The Flexor Hallucis Longus (FHL) transfer is an orthopedic surgical procedure used to reconstruct a severely damaged or weakened Achilles tendon. This technique involves repurposing a strong, nearby tendon from the foot to substitute for the function lost in the heel. The FHL tendon is moved to reinforce the Achilles. The goal of the transfer is to provide mechanical stability and restore the powerful push-off motion necessary for walking and running. This reconstructive approach is reserved for complex cases where a simple repair of the Achilles tendon is not possible.
Understanding the FHL Muscle and Function
The Flexor Hallucis Longus (FHL) is one of the deep muscles located in the posterior compartment of the lower leg, situated behind the shin bone and calf muscles. It originates primarily from the fibula. The muscle extends down to the ankle, where its tendon curves around the inner side of the heel and runs along the bottom of the foot before inserting into the base of the big toe.
The primary role of the FHL is to flex the big toe, pulling it downward toward the sole of the foot. It also assists in the ankle’s plantarflexion, the motion of pointing the foot downward. During the final phase of the walking cycle, known as the “toe-off,” the FHL provides a significant final thrust to propel the body forward. The proximity of the FHL tendon to the Achilles tendon makes it a suitable candidate for transfer, as its removal does not result in a substantial, long-term functional deficit in the foot.
Conditions Requiring FHL Transfer
The decision to perform an FHL transfer is made when the Achilles tendon damage is too extensive for a standard end-to-end repair. This procedure is indicated for chronic ruptures, which are tears that have been present for several weeks or months, leading to retraction and scarring of the tendon ends. When the gap between the torn ends of the Achilles tendon is large, often more than two centimeters, an FHL transfer provides the necessary length and material to bridge the defect without excessive tension.
This transfer is also used to augment the repair of severe Achilles tendinopathy, a degenerative condition that weakens the tendon tissue. In these cases, the surgeon must remove a large portion of the diseased tendon, and the FHL serves as a biological graft to reinforce the remaining Achilles structure. The FHL transfer is also considered in revision surgeries when a previous Achilles repair has failed. The procedure is valuable because the transferred tendon brings its own blood supply and muscle belly, which can enhance the healing potential of the area.
The Mechanics of the Surgical Transfer
The FHL transfer is performed through an incision made along the back of the ankle, often over the site of the damaged Achilles tendon. The surgeon first exposes the Achilles tendon and removes any damaged or degenerative tissue, determining the size of the defect that needs to be filled. The FHL tendon is then identified, which runs deep and slightly to the inside of the ankle, and is carefully detached from its insertion point at the base of the big toe.
Once harvested, the FHL tendon is prepared with strong sutures, often in a specialized pattern called a Krackow stitch, to allow for secure fixation. A bone tunnel is then drilled into the calcaneus (heel bone) at a location just in front of the Achilles tendon’s natural attachment point. The FHL tendon is passed through this newly created tunnel and fixed securely into the bone.
The surgeon uses anchors, interference screws, or specialized locking buttons to secure the transferred tendon inside the bone tunnel, ensuring it is held under the correct tension. This repositioning effectively reroutes the FHL muscle’s power to the heel, allowing it to take over the function of the compromised Achilles tendon. The remaining portion of the damaged Achilles tendon is then sutured around the newly transferred FHL to further reinforce the repair construct.
Post-Surgical Recovery and Rehabilitation
Recovery following an FHL transfer is a prolonged process that spans six to twelve months before a return to full activity. Immediately after surgery, the foot is immobilized in a cast or splint with the ankle pointed downward (plantarflexion) to protect the repair site from stretching. The initial phase involves a period of non-weight bearing, lasting two to six weeks, allowing the tendon to begin healing to the bone.
Around six to eight weeks post-surgery, the patient transitions to a walking boot, and a gradual introduction to partial weight-bearing begins. This middle phase focuses on controlled ankle movement. Physical therapy is initiated to prevent stiffness and begin regaining strength. The exercises are managed carefully to avoid aggressive dorsiflexion, which could overstretch the transferred tendon.
The focus shifts to advanced strengthening between three and six months, concentrating on rebuilding the calf muscle and restoring the ability to perform a single-leg heel raise. Full functional recovery, including a return to demanding athletic activities, is achieved between nine and twelve months post-operation. Consistent adherence to the physical therapy program is necessary to ensure the transferred tendon adapts to its new function and the full strength of the ankle is restored.