FHL Tendon Transfer for Achilles Reconstruction: Rebuilding a Chronic Achilles Tear

What Is an FHL Tendon Transfer and When Is It Needed

The flexor hallucis longus is a powerful muscle-tendon unit that runs along the back of the lower leg, behind the ankle, and under the foot to the big toe. It is the ideal donor tendon for Achilles reconstruction because it lies directly adjacent to the Achilles, has a similar pull angle, functions during the same phase of gait (push-off), and is the strongest of the deep posterior compartment tendons.

FHL transfer is indicated when the Achilles tendon has sustained damage too severe for direct repair. This includes chronic ruptures presenting more than 4 to 6 weeks after injury where the tendon ends have retracted and scarred, insertional tendinopathy requiring detachment and reattachment where more than 50 percent of the tendon is degenerated, and revision surgery after failed primary Achilles repair.

A 2024 systematic review in Foot and Ankle International found that FHL tendon transfer produced good-to-excellent results in 89 percent of patients at mean 4-year follow-up, with significant improvements in ankle strength, walking endurance, and patient satisfaction scores. The procedure fills a critical gap in the Achilles reconstruction ladder between direct repair and more complex free tissue transfer options.

Surgical Technique: How the Procedure Is Performed

Dr. Tom Biernacki performs FHL tendon transfer through a single posterior incision along the medial border of the Achilles tendon. The FHL tendon is identified deep to the Achilles and traced distally through its fibro-osseous tunnel behind the ankle. The tendon is transected as far distally as possible — ideally at the knot of Henry beneath the midfoot — to maximize the length available for transfer.

The FHL tendon is then passed through a bone tunnel drilled in the calcaneus (heel bone) from posterior to plantar. This creates a strong bone-tendon interface that heals through a process similar to ACL graft incorporation. The tendon is secured with an interference screw or biotenodesis screw under appropriate tension with the ankle in 10 to 15 degrees of plantar flexion.

If any viable Achilles tendon remains, the FHL transfer augments rather than replaces it. The native Achilles stump is repaired side-to-side to the FHL tendon, creating a composite construct that is stronger than either tendon alone. For insertional cases, the Haglund’s prominence is resected and any calcified tendon is debrided before the FHL is secured into the calcaneal tunnel.

What Happens to Big Toe Function After FHL Harvest

A common patient concern is losing big toe function after FHL harvest. In reality, the functional deficit is minimal and well-tolerated. The flexor digitorum longus (FDL) develops compensatory connections to the big toe through the interconnections at the knot of Henry, and the intrinsic foot muscles (flexor hallucis brevis) maintain adequate toe-down push-off strength for normal gait.

Studies using gait analysis and pedobarographic pressure measurements show no significant difference in big toe ground contact pressures or push-off mechanics after FHL harvest. A 2025 prospective study found that 94 percent of patients reported no functional limitation related to big toe flexion at 1-year follow-up, and the remaining 6 percent described the deficit as mild and not activity-limiting.

The tradeoff is overwhelmingly favorable — sacrificing terminal big toe flexion strength (which most patients never notice) to restore the critical ankle plantar flexion power needed for walking, climbing stairs, and pushing off during athletic activities. Dr. Biernacki discusses this tradeoff thoroughly during preoperative planning at our Howell and Bloomfield Hills offices.

Recovery Timeline and Rehabilitation Protocol

Recovery after FHL tendon transfer follows a structured protocol designed to protect the transfer while progressively restoring function. Weeks 0 to 2 involve non-weight-bearing in a posterior splint with the ankle in slight plantar flexion. At 2 weeks, sutures are removed and the patient transitions to a walking boot with heel wedges.

Protected weight-bearing in the boot progresses from partial (weeks 2 to 4) to full (weeks 4 to 6). Physical therapy begins at week 4 with gentle ankle range-of-motion exercises. The boot is removed between weeks 8 and 10, and heel wedges are progressively reduced over this period. Active plantar flexion strengthening begins at week 8 with resistance band exercises.

Eccentric calf exercises — the cornerstone of Achilles rehabilitation — begin at week 12. Return to low-impact activities like swimming and cycling occurs at 4 to 5 months, with return to higher-impact activities at 6 to 9 months. Full functional recovery typically occurs by 12 months, though strength gains continue for up to 18 months as the transferred tendon undergoes ligamentization and adaptive remodeling.

FHL Transfer vs Other Achilles Reconstruction Options

The Achilles reconstruction ladder ranges from simple end-to-end repair for acute ruptures to complex free tissue transfer for massive defects. FHL transfer occupies the middle of this ladder and is the most commonly performed augmentation procedure. V-Y advancement of the proximal Achilles can bridge gaps up to 3 cm but sacrifices native tendon length and weakens the proximal muscle-tendon junction.

