Overview
The cat's forepaw has five digits. Digit I (the dewclaw) is short, does not touch the ground and has two phalanges; digits II to V have three phalanges: proximal, middle and distal. The cat bears weight on digits II to V through the digital pads.
Key features
- Proximal phalanx (phalanx proximalis): the longest; articulates with the head of the metacarpal and with the proximal sesamoid bones.
- Middle phalanx (phalanx media): shorter and asymmetrical. At rest the distal phalanx folds dorsally alongside it.
- Distal phalanx (phalanx distalis): carries the claw. It has:
- Ungual process (processus unguicularis): curved and laterally compressed, it fits inside the horny sheath of the claw.
- Ungual crest (crista unguicularis): a bony fold that wraps the base of the claw like a sheath; the germinal tissue that produces the claw lies beneath it.
- Flexor tubercle: on the palmar surface, insertion of the deep digital flexor tendon.
- Dorsal elastic ligaments: run from the middle phalanx to the ungual crest of the distal phalanx and hold the claw retracted with no muscular effort. When the deep digital flexor contracts, the distal phalanx rotates palmarly, the ligaments stretch and the claw comes out (protraction).
- Sesamoid bones: proximal (two per digit, at the metacarpophalangeal joint) and dorsal.
Clinical relevance
- Onychectomy ("declawing"): it is not a nail trim but the amputation of the distal phalanx of each digit. If germinal tissue of the ungual crest is left behind, the claw can regrow abnormally. It causes pain, lameness and retained bone fragments; it is banned in many countries and discouraged by the main veterinary associations.
- High-rise syndrome: the distal limbs are among the regions injured, together with the face and thorax.
- Lung-digit syndrome: primary lung carcinoma of the cat can metastasise to the distal phalanges, with swelling and bone lysis in one or several digits.
- Polydactyly: inherited extra digits, with a higher risk of ingrown claws.