THE SKELETON AND JOINTS

The skeleton comprises the metacarpal bones and the proximal, middle, and distal phalanges. The metacarpo-

Figure 23-19 Transverse section of the right forearm 6 cm proximal to the proximal border of the accessory carpal, to demonstrate the topography of the accessory (check) ligament (4) of the superficial digital flexor; looking distally.

phalangeal and the proximal and distal interphalangeal joints linking these bones are commonly referred to as the fetlock, pastern, and coffin joints. A pair of proxi­mal sesamoid bones enlarges the concavity of the fetlock joint, and a single distal sesamoid bone enlarges that of the coffin joint.

The metacarpal skeleton comprises second, third, and fourth metacarpal bones. The third bone, the cannon bone, is much stronger than the other two and is the functional element. It carries a prominent tuberosity on its dorsal surface just distal to the joint. The bones to each side, generally known as the splint bones, are much reduced in size. Each has a small proximal base that continues into a tapering shaft. In young animals the splint and cannon bones are joined by fibrous tissue; this generally later ossifies, and the upper parts of the shafts are then fused together. The process is often accompanied by an acute inflammation (a condition known as “splints”), which leaves a palpable—and often visible—blemish on the dorsal surface.

The tapering second and fourth metacarpals end in slight but easily palpable buttons three quarters of the way down the cannon (see Figure 2-49, B). The lower parts of their shafts are free, and when a break occurs, it is a simple matter to remove the fragment below the fracture line.

The third metacarpal bone is exceptionally robust. It is oval in cross section (which distinguishes it from the longer but more rounded cannon bone of the hindlimb), and its thick compacta attests to its tremendous strength; it is in fact one of the strongest elements of the skeleton (see Figure 23-45/2).

The distal extremity presents an axially keeled condyle that articulates with the proximal phalanx and the paired sesamoid bones. When viewed from the side, the condyle encompasses some 220° of a circle, which is evidence of the great range of flexion and extension— the only movements allowed. The articular surface to each side of the keel is interrupted by a slight ridge that separates the more strongly curved palmar area from the larger dorsal one. Despite the obvious strength of the cannon bone, longitudinal fractures of the distal extremity are common racing injuries, more often involving the lateral than the medial side and the fore­limb rather than the hindlimb. The degree of involve­ment of the joint surface is an important factor in prognosis.

The proximal sesamoid bones are three-sided pyra­mids whose bases face distally (Figure 23-20/20). The dorsal (articular) surface of each lies against the condyle, the palmar (flexor) surface tilts axially and faces the flexor tendons that ride over it, and the abaxial surface is hollowed for the reception of the thick branch of the interosseous (see further on). The palmar aspects of the bones are converted by thick fibrous tissue (palmar liga­ment) into a single bearing surface over which the flexor tendons change direction. Although close to the proxi­mal phalanx, the sesamoid bones do not articulate with it.

The proximal sesamoids fracture most often of all the bones in the forelimb, followed in frequency by the metacarpal and carpal bones.

The strong proximal phalanx (PI for short) is com­pressed from front to back and is wider proximally than distally. Its proximal extremity is hollowed and deep-

Figure 23-20 Skeleton of the distal part of the forelimb. A, Left limb, dorsal view. B, Palmar view. 1, Radius; 2, radial carpal; 3, intermediate carpal; 4, ulnar carpal; 5, 6, 7, second, third, and fourth carpals; 8, large metacarpal bone; 8', meta­carpal tuberosity; 9, 9', medial and lateral splint bones; 10, proximal sesamoid bones; 11, proximal phalanx; 11', proximal tubercle; 11", attachment of distal digital annular and abaxial palmar ligaments; 11'", attachment of axial palmar and oblique sesamoidean ligaments; 12, middle phalanx; 12’, attachments of collateral ligament of coffin joint; 12", bearing surface for deep flexor tendon; 13, distal phalanx; 13', exten­sor process; 13", parietal groove; 14, navicular bone; 15, sole foramen and semilunar crest for attachment of deep flexor tendon; 16, palmar process and attachment of distal navicular ligament.

ened axially by a groove that allows it to conform to the condyle of the large metacarpal bone. Palpable tuber­cles to each side receive the collateral ligaments of the fetlock joint. The distal end is shaped as two condyles separated by a shallow axial groove and presents similar but smaller tubercles for the collateral ligaments of the pastern joint. The palmar surface of the bone is rough­ened for the attachment of several ligaments; a large triangular area and various smaller ones to each side stand out (see Figure 23-20H1,1Γ,1Γ',11'").

