Abstract
Abductor digiti minimi opponensplasty is a well-established method of restoring opposition following median nerve trauma or in congenital deficiency of thenar intrinsic musculature. The abductor digiti minimi is detached distally and rerouted through a subcutaneous tunnel for insertion into the area of the thumb metacarpophalangeal joint. The pivot point for this rotation is the pisiform bone and it is at this site that the ulnar nerve is vulnerable to compression by the transferred muscle. We describe a modification of technique which involves complete islanding of the muscle without re-insertion of its proximal end. This eliminates the possibility of this complication without detracting from the efficacy of opposition produced by the transferred muscle.
INTRODUCTION
The requirements for opponensplasty were first set out by Bunnell (1938):
The transferred muscle should be adequate in strength to achieve opposition;
Its original function should not be sacrificed;
The line of pull should follow as closely as possible the axis connecting the base of the proximal phalanx of the thumb to the area of the pisiform;
The distal insertion should be dorso-ulnar on the proximal phalanx so as to restore an element of pronation.
The use of the ulnar nerve innervated abductor digiti minimi (ADM) transfer for restoration of thumb opposition was first described by Huber (1921) and later by Nicolaysen (1922). It fulfills all but the second of these criteria, but the loss of abduction of the little finger is acceptable in most cases. The original account described dissection of the abductor digit minimi by detachment distally, leaving its proximal insertion attached. The distal end of the muscle was then rerouted through a subcutaneous tunnel beneath thenar skin so as to enable attachment of its distal tendon to the tendon of opponens pollicis (Fig 1).
The technique was modified to increase the length of the transfer by Littler and Cooley (1963), who detached the muscle proximally from the pisiform and left it attached only to the flexor carpi ulnaris. Distal insertion was performed to the abductor pollicis brevis tendon. Ogino et al. (1986) described detachment of the proximal origin of the abductor digiti minimi and reattachment to the palmaris longus tendon or the flexor retinaculum.
Rotation of the abductor digit minimi about the axis of the pisiform may produce compression of the ulnar nerve, which lies superficial in Guyon’s canal at this level (Fig 2). Though unpublished in the literature, this is a recognized complication and may require early decompression or reversal of the muscle transfer. We report a modification in technique, with which the possibility of ulnar nerve compression is eliminated.
PATIENTS AND TECHNIQUE
Surgical method
The ADM is exposed via a curvilinear palmar incision in the axis of the little finger, starting proximally over Guyon’s canal and ending with a midlateral extension on the ulnar aspect of the little finger (Fig 3). This permits easy dissection of the muscle and its distal tendon and, importantly, provides ample exposure of the ulnar nerve as it curves around the pisiform and gives origin to the motor supply to ADM. The muscle is dissected distal to proximal, taking care to harvest a significant amount of distal tendon to facilitate attachment to the thumb. The plane of dissection is clearly followed to the base of the muscle where the neurovascular pedicle is encountered on its radial aspect (Fig 4). Once this is seen, it can be protected as the entire origin of the muscle is detached by sharp dissection from the pisiform, the roof of Guyon’s canal, the flexor retinaculum and the flexor carpi ulnaris. It is possible to safely island the muscle completely on its neurovascular pedicle (Fig 5).
A subcutaneous tunnel beneath the thenar skin is dissected, aiming for the area of the metacarpophalangeal joint of the thumb. The distal tendon of the ADM is grasped and passed through the tunnel. It is our practice, at this stage, to place the thumb in full abduction and secure this position with a K-wire across the trapeziometacarpal joint. This ensures that no inadvertent strain is placed on the muscle, or the sutures used to secure it, throughout the rest of the procedure or during the immediate postoperative period.
The distal end of the muscle is sutured to the insertion of abductor pollicis brevis. No attempt is made to secure the proximal end of the muscle. It has been our experience that the divided muscle rapidly adheres to the deep structures particularly since the abducted thumb position is maintained by the Kirschner wire. The wounds are closed in a single layer. No drain is used. The thumb position is further secured with application of a lightweight cast.
CLINICAL MATERIAL
A pilot study assessing this technique in four patients has been performed.
All patients in the study were subjected to measurements of efficacy of opposition, using three separate indices (Foucher et al., 1991):
Kapandji score 1–9 (Fig 6).
Van Wetter’s apogée – a measure of maximal thumb abduction (Fig 7).
Duparc’s arc – a measure of circumduction (Fig 8).
Demographic details and performance are shown in Table 1.
The technique described above was followed precisely in each case. No case of ulnar nerve compression was recorded. In all cases the unoperated hand was used as control for measurement of the indices of opposition. The range of opposition measured with our three indices reached a level of 79% expressed as a percentage of the opposition range of the unoperated hand. This is comparable to the range we would expect utilizing other, longer established, transfers (flexor digitorum superficialis, palmaris longus, extensor indicis). This was a small series and we would caution against utilizing percentages as an indication of performance. Our series shows, however, that the technique is safe and does not carry a significant reduction in function.
DISCUSSION
The abductor digiti minimi (Huber) opponensplasty is an effective means of restoring opposition. It utilizes a dispensable muscle innervated by the ulnar nerve. The line of pull extends from pisiform to thumb metacarpophalangeal joint, the axis with the best mechanical advantage to abduct and oppose the thumb. It is unique amongst the methods of opponensplasty in restoring muscle bulk to the thenar eminence (Fig 9; Wissinger and Singsen, 1977). However, the path of the rotated ADM crosses the line of the ulnar nerve at the distal margin of the pisiform and immediate transient post-operative oedema may compress the ulnar nerve and long-term compressive scarring is a possibility.
Littler and Cooley (1963) and, more recently, Ogino (1986), have modified the method in order to minimize the possibility of compressing the ulnar nerve, and also lengthen the transfer. These workers considered it important to retain a proximal attachment and Littler’s method still leaves fibres of muscle crossing over the ulnar nerve. Ogino’s method avoids this crossover but displaces the entire muscle in a radial direction and loses some of the excellent mechanical advantage consequent of the line of pull from the area of the pisiform.
It would appear that our method of complete detachment of the muscle, which allows the proximal end to adhere in situ, maintains all the advantages of the ADM transfer without compromising the nerve. It can be criticized for requiring a more precise technique than the classical Huber transfer and the neurovascular bundle is more vulnerable during the surgery. However, the incision we have utilized, similar to that described by Littler and Cooley (1963), affords excellent exposure and minimizes this risk.