Dominic Stone
The human hand is a complex structure, with several muscles that work together to allow for movement and flexibility. One of the most important functions of the hand is its ability to extend, which is facilitated by various extensor muscles. These muscles are located in the forearm and are connected to the hand by long tendons. This network of muscles and tendons includes the extensor carpi radialis brevis, extensor carpi radialis longus, extensor digitorum, and extensor indicis, each playing a specific role in extending different parts of the hand and fingers. Understanding the anatomy of these extensors is crucial, as they are susceptible to injuries that can impact the hand's functionality.
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What You'll Learn
Extensor carpi radialis longus (ECRL)
ECRL is a wrist extensor that is innervated by the radial nerve, from spinal roots C6 and C7. It is supplied with blood by the radial artery. The muscle, like all extensors of the forearm, can be strengthened by exercise that resists its extension. The best exercises to improve strength in the ECRL are exercises that require you to move your wrist, hands, elbows, and forearms.
The ECRL is in the same muscle family as the extensor carpi radialis brevis (ECRB). Both muscles need to be strong to move the arm, wrist, and hand appropriately. Together, they produce wrist extension and abduction (radial deviation). They form the lateral compartment. The ECRL and ECRB receive blood from the radial artery.
The ECRL is a fusiform muscle that forms a flattened tendon that runs distally over the lateral surface of the radius. In the lower third of the forearm, the tendon is crossed by the tendons of the abductor pollicis brevis and extensor pollicis brevis. The tendons of the ECRL and ECRB pass deep into the extensor retinaculum in a common synovial sheath.
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Extensor digitorum (ED)
ED is a long muscle located in the posterior compartment of the forearm. It belongs to the group of superficial extensors of the forearm. These muscles can be easily palpated in the lateral aspect of the posterior forearm, especially during the extension of the hand when they are contracted. ED runs from the lateral epicondyle of the humerus to the medial four phalanges of the hand. ED is the most superficial muscle of the posterior forearm.
ED is situated medial to the extensor carpi radialis brevis muscle and lateral to the extensor digiti minimi and extensor carpi ulnaris muscles. Its tendons are placed deep to the extensor retinaculum, between the tendons of extensor digiti minimi on its medial side and extensor pollicis longus on its lateral side. Along with extensor indicis, the tendons of ED occupy the fourth extensor (dorsal) compartment. In the dorsum of the hand, the tendons of ED run superficial to the dorsal interossei muscles.
ED is supplied by the posterior interosseous artery and the radial recurrent artery. It is primarily responsible for extending the medial four digits at the metacarpophalangeal joints and secondarily at the interphalangeal joints. It also acts to extend the wrist joint. ED is innervated by the posterior interosseous nerve, which is a continuation of the deep branch of the radial nerve (root value C7 and C8).
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Extensor digiti minimi (EDM)
Extensor digitorum communis (EDC) muscles are responsible for the extension of the second (index finger), third (long finger), fourth (ring finger), and fifth (small finger) digits. The independent extension of the small finger is facilitated by the extensor digiti minimi (EDM) muscle. The EDM is a long, slender skeletal muscle situated in the posterior compartment of the forearm. It is also referred to as the extensor digiti quinti proprius.
The EDM is a part of the extrinsic muscles of the hand, located outside the hand. It is connected to the extensor digitorum and is situated between Extensor Digitorum and Extensor Carpi Ulnaris. The extensor digitorum and the extensor digiti minimi may occasionally be partially fused. The extensor digiti minimi is innervated by the posterior interosseous nerve (C7 and C8), a branch of the deep radial nerve.
The EDM originates from the common extensor tendon attached to the lateral epicondyle of the humerus. It extends from the distal humerus to the fifth digit. The muscle courses inferiorly, and its fibres separate from the extensor digitorum in the lower half of the forearm. The muscle then continues as a long tendon that passes through the extensor retinaculum near the wrist and into the fifth extensor compartment of the wrist.
The extensor digiti minimi helps in extending the fifth finger, also known as the little finger, at the metacarpophalangeal joint. It also contributes to the extension of the wrist and all the fifth finger joints from the ulnar side of the hand. The blood supply to the extensor digiti minimi comes from two sources: the radial recurrent artery and the posterior interosseous artery.
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Extensor carpi ulnaris (ECU)
The ECU originates from the lateral epicondyle of the humerus and the posterior border of the ulna. It crosses the forearm to the ulnar (medial) side and inserts at the base of the fifth metacarpal. The ECU extends the wrist and, when acting alone, inclines the hand towards the ulnar side. Through continued action, it extends the elbow joint.
ECU injuries are common in people who participate in activities requiring repetitive arm, elbow, and wrist movements, especially when tightly gripping an object. Symptoms of an ECU injury include pain when shaking hands or squeezing/gripping an object. The pain worsens with forceful wrist movement. Treatment options depend on the severity of the injury and can range from conservative treatments such as immobilization and stabilization to invasive procedures like steroid injections and, in severe cases, surgical repair or reconstruction.
ECU tendon instability can be challenging to diagnose due to normal changes in tendon position during wrist motion. Ultrasound imaging is a simple and practical method to evaluate ECU tendon displacement relative to the ulnar groove, aiding in the assessment of ulnar-sided wrist pain. MRI is also effective in detecting subtle changes in the ECU tendon and surrounding structures.
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Extensor indicis (EI)
The EI muscle's tendon passes through the fourth extensor compartment of the wrist, joining the ulnar side of the extensor digitorum tendon of the index finger. The extensor digitorum has four tendons that diverge on the back of the hand to follow the fingers. The EI tendon inserts into the dorsal aponeurosis of the index finger. The extensor indicis proprius (EIP) is a variant of the EI tendon that allows for independent index finger extension.
The EI muscle arises from the distal third of the dorsal part of the ulna's body and the interosseous membrane. It is innervated by the posterior interosseous nerve (C7 and C8), a continuation of the deep branch of the radial nerve. The EI muscle is one of the extrinsic muscles of the hand, which are located in the back of the forearm and have long tendons connecting them to bones in the hand.
The EI tendon is frequently used in hand surgery to restore the loss of function in other digits. It shows variations in different populations, with Indian populations having the highest rate of single-slip EIP and the lowest rate of double-slip EIP compared to Japanese, Europeans, and North Americans. Knowledge of the EI tendon's variants is crucial for surgeons when choosing the tendon to transfer during surgery.
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Frequently asked questions
The extrinsic extensor muscles of the hand are located in the back of the forearm and include the extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor digitorum (ED), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU), abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), and extensor indicis (EI).
The extensor digitorum muscle, also known as the extensor digitorum communis (EDC), extends the medial four digits of the hand. It arises from the lateral epicondyle of the humerus and divides into four tendons, which pass through a separate compartment of the dorsal carpal ligament.
The extensor indicis muscle (EI) acts upon the index finger, allowing for independent index finger extension.
The extensor pollicis longus (EPL) muscle acts upon the thumb, helping to extend the distal phalanx.