The middle phalanx (PII) is generally similar to PI but, being only half as long, is proportionately very robust. Both extremities are of equal width. The proxi­mal articular surface—hollowed with a slight axial ridge—is the reciprocal of the lower end of PI, whereas the distal one—two condyles separated by a groove— mimics that of PI.

The distal phalanx (PIII, coffin bone) generally con­forms to the interior of the hoof in which it resides, “as in a coffin.” It is wedge shaped: sharp distally and to the sides and blunt proximally and toward the back. The dorsal (parietal) surface is convex from side to side and lies against the dermis that unites it to the inner surface of the hoof wall. It tapers caudally into medial and lateral palmar processes that are notched (or per­forated) and grooved for the dorsal terminal branches of the digital arteries and accompanying nerves (Figure 23-20/13"). Depressions for the collateral ligaments of the coffin joint are present proximodorsal to the pro­cesses. The palmar (sole) surface is slightly concave to fit the domed sole of the hoof. Both parietal and sole surfaces are very porous to allow the passage of numer­ous small arteries from the interior of the bone into the overlying dermis. The articular surface faces proximally; it is very similar to the proximal articular surface of PII, consisting of two fossae separated by an axial ridge. Its dorsal border tapers to an extensor process, the highest point of the bone, where the common digital extensor tendon is attached. The palmar border is extended by a narrow articular zone for the distal sesamoid bone, which, in contrast to the proximal sesamoids, articulates with both major bones of the joint.

Figure 23-21 Hoof cartilage attached to palmar process of distal phalanx. 1,2,3, Proximal, middle, and distal phalanges; 4, hoof cartilage.

within the bone; this contains the anastomosis of the terminal palmar branches of the digital arteries. The deep flexor tendon ends on the semilunar crest just distal to the foramina (Figure 23-20/75).

The flat cartilages (of the hoof), which surmount and continue the palmar processes, lie mainly against the inner wall of the hoof, but their proximal borders are free, subcutaneous, and palpable to each side of the pastern joint (Figure 23-20, B/14).

The distal sesamoid (navicular) bone (Figure 23­21/5) is boat shaped with straight proximal and convex distal borders. Its dorsal (articular) surface contacts the distal end of PII; a narrow distal facet touches PIII. The palmar (flexor) surface faces the wide tendon of the deep flexor, providing it with yet another bearing surface as it bends toward the semilunar crest on the undersur­face of PIII. The navicular bone enlarges the distal articular surface of the coffin joint (see Figure 23-24/7',7").

The fetlock joint is formed between the large meta­carpal bone, PI, and the proximal sesamoid bones (Figure 23-22). The large bones are connected by medial and lateral collateral ligaments, while additional smaller and triangular (collateral) ligaments anchor the sesa­moid bones to the sides of the metacarpal condyle and the proximal tubercles of PI. A series of sesamoidean

Figure 23-22 Lateral radiograph of fetlock joint and digit. 1, Large metacarpal bone; 2, proximal sesamoid bones; 3, proximal phalanx; 4, middle phalanx; 5, distal phalanx; 5', extensor process; 5", canal containing terminal arterial arch; 6, navicular bone; 7, wall of hoof.

ligaments connects the bases of the sesamoid bones to the first phalanx and ensures that the sesamoids move against the metacarpal condyle in unison with PI. The deepest ligaments are short and pass to the proximopal- mar border of PI; they are overlain by rather longer cruciate ligaments that end a little more distally, and these in turn are overlain by oblique ligaments that attach broadly to the central triangular area of the palmar surface of the same bone. Finally, an additional straight sesamoidean ligament, arising from the bases of the sesamoids, connects with the complementary fibrocartilage of PII (Figure 23-23/4). The cruciate, oblique, and straight ligaments are mentioned again in connection with the action of the interosseus.

