Noninvasive ultrasound of the tibial muscle for longitudinal analysis of nerve regeneration in rats

Background: Today's criterion standards for measuring functional recovery after nerve trauma in experimental studies are the muscle mass ratio and the isometric tetanic force; both tests are invasive and require a sacrificial procedure. The authors propose ultrasound as a noninvasive method to determine muscle atrophy, and evaluate its validity and reliability by comparing it to muscle mass ratio, isometric tetanic force, and histology. Methods: Fifty rats sustained a 10-mm autograft sciatic nerve reconstruction. With a 2-week interval, five animals were tested with a total follow-up of 20 weeks. The functional recovery of the hind-limb muscles was measured with ultrasound, muscle mass ratio, and isometric tetanic force. In addition, neuromuscular junctions were analyzed histologically. The different evaluation techniques were compared and the reliability of the ultrasound was determined. Results: Four weeks after denervation, extensive muscle atrophy resulted in a decrease of muscle mass up to 30 percent. Ultrasound showed good correlations with muscle mass ratio for both tibial (r = 0.85) and gastrocnemius muscles (r = 0.89). Both intrarater reliability (r = 0.97) and interrater reliability (r = 0.88) of the ultrasound were high. The correlation with force was lower (0.62) but still statistically significant. Conclusions: Ultrasound measurement of muscle atrophy was highly correlated with the criterion standard muscle mass ratio and was also significantly correlated with isometric tetanic force. Histologic evaluation confirmed the regeneration pattern observed with ultrasound. The authors propose that ultrasound can be used as a valid alternative to muscle mass ratio to study muscle atrophy after nerve injury in a less-invasive and more animal-friendly manner.

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