The efferent supply of the trapezius muscle: a neuroanatomic basis for the preservation of shoulder function during neck dissection.

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The hypothesis that there is an alternative motor input to the trapezius muscle through cervical spinal nerves is evaluated through a review of the literature and through experimental studies performed by the author. Electrophysiologic testing of the spinal accessory nerve and of the ventral rami of the cervical spinal nerves was performed in cats and humans. These studies indicated that motor potentials recorded in the trapezius muscle after stimulation of the ventral rami of cervical spinal nerves in general represented the spread of current to the contiguous portions of the spinal accessory nerve or directly to the trapezius muscle. Rarely did there seem to be evidence of motor innervation to the upper and lower trapezius by fibers conveyed by the ventral rami of cervical spinal nerves. The predominant motor input to the trapezius muscle is through the spinal accessory nerve. Subjective evaluation, objective strength testing, and electromyographic evaluation were performed on patients who had undergone neck dissections. Three types of neck dissection were employed. These were modified neck dissection with preservation of the spinal accessory nerve, radical neck dissection with sacrifice of the spinal accessory nerve, and reconstruction using a cable graft and classical radical neck dissection. Regarding all of the methods of evaluation and comparison, the patients who retained function of the spinal accessory nerve in the postoperative period fared better than those who had a classical radical neck dissection with sacrifice of the spinal accessory nerve. This again supports the concept that the most important and predominant motor supply to the trapezius muscle is through the spinal accessory nerve. All of this information was used to construct a model of the spinal portion of the accessory nerve and of the innervation of the trapezius muscle. The clinical implications of this model are stated.