becomes spatially related to the upper member of the articulator in the same relationship that the maxilla has to the hinge axis and the Frankfort plane. Similarly, when a centric relation record is made at an established vertical dimension, the mandible is in its most retruded relation to the maxilla. Therefore when the maxillary cast is correctly oriented to the axis of the articulator, the mandibular cast automatically becomes correctly oriented to the opening axis, when attached to and mounted with an accurate centric relation record.

Unlike recording the fixed relationship of the maxilla to the mandibular opening axis (using the facebow transfer record), the mandibular position is recorded in space and is not a fixed point. Consequently, it is necessary to prove that the relationship of the mounted casts is correct. This can be done simply by making another interocclusal record, at centric relation, fitting the casts into the record, and checking to see that the condylar elements of the articulator are snug against the condylar housings. If this is not seen, another record is made until duplicate records are produced. Because centric relation is the only jaw position that can be repeated by the patient, mountings in this position can be replicated and verified for correctness.

A straightforward protrusive record is made to adjust the horizontal condylar inclines on the articulator. Lateral eccentric records are made so that the lateral condylar inclinations can be properly adjusted. All interocclusal records should be made as near the vertical relation of occlusion as possible. Opposing teeth or occlusion rims must not be allowed to contact when the records are made. A contact of the inclined planes of opposing teeth will invalidate an interocclusal record.

In some instances, a mounting of a duplicate diagnostic cast in the maximum intercuspal position may also be desirable to definitively study this relationship on the articulator. Because articulators simulate only jaw movements, it is not unreasonable to assume that the relationship of the casts mounted in centric relation may differ minutely from the maximum intercuspal position seen on the articulator and observed in the mouth. When diagnostic casts are hand related by maximum intercuspation for purposes of mounting on an articulator, it is essential that three (preferably four) positive contacts of opposing posterior teeth are present, having widespread molar contacts on each side of the arch. If occlusion rims are necessary to correctly orient casts on an articulator, a centric relation should usually be the horizontal jaw relationship to which the removable partial denture will be constructed.

Materials and Methods for Recording Centric Relation

Materials available for recording centric relation are (1) wax; (2) modeling plastic; (3) quick-setting impression plaster; (4) metallic oxide bite registration paste; (5) polyether impression materials; and (6) silicone impression materials. Of these, wax is likely to be least satisfactory unless carefully handled. If not uniformly softened when introduced into the mouth, it can record a position with unequal tissue placement. Also, it does not remain rigid and dimensionally stable after removal unless carefully chilled and handled upon removal (Figure 12-12).

Figure 12-12 A wax interocclusal record made on a cast framework. The modification spaces first had baseplate wax added; these were adjusted intraorally to provide space at the occlusal vertical dimension for recording wax, the wax was softened using a wax spatula and a hot water bath, the framework was placed in the mouth, and care was exercised to guide the patient to close into a previously verified (and practiced) interocclusal position deemed appropriate (in this instance, centric relation position). The record was recovered from the mouth, excess wax was removed with a warm scalpel, and the wax was chilled and replaced in the mouth to verify the record. If not verified, the wax was resoftened (with additional wax added as needed) and the procedure was repeated. B, Immediately after verification, the framework with interocclusal registration was replaced on the mandibular cast and inverted on the maxillary cast for mounting.

Modeling plastic is a satisfactory record medium because it can be flamed and tempered until uniformly soft before it is placed into the mouth. After modeling plastic is chilled, it is sufficiently stable to permit the mounting of casts with accuracy. For these reasons, it is a satisfactory medium for recording occlusal relations for complete or partial dentures. It also can be used with opposing natural teeth.

Impression plaster has advantages of softness when introduced and rigidity when set, which make it a satisfactory material for recording jaw relations. Its use is highly recommended when occlusion rims are used to mount casts correctly or to adjust articulators with interocclusal eccentric records.

Metallic oxide bite registration paste offers many of the advantages of plaster, with less friability. Although not strong enough to be used alone, when supported by a gauze mesh attached to a metal frame, it is a satisfactory recording medium. Also it may be used with occlusion rims. After the paste sets, the frame is removed from the mouth and the buccal side of the gauze released where it was secured with sticky wax. The tube on the lingual side may then be slid off the lingual extension of the frame. The frame is not needed when casts are mounted with this type of registration because the tube alone lends sufficient support to the interocclusal record.

Elastomeric materials are excellent for recording interocclusal relationships (Figure 12-13). Some are specially formulated for this purpose and have the qualities of extremely low viscosity, minimal resistance to closure, rapid set, low rebound, lack of distortion, and stability after removal from the mouth. Care should be exercised to ensure that no elastic rebound results when the record is related to the cast during the mounting procedure.

