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.

Fig. 9-1 Sagittal anatomical section through the carpus, transecting the extensor carpi radialis tendon (ECRT). C3, Third carpal bone; ICB, intermediate carpal bone.

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).

Fig. 9-2 Sagittal views of the palmar metacarpal region. Proximal is to the right. A, FreeStyle Extended Imaging (Sequoia model, Acuson, Mountain View, California, United States) ultrasonographic image of the palmar metacarpal region. B, Corresponding anatomical section. DDFT, Deep digital flexor tendon; ICL, accessory ligament of the DDFT (inferior check ligament); SDFT, superficial digital flexor tendon; SL, suspensory ligament.

Fig. 9-3 Transverse sections through the proximal metacarpal region. Dorsal is to the top and lateral is to the left. A, Anatomical specimen. B, Computed tomographic scan using soft tissue windowing. DDFT, Deep digital flexor tendon; ICL, accessory ligament of the deep digital flexor tendon (inferior check ligament); SDFT, superficial digital flexor tendon; SL, suspensory ligament.

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.

Fig. 9-4 Lateral scintigraphic image of the metacarpal region acquired before the end of the flow (vascular) phase and at the beginning of the pool phase. This jumper had a history of low-grade chronic desmitis of the proximal aspect of the suspensory ligament (SL), and mild diffuse superficial digital flexor (SDFT) tendonitis. An acute interference injury to the midmetacarpal region was evident on the skin. Note the proximal location of the acute injury to the SDFT. The linear area of uptake between the SL and SDFT is vascular artifact related to the time of acquisition.

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.

Fig. 9-5 Sagittal anatomical section through the pastern demonstrating a common location for the nutrient foramen (NF) entering the proximal phalanx (P1). MC3, Third metacarpal bone; P2, middle phalanx.

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.

Stay updated, free articles. Join our Telegram channel

Join