FHL Tendon Transfer for Achilles Tendon Reconstruction: Surgical Overview

Medically reviewed by Dr. Tom Biernacki, DPM — Board-Certified Podiatric Surgeon — Balance Foot & Ankle, Howell & Bloomfield Hills, MI. Last updated April 2026.

When the Achilles Tendon Requires Augmentation

Acute Achilles tendon ruptures managed surgically with primary repair generally have excellent outcomes. However, a subset of patients present with chronic or neglected ruptures, failed primary repairs, or severely degenerative tendons with poor tissue quality that cannot support primary repair alone. In these situations, the tendon gap is too large or the remaining tendon too compromised to achieve a durable end-to-end repair. Biological augmentation using a tendon transfer — most commonly the flexor hallucis longus (FHL) — provides the additional tissue needed to reconstruct a functional Achilles tendon.

At Balance Foot & Ankle in Howell and Bloomfield Township, Michigan, Dr. Biernacki performs Achilles tendon reconstruction including FHL augmentation for patients with complex Achilles tendon pathology requiring more than primary repair.

What Is the FHL Tendon?

The flexor hallucis longus (FHL) tendon is a powerful flexor of the big toe that originates in the deep posterior compartment of the lower leg and runs through a groove in the posterior talus to insert on the distal phalanx of the hallux. It is an ideal donor tendon for Achilles augmentation for several reasons: its line of pull is nearly parallel to the Achilles tendon; it is anatomically adjacent to the Achilles at the posterior ankle; it is a strong, large-caliber tendon; sacrifice of its distal insertion causes minimal functional deficit (big toe pushoff power is slightly reduced but interphalangeal flexion is preserved); and it has been used reliably in this application for decades with well-established outcomes.

Indications for FHL Transfer

FHL tendon transfer for Achilles reconstruction is indicated in several clinical scenarios:

• Chronic Achilles rupture — missed acute rupture presenting weeks to months later, where significant gap and muscle retraction prevent primary repair
• Failed primary Achilles repair — re-rupture or non-healing after initial surgical repair, requiring revision with additional tissue
• Insertional Achilles tendinopathy with massive degeneration — when debridement of degenerative tendon tissue leaves insufficient healthy tissue to close the defect
• Infected or compromised Achilles repair — when infection has compromised the primary repair and reconstruction requires healthy vascularized tissue

Surgical Technique

FHL tendon transfer for Achilles reconstruction is performed under general or regional anesthesia. The approach involves a posteromedial incision providing access to both the Achilles tendon and the FHL. The FHL tendon is identified medial to the Achilles, mobilized, and harvested at its most distal accessible level (at the master knot of Henry in the plantar foot, or at the posterior ankle — harvest point affects the length and excursion of tendon available). The harvested FHL tendon is then woven through or secured into the Achilles tendon repair site, providing biological augmentation with vascular tissue. The FHL may also be routed through a bone tunnel in the calcaneus to reinforce insertional repairs.

In larger defects, the FHL transfer is combined with other augmentation strategies including allograft (cadaveric) tendon, the V-Y musculotendinous advancement of the Achilles complex, or turndown flap techniques.

Recovery After FHL Transfer

Recovery from FHL Achilles reconstruction is typically longer than recovery from primary repair due to the underlying condition severity. A general timeline includes:

• Weeks 1–4: Non-weight-bearing in splint/cast; wound protection
• Weeks 4–8: Transition to boot; begin gentle weight-bearing with equinus positioning to protect repair
• Weeks 8–12: Progressive dorsiflexion; increasing weight-bearing
• Months 3–6: Physical therapy focused on calf strength and gait normalization; progressive return to activity
• Months 6–12+: Return to sport; final functional outcomes

Outcomes

Published series report good to excellent functional outcomes in 80–90% of patients undergoing FHL transfer for chronic Achilles reconstruction. The transferred FHL gains co-contraction with the Achilles complex over time, contributing meaningfully to plantarflexion power. Residual loss of FHL harvest is generally mild — many patients report no significant change in big toe function. The most important predictor of outcome is quality of the reconstruction and patient adherence to rehabilitation protocol.

Consultation for Complex Achilles Problems

Patients with chronic Achilles tendon pain, previous failed repair, or complex Achilles pathology benefit from evaluation by a foot and ankle surgeon with specific experience in tendon reconstruction. Dr. Biernacki at Balance Foot & Ankle performs comprehensive Achilles evaluation and presents all surgical and non-surgical options for informed decision-making. Contact our Howell or Bloomfield Township office for a consultation.

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Advanced Achilles Tendon Surgery in Michigan

FHL tendon transfer is an advanced surgical technique for severe Achilles tendon damage. Dr. Tom Biernacki performs reconstructive Achilles procedures at Balance Foot & Ankle to restore function and get you back on your feet.

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Clinical References

1. Den Hartog BD. “Flexor hallucis longus transfer for chronic Achilles tendonosis.” Foot Ankle Int. 2003;24(3):233-237.
2. Hahn F, et al. “Clinical results of the FHL transfer for Achilles tendon repair.” Foot Ankle Int. 2008;29(8):794-802.
3. Maffulli N, et al. “Clinical diagnosis of Achilles tendinopathy with tendinosis.” Clin J Sport Med. 2003;13(1):11-15.

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Dr. Tom Biernacki, DPM is a double board-certified podiatrist and foot & ankle surgeon at Balance Foot & Ankle Specialists in Southeast Michigan. With over a decade of clinical experience, he specializes in heel pain, bunions, diabetic foot care, sports injuries, and minimally invasive surgery. Dr. Biernacki is a member of the APMA and ACFAS, and his patient education content on MichiganFootDoctors.com and YouTube has reached over one million views.