Split tendon transfer of the posterior tibialis for spastic equinovarus foot deformity: Does tendon routing impact post-operative ankle kinematics?

Background: Split tendon transfer of the posterior tibialis (SPOTT) is a surgical procedure in which the split posterior tibialis tendon is transferred posterior to the fibula (PO) with insertion on the peroneus brevis tendon to rebalance the forces across the hindfoot. Routing of the split tendon through the interosseous membrane (IO) is a variation with the potential benefit of augmenting ankle dorsiflexion in swing. Research question: Does IO routing improve ankle dorsiflexion in swing and/or varus in stance compared to PO routing? Methods: A retrospective chart review was completed to identify forty-two patients who underwent a SPOTT procedure for equinovarus foot deformity. 14 patients received IO routing and 28 received PO routing. Two main outcomes: maximum ankle dorsiflexion in swing and average ankle coronal angle in stance were analyzed using a mixed effects model. Results: Both the IO and PO routing significantly improved maximum dorsiflexion in swing (p = 0.018 and p = 0.001, respectively.) There was no significant difference in the maximum dorsiflexion during swing, postoperatively, between the two groups (p = 0.431). Pre-operatively, patients in the IO group exhibited significantly more average ankle varus during stance, with a median of 6.7 degrees of varus compared to a median of 1.8 degrees of varus in the PO group (p = 0.024). The IO group yielded a statistically significant reduction in average ankle varus during stance (p < 0.001) whereas there was no significant change in the PO group (p = 0.138). There was no difference between groups in proportion of patients that achieved an average ankle coronal angle during stance within one SD of normative data (between-8.7 and 0.7 degrees, p = 0.381). Significance: The SPOTT procedure utilized for correction of spastic equinovarus foot deformity, can improve ankle kinematics with routing of the tendon through the IO membrane or posterior to the fibula if performed concurrently with the appropriate tendon lengthening to address equinus contracture. More clinical studies are needed to better understand potential confounding factors.