Chapter 9 Applied Anatomy of the Musculoskeletal System

It is beyond the scope of this book to describe all aspects of musculoskeletal anatomy in depth, yet a detailed knowledge of anatomy is fundamental to a lameness diagnostician, as highlighted in the chapters on observation and palpation (see Chapters 5 and 6). Some aspects of anatomy are considered in depth in individual chapters dealing with conditions of specific areas. This chapter considers some philosophical aspects of the importance of anatomical knowledge and describes some basic principles. It also provides illustrations that we hope will help the reader to understand better the three-dimensional aspects of anatomy.

Accurate interpretation of what we see and feel during an examination requires knowledge of what structures we are looking at and palpating. For example, a swelling is noted over the dorsal aspect of the carpus. Is the swelling diffuse and possibly related to a hygroma, periarticular edema, or cellulitis, or is there a discrete swelling, horizontally oriented, reflecting distention of the middle carpal joint? Or is it a longitudinal swelling reflecting distention of the common digital extensor tendon sheath or the tendon sheath of the extensor carpi radialis? If the swelling is longitudinal, are any compressions in the swelling caused by normal retinaculum or adhesions within the sheath (Figure 9-1)? If we examine the sheath by ultrasonography, is the echogenic band extending from the sheath wall to the enclosed tendon normal mesotendon, or is it an adhesion? If diffuse swelling is present around the dorsal aspect of the carpus associated with lameness, how can we tell if the middle carpal joint capsule is distended? We need to know that there is a palmar outpouching of the middle carpal joint on the palmarolateral aspect of the carpus, just distal to the accessory carpal bone. Thus during visual inspection and palpation the clinician should be constantly asking, “What structure am I seeing or palpating, what are its functions, and what would be the consequences of loss of function?” If it has abnormal contour or size, is this the result of swelling of that structure or of an adjacent or underlying structure? Having established what structure is abnormal, the clinician then must consider the best imaging modality. If it is a tendonous or ligamentous structure, ultrasonography probably will provide the most information, but we must remember that it has bony attachments, and damage at those attachments might best be assessed by either radiology or nuclear scintigraphy. So we need to know not only what each structure is, but also the structures to which it is attached.

During visual inspection and palpation, we also need to think logically. We know that the superficial and deep digital flexor tendons (SDFT, DDFT), the accessory ligament of the DDFT (ALDDFT), and the suspensory ligament (SL) lie on the palmar aspect of the third metacarpal bone (McIII) (Figure 9-2). Swelling confined to just the medial aspect of the metacarpal region is far more likely to reflect direct trauma to the medial aspect of the limb than sprain or strain of any of the ligamentous or tendonous structures. We need to know that the proximal aspect of the SL lies between the bases (heads) of the second and fourth metacarpal bones and therefore is inaccessible to direct palpation, and that desmitis often may be present without discernible soft tissue swelling (Figure 9-3).

We must be aware of anatomy to realize the possible consequences of trauma to an area. The paucity of soft tissues over the cranial aspect of the stifle makes the patella and the tibial tuberosity vulnerable to direct trauma, hence the risk of fracture after hitting a fixed fence. The lack of soft tissues also means that if the horse hits a thorn hedge, the possibility of a thorn penetrating the femoropatellar joint capsule, resulting in contamination and infection, is quite high. We also need to think about how structures move relative to one another while the horse is in motion. If a steeplechase horse sustains an interference injury on the palmar aspect of the metacarpal region while galloping, the position of the skin laceration probably will not coincide with the level of the laceration in the SDFT (Figure 9-4). We also need to know the relative positions of the laceration and the digital flexor tendon sheath to be aware of the likelihood that the sheath may have been traumatized, and thus the risk of infectious tenosynovitis. Faced with a contaminated wound on the dorsal aspect of a hind fetlock and severe lameness, and the possibility of infection of the metatarsophalangeal joint, we need to know where to expect to see distention of the plantar pouch of the joint capsule and to know that this site is safely accessible for arthrocentesis.

A fundamental principle of lameness investigation is the identification of the source or sources of pain. Although this may be possible through detailed clinical examination, in many instances it is essential to perform diagnostic analgesia (see Chapter 10). A detailed knowledge of the anatomy of nerves, joint capsules and the various outpouchings, tendon sheaths, and bursae is fundamental to safe, accurate performance of perineural and intrasynovial injections.

Given the knowledge of the close relationship among the distal interphalangeal joint capsule, the distal sesamoidean impar ligament, the collateral sesamoidean ligaments and the distal phalanx, and the close proximity of branches of the palmar digital nerve, it is not surprising that intraarticular analgesia is not specific and that other structures can be affected, especially if interpretation of the response is delayed or an excessively large volume of local anesthetic solution is used. Knowledge that the medial and lateral femorotibial joints do not normally communicate and that the cruciate ligaments usually are extraarticular structures is crucial for an understanding of why these joint compartments must be injected separately, and why the response to intraarticular analgesia may be both incomplete and delayed if a cruciate ligament is damaged.

Knowledge of functional neuroanatomy also is important for interpretation of specific gait abnormalities. Inability to bear weight on a hindlimb after general anesthesia may be the result of myopathy, but in the absence of marked pain and distress, it is more likely that the horse has lost extensor function and is unable to extend any of the hindlimb joints because of femoral nerve paresis. Loss of ability to extend the elbow may result from loss of triceps function associated with a fracture of the olecranon but may also be caused by radial nerve paresis.

