Achilles tendon injuries can be difficult to manage, especially when the tendon is severely damaged or ruptured for an extended period. When direct repair is not possible due to poor tissue quality or a large defect, flexor hallucis longus (FHL) tendon transfer surgery offers a strong, reliable reconstructive option. The FHL tendon, which normally flexes the big toe, is repurposed to reinforce or replace the Achilles tendon, restoring ankle strength, function, and mobility.
How Common It Is and Who Gets It? (Epidemiology)
Chronic Achilles tendon ruptures are relatively uncommon, accounting for approximately 10–25% of all Achilles tendon injuries. They often develop when an acute rupture is missed or inadequately treated. The condition is most prevalent in middle-aged or older adults—especially men aged 40 to 60—who experience a sudden increase in activity or return to sports after a period of inactivity. Individuals with metabolic conditions such as diabetes, obesity, or chronic corticosteroid use are also at greater risk of tendon degeneration and delayed healing.
Flexor hallucis longus tendon transfer is typically used in these chronic or neglected cases, representing about 5–10% of surgical Achilles tendon reconstructions.
Why It Happens – Causes (Etiology and Pathophysiology)
Chronic Achilles tendon rupture occurs when the tendon fibers degenerate or fail to heal properly after an initial injury. Over time, scar tissue replaces normal tendon fibers, leading to weakness and elongation. The tendon retracts, creating a large gap that cannot be bridged by direct repair. Repetitive microtrauma, delayed treatment, or systemic illnesses further compromise tendon strength.
The FHL tendon is an ideal donor because it shares the same line of pull and provides strong plantarflexion (downward push-off power), effectively compensating for the damaged Achilles tendon.
How the Body Part Normally Works? (Relevant Anatomy)
The Achilles tendon connects the calf muscles (gastrocnemius and soleus) to the heel bone (calcaneus), enabling plantarflexion—the motion required for walking, running, and jumping. The FHL muscle lies deep in the posterior leg and originates from the fibula, running behind the ankle and along the underside of the foot to insert into the distal phalanx of the big toe. The FHL tendon aids in toe flexion and ankle stabilization. Because of its strength, length, and close proximity to the Achilles, it can be safely transferred to reconstruct the ruptured tendon.
What You Might Feel – Symptoms (Clinical Presentation)
Patients with chronic Achilles rupture often report:
- Weakness or inability to push off when walking.
- Difficulty climbing stairs or standing on tiptoe.
- Swelling, tenderness, or a palpable gap above the heel.
- Altered gait or limping.
If untreated, chronic cases lead to muscle atrophy, poor propulsion, and compensatory pain in other joints.
How Doctors Find the Problem? (Diagnosis and Imaging)
Diagnosis combines clinical evaluation with imaging:
- Physical exam: Reveals weakness, indentation in the Achilles region, and a positive Thompson test (absence of plantarflexion when squeezing the calf).
- Ultrasound: Identifies tendon discontinuity and scar tissue.
- MRI: Defines the extent of the rupture, degree of tendon retraction, and tissue quality.
- X-rays: May show calcification or bone spurs near the tendon insertion.
Imaging helps determine whether direct repair or tendon transfer is appropriate.
Classification
Chronic Achilles tendon ruptures are categorized by the Myerson classification:
- Type I: Small gap (<2 cm), repairable end-to-end.
- Type II: Moderate gap (2–5 cm), may require V-Y lengthening or local tissue transfer.
- Type III: Large defect (>5 cm) or poor tissue quality—requires FHL tendon transfer or graft augmentation.
Other Problems That Can Feel Similar (Differential Diagnosis)
- Acute Achilles rupture
- Chronic tendinosis or tendinopathy
- Posterior ankle impingement
- Retrocalcaneal bursitis
- Gastrocnemius or plantaris muscle tear
Treatment Options
Non-Surgical Care
Non-surgical management is rarely successful in chronic cases but may be attempted in low-demand or medically unfit patients.
- Functional bracing or orthotics to support gait.
- Physical therapy to strengthen surrounding muscles.
- Limited mobility devices to prevent re-injury.
Surgical Care
FHL tendon transfer is the preferred technique for large chronic ruptures or failed prior repairs. The procedure involves:
- Harvesting the FHL tendon through an incision near the ankle.
- Preparing the Achilles tendon by debriding scarred tissue.
- Creating a tunnel in the calcaneus to secure the FHL tendon.
- Transferring and anchoring the FHL tendon to bridge the Achilles gap.
The FHL tendon integrates into the calcaneus and remaining Achilles fibers, restoring normal function.
