Plantar forces in flexor hallucis longus versus flexor digitorum longus transfer in adult acquired flatfoot deformity.

BACKGROUND: Flexor hallucis longus (FHL) and flexor digitorum longus (FDL) tendon transfers are frequently used to restore the function of a deficient tibialis posterior tendon in stage II adult acquired flatfoot deformity (AAFD). Either transfer causes some loss in toe flexion force, although the decision to tenodese the cut tendon to restore associated function remains controversial. This study quantified changes in plantar force before and after tendon transfer and with or without distal tenodesis in a cadaveric model.

METHODS: The plantar force distribution of 10 matched pairs of statically loaded cadaveric lower extremities was investigated. Each foot was tested when it was intact, after FDL/FHL tendon transfer, and after tendon transfer + tenodesis.

RESULTS: Transfer of either FHL or FDL showed a statistically significant decrease in flexion force of the great toe (P < .01) and lesser toes (P < .001), respectively. Subsequent tenodesis in either tendon demonstrated an ability to restore flexion force in the great (P < .05) and lesser (P < .01) toes, respectively, with the FHL transfer + tenodesis restoring great toe loading to near pretransfer levels. Following either transfer, plantar force increased in the medial forefoot; this was sustained with FDL transfer + tenodesis but reduced under FHL transfer + tenodesis. Lateral forefoot force increased modestly (8%) with FHL transfer (P < .05) but returned to near intact levels with tenodesis. FDL transfer + tenodesis resulted in increased medial midfoot and heel loading.

DISCUSSION: FHL or FDL transfer notably reduces associated toe flexion force. This loss can be restored to near normal levels with tenodesis for FHL transfer. As increased lateral forefoot loading is commonly associated with AAFD corrective procedures, FHL tenodesis may mitigate the unintended increases caused by the tendon transfer. The medial midfoot and heel loading with FDL transfer + tenodesis underscores that tendon transfers alone do not reestablish the passive architecture of the foot but augment deficient subtalar inversion force.

CLINICAL RELEVANCE: This cadaveric study shows that the FHL is more biomechanically suitable for tibialis posterior tendon insufficiency than the FDL, which may be a basis for a study to investigate whether it is superior in a clinical situation.