The Use of Grayscale Muscle Ultrasound to Indicate Muscle Recovery After Peripheral Nerve Reconstruction

Mayo Clinic, Rochester MN USA

Sara Saffari MD, MSc, Andrea J. Boon, MD

AIM: Although muscle ultrasound is routinely used to visualize changes in muscle texture in neuromuscular diseases, it is infrequently used to longitudinally assess functional recovery after nerve reconstruction. This study aimed to validate the use of grayscale muscle ultrasound by measuring echo intensity to longitudinally evaluate functional muscle reinnervation in a rabbit peroneal nerve defect model.

METHODS: 18 New Zealand White rabbits underwent a 30-mm peroneal nerve reconstruction with autografts or decellularized allograft. Standardized ultrasound measurements of the bilateral tibialis anterior muscles were performed prior to surgery and every four weeks postoperatively, and included cross-sectional muscle area, mean gray value (MGV). The MGV was divided by cross-sectional area to quantify the mean gray value normalized for area (MGVA), to correct for weight gain during the survival period. At 24 weeks, functional motor recovery was evaluated using isometric tetanic force (ITF) and compound muscle action potential (CMAP). Outcomes were compared between operated (left) and unoperated (right, control) sides within groups and as a ratio (L/R) across groups, using One-way ANOVA, corrected by Bonferroni post hoc testing. All results were reported as the mean ± SEM, with the level of significance set at p ≤ 0.05. MGVA data was compared with ITF and CMAP measurements by calculating the Spearman correlation coefficient.

RESULTS: Muscle area (Left/Right Ratio (L/R)) of autografts was superior to allografts at 4, 12, 16, 20 and 24 weeks (p<0.03 for all comparisons). MGVs of the operated side were significantly higher for autografts at 4, 8, and 12 weeks and at 12, 16, 20, and 24 weeks for allografts (p<0.01 for all comparisons), compared to their unoperated sides. Similar patterns were seen in both groups for MGVA (operated versus control side). MGVA (L/R) demonstrated a strong correlation with ITF (L/R) for autografts (ρ = -0.7) and allografts (ρ = -0.87), but inconsistent with CMAPs (L/R).

CONCLUSION: Quantitative muscle ultrasound demonstrated a reliable, non-invasive tool for evaluating motor recovery in a rabbit peroneal nerve reconstruction model. Clinical translation of grayscale ultrasound holds great promise as an outcome measure and provides valuable insights into muscle health and structural changes following nerve reconstruction, aiding early detection of complications.