Functional Anatomy of the Equine Musculoskeletal System
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necessary to prevent overextension of the pastern joint. Contraction of the superficial digital flexor muscle tightens its tendon’s insertions on the proximal end of the middle phalanx, preventing the pastern joint from buckling. The suspensory apparatus of the fetlock and the digital flexor tendons ensure that overextension of the fetlock joint, i.e., decreasing the dorsal articular angle, is minimized when the hoof strikes the ground. Yet at a gallop, when all of the horse’s weight is on one forelimb momentarily, the palmar aspect of the fetlock comes very close to the ground. During this descent of the fetlock, the coffin joint is flexed by the deep digital flexor tendon.
Metacarpus The equine metacarpus consists of the large third metacarpal (cannon) bone, the second (medial) and fourth (lateral) small metacarpal bones (splint bones), and the structures associated with them. The shaft of each small metacarpal bone is united by an interosseous ligament to the large metacarpal bone. The cortex under the rounded dorsal surfaces of the metacarpal bones is thicker than the cortex under their concave palmar surfaces. Length and curvature of the shafts and the prominence of the free distal extremities (“buttons”) of the small metacarpal bones are variable. The proximal extremities of the metacarpal bones articulate with the distal row of carpal bones; the second metacarpal articulating with the second and third carpals; the third metacarpal articulating with the second, third, and fourth carpals; and the fourth metacarpal with the fourth carpal bone. Dorsal Aspect The skin, fascia, and digital extensor tendons on the dorsal aspect of the metacarpus receive their blood supply from small medial and lateral dorsal metacarpal arteries originating from the dorsal carpal rete and descending between the large metacarpal bone and the respective medial or lateral metacarpal bone. Innervation to this region is furnished by the medial cutaneous antebrachial nerve (Figures 1.14 and 1.20) and the dorsal branch of the ulnar nerve (Figures 1.18 and 1.21). Deep to the skin the main tendon of the common digital extensor muscle inclines proximolaterad from its central position at the fetlock across the dorsal surface of the third metacarpal bone. Proximally the main tendon and the accompanying tendon of the radial head of the common digital extensor lie lateral to the insertional tendon of the extensor carpi radialis muscle on the prominent metacarpal tuberosity of the third metacarpal bone (Figure 1.20). The tendon of the lateral digital extensor muscle is lateral to the common extensor tendon, and the small radial tendon of the latter usually joins the lateral digital extensor tendon. Occasionally the radial tendon pursues an independent course to the fetlock. A strong fibrous band from the accessory carpal bone reinforces the lateral digital extensor tendon as it angles dorsad in its descent from the lateral aspect of the carpus.
Figure 1.18. Caudal view of left carpus and metacarpus; most of the digital flexor tendons are removed.