THE INNERVATION OF THE TRAPEZIUS MUSCLE - THE NEUROANATOMICAL BASIS FOR MODIFIED NECK DISSECTIONS

Abstract

The radical neck dissection has been accepted as the standard operation to perform for cervical neck metastases. However it is well recognised that a significant proportion of the patients will develop the 'shoulder syndrome' consisting of a painful drooping shoulder with failure of shoulder abduction, winging of the scapula, electromyographic abnormalities and problems also at the sternoclavicular joint. Various modifications of the radical neck dissection have been suggested to avoid this syndrome. This syndrome is believed to be due to the denervation of the trapezius muscle during the radical neck dissection. There is little dispute in the literature that the accessory nerve provides the predominant motor input to the trapezius muscle. What was unclear in the literature was the extent of efferent contributions from the cervical plexus and other nerves. Logical and appropriate use of modified neck dissections demands an accurate and complete knowledge of the motor innervation of the trapezius muscle.

Research conducted by us has only used human cadavers and subjects. The obvious advantage unlike many others published on embryos or animal subjects is that it is not necessary to extrapolate our results from one species to another or even from embryo to adult within the same species. Our research is also unique in that it was a longitudinal study led by the candidate employing various scientific techniques such as cadaveric dissections, clinical assessment, electromyography, nerve conduction studies and histochemistry. The conclusions from these various studies often corroborated one another. Significant contributions were made to the understanding of the neuroanatomy of the trapezius muscle. In the study undertaking cadaveric dissections, we detailed the course of the accessory nerve and the position of the cervical contributions to the nerve. This knowledge is essential for planning modified neck dissections. We highlighted our observation of the cervical plexus branches entering the trapezius independent of the accessory nerve; something uncommented in most anatomy books. By tracing cervical plexus branch (C4) back to the ventral horn of the spinal cord, we suggested that there is probably efferent input from the cervical plexus to the trapezius. In our study of postoperative patients by clinical assessment and electromyography, we suggested that the cervical plexus does indeed provide efferents to the middle and lower portions of the trapezius. We also suggested that there is probably an undescribed motor supply to the trapezius. Additional cadaveric dissections of the lower trapezius have demonstrated nerve fibres running into and through the muscle. We suggested that these might be the undescribed motor supply. However further studies will be needed to confirm this. Results of our measurement of intra-operative motor action potentials in patients undergoing neck dissections, corrohorated the conclusions of our earlier studies that the cervical plexus provide motor innervation to the trapezius. It however highlighted the accessory nerve as the most important motor input to the trapezius muscle. Our histochemical study has not been reported by others. We were able to conclude from the study that the accessory nerve was nearly always a pure motor nerve and that branches of the cervical plexus had variable efferent fibres. With all our studies, we were then able to arrive at a comprehensive understanding of the motor innervation of the trapezius. Our studies have suggested that:

(i) The accessory nerve provides the most important efferent input to the trapezius; (ii) Motor innervation from the cervical plexus is unpredictable; (iii) The accessory nerve and the cervical plexus have overlapping motor innervation of the trapezius, with the upper portion of the muscle innervated by the accessory nerve and the middle and lower portions by both the accessory nerve and the cervical plexus; and (iv) There is probably an undescribed motor input to the trapezius in addition to that from the accessory nerve and the cervical plexus.

Based on this new understanding of the neuroanatomy of the trapezius, a hierarchy of neck dissections in which the shoulder function is least affected postoperatively, is suggested. The modifications from the classical radical neck dissections include the supraomohyoid neck dissection, preservation of the spinal accessory nerve or functional neck dissection, cable grafting of the cut ends of the spinal accessory nerve, grafting of the distal cut end of the spinal accessory to C3 or C4 stumps, preservation of the lower end of the spinal accessory and the rami communicantes from C2/C3 or C4 and preservation of C3 and/or C4 fibres running to the trapezius independent of the spinal accessory nerve. Our research while demonstrating conclusively that the cervical plexus contributes significant efferent input to the trapezius, highlighted the importance of the spinal accessory nerve in maintaining shoulder function.