Motor Innervation of the Trapezius Muscle：The IntraoperativeMotor Conduction Study

In radical neck dissection, sacrifice of the spinal accessory nerve creates a definite deficit of the shoulder function. To demonstrate the presence of motor input from the spinal accessory and the cervical plexus to the trapezius muscle, intra-operative motor nerve conduction study was performed. Subjects and Method：Twenty-four patients were studied on completion of the operation. In each patient, the entire length of the spinal accessory nerve, the upper cervical plexus and some cervical plexus branches running to the trapezius were preserved independently. Compound muscle action potentials were measured for each part of the trapezius muscle on stimulation of the spinal accessory, C2, C3 and C4 nerves. Results：By stimulating the spinal nerve, evoked responses were obtained from all 24 patients in the upper, middle and lower trapezius. C2 contributions were seen in 2 out of 24 patients, supplying all three parts of the muscle in 0. C3 contributions were seen in 11 out of 24 patients, supplying all three parts of the muscle in 8. C4 contributions were seen in 20 out of 24 patients, supplying all three parts of the muscle in 16. Conclusion：The spinal accessory nerve provides the most important and consistent motor input and C2, C3 and C4 also provide motor input to the trapezius muscle. But they were not consistently present and did not innervate all three parts of the trapezius muscle. Compared with other studies, it is interesting to note that C4 gives more consistent motor inputto the trapezius muscle than other cervical branches.