Point-Counterpoint: Is The Flexor Digitorum Longus Tendon Transfer Effective For Stage 2 Adult-Acquired Flatfoot?
These adjunctive procedures improve the longitudinal arch of the foot by decreasing the antagonist hindfoot eversion force and increase the overall hindfoot inversion force by increasing the moment arm of the transferred tendon.5 The typical adjunctive procedures with Stage 2 PTTD are a gastrocnemius-soleus recession or a medial displacement calcaneal osteotomy in conjunction with the flexor digitorum longus transfer. A medial displacement calcaneal osteotomy reverses the coronal plane hindfoot malalignment. This medial displacement helps medialize the pull of the Achilles tendon to reduce the antagonist’s pull on the relatively weak flexor digitorum longus tendon.12-14
Step-By-Step Surgical Insights
My approach to Stage 2 PTTD that has failed conservative treatment is a gastrocnemius-soleus recession with a medial displacement calcaneal osteotomy and transfer of the flexor digitorum longus tendon through a dorsal to plantar bone tunnel through the navicular. The flexor digitorum longus tendon transfer acts to support the longitudinal arch of the foot and augment the posterior tibial tendon. This transfer also gives a static support to the often attenuated spring ligament. Surgeons most often inferiorly reflect the posterior tibial tendon for later repair.
If the posterior tibial tendon disease process is extensive, resect it. This diseased tendon may cause persistent pain if one does not resect it, leading to dissatisfaction of the patient postoperatively.5 Trevino, Moseir and their respective colleagues showed that stage 2 posterior tibial tendons were diseased microscopically with tendinosis characterized by mucinous degeneration, fibroblast hypercellularity, chondroid metaplasia and neovascularization.15-17 This results in a disruption in the collagen bundle structure and orientation.
One can access the insertion of the posterior tibial tendon to determine if an accessory ossicle is present. If so, excise this ossicle. After either reflecting or resecting the posterior tibial tendon, identify the flexor digitorum longus tendon sheath and expose the tendon. Carry dissection distally to the knot of Henry. With the ankle in maximum plantarflexion and the lesser digits in maximum plantarflexion, to decrease the tautness of the flexor digitorum longus tendon, transect the tendon. Take care to avoid the medial plantar nerve, which lies just plantar to the tendon. Transecting the flexor digitorum longus at the knot of Henry leaves the interconnections between the flexor digitorum longus and flexor hallucis longus intact, allowing for preservation of the flexor hallucis longus function.18-20
Then pass the flexor digitorum longus from inferior to superior through the navicular bone tunnel. The forefoot is supinated and the surgeon sutures the flexor digitorum longus tendon in a side-to-side anastomosis back to itself. Once this transfer is complete, advance the posterior tibial tendon to augment the repair and suture the posterior tibial tendon into the transferred flexor digitorum longus tendon.
What Other Studies Reveal
Studies have shown an increase in residual strength of the transferred muscle and tendon secondary to hypertrophy.21,22 The decreased muscle mass of the antagonistic muscles as compensation from the medial displacement calcaneal osteotomy may also contribute to a relatively increased strength of the flexor digitorum longus tendon transfer. Wacker and colleagues compared magnetic resonance imaging (MRI) in 12 patients with unilateral stage 2 PTTD with the asymptomatic leg.21 The MRI showed a mean atrophy of the posterior tibial tendon of 10.7 percent and a hypertrophy of the flexor digitorum longus tendon of 17.2 percent. The study also showed hypertrophy of the flexor digitorum longus tendon to be 44 percent greater than the contralateral side when the posterior tibial tendon received resection.