Claw Hand, Ape Hand, and the Sign of Benediction: Animated Review

Summary

This video explores common hand deformities resulting from nerve injuries: claw hand, ape hand, and the sign of benediction. Claw hand, caused by ulnar nerve damage, leads to hyperextension of the metacarpophalangeal joints and flexion of the interphalangeal joints. Ape hand, resulting from median nerve injury, pulls the thumb into the plane of the fingers. The sign of benediction, a key feature of high median nerve injuries, shows a partial flexion of digits 4 and 5 while digits 2 and 3 remain extended when attempting to make a fist. The video also explains the underlying anatomy and muscle functions involved in these conditions.

Takeaways

• 😀 Claw hand, ape hand, and the sign of benediction are hand deformities caused by nerve injuries, which affect the muscles controlling digits 2 through 5.
• 😀 Claw hand results from ulnar nerve injury, leading to unopposed extension at the MCP joints and flexion at the IP joints of digits 4 and 5.
• 😀 Ape hand is caused by median nerve injury, which paralyzes the thenar muscles and causes the thumb to align with the other fingers, resembling the hand of lower primates.
• 😀 The sign of benediction is a specific sign of high median nerve injury, where only digits 4 and 5 can flex while digits 1, 2, and 3 remain extended during an attempted fist formation.
• 😀 The extensor digitorum is primarily responsible for extending the MCP joints but has limited power at the IP joints due to mechanical opposition from flexor muscles.
• 😀 Flexor digitorum superficialis and flexor digitorum profundus are powerful flexors at the IP joints, counteracting the extension from the extensor digitorum at these joints.
• 😀 The lumbricals and interossei muscles play a vital role in extending the IP joints by transmitting force through the extensor expansion, despite being flexor muscles themselves.
• 😀 Nerves involved in finger movement include the radial nerve (extensor digitorum), median nerve (flexor digitorum superficialis, radial half of flexor digitorum profundus, lumbricals, thenar muscles), and ulnar nerve (interossei, ulnar half of flexor digitorum profundus, lumbricals).
• 😀 Loss of function of the interossei and lumbricals due to ulnar nerve injury leads to a characteristic claw hand appearance with hyperextension at the MCP joints and flexion at the IP joints.
• 😀 The difference between claw hand and the sign of benediction is that claw hand is a resting deformity due to ulnar nerve injury, while the sign of benediction occurs actively when attempting to flex the digits after median nerve injury.

Q & A

What are the key muscles involved in finger extension?

The key muscle involved in finger extension is the extensor digitorum, which acts on digits 2 through 5. It is assisted by the extensor indices and extensor digit minimi muscles, although the mechanics of finger extension are primarily driven by the extensor digitorum.

Why is the extensor digitorum ineffective at extending the interphalangeal joints by itself?

The extensor digitorum is ineffective at extending the interphalangeal joints because of mechanical opposition from the flexor compartment muscles, specifically the flexor digitorum superficialis and flexor digitorum profundus, which powerfully flex the interphalangeal joints.

How do the lumbricals and interossei contribute to finger extension?

The lumbricals and interossei muscles assist in extending the interphalangeal joints by transmitting force from the extensor digitorum via the extensor expansion. This allows for coordinated extension at the metacarpophalangeal and interphalangeal joints, even though these muscles are located in the flexor compartment.

What role does the extensor expansion play in finger movement?

The extensor expansion, or extensor hood, is a tendonous structure that allows muscles like the lumbricals and interossei to transmit force that aids in extending the interphalangeal joints. It facilitates the coordination of these muscles with the extensor digitorum to achieve full extension.

What is the significance of the radial, median, and ulnar nerves in hand function?

The radial nerve innervates the extensor muscles like the extensor digitorum. The median nerve innervates muscles like the flexor digitorum superficialis, flexor pollicis brevis, and the thenar eminence muscles. The ulnar nerve controls the other half of flexor digitorum profundus, the interossei, and the adductor pollicis, which are critical for fine motor control in the hand.

What causes claw hand deformity and how does it occur?

Claw hand occurs due to an injury to the ulnar nerve, resulting in the loss of function of the lumbricals and interossei in digits 4 and 5. This causes hyperextension of the metacarpophalangeal joints and flexion at the interphalangeal joints of these digits, leading to a clawed appearance.

How does median nerve injury lead to ape hand deformity?

Ape hand results from median nerve injury, which paralyzes the thenar muscles (abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis). This leads to the thumb being pulled into the same plane as the other digits, mimicking the hand posture of some lower primates.

What is the difference between the sign of benediction and claw hand?

The sign of benediction is seen when someone with a high median nerve injury attempts to make a fist, with digits 4 and 5 partially flexed while the others remain extended. In contrast, claw hand is a resting deformity caused by ulnar nerve damage, resulting in hyperextension of the metacarpophalangeal joints and flexion at the interphalangeal joints of digits 4 and 5.

What causes the 'sign of benediction' to occur only during an active attempt to flex the fingers?

The 'sign of benediction' occurs due to the paralysis of the flexor digitorum superficialis and the radial half of flexor digitorum profundus following a high median nerve injury. This leaves only the ulnar half of flexor digitorum profundus functional, causing partial flexion of digits 4 and 5 while the other digits remain extended.

Why do the thumb and digits align in the same plane in ape hand?

In ape hand, the thumb aligns with the other digits due to the loss of function in the thenar muscles following a median nerve injury. The unopposed action of the adductor pollicis muscle pulls the thumb into the same plane as the other fingers, giving it a characteristic primate-like appearance.