FHL Transfer: Procedure, Recovery, and Muscle Adaptations

Surgical tendon transfers restore function in cases of muscle or tendon dysfunction. The flexor hallucis longus (FHL) transfer is commonly performed to compensate for Achilles tendon injuries or chronic degeneration. By redirecting the FHL tendon, surgeons reinforce plantarflexion while preserving foot function.

Understanding this surgery requires examining its impact on biomechanics, recovery timelines, and long-term muscular adaptations.

Anatomy Of The Flexor Hallucis Longus

The flexor hallucis longus (FHL) is a deep posterior compartment muscle of the lower leg, crucial to foot mechanics. Originating from the distal two-thirds of the fibula and adjacent interosseous membrane, it courses down the posterior leg, passing behind the medial malleolus before inserting onto the distal phalanx of the great toe. This trajectory enables it to flex the hallux powerfully and stabilize the foot during movement.

As the tendon travels through the tarsal tunnel, a synovial sheath reduces friction for smooth gliding. The FHL shares this space with the flexor digitorum longus (FDL) and tibialis posterior, making it susceptible to entrapment or irritation. At the plantar foot, it crosses the FDL at the Knot of Henry, allowing some force distribution, though the FHL remains the dominant hallux flexor.

The FHL plays a key role in weight-bearing activities, particularly during the push-off phase of gait, where it provides propulsion and stabilizes the medial longitudinal arch. This function is especially important in sprinting and jumping, where high activation levels have been observed in electromyographic (EMG) studies.

Indications For Transfer

The FHL transfer restores plantarflexion strength and compensates for posterior ankle tendon insufficiency. A common indication is chronic Achilles tendinopathy, particularly when conservative management fails. Degenerative changes from repetitive microtrauma can lead to partial or full-thickness tears, impairing push-off strength and gait efficiency. When the Achilles tendon is irreparable, the FHL transfer reinforces plantarflexion, supporting load-bearing activities.

FHL transfer is also used in Achilles tendon rupture cases where primary repair is unfeasible due to extensive tissue loss. The FHL’s proximity to the Achilles insertion and robust structure make it an effective graft for bridging tendon defects. Studies have shown that patients with chronic Achilles ruptures experience improved plantarflexion strength and gait mechanics after FHL transfer.

Another indication is posterior tibial tendon dysfunction (PTTD), a progressive condition leading to medial arch collapse and foot instability. In advanced stages, when primary repair is insufficient, the FHL can be redirected to support the hindfoot. Research indicates that this transfer improves medial arch integrity and reduces pain in severe dysfunction cases.

In neuromuscular disorders like Charcot-Marie-Tooth disease or post-stroke spasticity, FHL transfer addresses foot muscle imbalances. These conditions often cause foot drop or claw toe deformities, leading to gait abnormalities. By rerouting the FHL, surgeons enhance foot positioning and mobility. Electromyographic studies suggest the procedure improves foot control and reduces compensatory strain.

Procedural Techniques

Performing an FHL transfer requires careful surgical planning to optimize tendon integration and biomechanics. The procedure begins with patient positioning in a prone or supine orientation. A posteromedial incision along the ankle provides access to the FHL tendon. Careful dissection preserves nearby neurovascular structures, particularly the tibial nerve and posterior tibial artery. Once identified, the tendon is released from its distal insertion to allow mobilization toward the calcaneus.

Fixation to the calcaneus is achieved using a bone tunnel, interference screw, or suture anchor. Biomechanical studies favor interference screws for their superior pullout strength and reduced risk of elongation under load. Proper tensioning is critical—excessive tightness restricts dorsiflexion, while inadequate tension fails to restore plantarflexion strength. Fluoroscopic guidance ensures optimal positioning before final fixation.

Graft preparation is essential for success. The FHL tendon may be reinforced with biological scaffolds or synthetic constructs, particularly in revision surgeries. Surgeons must align the transferred tendon’s pull with the native Achilles function. Suturing techniques, such as the Krakow or whipstitch method, provide strong fixation while minimizing bulk at the repair site. Soft tissues are meticulously reapproximated to promote seamless tendon integration.

Effects On Foot Biomechanics

Redirecting the FHL tendon alters foot mechanics, particularly in plantarflexion and propulsion. The FHL assists the gastrocnemius-soleus complex in generating push-off force. When transferred, its contribution at the hallux is lost, shifting the load to the remaining flexor tendons and intrinsic foot muscles. This redistribution can subtly affect gait, especially in activities requiring forceful toe-off, such as running or jumping.

The absence of FHL function at the great toe may reduce balance and toe grip strength, though compensatory mechanisms emerge. The flexor digitorum longus (FDL) assumes a greater role in forefoot stabilization. Kinematic analyses show minor reductions in hallux plantar pressure post-transfer, though these typically do not impair overall foot function. The extent of these changes depends on individual biomechanics, with athletes experiencing more pronounced adaptations.

Rehabilitation And Recovery Dynamics

Post-operative management focuses on tendon integration while minimizing complications. Immobilization in a plantarflexed position using a cast or walking boot lasts four to six weeks, allowing early tendon healing. Weight-bearing protocols vary, with some approaches permitting partial weight-bearing within weeks, while others require a more conservative progression. Patients are advised to avoid dorsiflexion beyond neutral to prevent strain on the transferred tendon.

As healing progresses, rehabilitation restores range of motion, strength, and mobility. Physical therapy begins with passive and active-assisted exercises to prevent stiffness. By eight to twelve weeks, gradual strengthening exercises target the gastrocnemius-soleus complex and remaining foot flexors. Eccentric loading protocols enhance tendon resilience. Return to full activity, including high-impact sports, typically occurs between six to twelve months. Long-term outcomes show most patients regain near-normal gait, though minor deficits in toe flexion strength may persist.

Muscular Adaptations Over Time

Following FHL transfer, the body undergoes neuromuscular adjustments to compensate for altered tendon function. The flexor digitorum longus (FDL) assumes greater responsibility for toe flexion, as demonstrated in electromyographic studies. This shift mitigates the loss of the FHL at the great toe, though minor reductions in fine motor control may occur. The intrinsic foot muscles, including the abductor hallucis and lumbricals, also contribute more actively to stability. Over time, these compensatory patterns become efficient, reducing functional deficits.

At the ankle, the gastrocnemius-soleus complex adapts to the additional workload. Muscle hypertrophy in this region has been observed in long-term follow-ups, especially in individuals engaged in strength training. This adaptation bolsters plantarflexion force, ensuring propulsion remains sufficient for walking and athletic performance. Despite these adjustments, subtle biomechanical differences may persist, particularly in tasks requiring fine toe coordination. Longitudinal studies indicate that while most patients achieve satisfactory function, some experience mild foot fatigue during prolonged activity. Targeted rehabilitation remains essential for optimizing long-term performance.