Vascular anatomy is important because many nerves lie close to vessels. With superficial nerves and vessels, identification of the vessel may facilitate palpation of the nerve and thus aid accurate perineural injection. Avoiding penetration of the vessel and causing hematoma formation also is desirable. With regard to deeper nerves the veterinarian may benefit by knowing that the needle must be in close proximity to the nerve if blood appears in the needle hub. This information can be helpful when performing perineural analgesia of the deep branch of the fibular nerve.

Assessment of digital pulse amplitudes is an integral part of palpation. Increased pulse amplitude usually signifies a site of inflammation at, or distal to, the region of palpation, especially in association with inflammatory conditions of the foot, such as subsolar abscessation or laminitis. Palpation of the pulse in the dorsal metatarsal artery and assessment of saphenous vein filling can be helpful in the evaluation of a horse with suspected aortoiliac thrombosis.

Knowledge of the sites of major vessels is important when considering the consequences of major laceration to an area and possible avascular areas, and in planning a surgical approach to an area. All bones have one or more nutrient foramina through which major vessels enter. These usually are in standard locations (Figure 9-5). Knowledge of these sites is critical for accurate radiological interpretation because a nutrient foramen appears as a radiolucent area, which should not be confused with a pathological lesion. The position of these intraosseous vessels also has important consequences in considering repair of major long bone fractures.

Thermography relies on the detection of surface heat and is obviously greatly influenced by the position of superficial vessels. Interpretation may be misleading without knowledge of location. Thus it should be absolutely clear that anatomy is a dynamic subject and is not merely a function of knowing the origins and insertions of numerous structures.

We also need to know some fundamentals of biomechanics. What is the biomechanical function of the SL? What are the implications of loss of function? For example, how may function be altered by a change in foot angle after application of a heel wedge? How is load in the distal limb joints affected by mediolateral foot imbalance? If the accessory ligament of the SDFT is cut (superior check desmotomy), how does this alter the function of not only the SDFT but also other tendonous and ligamentous structures? Does consequent overload of the SL predispose to an increased risk of suspensory desmitis? When orthopedic surgery is being considered, which is the tension side of the bone, to which a bone plate should be applied to take advantage of the tension band principle?

In more general terms, how will lameness in the left hindlimb alter forces in the other limbs, and does this vary with the gait? Given the reciprocal apparatus of the hindlimb and the inability to flex and extend the limb joints independently, it is not surprising that the gait characteristics of hindlimb lameness are so similar, irrespective of the source of pain causing lameness. Understanding the reciprocal apparatus in addition to the results of loss of its function (e.g., after damage to the fibularis tertius) is hugely important for an understanding of hindlimb lameness.

After the source of pain causing lameness has been isolated, then it is necessary to establish what is causing pain; this requires one of a number of imaging modalities: radiography, ultrasonography, nuclear scintigraphy, magnetic resonance imaging (MRI), computed tomography (CT), and exploratory arthroscopy, bursoscopy, or tenoscopy. Accurate interpretation of the findings from any of these techniques requires specialist anatomical knowledge. With radiographic images, various structures are superimposed, resulting in potentially confusing radiolucent lines that can mimic a fracture (e.g., in the relatively complex carpus and tarsus). A frog shadow superimposed over the navicular bone may mimic a fracture. We must be cognizant of anatomical variations, for example, the shape and size of the crena of the distal phalanx. We have to know how best to image a specific anatomical location, such as the sustentaculum tali of the calcaneus (fibular tarsal bone) using a skyline projection. For interpretation of the clinical significance of periosteal or entheseous new bone, detailed knowledge of the soft tissue structures that do (or do not) attach in that area is vital. Particularly in the fetlock and pastern areas, numerous ligamentous structures have discrete areas of attachment (Figure 9-6).

Radiography requires the awareness that we are looking at a three-dimensional structure in two dimensions, and thus images of the area must be obtained from several different angles. With ultrasonography, and more particularly with MRI and CT, structures can be imaged in three dimensions; this requires detailed knowledge of the shape, size, and relationships among structures. In the proximal metatarsal region the DDFT lies more medial than the SDFT and SL, and thus these structures cannot be imaged adequately by ultrasonography at the same time from the plantar aspect of the limb (Figure 9-7). The transducer must be moved to a plantaromedial site to evaluate the DDFT in its entirety. A large vessel on the plantarolateral aspect of the SL can cause shadowing artifacts in the SL.

The internal architecture of the joint becomes important during exploratory arthroscopy. What are the normal variations in cartilage thickness? Where do you expect to see a synovial fossa? Which parts of the synovial membrane are usually more vascular? Is it normal that the cranial cruciate ligament can be seen without synovial covering from the medial femoral tibial joint?

A textbook of this type cannot possibly provide detailed descriptions of all aspects of anatomy, functional anatomy, and biomechanics, nor answer all of the questions posed earlier in this chapter. It is hoped that this overview will stimulate readers to have a thirst for more knowledge of these subjects, in the understanding of their huge importance.

Lameness clinicians are encouraged to acquire a set of boiled-out bones for reference and perform detailed dissections of cadaver limbs to improve knowledge of anatomy. Practicing nerve block techniques on cadaver limbs is very important for inexperienced clinicians or those performing a new block for the first time. If a lame horse must be humanely destroyed, clinicians should take the opportunity, whenever possible, to perform a postmortem examination to correlate clinical findings with the actual lesions and revise anatomy at the same time. Each time a dissection is performed, new anatomical detail becomes apparent that previously may have been missed.

The remainder of this chapter provides some basic definitions of anatomical terms used elsewhere in the book, describes the reciprocal apparatus of the forelimb and hindlimb, and presents correlative illustrations of anatomical specimens and images of those areas to assist in the understanding of three-dimensional anatomy.