Overview
The claw of the dog (unguicula) is a modified skin structure: a capsule of hard keratin that wraps the ungual process (processus unguicularis) of the distal phalanx. At its base, the ungual crest (crista unguicularis) of the phalanx forms a bony shelf that covers the proximal edge of the claw.
The thoracic limb has five clawed digits. Digit I is the dewclaw: it sits on the medial side, above the ground, and does not bear weight. The pelvic limb usually has four digits; digit I is absent or vestigial, although some breeds have hind dewclaws, sometimes double.
Structure
- Wall: forms the dorsal and side surfaces of the claw; it is curved and laterally compressed.
- Sole (solea unguiculae): palmar or plantar surface of softer keratin, between the edges of the wall.
- Claw dermis: highly vascular, innervated connective tissue between the phalanx and the keratin. It is the living part of the claw (the "quick"). In unpigmented claws it shows as a pink zone.
- Growth: the claw grows continuously from its base, under the fold of skin around it, and is worn down by use.
Clinical relevance
- Overgrowth: in dogs with little exercise or on soft surfaces the claw does not wear, curls and alters how the digit bears weight. The dewclaw, which never touches the ground, can grow round until it pierces the pad.
- Nail trimming: the cut must stay distal to the quick. In white claws it can be seen; in pigmented claws trim in small steps. The quick lengthens as the claw grows, so frequent trims let it recede gradually. Cutting into the quick causes pain and bleeding, controlled with pressure or a styptic powder.
- Other disorders: claw fractures and avulsions (common in the dewclaw), paronychia (infection of the claw fold), symmetric lupoid onychodystrophy and tumours of the nail bed such as squamous cell carcinoma and digital melanoma.