Figure 12-13 Elastomeric interocclusal registration material used to record mandibular position.

The mandibular cast should be mounted on the lower arm of the articulator, with the articulator inverted (Figure 12-14A). The articulator is first locked in centric position, and the incisal pin is adjusted so that the anterior distance between the upper and lower arms of the articulator will be increased 2 to 3 mm greater than the normal parallel relationship of the arms. This is done to compensate for the thickness of the interocclusal record so that the arms of the articulator will again be nearly parallel when the interocclusal record is removed and the opposing casts come into contact.

Figure 12-14 A, Mandibular cast inverted on the mounted maxillary cast, making sure that the cast is fully seated into the interocclusal record and stabilized to the opposing cast. It is important to check the posterior occlusion rim contact to ensure that no interfering contact has altered the record. Space should be observed between the opposing record bases (or record base and opposing occlusion). B, Mounted casts demonstrating the occlusal plane as found in the mouth. The Frankfort plane of the patient is oriented parallel to the articulator base and the floor. Also, inspection of the posterior rims demonstrates space between the rims, which ensures that the recorded position was registered without influence from rigid contacting components, only from softened wax.

The base of the cast should be keyed and lightly lubricated for future removal. With the diagnostic casts accurately seated and secured in the occlusal record, the mandibular cast is affixed with stone to the lower member of the inverted articulator.

An articulator mounting thus made will have related the casts in centric relation (Figure 12-14B). The dentist then can proceed to make an occlusal analysis by observing the influence of cusps in relation to one another after the articulator has been adjusted by using eccentric interocclusal records.

After an occlusal analysis has been made, the casts may be removed from their mounting for the purpose of surveying them individually and for other purposes as outlined previously. The indexed mounting ring record also should be retained throughout the course of treatment in the event that further study should be needed. It is advisable that the mounting be identified with the articulator that is used, so that it may always be placed back onto the same articulator.

Radiographic Interpretation

Many of the reasons for radiographic interpretation during oral examination are outlined herein and are considered in greater detail in other texts. Aspects of such interpretation that are most pertinent to removable partial denture construction are those relative to the prognosis of remaining teeth that may be used as abutments.

Index Areas

Index areas are those areas of alveolar support that disclose the reaction of bone to additional stress. Favorable reaction to such stress may be taken as an indication of future reaction to an added stress load. Teeth that have been subjected to abnormal loading because of the loss of adjacent teeth or that have withstood tipping forces in addition to occlusal loading may be better risks as abutment teeth than those that have not been called on to carry an extra occlusal load (Figures 12-15 and 12-16). If occlusal harmony can be improved and unfavorable forces minimized by the reshaping of occlusal surfaces and the favorable distribution of occlusal loading, such teeth may be expected to support the prosthesis without difficulty. At the same time, other teeth, although not at present carrying an extra load, may be expected to react favorably because of the favorable reaction of alveolar bone to abnormal loading elsewhere in the same arch.

Figure 12-15 The reaction of bone adjacent to teeth that have been subjected to abnormal stress serves as an indication of probable reactions of that bone when such teeth are used as abutments for fixed or removable restorations. Such areas are called index areas.

Figure 12-16 A, The canine has provided support for the distal extension removable partial denture for 10 years. There has obviously been positive bone response (arrow) to increased stress generated by the removable partial denture. B, The mandibular first premolar has provided support for the distal extension denture for 3 years. Bone response (arrow) to past additional stress has been unfavorable.

Other index areas are those around teeth that have been subjected to abnormal occlusal loading; that have been subjected to diagonal occlusal loading caused by tooth migration; and that have reacted to additional loading, such as around existing fixed partial denture abutments. The reaction of the bone to additional stresses in these areas may be either positive or negative, with evidence of a supporting trabecular pattern, a heavy cortical layer, and a dense lamina dura, or the reverse response. With the former, the patient is said to have a positive bone factor, which means the ability to build additional support wherever needed. With the latter, the patient is said to have a negative bone factor, which means the inability to respond favorably to stress.

Root Morphology

The morphologic characteristics of the roots determine to a great extent the ability of prospective abutment teeth to resist successfully additional rotational forces that may be placed on them. Teeth with multiple and divergent roots will resist stresses better than teeth with fused and conical roots, because the resultant forces are distributed through a greater number of periodontal fibers to a larger amount of supporting bone (Figure 12-17).

Figure 12-17 A, The prognosis for abutment service is more favorable for a molar with divergent roots (shaded) than for the same tooth if its roots were fused and conical. B, Evidence that prospective abutment has conical and fused roots indicates the necessity for formulating a framework design that will minimize additional stresses placed on the tooth by the abutment service.

