Introduction

Flexor Pollicis Longus muscle is a unipennate muscle^[1] and one among deep muscles of anterior compartment of forearm. This muscle originates from the upper three fourths of anterior surface of the shaft of radius, extending between its tuberosity to within a short distance of pronator quadratus and adjoining part of the anterior surface of the interosseous membrane.^[2] The muscle fibres end in a flattened tendon, lies superficial to the pronator quadratus and lateral to the flexor digitorum profundus. The tendon of this muscle passes deep to the flexor retinaculum, between the opponens pollicis and the oblique head of the adductor pollicis and is enclosed in a synovial sheath(osseo-aponeurotic) to facilitate smooth gliding.^[3] Tendon gets inserted to the palmar surface of the base of distal phalanx of the thumb. This muscle is innervated by anterior interosseous nerve.^[4] Blood supply to this muscle is by anterior interosseous artery which is a branch of ulnar artery.^[5] The main action of this muscle is flexion of distal phalanx of the thumb. It also flexes proximal phalanx of thumb and first metacarpal at the metacarpophalangeal and carpometacarpal joints respectively.^[6] Embryologically all skeletal muscles of the limb arise from myogenic precursor cells derived from hypaxial division of myotomes, which in turn come from somites. The upper limb muscles begin to form around the fifth week of embryonic development. As the limb bud grows, these myogenic cells migrate into the developing limb and split into dorsal and ventral masses. Flexor pollicis longus muscle arises from the ventral muscle mass of the forearm.^[7]

Methods

During the routine dissection on the right upper limb of a male cadaver of 72years old in the department of Sharira Rachana, after careful removal of skin, superficial fascia, superficial muscles of forearm, variation is identified.

Result

After dissection of skin, superficial fascia and superficial muscle of forearm, it is observed that flexor pollicis longus muscle had an accessory tendinous head which originates from the lateral aspect of coronoid process of ulna, lateral to ulnar tuberosity above insertion of Brachialis muscle.

Discussion

Flexor pollicis longus muscle plays a vital role in the functional anatomy of the hand especially in thumb movement and precision grip. The presence of an accessory head of this muscle is an anatomical variation, commonly referred to as Gantzer’s muscle. The accessory head is found in approximately 25%-73.6% of individuals in cadaveric studies, typically arises from medial epicondyle of the humerus, the coronoid process of the ulna, or the shaft of the ulna. It usually inserts into the main belly or tendon of the muscle.^[8] This may assist in enhancing strength and stability of the thumb. Clinically the accessory head can compress the anterior interosseous nerve, leading to the condition called anterior interosseous nerve syndrome. This condition causes weakness or paralysis of flexor pollicis longus, flexor digitorum profundus, pronator quadratus.^[9]

Conclusion

An accessory muscle slip of flexor pollicis longus exhibits considerable anatomical variation. Recognising its presence is vital in clinical practice, particularly in cases of anterior interosseous nerve syndrome which is usually developed due to injuries, compressions or during forearm surgeries. Knowledge about these anatomical variations not only aids in accurate diagnosis, surgical planning but also enhances our understanding about functional anatomy of forearm.

References