Recovery and What to Expect After Treatment
After surgery, the ankle is immobilized in a plantarflexed position using a cast or boot for 4–6 weeks. Weight-bearing is avoided during this period. Physical therapy begins around 6–8 weeks, progressing to strengthening and balance exercises. Most patients resume normal walking within 3 months and return to higher-impact activities within 6–12 months.
Possible Risks or Side Effects (Complications)
- Infection or delayed wound healing.
- Weakness in big toe flexion (usually mild and without functional impact).
- Scar tenderness or stiffness.
- Rarely, re-rupture or tendon elongation.
- Deep vein thrombosis or nerve irritation.
Long-Term Outlook (Prognosis)
FHL tendon transfer provides excellent long-term outcomes, with significant improvements in pain, strength, and mobility. Studies show average AOFAS ankle-hindfoot scores above 90, indicating high function and satisfaction. Most patients experience minimal loss of toe strength and can return to normal daily activities or recreational sports.
Out-of-Pocket Costs
Medicare
CPT Code 27691 – Flexor Hallucis Longus (FHL) Tendon Transfer: $175.25
Medicare Part B typically covers 80% of the approved cost for this procedure once your annual deductible has been met, leaving you responsible for the remaining 20%. Supplemental Insurance plans such as Medigap, AARP, or Blue Cross Blue Shield generally cover that remaining 20%, minimizing or eliminating out-of-pocket expenses for Medicare-approved procedures. These plans coordinate with Medicare to fill the coverage gap and reduce the patient’s financial responsibility.
If you have Secondary Insurance such as TRICARE, an Employer-Based Plan, or Veterans Health Administration coverage, it acts as a secondary payer. These plans usually cover any remaining balance, including coinsurance or small deductibles, which typically range between $100 and $300 depending on your plan and provider network.
Workers’ Compensation
If your FHL tendon transfer is required due to a work-related injury, Workers’ Compensation will cover all related medical expenses, including surgery, postoperative care, and rehabilitation. You will not have any out-of-pocket expenses, as the employer’s insurance carrier directly covers all approved costs.
No-Fault Insurance
If your FHL tendon injury or surgery is related to an automobile accident, No-Fault Insurance will typically cover the full cost of your treatment, including surgery and recovery. The only potential out-of-pocket expense may be a small deductible or co-payment, depending on your insurance policy.
Example
Michael Johnson underwent an FHL tendon transfer (CPT 27691) to restore strength and stability in his ankle after a chronic injury. His estimated Medicare out-of-pocket cost was $175.25. Since Michael had supplemental coverage through AARP Medigap, his remaining balance was fully covered, leaving him with no out-of-pocket expenses for the surgery.
Frequently Asked Questions (FAQ)
Q. What is Flexor Hallucis Longus (FHL) Tendon Transfer for Achilles Tendon Repair?
A. FHL tendon transfer is a surgical procedure used to treat chronic Achilles tendon ruptures by rerouting the flexor hallucis longus tendon from the big toe to replace the damaged Achilles tendon, improving foot and ankle function.
Q. How is FHL Tendon Transfer surgery performed?
A. The surgery involves harvesting the FHL tendon from near the big toe, creating a tunnel in the heel bone, and reattaching the tendon to the damaged Achilles tendon, allowing the FHL tendon to compensate for the Achilles tendon’s function.
Q. Why is FHL Tendon Transfer used?
A. It is used when the Achilles tendon is too damaged to heal through traditional repair methods, typically after chronic ruptures or when there is a large gap between the two ends of the Achilles tendon.
Q. What are the benefits of FHL Tendon Transfer?
A. The procedure restores the function of the Achilles tendon, improving plantar flexion and helping with walking, running, and everyday activities. Patients often report regaining nearly normal strength after surgery.
Q. What are the risks and complications of FHL Tendon Transfer?
A. Risks include infection, weakness of the big toe, and possible clawing of smaller toes. These complications are generally manageable, but additional surgery may be needed in some cases.
Q. How long does it take to recover from FHL Tendon Transfer surgery?
A. Recovery involves avoiding weight-bearing for several weeks, followed by gradual rehabilitation. Most patients regain full function within several months, though full recovery may take up to a year.
Q. Can I walk immediately after FHL Tendon Transfer surgery?
A. No, you will need to rest the foot and avoid putting weight on it for several weeks. After that, gradual weight-bearing and physical therapy will help restore function.
Q. Is physical therapy necessary after FHL Tendon Transfer?
A. Yes, physical therapy is essential to help restore strength, flexibility, and proper mobility to the foot and ankle after surgery.