Third Molars

Unerupted third molars should be considered as prospective future abutments to eliminate the need for a distal extension removable partial denture (Figure 12-18). The increased stability of a tooth-supported denture is most desirable to enhance the health of the oral environment.

Figure 12-18 First and second molars have been lost by this 18-year-old patient. A distal extension removable partial denture may be constructed until the third molar erupts and is fully formed. A tooth-supported restoration may then be considered.

Periodontal Considerations

An assessment of the periodontium in general and abutment teeth in particular must be made before prosthetic restoration. One must evaluate the condition of the gingiva, looking for adequate zones of attached gingiva and the presence or absence of periodontal pockets. The ideal periodontal condition is a disease-free periodontium with adequate attached mucosa in regions at or adjacent to removable partial denture component parts that cross the gingival margins to best resist the mechanical challenges posed as the result of function and use. The condition of the supporting bone must be evaluated, with specific attention to reduced bone support and mobility patterns recorded. If mucogingival involvements, osseous defects, or mobility patterns are recorded, the causes and potential treatment must be determined.

Oral hygiene habits of the patient must be determined, and efforts made to educate the patient relative to plaque control. The most decisive evidence of oral hygiene habits is the condition of the mouth before the initial prophylaxis. Good or bad oral hygiene is basic to the patient’s nature, and although it may be influenced somewhat by patient education, the long-range view must be taken. It is reasonably fair to assume that the patient will do little more in the long-term future than he has done in the past. In making decisions as to the method of treatment based on oral hygiene, the future in years, rather than in weeks and months, must be considered. It is probably best not to give the patient the benefit of any doubt as to future oral hygiene habits. Rather, the benefit should come from protective measures where any doubt exists about future oral hygiene habits. Therefore for patients at greatest risk, an oral prophylaxis with continued oral hygiene instructions should be scheduled for 3- to 4-month intervals. In addition, the patient must be advised of the importance of regular maintenance appointments for tissue-supported prostheses to maintain occlusal relationships. When these ongoing observations and prophylactic requirements are described, the patient is faced with the realization that he or she must be willing to share responsibility for maintaining the health of the mouth after restorative and prosthodontic treatment.

The remaining teeth and prosthesis will require meticulous plaque control after placement of a removable partial denture. Because of the nature of material coverage of oral tissues, the oral microflora can change with the use of a removable prosthesis. Coupled with this microbial change is the potential for a mechanical challenge to tissue integrity if the appropriate relationship of the prosthesis and soft tissues of the residual ridge, as well as the marginal gingival, is not maintained.

Caries Risk Assessment Considerations

Caries activity in the mouth, past and present, and the need for protective restorations must be considered. The decision to use full coverage is based on a need to reshape abutment teeth to accommodate the components of the removable partial denture, prevention of restoration breakdown when abutments have large direct restorations, or evidence of recurrent caries risk. Occasionally, three-quarter crowns may be used where buccal or lingual surfaces are completely sound, but intracoronal restorations (inlays) are seldom indicated in any mouth with evidence of past extensive caries or precarious areas of decalcification, erosion, or exposed cementum.

Frequent consumption of sugars can lead to carious involvement of roots, caries around restorations, or caries associated with clasps of removable partial dentures. Intelligent consumption of sweets (smaller amounts and less frequent consumption) and frequent plaque removal are the recommended countermeasures. Excellent protection from caries can be provided by fluoride applications via toothpastes, mouth rinses, or (in extreme cases, such as postradiation xerostomia) 1% NaF gels applied daily with plastic trays.

Xerostomia, caused by degeneration of salivary glands (Sjögren’s syndrome) or various medications, will enhance the occurrence and severity of caries, as well as contribute to irritation of the oral mucosa. A possible way to alleviate xerostomia is the use of synthetic saliva, with a carboxymethylcellulose base, which can be enriched with fluoride in an effort to counteract caries. Frequent use provides an excellent means of maintaining high fluoride intraorally for long periods of time, thus enhancing the remineralization of incipient caries. Although providing instructions for improvement of oral hygiene is a duty of the dental team, suspected problems of dietary deficiencies should be referred to a nutritionist.

Evaluation of the Prosthesis Foundation—Teeth and Residual Ridge

An evaluation of the prosthesis foundation is required to ensure that an appropriately stable base of sound teeth and/or residual ridge(s) is provided to maximize prosthesis function and patient comfort. To that end, the evaluation focuses on the identification of conditions that are inconsistent with sound support and predictably stable function.

Surgical Preparation

The need for pre-prosthetic surgery or extractions must be evaluated. The same criteria apply to surgical intervention in the partially edentulous arch as in the completely edentulous arch. Grossly displaceable soft tissues covering basal seat areas and hyperplastic tissue should be removed to provide a firm denture foundation. Mandibular tori should be removed if they will interfere with the optimum location of a lingual bar connector or a favorable path of placement. Any other areas of bone prominence that will interfere with the path of placement should be removed also. The path of placement will be dictated primarily by the guiding plane of the abutment teeth. Therefore some areas may present interference to the path of placement of the removable partial denture by reason of the fact that other unalterable factors such as retention and esthetics must take precedence in selecting that path.

