The Value of the Tender Muscle Sign in Detecting Motor Recovery After Peripheral Nerve Reconstruction

Purpose Squeezing a denervated muscle a few weeks after nerve repair produces a characteristic response in patients. This response is observed before any clinical evidence of motor recovery. We called this response the tender muscle sign (TMS) and wanted to determine whether this sign was related to the recovery of motor power. Methods We studied 31 adults with unilateral brachial plexus injuries who underwent 50 procedures for reinnervation of the supraspinatus, deltoid, and biceps. Follow-up was monthly for the first year and at 3-monthly intervals thereafter. Average duration of follow-up was 3.3 years. The TMS was sought at each visit. The presence of the TMS, when it was first observed, and time to Medical Research Council (MRC) grade 1 and 3 recoveries were recorded. The sensitivity, specificity, and predictive values of TMS for motor recovery were calculated. Results The TMS was always detected earlier than palpable muscle contraction. It was significantly associated with recovery of MRC grade 1 and 3 motor power. The sensitivity of TMS for MRC grade 1 recovery was 96% and specificity was 100%. For MRC grade 3 recovery, it had 97% sensitivity and 27% specificity. The positive predictive value was 100% for MRC grade 1 recovery and 83% for MRC grade 3. The negative predictive value was 50% for MRC grade 1 recovery and 75% for MRC grade 3. Conclusions Previous studies have demonstrated the presence of nocioceptive receptors in human skeletal muscle. The reinnervation of these receptors by the regenerating axons results in cramp-like tenderness when the muscle is squeezed. This response is specific to a reinnervated muscle and cannot be elicited in denervated or normally innervated muscle. The TMS is a simple, clear, and early indicator of muscle reinnervation that is useful in monitoring motor recovery after nerve regeneration. Type of study/level of evidence Diagnostic IV.