Ulnar claw

An ulnar claw is an abnormal hand position that develops due to a problem with the ulnar nerve. It should not be confused with the hand of benediction which results from a median nerve injury, and presents as an inability to flex the three most radial digits when asked to make a fist. The "hand of benediction" is so named because a similar posture appears in some Christian paintings.

Presentation

The hand will show hyper-extension of the metacarpophalangeal joints (MCP) and flexion at the distal and proximal Interphalangeal (IP) joints of the 4th and 5th digits (ring and little finger) [http://www.wheelessonline.com/ortho/ulnar_nerve Wheeless' Textbook of Orthopaedics - Ulnar Nerve] ] .The clawing will become most obvious when the person is asked to straighten their fingers.

Pathogenesis

An ulnar claw will follow an ulnar nerve lesion. The clawing comes about due to the actions of the extrinsic muscles of finger movement (long flexors and extensors of the forearm) being unopposed by the non-functioning medial two lumbricals which were under ulnar innervation. To help understand this think of what it is the lumbricals do. They flex the MCP joint and extend both IP joints (the so-called "Writer's Upstroke"), so it makes sense that the loss of the lumbricals will manifest as the opposite. The index and middle fingers remain normal because the lateral two lumbricals are under median nerve control.

Ulnar Paradox

The ulnar paradox is that with a more proximal lesion to the nerve, where more muscles lose innervation, the clawing is not as severe.

Pathogenesis

In distal ulnar nerve lesions (as described above) we only loose the function of the intrinsic muscles of the hand which are supplied by the ulnar nerve. If the ulnar nerve is damaged at the elbow, as well as losing most intrinsic muscles of the hand, we also lose the function of the forearm flexors which are innervated by the ulnar nerve (namely the medial half of Flexor Digitorum Profundus (FDP) and Flexor Carpi Ulnaris). The loss of function of the medial half of FDP results in less flexion being exerted on the 4th and 5th digits, thus less clawing, when the ulnar nerve is damaged at the elbow.

Other claw hands

Median claw

Caused by median nerve lesions. The hand will show hyper-extension of the metacarpophalangeal joints (MCP) and flexion at the Interphalangeal (IP) joints of the 2th and 3rd digits (index and middle). The pathogenesis is similar to that of ulnar clawing (loss of the relevant lumbricals and unopposed action of forearm flexors and extensors).

Dupuytren's contracture

Dupuytren's contracture is a deformity of the hand due to thickening and fibrosis of the palmar aponeurosis and eventual contracture of the 4th and 5th digits. Presenting as a small hard nodule in the base of the ring finger, it tends to affect the ring and little finger as puckering and adherence of the palmar aponeurosis to the skin. Eventually the MCP and IP joints of the 4th and 5th digits become permanently flexed. This claw appearance can be distinguished from an ulnar claw in that the MPC is flexed in Dupuytren’s but hyperextended in ulnar nerve injuries.

References

#Gray's Anatomy for Students
#Illustrated Clinical Anatomy
#McMinn's Clinical Atlas of Human Anatomy