Clinical research in pre-prosthetic surgical concepts has contributed significant developments to management of the compromised partially edentulous patient. Bone augmentation and guided bone regeneration procedures have been used with varying degrees of success as an alternative method of improving ridge support for the denture base areas. Skill and judgment must be exercised in patient selection, procedural planning, and surgical and prosthetic management to optimize clinical results. Use of osseointegrated implants can provide a foundation for developing suitable abutment support for removable partial dentures. As in any surgical procedure, results depend on careful treatment planning and cautious surgical management.

Extraction of teeth may be indicated for one of the following three reasons:

Another consideration for pre-prosthetic surgery involves the decision between use of a removable partial denture and an implant-supported prosthesis. The following categories of tooth loss are presented with comparative comments germane to such decisions.

Analysis of Occlusal Factors

From the occlusal analysis made by evaluating the mounted diagnostic casts, the dentist must decide whether it is best to accept and maintain the existing occlusion or to attempt to improve on it by means of occlusal adjustment and/or restoration of occlusal surfaces. It must be remembered that the removable partial denture can supplement the occlusion that exists only at the time the prosthesis is constructed. The dominant force that dictates the occlusal pattern will be the cuspal harmony or disharmony of the remaining teeth and their proprioceptive influence on mandibular movement. The goal of artificial tooth placement is to harmonize with the functional parameters of the existing occlusion providing bilateral, simultaneous functional contact.

Chapter 17 identifies schemes of occlusion recommended for partially edentulous configurations. A review of these recommendations will provide a guide for modifying the existing occlusion or developing the appropriate occlusal scheme for each partially edentulous configuration.

Improvements in the natural occlusion must be accomplished before the prosthesis is fabricated, not subsequent to its fabrication. The objective of occlusal reconstruction by any means should be occlusal harmony of the restored dentition in relation to the natural forces already present or established. Therefore one of the earliest decisions in planning reconstructive treatment must be whether to accept or reject the existing vertical dimension of occlusion and the occlusal contact relationships in centric and eccentric positions. If occlusal adjustment is indicated, cuspal analysis always should precede any corrective procedures in the mouth by selective grinding. On the other hand, if reconstruction is to be the means of correction, the manner and sequence should be outlined as part of the overall treatment plan.

Fixed Restorations

There may be a need to restore modification spaces with fixed restorations rather than include them in the removable partial denture, especially when dealing with isolated abutment teeth. The advantage of splinting must be weighed against the total cost, with the weight of experience always in favor of using fixed restorations for tooth-bounded spaces unless the space will facilitate simplification of the removable partial denture design without jeopardizing the abutment teeth. One of the least successful removable partial denture designs is seen when multiple tooth-bounded areas are replaced with removable partial dentures in conjunction with isolated abutment teeth and distal extension bases. Biomechanical considerations and the future health of the remaining teeth should be given preference over economic considerations when such a choice is possible.

Orthodontic Treatment

Occasionally, orthodontic movement of malposed teeth followed by retention through the use of fixed partial dentures makes possible a better removable partial denture design mechanically and esthetically than could otherwise be used. Although adequate anchorage for tooth movement can be a major limitation in partially edentulous arches, carefully placed implants that subsequently can be used for prosthesis support have been used to expand orthodontic applications for this patient group.

Need for Determining Type of Mandibular Major Connector

As was discussed in Chapter 5, one of the criteria used to determine the use of the lingual bar or linguoplate is the height of the floor of the patient’s mouth when the tongue is elevated. Because the inferior borders of the lingual bar and the linguoplate are placed at the same vertical level, and because subsequent mouth preparations depend in part on the design of the mandibular major connector, determination of the type of major connector must be made during the oral examination. This determination is facilitated by measuring the height of the elevated floor of the patient’s mouth in relation to the lingual gingiva with a periodontal probe and recording the measurement for later transfer to diagnostic and master casts. It is most difficult to make a determination of the type of mandibular major connector to be used solely from a stone cast that may or may not accurately indicate the active range of movement of the floor of the patient’s mouth. Too many mandibular major connectors are ruined or made flexible because subsequent grinding of the inferior border is necessary to relieve impingement of the sensitive tissues of the floor of the mouth.

