Clinically practical formula for preoperatively estimating the cutting rate of the spinal nerve root in a functional posterior rhizotomy

A functional posterior rhizotomy (FPR) ideally involves minimal cutting of the posterior root while providing maximal reduction of disabling spasticity. However, the ideal cutting rate has yet to be determined. It was hypothesized that the cutting rate of the posterior root would negatively correlate with preoperative motor function in children with spasticity.

Children who underwent an FPR between March 1996 and March 2017 and whose pre- and postoperative data were followed more than a year were enrolled. The preoperative Gross Motor Function Measure (GMFM) score and the overall cutting rate of the posterior root were plotted on a scatter plot, and a simple linear regression analysis was performed. The rationale for the cutting rate of the posterior root was tested by postoperative chronological changes in the GMFM score up to 5 years after the FPR. The Gross Motor Function Classification System (GMFCS) was used to group the children. The postoperative and preoperative GMFM were compared at each GMFCS level.

One hundred thirty-seven children (aged 2 to 19 years old, mean 5.9 years old) met the selection criteria. The cutting rate of the posterior root ranged from 17 to 83%, (mean 48.3%). A scatter plot was then made using GMFM scores between 10 and 90. The formula for the simple linear regression analysis was y = − 0.5539x + 73.896 (x, GMFM score; y, overall cutting rate (%)). The formula was further approximated based on the scatter plot findings as y = 100 - x. The postoperative GMFM showed an improved average score for all GMFCS levels although statistically significant improvement at postoperative 5 years was confirmed in only the GMFCS level 1 group.

The findings of this study supported the hypothesis of the negative correlation of the cutting rate of the posterior root with preoperative motor function in children with spasticity. The amount of posterior nerve root/rootlet cutting during FPR negatively correlated with the preoperative GMFM score. The approximated formula is simple, practical for clinical use, and helpful for preoperatively estimating the required overall cutting rate for the posterior root. The suggested cutting rate induced by the approximated formula should be used as a reference value and be modified according to preoperative motor function, severity and distribution of spasticity, the result of intraoperative neurophysiology, and other factors.