The sesamoid bones are connected to each other by a thick palmar ligament that extends the bearing surface for the flexor tendons proximally by about 2 cm (Figure 23-23/2). This extension supports the tendons when the sesamoids themselves slip below the condyle in maximal overextension of the fetlock joint (when the dorsal angle can be as small as 90°). When the joint is fully flexed, the sesamoid bones lose contact with the condyle and ride up on the back of the metacarpal bone, where bone-to-bone contact is prevented by the proximal extension of the palmar ligament.

The joint capsule is capacious, and to allow for the fetlock’s mobility, it extends large dorsal and palmar pouches proximally (see Figure 23-27/7). These lie against the shaft of the metacarpal bone and are easily punctured from the side; the end of the splint bone, the

Figure 23-23 A, Structures supporting the fetlock joint. 1, Interosseus; 2, proximal sesamoid bones connected by thick palmar ligament; 3, collateral sesamoidean ligament; 4, straight sesamoidean ligament; 5, oblique sesamoidean liga­ment; 6, stump of superficial flexor; 7, 7’, axial and abaxial palmar ligaments of pastern joint; 8, hoof cartilage; 9, stump of deep flexor. B, Real specimen.

interosseus, and the sesamoid bone are convenient (almost visible) landmarks for entry into the palmar pouch. An other perhaps better place in the bowed limb is between the sesamoid bone and the metacarpus, directly through the collateral ligament of the flexed joint (see Figure 23-26, B-C/11). Distentions of the joint known as “wind puffs” or “galls” manifest them­selves at this site. The interior of the dorsal pouch con­tains a so-called capsular fold (see Figure 23-27/7'). This arises from the shaft of the metacarpal bone and projects distally into the center of the pouch; its inflam­mation and enlargement can cause lameness. Short distal palmar pouches are palpable as small depressions in the angles between PI and the bases of the sesamoid bones.

The movement of the pastern joint is much more restricted. Paired (axial and abaxial) palmar ligaments connect the palmar aspect of PI with the complemen­tary Abrocartilage of PII (Figure 23-23/7,7'); together with the straight sesamoidean ligament (Figure 23-23/4) they limit overextension. The capsule is similar to that of the fetlock joint, but the pouches are smaller and only the dorsal one is accessible for puncture, again from the side. The radiographic appearance of the pastern and coffin joints is shown in Figure 23-24, A).

The coffin joint allows flexion and extension to about the same degree as the pastern joint. The collateral liga­ments are short and thick and are solidly anchored at both ends to depressions in the bones. The navicular bone, an integral part of the joint, is suspended from the distal extremity of PI by the collateral navicular ligaments (Figure 23-25/2). These cross the medial and lateral borders of PII and attach to the ends and proxi­mal border of the navicular bone in a U-shaped fashion. A very short but wide distal navicular ligament (Figure 23-25/3) connects the distal border of the bone with PIII, attaching proximal to the prominent sole foram­ina. The capsule attaches to the articular margins of the three bones and resembles those of the other digital joints in having dorsal and palmar pouches. The pouches are small, and only the dorsal one is accessible for punc­ture (at the proximal border of the hoof); the procedure is not easy (see Figure 23-27, C-D).

The incorporation of sesamoid bones in the fetlock and coffin joints divides the weight pressing onto the lower part of each joint over two bones, phalanx and sesamoid. The elasticity of the sesamoid ligaments and the flexor tendons behind them allows the joint to yield slightly during foot impact. This is but one of several mechanisms designed to dissipate the concussion gener­ated by so heavy and swift an animal. The concussive effects may be accentuated by poor conformation: upright pasterns and small feet (in relation to body size) are a combination encountered frequently in animals afflicted with navicular disease, a relatively common cause of lameness. This condition is characterized by erosion at the margins of the navicular bone, where its ligaments attach, and by inflammation and degen­eration of the navicular bursa (Figure 23-27/10) and the related part of the deep flexor tendon (Figure 23­27/13). However, the exact pathogenesis is still debated, and different authorities give quite contradictory explanations.