FHL Transfer for Achilles Tendinosis: An Intraoperative Masterclass

Introduction and Epidemiology

Insertional and midsubstance Achilles tendinosis represents a painful, progressive degenerative process characterized by mechanical and vascular compromise affecting the paratenon and intrinsic collagen fibers. Unlike acute inflammatory tendinitis, tendinosis is a chronic, non-inflammatory degeneration marked by fibroblastic hypertrophy, disorganized collagen, and mucoid degeneration.

Epidemiologically, this pathology is most frequently observed in patients in their mid-40s to late 60s. While historically associated with "weekend warriors" and athletes subjected to repetitive eccentric loading, a significant proportion of patients present with a sedentary lifestyle. Systemic risk factors substantially increase the incidence and severity of Achilles tendinopathy. Patients presenting with seronegative arthropathies, spondyloarthropathies, hypercholesterolemia, sarcoidosis, chronic renal failure, or those with a history of fluoroquinolone or systemic corticosteroid use exhibit a markedly higher propensity for advanced tendon degeneration and subsequent rupture.

When conservative measures fail and the degenerative burden exceeds 50% of the tendon's cross-sectional area, isolated debridement is insufficient. In these advanced cases, augmentation or reconstruction utilizing a flexor hallucis longus (FHL) tendon transfer is the gold standard, providing both mechanical reinforcement and a robust vascular supply to the compromised watershed region.

Surgical Anatomy and Biomechanics

Achilles Tendon Anatomy

The Achilles tendon is the largest and strongest tendon in the human body, serving as the common distal insertion for the gastrocnemius and soleus muscles (the triceps surae) onto the posterior calcaneal tuberosity. The tendon fibers undergo a 90-degree lateral rotation as they descend; consequently, the gastrocnemius fibers insert laterally, while the soleus fibers insert medially.

The Achilles lacks a true synovial sheath, relying instead on a highly vascularized paratenon for its extrinsic blood supply. The intrinsic blood supply is derived proximally from the muscular branches and distally from the osseous insertion. This vascular architecture results in a relative hypovascular zone, or "watershed area," located 2 to 6 cm proximal to the calcaneal insertion. This zone is highly susceptible to degenerative mucoid changes and spontaneous rupture.

Flexor Hallucis Longus Anatomy

The FHL is the ideal tendon for Achilles augmentation due to its anatomical proximity, biomechanical properties, and vascular contributions. The FHL muscle belly extends remarkably distal, often reaching the level of the tibiotalar joint.

The FHL tendon courses posterior to the medial malleolus, lateral to the neurovascular bundle, and enters the foot beneath the sustentaculum tali. At the midfoot, it crosses deep to the flexor digitorum longus (FDL) at the Master Knot of Henry before inserting on the distal phalanx of the hallux.

Biomechanical Rationale for Transfer

The FHL is an in-phase plantarflexor, firing synergistically with the gastrocsoleus complex during the gait cycle. It is the second strongest plantarflexor of the ankle, though its absolute strength is only a fraction of the native Achilles. The primary biomechanical advantage of the FHL transfer is its axis of contractility, which closely parallels that of the Achilles. Furthermore, the low-lying, highly vascular muscle belly of the FHL is transposed directly into the avascular watershed zone of the Achilles, promoting angiogenesis and healing of the debrided tendon bed.

Indications and Contraindications

The decision to proceed with an FHL transfer relies on a comprehensive assessment of the tendon's structural integrity, the chronicity of the symptoms, and the patient's functional demands. The primary indication for FHL transfer is chronic Achilles tendinosis or chronic rupture where thorough debridement results in a defect comprising greater than 50% of the tendon diameter, or a gap exceeding 3 cm.

Pre Operative Planning and Patient Positioning

Clinical Evaluation

The patient must be assessed for hyperpronation or heel varus deformities, which induce eccentric Achilles tendon loading. Ankle dorsiflexion is measured utilizing the Silfverskiöld test (knee flexed versus extended) to isolate gastrocnemius tightness from global Achilles contracture. If isolated gastrocnemius tightness is present, a gastrocnemius recession (Strayer or Baumann procedure) may be considered adjunctively. Palpation identifies the point of maximal tenderness, and the size of the posterolateral calcaneal tuberosity is assessed to determine the need for a concurrent Haglund resection.

