Question

Which muscle is paralyzed if there is hyperextension of metacarpophalangeal joint and flexion of the interphalangeal joint?

• A. Extensor digitorum
• B. Interossei and lumbricals
• C. Adductor pollicis
• D. Pronator quadratus muscle

Answer 1

The Correct Option is B

The condition described—hyperextension of the metacarpophalangeal (MCP) joint and flexion of the interphalangeal (IP) joint—is indicative of a classic clinical sign known as "claw hand." This occurs when there is paralysis of specific muscles responsible for flexion and extension balance in the fingers.

In a normal functioning hand, the interossei and the lumbrical muscles play a crucial role. The interossei muscles, located between the metacarpal bones, primarily assist with finger abduction and adduction, while the lumbricals help flex the MCP joints and extend the IP joints.

When these muscles (interossei and lumbricals) are paralyzed, they lose their ability to counteract the action of the extensor digitorum muscle, which still exerts force on the MCP joints. This leads to unopposed extension at the MCP joints and subsequent flexion at the IP joints due to the passive tension created by the long flexors.

Therefore, the correct muscle group that is paralyzed, leading to the described joint positioning, is the interossei and lumbricals.

Muscle	Function
Extensor digitorum	Extends fingers at MCP joints
Interossei and lumbricals	Flexes MCP and extends IP joints
Adductor pollicis	Adducts thumb
Pronator quadratus muscle	Pronates forearm

The paralysis of the interossei and lumbricals muscles disrupts normal digital mechanics, confirming them as the affected muscles in the question scenario.