Need for Reshaping Remaining Teeth

The clinical crown shapes of anterior and posterior teeth are not capable of supporting a removable partial denture framework without appropriate modification. Without the required modifications, the prosthesis does not adequately benefit from the support and stability offered by the teeth and consequently will not be comfortable to the patient. Many failures of removable partial dentures can be attributed to the fact that the teeth were not reshaped properly to establish guiding planes or to receive clasp arms and occlusal rests before the impression for the master cast was made. Of particular importance are the paralleling of proximal tooth surfaces to act as guiding planes, the preparation of adequate rest areas, and the reduction of unfavorable tooth contours (Figure 12-19). To neglect planning such mouth preparations in advance is inexcusable and leads to unsuccessful removable prosthesis service.

Figure 12-19 A, Unmodified buccal surface of the mandibular premolar illustrates a typical height of contour location natural for this tooth (middle and occlusal thirds of the tooth). B, Proximal surface modification is required (hatched region) to produce a guide-plane surface. C, Buccal surface modification is needed to position the height of contour for favorable clasp location. The tooth modification is a continuation of the proximal surface modification onto the buccal surface, and generally requires less than 0.5 mm of tooth removal.

The design of clasps is dependent on the location of the retentive, stabilizing, reciprocal, and supporting areas in relation to a definite path of placement and removal. Failure to reshape unfavorably inclined tooth surfaces and, if necessary, to place restorations with suitable contours not only complicates the design and location of clasp retainers but also often leads to failure of the removable partial denture because of poor clasp design.

A malaligned tooth or one that is inclined unfavorably may make it necessary to place certain parts of the clasp so that they interfere with the opposing teeth. Unparallel proximal tooth surfaces not only will fail to provide needed guiding planes during placement and removal but also will result in excessive blockout. This inevitably results in placement of the connectors so far out of contact with tooth surfaces that food traps are created. To pass lingually inclined lower teeth, clearance for a lingual bar major connector may have to be so great that a food trap will result when the restoration is fully seated. Such a lingual bar will be located so that it will interfere with tongue comfort and function. These are only some of the objectionable consequences of inadequate mouth preparations.

The amount of reduction of tooth contours should be kept to a minimum, and all modified tooth surfaces not only should be repolished after reduction but also should be subjected to fluoride treatment to lessen the incidence of caries. If it is not possible to produce the contour desired without perforating the enamel, then the teeth should be recontoured with an acceptable restorative material. The age of the patient, caries activity evidenced elsewhere in the mouth, and apparent oral hygiene habits must be taken into consideration when one is deciding between reducing the enamel or modifying tooth contours with protective restorations.

Some of the areas that frequently need correction are the lingual surfaces of mandibular premolars, the mesial and lingual surfaces of mandibular molars, the distobuccal line angle of maxillary premolars, and the mesiobuccal line angle of maxillary molars. The actual degree of inclination of teeth in relation to the path of placement and the location of retentive and supportive areas are not readily interpretable during visual examination. These are established during comprehensive analysis of the diagnostic cast with a surveyor, which should follow the visual examination.

Indications for Use of Fixed Restorations

Anterior Modification Spaces

Usually any missing anterior teeth in a partially edentulous arch, except in a Kennedy Class IV arch in which only anterior teeth are missing, are best replaced by means of a fixed restoration. There are exceptions. Sometimes a better esthetic result is obtainable when the anterior replacements are supplied by a removable partial denture, at other times treatment is simplified by inclusion of an anterior modification space into the removable partial denture (Figure 12-21). This is also true when excessive tissue and bone resorption necessitates placement of the pontics in a fixed partial denture too far palatally for good esthetics or for an acceptable relation with the opposing teeth. However, in most instances, from mechanical and biological standpoints, anterior replacements are best accomplished with fixed restorations. The replacement of missing posterior teeth with a removable partial denture is then made much less complicated and gives more satisfactory results.

Figure 12-21 A, Diagnostic waxing of this complex case revealed the best means to manage replacement of tooth #’s 6 and 7 was with a fixed prosthesis, especially since the ridge defect was not severe and the adjacent teeth offered good retainer support. B, In contrast, this complex situation requires the maxillary anterior to be repositioned palatally to address an esthetic concern caused by the condition of the maxillary canines and the need to replace the posterior teeth as well. C, The anterior teeth will be more easily managed as part of the removable partial denture.

(Courtesy Dr. M. Alfaro, Columbus, OH.)

Replacement of Unilaterally Missing Molars (Shortened Dental Arch)

Often the decision must be made to replace unilaterally missing molars (Figure 12-22). The decision must balance the impact of the treatment on the remaining oral structures with the potential benefit to the patient long term. To restore the missing molars with a fixed partial denture would require a cantilever prosthesis or the use of dental implants. A cantilever-fixed prosthesis is most applicable if the second molar is to be ignored, then only first molar occlusion need be supplied with the use of a cantilever-type fixed partial denture. Occlusion need be only minimal to maintain occlusal relations between the natural first molar in the one arch and the prosthetic molar in the opposite arch. The cantilevered pontic should be narrow buccolingually and need not occlude with more than one half to two thirds of the opposing tooth. Often such a restoration is the preferred method of treatment. However, at least two abutments should be used to support a cantilevered molar opposed by a natural molar.

