Electrophysiological study of the dominant motor innervation to the extensor digitorum communis muscle and long head of triceps brachii at posterior divisions of brachial plexus

BACKGROUND: Restoration of elbow and finger extension function is still challenging in management of complete brachial plexus avulsion injury, mainly because of fewer available donor nerves for transfer to the radial nerve. Selective neurotization could be a potentially alternative for overcoming this dilemma. This study was designed to identify the innervation dominance of the extensor digitorum communis muscle (EDCM) and long head of the triceps brachii (LTB) at the level of division of brachial plexus. METHODS: From February 2008 to October 2009, 17 patients with complete brachial plexus avulsion injury underwent the procedure of contralateral C7 nerve root transfer. The posterior divisions of brachial plexus on the healthy donor side were intraoperatively stimulated and the compound muscle action potentials (CMAPs) from the extensor digitorum communis muscle and long head of triceps brachii were recorded by an electrophysiological device. RESULTS: In 13 out of 17 patients (76.5%), the maximal amplitude of CMAP from EDCM was induced by stimulation of the posterior division of lower trunk (PDLT). The mean amplitudes of CMAP from EDCM with stimulation of the posterior division of upper trunk (PDUT), middle trunk (PDMT), and PDLT were 0.64 ± 0.95, 1.64 ± 1.56, and 5.32 ± 4.67 mV (P < 0.05), respectively. The maximal amplitude of CMAP from LTB was induced mainly by stimulation of the PDMT) and PDLT (6 out of 11 and 5 out of 11 patients). The mean amplitudes of CMAP from LTB with stimulation of the PDUT, PDMT, and PDLT were 0.15 ± 0.24, 5.20 ± 4.27, and 7.48 ± 9.90 mV, respectively. The differences of CMAPs between stimulation of PDUT and other two divisions were significant (P < 0.05). CONCLUSIONS: From the electrophysiological point of view, this study showed that the PDLT was the major motor division innervating EDCM, and the PDMT and PDLT shared the similar proportion of LTB innervation.