Ape hand deformity is a deformity in humans who cannot move the thumb outside of the plane of the palm. It is caused by inability to oppose the thumb and the limited abduction of the thumb. It is also caused by damage to the median nerve.

It can occur with an injury of the median nerve

Median nerve

The median nerve is a nerve in humans and other animals. It is in the upper limb. It is one of the five main nerves originating from the brachial plexus....

 either at the elbow

Elbow

The human elbow is the region surrounding the elbow-joint—the ginglymus or hinge joint in the middle of the arm. Three bones form the elbow joint: the humerus of the upper arm, and the paired radius and ulna of the forearm....

 or the wrist

Wrist

In human anatomy, the wrist is variously defined as 1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand;...

, impairing the thenar muscles.

Ape hand deformity is closely related to Median Nerve Palsy

Median nerve palsy

Injuries to the arm, forearm or wrist area can lead to various peripheral nerve disorders. One such disorder is median nerve palsy. If the median nerve is damaged, the ability to abduct and oppose the thumb may be lost due to paralysis of the thenar muscles. Various other symptoms can occur which...

, which is usually caused by deep injuries to the arm, forearm and wrist area.