Overview
The hind paw of the dog normally has four functional digits (II to V), each with three phalanges: proximal, middle and distal. Digit I (the dewclaw) is often absent on the pelvic limb; when present it usually has only two phalanges and may be attached to the rest of the paw by skin alone. Digits III and IV are the longest and take most of the load. The arrangement is the same as in the forepaw, and the NAV terms for the hind paw use pedis instead of manus.
Proximal phalanx (phalanx proximalis)
The longest. Its base has a concave articular surface that receives the head of the metatarsal; its distal head carries a trochlea for the middle phalanx. The collateral ligaments and the proximal sesamoids attach at the metatarsophalangeal joint.
Middle phalanx (phalanx media)
Shorter than the proximal one, of similar shape. The tendon of the superficial digital flexor inserts on its base after splitting into two branches to let the deep flexor pass.
Distal phalanx (phalanx distalis)
Housed inside the claw. It shows:
- Ungual process (processus unguicularis): curved, conical projection over which the claw (unguis) is moulded.
- Ungual crest (crista unguicularis): bony rim surrounding the base of the process, where the claw fold of the skin attaches.
- Flexor tubercle (tuberculum flexorium): on the plantar surface, insertion of the deep digital flexor tendon.
- Extensor process: dorsal, insertion of the extensor tendon.
Sesamoid bones
- Proximal (plantar) sesamoids: one pair for each of digits II to V, on the plantar side of the metatarsophalangeal joint, embedded in the tendons of the interosseous muscles. They form a groove for the flexor tendons.
- Dorsal sesamoids: small, in the extensor tendon over the same joint; often cartilaginous.
- At the distal interphalangeal joint there is a plantar sesamoid cartilage, not a bone.
Joints and movement
The interphalangeal joints are hinges. The distal one is held in slight hyperextension (claw raised) by elastic dorsal ligaments, a mechanism far less developed than in the cat, so the dog's claw is not retractile.
Clinical relevance
- Digital fractures and luxations: common in working dogs and greyhounds ("knocked-up toe" from proximal interphalangeal luxation).
- Sesamoid disease: fragmentation or fracture of the proximal sesamoids, especially in Rottweilers and greyhounds; better documented in the forepaw than in the hind paw.
- Subungual tumours: squamous cell carcinoma and melanoma; they present as a broken claw that does not heal or a swollen digit, and radiographs show lysis of the distal phalanx. Treatment is digit amputation.
- Hind dewclaws: often removed in puppies because they tend to catch and tear.