Functional compensative mechanism of upper limb with root avulsion of C-5-C-6 of brachial plexus after ipsilateral C-7 transfer

Objective: To investigate the compensative mechanism of no further impairment of the upper limb after ipsilateral C-7 transfer for treatment of root avulsion of C-5-C-6 of the brachial plexus. Methods: Sixty Sprague Dawley (SD) rats were randomly divided into a C-7-transection group and a control group, 30 rats each. In the C-7-transection group, the left forelimbs of the animals underwent transection of ipsilateral C-7 nerve root while C-5 and C-6 nerve roots were avulsed. In the control group, the left forelimbs only underwent C-5 and C-6 root avulsion. The representative muscles of C-7 (innervated mainly by C-7) including latissimus dorsi, triceps, extensor carpi radialis brevis and extensor digitorum communis were evaluated with neurophysiological investigation, muscular histology and motor end plate histomorphometry 3, 6 and 12 weeks after operation. The right forelimbs of all rats were taken as the control sides. Results: Three weeks after operation, the recovery rates of amplitudes of compound muscle action potential (CMAP) and CMAP latency, muscular wet weight and cross-sectional area of muscle fibers, and area of postsynaptic membranes of those four representative muscles in the C-7 transection group were significantly lower than those of the control group (P <0.05 or P<0.01). Six weeks postoperatively, the recovery rates of CMAP amplitude and latency of the triceps showed no significant difference between the C-7 transection group and the control group (P>0.05). For the extensor carpi radialis brevis and the extensor digitorum communis, the recovery rates of the cross-sectional area of muscle fibers, the amplitude and latency of CMAP and the area of postsynaptic membranes showed no significant difference between the two groups (P >0.05), while the rest parameters were still significantly different between the two group (P<0.05 or P<0.01). As far as the ultramicrostructure was concerned in the C-7-transection group, more motor end plates of four representative muscles were observed and their ultramicrostructure also had a tendency to mature as compared with those of 3 weeks postoperatively. Twelve weeks after operation, all parameters of the C-7-transection group were not significantly different from those of the control group (P>0.05). In the C-7-transection group, the motor end plates were densely distributed and their ultramicrostructure in four representative muscles appeared to be mature as compared with those of the control group. Conclusions: After ipsilateral C(7)transfer for treatment of root avulsion of C-5-C-6 of the brachial plexus, the nerve fibers of the lower trunk can compensatively innervate fibers of C-7-representative muscles by means of motor end plate regeneration, so there is no further impairment on the injured upper limb.