Imaging Studies

Standard weight-bearing radiographs of the foot and ankle are mandatory to evaluate the extent of intra-tendinous calcification, the presence of a Haglund deformity (measured via parallel pitch lines or Fowler-Philip angle), and overall hindfoot alignment.

While radiographs demonstrate osseous pathology, magnetic resonance imaging (MRI) is the gold standard for evaluating the soft tissue envelope. MRI delineates the precise proximal and distal extent of tendinosis, quantifies the percentage of cross-sectional degeneration, and accurately measures the gap in chronic ruptures. Fluid-sensitive sequences (T2/STIR) will highlight intratendinous mucoid degeneration and retrocalcaneal bursitis.

Patient Positioning

The patient is positioned prone on the operating table. Chest rolls and pelvic padding are utilized to ensure adequate pulmonary excursion and prevent pressure necrosis. The operative lower extremity is prepped and draped freely to allow for intraoperative assessment of ankle resting tension and equinus. A thigh tourniquet is applied. Prophylactic intravenous antibiotics are administered prior to exsanguination and tourniquet inflation.

Detailed Surgical Approach and Technique

Surgical Incision and Exposure

A posteromedial longitudinal incision is preferred to avoid the sural nerve, which courses laterally. The incision begins 1 cm medial to the Achilles tendon, extending from the musculotendinous junction to the distal calcaneal insertion.

Careful dissection is carried down through the subcutaneous tissues. The paratenon is identified and incised sharply in line with the skin incision. It is critical to elevate the paratenon as a distinct, full-thickness layer to facilitate robust closure and prevent skin tethering postoperatively.

Tendon Debridement and Calcaneal Preparation

The Achilles tendon is inspected. A central longitudinal tenotomy is performed to expose the core of the tendon. All necrotic, mucoid, and disorganized fibrotic tissue is aggressively debrided until healthy, parallel collagen fibers are encountered.

If the pathology is insertional, the tendon is detached from its central and deep insertions to expose the retrocalcaneal bursa and the posterosuperior calcaneal tuberosity. The inflamed bursa is excised entirely. A sagittal saw or sharp osteotome is utilized to resect the Haglund deformity. The resection must be aggressive enough to prevent impingement during maximal ankle dorsiflexion, typically angled from posterior-superior to anterior-inferior.

Once debridement is complete, the remaining tendon volume is reassessed. If greater than 50% of the tendon is compromised, the surgeon proceeds with the FHL transfer.

Flexor Hallucis Longus Harvest

The FHL can be harvested via a single-incision posterior approach or a dual-incision technique (adding a medial midfoot incision for maximum length). For most Achilles reconstructions, the single-incision posterior harvest provides adequate length.

Deep to the Achilles, the deep posterior compartment fascia is incised. The FHL muscle belly is identified by its low-lying lateral position relative to the posterior tibial neurovascular bundle.

CRITICAL STEP: The posterior tibial artery and tibial nerve lie immediately medial to the FHL. These structures must be identified, protected, and gently retracted medially.

The FHL tendon is isolated as it enters the fibro-osseous tunnel beneath the sustentaculum tali. The ankle and hallux are maximally plantarflexed to deliver as much tendon proximally as possible. The tendon is transected sharply at the most distal accessible point within the posterior wound.

The harvested FHL tendon is brought into the operative field and prepared with a locking Krackow or whipstitch using high-tensile, non-absorbable suture.

Tendon Transfer and Fixation

A guide pin is placed into the superior aspect of the calcaneal tuberosity, directed plantarward and slightly anteriorly. The trajectory is confirmed under fluoroscopy. The pin is over-reamed with a cannulated drill bit matched to the diameter of the prepared FHL tendon (typically 7 to 9 mm).

The FHL tendon is passed through the remaining native Achilles tendon (if bridging a gap) and drawn into the calcaneal tunnel.

