Anatomic Variations of the Hand Extensor Muscle Tendons

Variations of muscles on the dorsal aspect of the forearm are seen very common. These variations can be symptomatic but generally they are diagnosed incidentally. On the posterior compartment of forearm, extensor carpi radialis brevis related to abduction of the wrist and extensor digitorum related to extension of the wrist and fingers. This case report showed rare variations of extensor carpi radialis brevis and extensor digitorum muscles. During routine dissections of the extensor compartment of the forearm for anatomy education, it was observed that extensor carpi radialis brevis muscle had an accessory head which attached to dorsum and proximal phalanx of the index finger. Besides, it was found that extensor digitorum muscle divided into three tendons distally. When we followed these tendons, it was observed that tendon of index finger was absent and other three tendons went to middle, ring and little fingers. Accessory muscles located on the extensor compartment of the forearm are seen common. They can be reason of the symptoms of the nerve entrapment. Besides, these unusual muscles may confuse radiologists, surgeons and anatomists. In order to prevent diagnostic errors and avoid surgical complications, these anatomical variations should be well-known.


INTRODUCTION
Extension of the fingers and the wrist is carried out by the muscles on the dorsal aspect of the forearm (1).Variations of these muscles are seen very common.Generally, these variations are often diagnosed incidentally during surgery, but sometimes they can be symptomatic.Knowledge of such variations is important, especially in clinical practice such as surgical operations.
Extensor carpi radialis brevis originates from the lateral epicondyle of the humerus via a tendon and it shares this tendon with other forearm extensor muscles.The belly of the muscle ends in a flat tendon at about mid-forearm.This tendon passes under the extensor retinaculum and then it is attached to the dorsal surface of the base of the third metacarpal on its radial side, distal to its styloid process of radius, and on adjoining parts of the second metacarpal base.The extensor carpi radialis brevis is innervated by the posterior interosseous nerve (2).
Extensor digitorum originates from the lateral epicondyle of the humerus via the common extensor tendon.Distally, it divides into four tendons.These tendons pass under the extensor retinaculum (2).Each of these tendons goes to a finger: the index, the middle, the ring and the little fingers (3).The extensor indicis tendon accompanies to the tendon of the index finger of the extensor digitorum.The extensor indicis tendon lies at its ulnar side.Similarly, the extensor digiti minimi tendon accompanies to the tendon of the fifth finger of the extensor digitorum and lies at its ulnar side (2).
The anatomic variations and prevalence of the extensor muscle tendons have been reported in many studies (3,4).Similarly to these studies, in this case report, we showed rare variations of extensor carpi radialis brevis and extensor digitorum muscles.
206 Case Report / Olgu Sunumu CASE REPORT At Gazi University, Department of Anatomy, Faculty of Medicine, variations of extensor carpi radialis brevis and extensor digitorum were found in right upper extremity of a 66-years-old male cadaver, during routine dissections of the extensor compartment of the forearm for anatomy education.The incision was performed on right forearm from interepicondylar line to extensor retinaculum.Skin, subcutaneous fat, superficial and deep fascias were removed and extensor muscles were exposed.After extensor muscles were defined, variations of the extensor carpi radialis brevis and extensor digitorum were identified.Because of determination of these variations, dorsum of the hand was also dissected.
In our case, it was observed that extensor carpi radialis brevis muscle had an accessory head.Tendon of this accessory head was pursued and it was confirmed that it attached to dorsum and proximal phalanx of the index finger.In addition to this variation, we found that extensor digitorum muscle divided into three tendons distally.When we followed these tendons, it was observed that tendon of index finger was absent and other three tendons went to middle, ring and little fingers (Figure 1 and 2).

DISCUSSION
Extensor carpi radialis brevis related to abduction of the wrist and extensor digitorum related to extension of the wrist are superficial muscles on the posterior compartment of forearm (1).In this region, variations are seen very common.In order to understand anatomical variations, knowledge of human embryology is important.In the forearm, the precursor extensor muscle mass differentiates into a radial portion.Then, this radial portion divides into superficial and deep portions.Extensor digitorum, extensor carpi ulnaris, and extensor digiti minimi muscles are originated from the superficial portion while abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus and extensor indicis muscles are originated from the deep portion (5).Developmental defect in different stages of gestation is one of the reasons of the anatomical variations.Besides, various changes of extensor limb myotomes involved in the transformation of the somatic layer of lateral mesoderm; such as regression, retention, or reappearance may be the cause of variations of the tendons (6).
Different variations related to tendon of the extensor digitorum and additional head of the extensor carpi radialis brevis were reported by various authors.Arora et al. (7) reported same variation observed in our case.In their study, researchers observed that the extensor carpi radialis brevis had an accessory belly attaching to the dorsum of index finger.Besides, they stated that extensor digitorum showed trifurcation distally.Similarly, Melling et al. (8) also found an accessory muscle arised from the extensor carpi radialis brevis and attached to dorsum of the index finger in both forearms of a 55-years-old male cadaver.Agarwal and Tirthani (4) examined extensor digitorum tendons and determined absence of the tendon of the index finger in one specimen in 120 upper extremities in their study about the long extensor tendons of the finger.
Gumusalan et al. (9) reported an accessory muscle arised from extensor radialis brevis and attached to second metacarpal bone.Vessal and Rai (1) detected an accessory extensor carpi radialis brevis on the right forearm of a 39-years-old Caucasian female patient.
Location of the extensor tendons is very superficial so that subcutaneous tissue protection is very inadequate.Therefore, the tendons of the extensor muscles are more frequently injured than the tendons of flexor muscles (10).At origin and insertion of muscles, multiple variations are common.Knowledge of the existence of these variations is necessary for radiologists to help to formulate a differential diagnosis of cases (1).
Accessory muscles located on the extensor compartment of the forearm are seen common.They can be reason of the symptoms of the nerve entrapment (1).Besides, these abnormal muscles may confuse surgeons and anatomists.In order to prevent diagnostic errors and avoid surgical complications, these anatomical variations should be well-known.