Figure 12-22 A, Unilaterally missing molars. If the patient exhibits opposing contacts to the remaining six posterior teeth (bilateral premolars, right first and second molars), functional gain attained by replacing the left molars may be minimal. B, By contrast, the functional gain resulting from replacement of the posterior occlusion in this patient is likely significant.

To replace unilaterally missing molars with a removable partial denture necessitates the use of a distal extension prosthesis. This involves the major connector joining the edentulous side to retentive and stabilizing components located on the non-edentulous side of the arch. Leverage factors are frequently unfavorable, and the retainers used on the non-edentulous side are often unsatisfactory. Two factors important to consider in making the decision to provide a unilateral, distal extension removable partial denture include the opposing teeth and the future effect of the maxillary tuberosity.

First, the opposing teeth must be considered if it is considered important to prevent extrusion and migration. This influences replacement of the missing molars far more than any improvement in masticating efficiency that might result. Replacement of missing molars on one side is seldom necessary for reasons of mastication alone.

Second, the future effect of a maxillary tuberosity must be considered if concern exists for tuberosity enlargement. Often when left uncovered, the tuberosity increases in size, making future occlusal treatment difficult. However, covering the tuberosity with a removable partial denture base, in combination with the stimulating effect of the intermittent occlusion, helps maintain tuberosity size and position. In such an instance, it may be better to make a removable partial denture with cross-arch stabilization and retention than to leave a maxillary tuberosity uncovered.

Indications for Removable Partial Dentures

Although a removable partial denture should be considered only when a fixed restoration is contraindicated, there are several specific indications for the use of a removable restoration.

Excessive Loss of Residual Bone

The pontic of a fixed partial denture must be correctly related to the residual ridge and in such a manner that the contact with the mucosa is minimal. Whenever excessive resorption has occurred, teeth supported by a denture base may be arranged in a more acceptable buccolingual position than is possible with a fixed partial denture (Figure 12-23).

Figure 12-23 A, Occlusal view of the anterior ridge defect (Kennedy Class IV) shows the palatal position of the ridge crest. Incisal edges of opposing dentition require a more labial position, which would create a difficult pontic form. B, Labial view of the same cast shows the significance of the vertical bone loss. Replacement of the teeth and ridge anatomy is best accomplished with a removable partial denture.

Unlike a fixed partial denture, the artificial teeth supported by a denture base can be located without regard for the crest of the residual ridge and more nearly in the position of the natural dentition for normal tongue and cheek contacts. This is particularly true of a maxillary denture.

Anteriorly, loss of residual bone occurs from the labial aspect. Often the incisive papilla lies at the crest of the residual ridge. Because the central incisors are normally located anterior to this landmark, any other location of artificial central incisors is unnatural. An anterior fixed partial denture made for such a mouth will have pontics resting on the labial aspect of this resorbed ridge and will be too far lingual to provide desirable lip support. Often the only way the incisal edges of the pontics can be made to occlude with the opposing lower anterior teeth is to use a labial inclination that is excessive and unnatural, and both esthetics and lip support suffer. Because the same condition exists with a removable partial denture in which the anterior teeth are abutted on the residual ridge, a labial flange must be used to permit the teeth to be located closer to their natural position.

The same method of treatment applies to the replacement of missing mandibular anterior teeth. Sometimes a mandibular anterior fixed partial denture is made six or more units in length, in which the remaining space necessitates leaving out one anterior tooth or using the original number of teeth but with all of them too narrow for esthetics. In either instance, the denture is nearly in a straight line because the pontics follow the form of the resorbed ridge. A removable partial denture will permit the location of the replaced teeth in a favorable relation to the lip and opposing dentition regardless of the shape of the residual ridge. When such a removable prosthesis is made, however, positive support must be obtained from the adjacent abutments.

Abutments With Guarded Prognoses

If the prognosis of an abutment tooth is questionable or if it becomes unfavorable while under treatment, it might be possible to compensate for its impending loss by a change in denture design. The questionable or condemned tooth or teeth may then be included in the original design and, if subsequently lost, the removable partial denture can be modified or remade (Figure 12-24). Most removable partial denture designs do not lend themselves well to later additions, although this eventuality should be considered in the design of the denture.

