The Double Motor Innervation of the Opponens Pollicis Muscles: an Electromyographic Study

Home , Opponens pollicis muscle, Thenar eminence

J. Anat. (1974), 117, 2, pp. 239-331 329 Printed in Great Britaini

The double motor innervation of the opponens pollicis muscles: an electromyographic study

DAN HARNESS, ELIEZER SEKELES AND JOSHUA CHACO* The Departments of Orthopaedics, Physical Medicine and Rehabilitation-Hadassah University Hospital and the Department of Anatomy, The Hebrew University-Hadassah Medical School (Accepted 6 December 1973)

INTRODUCTION It is generally accepted that the abductor pollicis brevis, the flexor pollicis brevis and the opponens pollicis are innervated by the median nerve in most people. Damage to the median nerve, therefore, should cause, amongst other things, loss of the power of opposition, which is crucial for the performance of many daily activities. But it is also widely known that the ulnar nerve may take part in the innervation of these muscles. Thus, Day & Napier (1961) found by dissection that the deep head of the flexor pollicis brevis was innervated by the ulnar nerve in 21 out of 24 hands. Furthermore, using electromyography (EMG) combined with percutaneous stimulation, Forrest (1967) showed that the superficial head of the flexor pollicis brevis had both median and ulnar innervation in 17 out of 25 hands examined, and in 2 hands had only ulnar innervation: moreover, in the same study, he showed that in 5 hands the opponens pollicis was doubly innervated. Cannieu (1896, 1897) and Riche (1897) reported the presence of a terminal ansa in the thenar eminence, uniting branches of the median and ulnar nerves. Recently Harness & Sekeles (1971) demonstrated such an ansa in 27 out of 35 dissected hands (77 oo). Since about two-fifths of a terminal muscular nerve may be composed of sensory fibres, it cannot be assumed that the ansa carries motor fibres from the ulnar nerve to the three thenar muscles mentioned. The present investigation was therefore carried out to determine whether or not the ansa, if present, carries motor fibres.

MATERIALS AND METHODS Seventy persons, 32 female and 38 male, were tested with the Hewlett Packard EMG apparatus, using bicoaxial needle electrodes. The needle was inserted into the opponens pollicis muscle, potentials appearing on the oscilloscope during soft opposition of the thumb being taken to indicate that the needle had been correctly located (on the assumption that the flexor pollicis brevis is not contributing to the potentials picked up from the opponens pollicis). The examinee was then instructed * Request for reprints: Dr E. Sekeles, Department of Anatomy, The Hebrew University-Hadassah Medical School, P.O.B. 1172, Jerusalem, Israel. 330 DAN HARNESS, ELIEZER SEKELES AND JOSHUA CHACO to relax, and the median and ulnar nerves were stimulated through the skin in the wrist region. Stimulation was supramaximal for 0 1 msec, and repeated at 1 second intervals. The bicoaxial needle served as the recording electrode. The evoked potential from the opponens pollicis muscle was depicted on the oscilloscope and photographed.

RESULTS AND DISCUSSION The results are quickly stated. Median nerve stimulation evoked potentials from opponens pollicis in all 70 examinees (140 hands). When the ulnar nerve was stimulated, evoked potentials were recorded bilaterally in 54 examinees (77 %) but in the remaining 16 examinees (23 %) no response was recorded in either hand. This means that in 77 % of the examinees the opponens pollicis of both hands had motor innervation from the ulnar nerve as well as from the median nerve. In the other 23 % of examinees the motor innervation was from the median nerve only. Previous reports of EMG studies of these muscles, such as those of Sala (1958) and Forrest (1967), estimated the frequency of the double motor innervation of the flexor pollicis brevis at about 75 %. According to Basmajian (1967) this short thumb flexor comes into action during powerful opposition, while the opponens pollicis is mostly responsible for soft opposition, associated, for example, with fine movements of the thumb. Forrest (1967) found that the opponens pollicis had double motor innervation in 20 % of his cases, while Sala (1958) found it in one third of his cases. We find, however, that the frequency of double innervation of the opponens pollicis is apparently very much higher (77 %) than this. The reasons for this large dis- crepancy are not clear, but perhaps it should be stated that the validity of our results depends rather critically on whether or not impulses are picked up from the flexor pollicis brevis. The implications of our findings may be most important for clinicians. First, they may explain the numerous cases in which median nerve lesions are not followed by signs of dysfunction, and atrophy of the thenar muscles. Secondly, electromyography of thenar muscles will find increasing use for proper evaluation of median function, for example, in the carpal tunnel syndrome; for if it is found that the flexor pollicis brevis and the opponens pollicis are doubly innervated, surgery may not be necessary. Of course one is thinking of a substantial contribution of motor fibres from the ulnar nerve, but in truth the relative contributions are not known. The presence of double motor innervation of the flexor pollicis brevis in about 75 % of cases (Forrest, 1967) and of the opponens pollicis in about 77 %, as found by us, indicates that this pattern of innervation should be considered the standard one. The fact that a median-ulnar ansa can be shown by dissection in almost the same percentage of cases (Harness & Sekeles, 1971) supports the view that the ansa is a motor anastomosis, because it is the only plausible nervous pathway in the region that could convey motor fibres from the uinar nerve into the thenar eminence. Double motor innervation of opponens pollicis 331

SUMMARY In 70 persons electromyography and percutaneous nerve stimulation were used to investigate the innervation of the opponens pollicis muscle of both hands. A double motor innervation (median and ulnar nerves) was found bilaterally in 54 out of the 70 examinees, while the remaining 16 had only median innervation to the opponens pollicis of both hands. In the light of our findings, and those of other workers, it is proposed that the double innervation of opponens pollicis and flexor pollicis brevis should be regarded as the normal pattern. Furthermore, these findings suggest that the terminal ansa in the thenar eminence is a motor anastomosis, and that it serves to convey ulnar motor fibres into the muscles.

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