Q. Who is a good candidate for FHL Tendon Transfer?
A. This procedure is ideal for patients with chronic Achilles tendon ruptures who have not had successful outcomes with other treatments, particularly when the tendon is too damaged to repair using traditional methods.
Q. What is the long-term outcome of FHL Tendon Transfer?
A. Most patients experience significant improvements in function, with many able to return to normal activities. However, the long-term success depends on the patient’s adherence to rehabilitation and recovery protocols.
Q. How long does the FHL Tendon Transfer procedure take?
A. The surgery typically takes 1 to 2 hours, depending on the complexity of the Achilles tendon injury and the specific surgical approach used.
Q. Will there be any visible scars after FHL Tendon Transfer?
A. Yes, there will be a small scar where the tendon is harvested and another where the tendon is reattached to the Achilles. These scars typically heal well and become less noticeable over time.
Q. Can FHL Tendon Transfer be performed on both feet?
A. While it is possible to perform the procedure on both feet, it is generally done one foot at a time to allow for proper healing and minimize complications.
Q. Will I need to wear a boot or cast after surgery?
A. Yes, you will likely need to wear a walking boot or cast for several weeks to protect the foot and allow the tendon to heal properly.
Q. Is FHL Tendon Transfer a permanent solution for Achilles tendon rupture?
A. For most patients, FHL tendon transfer provides a long-term solution by restoring functionality to the Achilles tendon. However, full recovery depends on proper rehabilitation, and some patients may require additional treatments for optimal results.
Summary and Takeaway
Flexor Hallucis Longus tendon transfer is a highly effective surgical solution for chronic Achilles tendon ruptures when direct repair is not feasible. It restores plantarflexion strength, enables normal gait, and provides long-term durability. With proper rehabilitation and follow-up, most patients regain near-normal function with minimal complications.
Clinical Insight & Recent Findings
A recent open-access study by Meter et al. (2022) describes an advanced technique for treating chronic or revision Achilles tendon ruptures, particularly when there is a large tendon gap or poor tissue quality. The authors combined flexor hallucis longus (FHL) tendon transfer with a posterior tibial tendon (PTT) allograft to reconstruct an 8.5 cm Achilles tendon defect.
This dual-tendon approach provided a stable repair while maintaining natural foot mechanics. The FHL tendon was anchored through the heel bone using an Endobutton system, while the PTT allograft bridged the remaining gap between the proximal and distal Achilles stumps. Postoperative care included non-weightbearing for four weeks followed by gradual rehabilitation. The technique’s main advantage lies in preserving native anatomy and improving tendon strength in complex revision cases.
According to the authors, this method offers surgeons a reliable reconstructive option that reduces the risk of re-rupture and restores functional mobility. (“Study on combining FHL tendon transfer with posterior tibial tendon graft for Achilles tendon repair – see PubMed.“)
Who Performs This Treatment? (Specialists and Team Involved)
This surgery is performed by orthopedic foot and ankle surgeons or podiatric surgeons specializing in tendon reconstruction. Postoperative management involves physical therapists and rehabilitation specialists.
When to See a Specialist?
Consult a specialist if you have chronic Achilles pain, weakness, or difficulty pushing off when walking. A visible gap or a “divot” above the heel after injury indicates the need for evaluation.
When to Go to the Emergency Room?
Go to the ER immediately after a sudden sharp pain, snapping sensation, or inability to push off your foot — these are signs of an acute rupture that require urgent attention.
What Recovery Really Looks Like?
Recovery is gradual and requires patience. After immobilization, therapy focuses on progressive stretching and strengthening. Full recovery may take 6–12 months, but most patients regain excellent strength and mobility.
What Happens If You Ignore It?
Untreated chronic Achilles ruptures lead to persistent weakness, calf atrophy, and difficulty walking. Delayed reconstruction becomes more complex and may yield less predictable results.
How to Prevent It?
Warm up before exercise, stretch the calf muscles regularly, and avoid sudden increases in activity. Proper footwear and early treatment of Achilles pain reduce the risk of rupture.
Nutrition and Bone or Joint Health
Adequate protein intake, vitamin D, calcium, and hydration promote tendon healing and bone health. Smoking cessation improves circulation and recovery outcomes.
Activity and Lifestyle Modifications
After surgery, start with low-impact activities like cycling and swimming before resuming running or jumping. Maintain flexibility and strength in the calf and ankle to prevent re-injury.
Dr. Mo Athar