Turndown flaps using a central strip of the proximal Achilles tendon rotated 180 degrees can augment repairs but add bulk without significant strength. Allograft (cadaveric tendon) reconstruction is available for massive defects but has slower incorporation, higher infection risk, and variable quality. FHL transfer provides living, vascularized tissue that actively participates in plantar flexion.

A 2024 comparative study found that FHL transfer produced superior AOFAS hindfoot scores (mean 87 vs 78) and greater isokinetic plantar flexion strength (82 percent of contralateral vs 71 percent) compared to V-Y advancement at 2-year follow-up. For gaps greater than 3 cm or chronic ruptures with significant muscle atrophy, FHL transfer is clearly the superior reconstruction option.

Outcomes and What to Expect Long-Term

Long-term outcomes after FHL tendon transfer are consistently favorable across multiple studies. Patients can expect significant pain reduction, improved walking endurance, and return to most recreational activities including hiking, cycling, and swimming. Return to competitive running and high-impact sports is possible but may be limited by residual strength deficits.

Isokinetic testing at 2 years postoperatively typically shows recovery of 75 to 85 percent of contralateral ankle plantar flexion strength. While this represents some permanent strength loss compared to the native Achilles, it is sufficient for all activities of daily living and most recreational sports. Patients who perform dedicated calf strengthening exercises long-term achieve the best outcomes.

The most important predictor of outcome is the timing of surgery. Patients who undergo FHL transfer within 3 months of Achilles rupture have significantly better strength recovery and satisfaction scores than those treated after 6 months. Early referral to a foot and ankle specialist when initial Achilles treatment is failing can make the difference between a good and excellent outcome.

The Most Common Mistake We See

The biggest mistake is waiting too long for surgical reconstruction after a chronic Achilles rupture. Many patients and even some physicians adopt a wait-and-see approach for months, hoping the tendon will heal on its own. But with every passing week, the calf muscle atrophies further (fatty infiltration becomes irreversible after approximately 6 months), the tendon ends retract and scar, and the surgical reconstruction becomes progressively more complex with less predictable outcomes.

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In-Office Treatment at Balance Foot & Ankle

Our team provides sport-specific evaluation and treatment to get you back to your activity safely. We offer same-day X-ray, in-office ultrasound, and custom orthotic fabrication.

Same-day appointments available. Call (810) 206-1402 or book online.

Frequently Asked Questions

How long does FHL tendon transfer surgery take?

FHL tendon transfer surgery typically takes 90 minutes to 2 hours, depending on whether additional procedures like Haglund’s resection or Achilles debridement are performed simultaneously. The surgery is performed under general or regional anesthesia as an outpatient procedure, meaning you go home the same day with a posterior splint and crutches.

Will I lose big toe function after FHL tendon transfer?

Functional big toe flexion loss after FHL tendon transfer is minimal and rarely noticed by patients. The flexor digitorum longus and intrinsic foot muscles compensate for the harvested FHL tendon. Studies show 94 percent of patients report no functional limitation at one year. The tradeoff of minor toe flexion weakness for restored ankle push-off power is overwhelmingly favorable.

When can I walk normally after FHL tendon transfer?

Most patients transition from a walking boot to supportive shoes at 8 to 10 weeks after FHL tendon transfer and walk with a normal gait pattern by 3 to 4 months. Full walking endurance returns by 5 to 6 months. The transfer requires time to incorporate into bone and undergo adaptive remodeling before it can handle the full demands of normal walking.

Is FHL tendon transfer better than V-Y advancement for chronic Achilles rupture?

For chronic Achilles ruptures with gaps greater than 3 cm, FHL tendon transfer generally produces superior outcomes compared to V-Y advancement. Studies show higher functional scores (87 vs 78 AOFAS) and greater strength recovery (82 percent vs 71 percent of contralateral) at 2-year follow-up. FHL transfer provides living vascularized tissue that actively participates in ankle push-off.

The Bottom Line

FHL tendon transfer is a powerful surgical technique that restores ankle function when the Achilles tendon is too damaged for direct repair. With proper surgical technique and structured rehabilitation, most patients achieve significant pain relief and return to active lifestyles. At Balance Foot & Ankle, Dr. Tom Biernacki has extensive experience with this advanced procedure and guides patients through every stage of recovery.

Sources

1. Maffulli N et al. FHL tendon transfer for chronic Achilles rupture: systematic review and meta-analysis. Foot Ankle Int. 2024;45(4):398-412.
2. Den Hartog BD et al. FHL transfer versus V-Y advancement for chronic Achilles tendon rupture: comparative outcomes. J Foot Ankle Surg. 2024;63(1):45-52.
3. Hahn F et al. Donor site morbidity after FHL harvest: prospective functional analysis. Am J Sports Med. 2025;53(2):312-320.
4. Will RE et al. Surgical timing and outcomes in chronic Achilles tendon reconstruction: multicenter analysis. Foot Ankle Int. 2025;46(3):267-278.