Figure 12-24 Kennedy Class II, modification 1, where the molar abutment has a guarded prognosis. A, Anterior abutment of the modification space has a clasp assembly that accommodates for potential future loss of the distal molar while currently providing adequate support, stability, and retention. B, Premolar clasp assembly comprises a mesial rest, a distal guide plane, and a wrought-wire retainer design, which will accommodate future distal extension movement. C, Buccal view shows guide-plane contact and a wrought-wire location that is appropriate for a distal extension.

When the tooth in question will be used as an abutment, every diagnostic aid should be used to determine its prognosis as a prospective abutment. It is usually not as difficult to add a tooth or teeth to a removable partial denture as it is to add a retaining unit when the original abutment is lost and the next adjacent tooth must be used for that purpose.

It is sometimes possible to design a removable partial denture so that a single posterior abutment, about which there is some doubt, can be retained and used at one end of the tooth-supported base. Then if the posterior abutment is lost, it could be replaced by adding an extension base to the existing denture framework. Such an original design must include provisions for future indirect retention, flexible clasping of the future abutment, and provisions for establishing tissue support. Anterior abutments that are considered poor risks may not be so freely used because of the problems involved in adding a new abutment retainer when the original one is lost. It is rational that such questionable teeth should be condemned in favor of more suitable abutments, even though the original treatment plan must be modified accordingly.

Comparative Physical Properties of Gold and Chromium-Cobalt

Chromium-cobalt alloys generally have less yield strength when compared with gold alloys used for removable partial dentures. Yield strength is the greatest amount of stress an alloy will withstand and still return to its original shape in an unweakened condition. Possessing a lower proportional limit, the chromium-cobalt alloys will deform permanently at lower loads than gold alloys. Therefore the dentist must design the chromium-cobalt framework so that the degree of deformation expected in a direct retainer is less than a comparable degree of deformation for a gold component. The modulus of elasticity refers to the stiffness of an alloy. Gold alloys have a modulus of elasticity approximately one-half that for chromium-cobalt alloys for similar uses. The greater stiffness of the chromium-cobalt alloy is advantageous but at the same time offers disadvantages. Greater rigidity can be obtained with the chromium-cobalt alloy in reduced sections in which cross-arch stabilization is required, thereby eliminating an appreciable bulk of the framework. Its greater rigidity is also an advantage when the greatest undercut that can be found on an abutment tooth is in the nature of 0.05 inch. A gold retentive element would not be as efficient in retaining the restoration under such conditions as would the chromium-cobalt clasp arm.

A high yield strength and a low modulus of elasticity produce greater flexibility. The gold alloys are approximately twice as flexible as the chromium-cobalt alloys; in many instances, this provides a distinct advantage in the optimum location of retentive elements of the framework. The greater flexibility of the gold alloys usually permits location of the tips of retainer arms in the gingival third of the abutment tooth. The stiffness of chromium-cobalt alloys can be overcome by including wrought-wire retentive elements in the framework.

The bulk of a retentive clasp arm for a removable partial denture is often reduced for greater flexibility when chromium-cobalt alloys are used as opposed to gold alloys. This, however, is inadvisable because the grain size of chromium-cobalt alloys is usually larger and is associated with a lower proportional limit, and so a decrease in the bulk of chromium-cobalt cast clasps increases the likelihood of fracture or permanent deformation. The retentive clasp arms for both alloys should be approximately the same size, but the depth of undercut used for retention must be reduced by one half when chromium-cobalt is the choice of alloys. Chromium-cobalt alloys are reported to work/harden more rapidly than gold alloys, and this, associated with coarse grain size, may lead to failure in service. When adjustments by bending are necessary, they must be executed with extreme caution and limited optimism.

Chromium-cobalt alloys have a lower density (weight) than gold alloys in comparable sections and therefore are about one half as heavy as gold alloys. The weight of the alloy in most instances is not a valid criterion for selection of one metal over another because after placement of a removable partial denture, the patient seldom notices the weight of the restoration. The comparable lightness of chromium-cobalt alloys, however, is an advantage when full palatal coverage is indicated for the bilateral distal extension removable partial denture. Weight is a factor that must be considered when the force of gravity must be overcome, so that usually passive direct retainers will not be activated constantly to the detriment of abutment teeth.

The hardness of chromium-cobalt alloys presents a disadvantage when a component of the framework, such as a rest, is opposed by a natural tooth or by one that has been restored. We have observed more wear of natural teeth opposed by some of the various chromium-cobalt alloys as contrasted with type IV gold alloys.

It has been observed that gold frameworks for removable partial dentures are more prone to produce uncomfortable galvanic shock to abutment teeth restored with silver amalgam than are frameworks made of chromium-cobalt alloy. This may not be a valid criterion for the selection of a particular alloy when the dentist has complete control over the choice of restorative materials.

