The word "thenar" means fleshy mounds. In human anatomy, the word thenar associates with the two fleshy mounds located in the palmar surface of the hand. These thenar mounds are called the thenar eminence and the hypothenar eminence. The thenar eminence is the fleshy mound at the base of the thumb. The hypothenar eminence is the mound located at the base of the fifth digit (little finger).
The eminences at either side of the hand are made up of muscles. The muscles located in the thenar eminence function primarily to control the thumb. The muscles that reside in the hypothenar eminence will contract and manifest motion through the little finger.
The hypothenar eminence is made up of four muscles, but there are only three primary muscles. The four muscles are the abductor digiti minimi muscle, the flexor digiti minimi brevis muscle, the opponens digiti minimi muscle, and the palmaris brevis muscle. All these muscles contract and create specific movements to the little finger.
Structure and Function
The hypothenar eminence is on the medial side of the hand. This group of muscles is considered intrinsic muscles of the hand. The mound-like structure formed from the abductor digiti minimi muscle, the flexor digiti minimi brevis muscle, the opponens digiti minimi muscle, and the palmaris brevis muscle will go on to serve many functions other than moving the little finger. The abductor digiti minimi muscle causes the little finger to abduct away from the hand (moves the little finger medially) when it contracts. The opponens digiti minimi muscle moves the little finger to allow the opposition of the hand. The motion created by the opponens digiti minimi muscles is a combination of palmar rotation, flexion, and abduction of the little finger. The flexor digiti minimi brevis muscle will flex the little finger at the metacarpophalangeal joint. The palmaris brevis muscle serves a minor function within the hypothenar eminence. The palmaris brevis muscle increases grip strength and protects the ulnar nerve as it enters the hand.
The origins and attachments of the hypothenar muscles vary slightly, but they have one thing in common. They manifest motions in the little finger. The abductor digiti minimi muscle derives its origin from the pisiform carpal bone, dorsal aponeurosis, and the tendon of the flexor carpal ulnaris muscle. The abductor digiti minimi muscles will attach to the little finger at the base of the proximal phalanx. The origin of the flexor digiti minimi brevis muscle is from the hamate carpal bone and the transverse carpal ligament (flexor retinaculum). The flexor digiti minimi brevis muscle also attaches at the proximal phalanx of the little finger. The opponens digiti minimi muscle also originates from the hamate carpal bone and the transverse carpal ligament similar to the flexor digiti minimi brevis muscle. But the opponens digiti minimi muscle attaches to the medial side of the proximal phalanx. The palmaris brevis muscle originates from the transverse carpal ligament and inserts into the skin on the ulnar side of the palm.
The thenar eminence will develop mainly from the ectoderm and mesoderm germ layers. The overlying skin and nerve innervation derive from the ectoderm layer. The neural crest cells from the ectodermal layer will form the ulnar nerve. The development of the hypothenar muscles is similar to the development of most muscles. The mesenchymal tissue from the mesodermal germ layer will differentiate into the muscles. The mesenchymal tissue will also form the blood vessels that perfuse the hypothenar eminence. The mesenchyme will form the ulnar artery and its branches to perfuse the hypothenar eminence.
Blood Supply and Lymphatics
The blood supply to the hypothenar eminence comes from the ulnar artery. The ulnar artery will enter the hand and form the superficial palmar arch. This arch will be responsible as the main blood supply to the hypothenar eminence. While the radial artery also provides collateral blood flow. The radial artery will form the deep palmar arch in the hand. The deep palmar arch has anastomoses to the superficial palmar arch or the ulnar artery. These anastomoses will provide collateral blood flow to the hypothenar eminence if the ulnar artery's blood flow is insufficient.
The lymphatic drainage of the thenar eminence will drain toward the cubital fossa. The cubital fossa contains cubital lymph nodes. Then the lymph fluid will drain toward the axilla and eventually return to either the right lymphatic duct or the thoracic duct. The right lymphatic duct will drain the right hypothenar eminence while the thoracic duct will drain the left eminence.
The nerve innervation of the hypothenar eminence is entirely by the ulnar nerve (C8, T1 spinal roots). The ulnar will be travel in the Guyon canal (hook of the hamate and the pisiform). The ulnar nerve will travel superficially to the transverse carpal ligament but under the palmaris brevis muscle. The palmaris brevis muscle protects the ulnar nerve as the nerve enters the hand.
Muscles in the hypothenar eminence:
flexor digiti minimi muscle
abductor digiti minimi muscle
opponens digiti minimi muscles
palmaris brevis muscle
All these muscles in the hypothenar eminence contribute to the movements of the little finger and the mound-like structure on the palmar side of the hand.
The origins and attachments of the hypothenar muscles may vary slightly, but the hypothenar eminence itself is consistent. The hypothenar eminence may consist of three or four muscles depending on the individual. Some individuals lack the palmaris brevis muscle. The attachment of the palmaris brevis muscle can vary also. The most common attachment for the palmaris brevis muscle is the skin on the ulnar side of the hand. The palmaris brevis muscle can sometimes be found attached to the pisiform bone.
The anatomy of the hypothenar eminence is essential to know during hand surgeries. The knowledge of the relationship between the nerves, blood vessels, muscles, and bones allow for a safe and optimal approach when it comes to hand surgery.
When hand reconstruction involves the ulnar artery, the collateral blood supply should undergo evaluation for sufficiency — the assessment of the collateral blood to the hypothenar eminence occurs with the modified Allen's test. The reason collateral blood flow should be evaluated is that if the hypothenar eminence is left with inadequate perfusion post-operatively. The muscles, nerves, and tissues of that region may undergo ischemic necrosis.
The repair of the ulnar nerve in "handlebar palsy." This procedure requires the surgeon to know the relationship between the hook of the hamate bone to the pisiform bone. The ulnar nerve will pass between the hook of the hamate and the pisiform bone (Guyon’s canal). During the repair, it is crucial to release the compression of the ulnar nerve without damaging it. If damage to the ulnar nerve occurs. There will be atrophy of the hypothenar eminence along with loss of sensory and motor function in the region the ulnar nerve innervates.
The ulnar nerve innervates the hypothenar. If the ulnar is damage or compromise, the muscles in the hypothenar eminence will atrophy. The atrophy of the hypothenar eminence can indicate a compromise of the ulnar nerve proximal to the hand.
As the ulnar nerve passes between the hook of the hamate and pisiform bone (Guyon's canal), it can become compressed. Individuals that apply direct pressure to the Guyon canal such as cyclist can damage the ulnar nerve. Damage to the ulnar nerve will manifest as atrophy, numbness, tingling, and pain in the hypothenar eminence along with fourth and fifth digits. This condition is the same as "carpal tunnel syndrome," but it affects the ulnar nerve.
The hypothenar eminence can become damaged if the ulnar artery is compromised, and there is insufficient collateral blood flow. This condition is called the "hypothenar hammer syndrome: This condition tends to affect individuals that work with tools that need tight gripping and repetitive pounding of the tool. The tight grip and pounding of tools will cause recoil impact on the vascular blood supply in the hypothenar eminence. If the blood supply becomes compromised, there will be necrosis of the hypothenar eminence.
The ulnar artery predominantly perfuses the hypothenar eminence, and it receives innervation from the ulnar nerve. If there were to be any compromise to the nerve or artery proximally in the arm, the patient would present with symptoms of damage manifested through the resting and active presentation of the hand.
The hypothenar eminence also corresponds to the dermatome level for C8. If there is a loss or decrease of sensation and the feeling of pins and needles (paraesthesia). This neurological finding could indicate a lesion affecting the C8 spinal root on the ipsilateral side.