The hypothenar muscles
The hypothenar musculature is a group of four short muscles found at the ulnar side of the palm. Their muscle bellies form the prominent surface above the base of the little finger, also known as the hypothenar eminence.
The four hypothenar muscles are:
Abductor digiti minimi
This muscle ranges from the pisiform bone to the ulnar side of the proximal phalanx and the dorsal aponeurosis of the little finger.
Flexor digiti minimi
It has its origin surface at the flexor retinaculum and the hook of the hamate bone. Distally it inserts at the base of the proximal phalanx of the little finger. This muscle is often very small or even completely missing.
Opponens digiti minimi
This muscle arises also from both the flexor retinaculum and the hook of the hamate bone but inserts more proximally at the ulnar surface of the fifth metacarpal bone. The opponens digiti minimi is the strongest and deepest of all hypothenar muscles.
This muscle extends from the flexor retinaculum and the palmar aponeurosis to the skin at the ulnar side of the hands, lending it a rectangular appearance. Even though this muscle lies superficially, above all hypothenar muscles, it cannot be distinguished through palpation due to the fact that it is quite thin. The palmaris brevis differs from the other three muscles in many ways, which is why it is often not considered as one of the hypothenar muscles.
All hypothenar muscles are supplied by the ulnar nerve (C8-Th1) which courses along the flexor carpi ulnaris from the forearm to the wrist. From there, it runs through the ulnar tunnel between the pisiform bone and the hook of the hamate bone, also known as Guyon's canal. This pathway is formed by the flexor retinaculum and the pisohamate ligament on the dorsal side, and the palmar carpal ligament and the palmaris brevis muscle on the palmar side.
After exiting the tunnel the ulnar nerve divides into a superficial branch and a deep branch. While the deep branch courses towards the metacarpus along with the ulnar artery and supplies the hypothenar muscles, including others, the superficial branch carries on distally underneath the palmaris brevis.
The hypothenar musculature aids with movements of the little finger (with the exception of the palmar brevis). Hereby the main actions of each muscle can be easily associated to their names:
- The abductor digiti minimi moves the little finger away from the hand (abduction in the carpometacarpal [CMC] and metacarpophalangeal joints [MCP]) but it also does an extension in the proximal [PIP] and distal interphalangeal joints [DIP] due to its insertion at the dorsal aponeurosis.
- The flexor digiti minimi bends the little finger (flexion in the MCP).
- The contraction of the opponens digiti minimi leads to a combination of flexion, adduction and lateral rotation in the CMC. This so-called opposition movement plays an important role in gripping movements.
- Functionally, the palmaris brevis does not act as a “normal” muscle. Its special function is rather to protect the neurovascular pathway, which runs underneath it, from compression.
The Guyon’s canal is a typical place of entrapment of the ulnar nerve leading to a clinical picture referred to as the Guyon's canal syndrome. The most common causes are:
- chronic compression (e.g. resting the hand on the bicycle handlebar)
- rather rarely, tumors
The affected patients complain about pain and disturbed sensation at the ulnar side of the palm. In advanced stages, paralysis and atrophy of the hypothenar and metacarpal muscles occur.
In order not to mistake the Guyon’s canal syndrome with the more common carpal tunnel syndrome the following examinations should be performed:
- Tapping lightly on both the Guyon's canal and carpal tunnel. If there is local paraesthesia one should suspect an underlying compression. This test, referred to as Hoffmann-Tinel sign, can be performed at the site of any suspected peripheral nerve compression.
- Asking the patient to grasp a sheet of paper between thumb and index finger. If there is a weakness of the adductor pollicis muscle, the patient will try to compensate it by using the flexor pollicis longus muscle which is innervated by a branch of the median nerve (Froment's sign).