Occasional head of flexor pollicis longus muscle: a study of its morphology and clinical significance.

MV Hemmady, AV Subramanya, IM Mehta 
 Dept of Anatomy, TN Medical College, Bombay Central, Maharashtra.

Correspondence Address:
M V Hemmady
Dept of Anatomy, TN Medical College, Bombay Central, Maharashtra.

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::   Introduction

The flexor pollicis longus muscle is described as arising chiefly from the anterior surface of the radius below the anterior oblique line and above the insertion of pronator quadratus and from an adjoining strip o interosseous membrane (principle head). In addition, the flexor pollicis longus muscle may have an additional origin from the medial border of the coronoid process o the ulna or from the medial epicondyle of the humerus (variously referred to as Gantzer's muscle / accessory head/occasional head)[1],[2],[3].

The fibres of the principal head of this unipennate muscle descend obliquely to insert on a tendon, which forms high on the volar surface, on the uinar border of the muscle. At first broad, it becomes cylindrical near the wrist. The insertion of the fibres continues nearly to the point where the tendon passes under the transverse carpal ligament[4].

The occasional head runs distally and obliquely parallel to the oblique cord (phylogenetically degenerate fibres of the upper part of the flexor pollicis longus muscle) to join the principal head and its tendon[5].

In the current study, the authors are concerned with the frequency of occurence of the occasional head of flexor pollicis longus muscle, its morphology, its relation to the structures of the proximal forearm and the clinical significance thereof.

::   Methods

For this study, 54 upper extremities of adult Indian cadavers, (chiefly male) were examined, most of them in pairs. The dissection was carried out by using a volar incision extending from the distal arm to the insertion of the flexor pollicis longus. The occasional head was brought into view upon reflection of the flexor digitorum superficialis muscle in addition to the flexor carpi radialis and the palmaris longus. An attempt was made to trace the occasional head to its origin. Its relation with other structures of the forearm was studied.

::   Results

The proximal attachments of the occasional head of flexor pollicis longus muscle are tabulated in [Table:1].

In all cases (except two) it was found to be fused with the other muscles of the flexor-pronator group of the forearm, probably because phylogentically they have a common origin namely the pronator-flexor group of Humphrey [Figure:1] and [Figure:2]. In some cases they were so intimately blended that separation was extremely laborious. In the two exceptional cases, the occasional head arose by a flimsy tendon from the proximal part of the flexor digitorum superficialis and inserted by a thin tendon into the principal belly of the flexor pollicis longus muscle [Figure:2]. In cases where it had a dual origin, namely the medial epicondyle and the coronoid process of the ulna; it tended to blend with the articular capsule of the elbow joint.

The length of the occasional belly varied from 5-8 cm whereas the diameter was found to be in the range of 1-2 1/2 cm. The median nerve was found to run over the occasional head while the anterior interosseous nerve ran posteriorly. Hence the accessory head was sandwiched between the median and the anterior interosseous nerve [Figure:3]. The nerve supply to the occasional head was by a branch from the anterior interosseous nerve, which was given proximal to the other branches of the nerve.

In all the cases, the occasional head was found to be less bulky than the principal belly of the flexor pollicis longus muscle. Also, the occasional heads on the right side were bulkier than the left in a majority of the cases probably because in the general population the right extremity is the dominant one.

::   Discussion

The so-called occasional belly of the flexor pollicis longus muscle was seen to be present mori frequently (66.66%) than absent. The morphology and relations of this 'occasional belly' are of great interest from the clinical point of view.

The occasional belly per se may cause entrapment neuropathy of the anterior interosseous nerve6. Also, cicatricial contraction of the occasional belly of the flexor pollicis longus muscle (as seen in Volkmanin's ischemic contracture or following surgical or non-surgical trauma around the proximal forearm and elbow) may lead to entrapment of the median and anterior interosseous nerves since they are so closely related to this belly. In addition, the median nerve may get entrapped between the humeral origin of the occasional belly and the flexor carpi radialis.

The presence of the occasional head has to be borne in mind during anterior approaches to the proximal radius and the elbow joint, as also during a decompressive fasciotomy for compartment syndrome of the forearm.

The muscle-slide operation of Max Page for Volkmanin's ischemic contracture involves release of the forearm flexors including the flexor pollicis longus muscle. A similar surgery described by him in 1923 for spastic paralysis of the muscles of the forearm and wrist was vindicated by Inglis and Cooper[7] (1966). In the above surgeries the accessory belly of flexor pollicis longus may also have to be released. Similarly, in the surgery described for a thumb-in-paim deformity following spastic paralysis, the occasional belly may have to be length ened/released alongwith the principal belly of the flexor pollicis longus[8].

Kaplan[9] has described a case of a long standing flexion contracture of the inter- phalangeal joint of the thumb following a fracture dislocation of the elbow which was later found to be due to the cicatricial contraction of the accesory belly of the flexor pollicis longus and had to be subsequently elongated to correct the deformity. Hence in a flexion deformity of the thumb, involvement of the occasional head has to be kept in mind[10].

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