Tensioning is a critical variable. The ankle is held in 10 to 15 degrees of plantarflexion (matching the resting tension of the contralateral limb). The FHL is pulled under maximal tension into the tunnel. Fixation is achieved utilizing a bioabsorbable or biocomposite interference screw placed anterior to the tendon to compress it against the stronger posterior cortical bone of the calcaneus.

Following fixation, the FHL muscle belly is sutured directly to the medial and lateral borders of the native Achilles remnant. This provides the crucial vascular ingrowth to the watershed area. Any remaining native Achilles tendon is repaired with heavy non-absorbable sutures using a Krackow technique, anchoring it via suture anchors to the calcaneus.

Closure

Meticulous hemostasis is achieved following tourniquet deflation. The paratenon is closed over the reconstructed tendon using a continuous absorbable suture. Subcutaneous tissues are closed in layers, followed by a tension-free skin closure. The limb is placed in a well-padded, short-leg splint in 15 degrees of plantarflexion.

Complications and Management

Surgical intervention in the posterior ankle carries inherent risks, primarily related to the tenuous soft tissue envelope and proximity to critical neurovascular structures.

Post Operative Rehabilitation Protocols

Successful outcomes depend heavily on strict adherence to a phased rehabilitation protocol to protect the transfer while preventing severe arthrofibrosis.

Phase I: Maximum Protection (Weeks 0 - 2)

• Weight Bearing: Strictly Non-Weight Bearing (NWB).
• Immobilization: Short-leg splint or cast in 15-20 degrees of plantarflexion (equinus).
• Goals: Wound healing, edema control, pain management.

Phase II: Progressive Loading (Weeks 2 - 6)

• Weight Bearing: Transition to partial weight-bearing, progressing to full weight-bearing as tolerated.
• Immobilization: Controlled Ankle Motion (CAM) boot equipped with 2 to 3 heel wedges.
• Therapy: Initiate active range of motion (ROM) out of the boot (plantarflexion and dorsiflexion to neutral only). Avoid passive dorsiflexion stretching. One heel wedge is removed every 1 to 2 weeks.

Phase III: Strengthening and Normalization (Weeks 6 - 12)

• Weight Bearing: Full weight-bearing in regular footwear (often utilizing a small heel lift initially).
• Therapy: Aggressive focus on restoring full active and passive ROM. Initiate isometric and isotonic strengthening of the gastrocsoleus and FHL. Proprioceptive training and bilateral heel raises progress to unilateral heel raises.

Phase IV: Return to Activity (Months 3 - 6+)

• Therapy: Plyometric training, sport-specific agility drills.
• Milestones: Patient must demonstrate symmetrical ROM and the ability to perform a single-leg heel raise prior to returning to high-impact sports. Maximum hypertrophy of the FHL and functional recovery may take up to 12-18 months.

Summary of Key Literature and Guidelines

The utilization of the FHL for Achilles augmentation is supported by decades of robust orthopedic literature.

• Wapner et al. (1993) provided the foundational clinical series demonstrating the efficacy of the single-incision FHL transfer for chronic Achilles ruptures. They highlighted the dual benefit of mechanical bridging and vascular ingrowth, noting high patient satisfaction and minimal functional deficit at the hallux.
• Coull et al. (2003) evaluated the morbidity of FHL harvest. Their biomechanical and clinical analyses demonstrated that while peak plantarflexion power of the hallux is decreased, the flexor hallucis brevis (FHB) maintains adequate function for normal ambulation, and patients rarely notice the deficit.
• Den Hartog (2003) refined the technique for insertional tendinopathy, emphasizing the necessity of aggressive Haglund resection and thorough debridement, using the FHL to reconstruct the insertion site via interference screw fixation.
• Oksanen et al. (2014) utilized post-operative MRI to prove that the transferred FHL muscle belly undergoes significant work-induced hypertrophy, functionally adapting to its new role as a primary ankle plantarflexor.

Current consensus guidelines recommend FHL transfer as the procedure of choice for Achilles defects >3 cm, severe chronic midsubstance tendinosis requiring >50% debridement, and revision Achilles surgeries where the native tissue quality is insufficient for primary repair.

Clinical & Radiographic Imaging