Functional evaluation in the rat sciatic nerve defect model: a comparison of the sciatic functional index, ankle angles, and isometric tetanic force.

Background: The sciatic functional index has long been the standard method of assessing motor recovery in the rat sciatic nerve model. The relative subjective nature of the assessment has led to development of newer methods, including video gait analysis and quantitative measurement of isometric tetanic muscle force.

Methods: Forty male Lewis rats, each with a 10-mm segmental defect in the sciatic nerve, were divided randomly into two groups: rats in group I underwent repair with reversed autograft, and those in group II received a collagen conduit. Video gait analyses were performed at 0, 4, 8, and 12 weeks, and sciatic functional index and ankle angles in four different walking phases were recorded. Isometric tetanic force of the tibialis anterior muscle was also measured at 12 weeks and correlated with sciatic functional index and video gait analysis data.

Results: The sciatic functional index results did not correlate with isometric tetanic force. Significantly, the sciatic functional index could not be measured in 26 percent of the rats at 8 weeks and 59 percent of the rats at 12 weeks secondary to toe contractures. Among various ankle angle measurements, only the ankle angle in toe-off phase correlated well with isometric tetanic force.

Conclusions: Toe contractures occurred more frequently in rats with better nerve recovery, and interfered with evaluation of the motor recovery using the sciatic functional index method. Ankle angle in toe-off phase measured from video gait analysis is a useful parameter that reflects functional recovery of the muscle force.