Commercially pure (CP) titanium and titanium in alloys containing aluminum and vanadium, or palladium (Ti-O Pd), should be considered potential future materials for removable partial denture frameworks. Their versatility and well-known biocompatibility are promising; however, long-term clinical trials are needed to validate their potential usefulness. Currently, when CP titanium is cast under dental conditions, the material properties change dramatically. During the casting procedure, the high affinity of the liquid metal for elements such as oxygen, nitrogen, and hydrogen results in their incorporation from the atmosphere. As interstitial alloying elements, their deleterious effect on mechanical properties is a problem. Also, reactions between molten titanium metal and the investment refractory produce gases, which cause porosity. With alpha-beta alloys, such as Ti-6Al-4V, a surface skin of alpha titanium can form (alpha-case zone), which has a tremendous effect on electrochemical behavior and mechanical properties. This could be important for small thin structures, such as clasp assemblies and major and minor connectors. The CP grades of titanium have yield strengths that are too low for clinical use as clasps (450 MPa minimum), although the ductility is high. The much higher yield strengths of the Ti-6Al-4V alloys are the same as that of a typical bench-cooled cobalt-chromium alloy, but with far superior ductility. The typical Young’s modulus of elasticity of titanium alloy is half that of cobalt-chromium and just slightly higher than that of type IV gold alloys. This would require a different approach to clasp design than is used with cobalt-chromium alloys, and would present some advantages. Wrought titanium alloy wires are also flexible because of the same low elastic modulus. Beta alloys, which are used in orthodontics, have two-thirds the elastic modulus of CP titanium and Ti-6Al-4V. The joining of titanium by brazing is a problem because like-casting inert atmospheres must be used. The corrosion and fatigue behavior of brazed joints has yet to be tested for long-term corrosion resistance and clinical efficacy. Clinical use has demonstrated reasonable short-term results, but laboratory fabrication difficulties need to be addressed, and long-term advantages over existing alloys must be demonstrated before titanium will gain broad clinical use.

Wrought Wire: Selection and Quality Control

Wrought-wire direct retainer arms may be attached to the restoration by embedding a portion of the wire in a resin denture base, by soldering to the fabricated framework, or by casting the framework to a wire embedded in the wax pattern (Figure 12-25). The physical (mechanical) properties of available wrought wires are most important considerations when a proper wire for the desired method of attachment is selected. These properties include yield strength or proportional limit, percentage elongation, tensile strength, and fusion temperature. After the wire is selected, the procedures to which the wire is subjected in fabricating the restoration become critical. Improper laboratory procedures can diminish certain desirable physical properties of the wrought structure, rendering it relatively useless for its intended purpose. For example, when wrought wire is heated, as in a cast-to or soldering procedure, its physical properties and microstructure may be considerably altered, depending on temperature, heating time, and cooling operation. All manufacturers of wrought forms for dental applications furnish charts listing their products and the physical properties of each product. The percentage of noble metals is given. In addition, most manufacturers designate wires that may be used in a cast-to procedure. American Dental Association (ADA) Specification No. 7 addresses itself to wrought gold wire in terms of both content and minimum physical properties (Table 12-1).

Figure 12-25 The wrought-wire retainer arm has been contoured to design and incorporated into the wax pattern of this frame, where it will become an integral part of the framework. The wire is contoured in two planes and will be mechanically retained in the casting.

Table 12-1 Comparative Specifications Contained in ADA Specification No. 7

ADA, American Dental Association.

The tensile strength of the wrought structure is approximately 25% greater than that of the cast alloy from which it was made. The wrought structure’s hardness and strength are also greater. This means that a wrought structure that has a smaller cross section than a cast structure may be used as a retainer arm (retentive) to perform the same function. It has been suggested that a minimum yield strength of 60,000 psi is required for the retentive element of a direct retainer. A percentage elongation of less than 6% is indicative that a wrought wire may not be amenable to contouring without attendant undesirable changes in microstructure.

Regardless of the method of attaching the wrought-wire retainer that is used, that is, embedding, soldering, or cast-to, tapering the wrought arm seems most rational. A retainer arm is in essence a cantilever that can be made more serviceable and efficient by tapering. Tapering to 0.8 mm permits more uniform distribution of service stresses throughout the length of the arm, being readily demonstrated by photoelastic stress analysis. Uniform tapering of an 18-gauge, round wire arm can be accomplished by rapidly rotating the wire in angled contact with an abrasive disk in the dental lathe. It is then polished by rotating the wire in angled contact with a mildly abrasive rubber disk in the dental lathe. The appropriate taper is shown in Figure 12-26.

Figure 12-26 Round, 18-gauge wrought wire for the retentive component of the direct retainer assembly (clasp) is uniformly tapered to 0.8 mm from its full diameter to its terminus. Tapering should precede contouring of the wire for the retainer arm.