High-yield overview

Flexor hallucis longus tendon transfer for neglected, re-ruptured or large-gap Achilles defects | advanced

Surgical Imaging

Critical Danger Structures and Exam Traps

Sural Nerve — Posteromedial Approach

Location: The sural nerve runs in the subcutaneous plane along the lateral border of the Achilles tendon, crossing from lateral to medial in the distal third.

Risk: During a posteromedial incision the nerve is encountered first in the subcutaneous layer. Transection causes lateral foot numbness and painful neuroma. Identify the nerve under loupe magnification before incising the paratenon.

Prevention: Use a slightly curved posteromedial incision staying medial to the midline; gently sweep the subcutaneous fat laterally to visualise the nerve before deepening.

Posterior Tibial Neurovascular Bundle

Location: The posterior tibial artery, veins and tibial nerve lie between the FHL and FDL tendons at the level of the ankle, deep to the flexor retinaculum.

Risk: The bundle is immediately medial to the FHL tendon. Aggressive medial retraction or blind dissection during harvest can injure the artery or nerve, causing ischaemia or plantar numbness.

Prevention: Identify the bundle by palpation or Doppler before dividing the flexor retinaculum; protect it with a vessel loop and harvest the FHL lateral to the bundle.

Medial and Lateral Plantar Nerves at Knot of Henry

Location: The knot of Henry lies 2-3 cm distal to the navicular tuberosity where FHL crosses FDL. The medial plantar nerve lies medial to FHL; the lateral plantar nerve lies lateral to FHL.

Risk: A plantar incision placed too medially or too laterally can transect a plantar nerve, causing permanent great-toe or lesser-toe numbness and neuropathic pain.

Prevention: Use a 3-4 cm longitudinal plantar incision centred over the knot of Henry; identify both plantar nerves before dividing the master knot and harvesting the FHL.

Calcaneal Tunnel Fracture or Blow-out

Location: The calcaneal tunnel is drilled from the posterior superior calcaneus directed distally and slightly medially.

Risk: An oversized tunnel or eccentric drilling can fracture the medial or lateral wall, especially in osteoporotic bone. A blow-out compromises interference screw purchase and risks loss of fixation.

Prevention: Use a 7-8 mm tunnel (matching tendon diameter); confirm tunnel position with fluoroscopy; avoid reaming beyond the anterior cortex.

Undercorrected or Overcorrected Tension

Location: Tension is set with the knee in 90 degrees flexion and the ankle in neutral to slight plantarflexion.

Risk: Over-tensioning produces a stiff plantarflexed ankle and weak great-toe push-off. Under-tensioning produces a calcaneus gait with persistent weakness and risk of re-rupture.

Prevention: Set resting tension so the ankle rests in 10-15 degrees plantarflexion with the knee flexed; confirm symmetric resting posture to the contralateral side before final fixation.

Wound Breakdown and Deep Infection

Location: The posteromedial wound lies over the reconstructed tendon and is under tension with early dorsiflexion.

Risk: Chronic Achilles skin is thin and poorly vascularised. Wound edge necrosis occurs in 5-15 percent; deep infection can seed the tendon-bone interface and destroy the reconstruction.

Prevention: Use a gently curved incision, minimise undermining, achieve meticulous haemostasis, close in layers without tension, and delay dorsiflexion stretching until wound is healed (4-6 weeks).

Mnemonic

F.H.L. — T.R.A.N.S.F.E.R.FHL TRANSFER — Rationale and Harvest Principles

Mnemonic

G.A.P. — B.R.I.D.G.E.ACHILLES GAP — Decision Algorithm for Reconstruction

Surgical Indications

Absolute Indications

Chronic Achilles rupture with a gap greater than 3 cm after debridement where primary end-to-end repair is impossible
Re-rupture after previous repair with retracted, degenerate tendon ends
Large insertional Achilles defect after debridement of calcific tendinopathy or Haglund resection
Failed conservative management of neglected rupture with persistent weakness and functional deficit

Relative Indications

Patient with high functional demand (sports, manual work) and a gap of 2-3 cm with poor tendon quality
Diabetic or vasculopathic patient where bringing vascularised tissue into the repair zone improves healing
Revision reconstruction after failed primary repair or augmentation

Contraindications

Absolute:

Active infection at the surgical site
Severe peripheral vascular disease with non-palpable posterior tibial pulse
Non-ambulatory patient or those unable to comply with protected weight-bearing

Relative:

Isolated great-toe pathology (hallux rigidus, prior FHL tenodesis) where donor morbidity would be unacceptable
Severe osteoporosis compromising calcaneal tunnel fixation
Heavy smoker unwilling to cease smoking perioperatively

Evidence for FHL Transfer

Rationale for FHL as Transfer of Choice

In-phase action with the Achilles (both plantarflex the ankle)
Independent vascularised muscle belly extends distally to the ankle joint, bringing blood supply into the repair zone
Lies immediately adjacent to the Achilles — minimal dissection required for harvest
Donor morbidity is low: great-toe push-off strength decreases by 20-30 percent but patients rarely report functional deficit
Superior outcomes compared with FDL or peroneal transfers in multiple series

Gap Bridging Techniques

V-Y turndown or gastrocnemius turndown flap for gaps of 3-5 cm
Direct FHL augmentation woven through the native tendon or secured into a calcaneal tunnel for larger defects
Combined techniques (turndown plus FHL) for gaps greater than 5 cm or when native tendon quality is extremely poor

Clinical Decision Scenarios

Practise clinical reasoning and management decisions out loud

Viva scenarioAdvanced

Clinical prompt

“A 52-year-old recreational runner presents with a 6-month history of a neglected Achilles rupture. MRI shows a 4.5 cm gap with retracted, degenerate tendon ends. He has a palpable gap and cannot perform a single-leg heel rise. How do you manage him?”

Practical approach

This patient has a chronic neglected Achilles rupture with a gap greater than 3 cm and poor tendon quality — primary repair is not feasible. I would offer reconstruction with FHL tendon transfer. **Pre-operative assessment**: I would confirm the diagnosis with MRI (already done) and assess vascular status (posterior tibial pulse), great-toe function, and patient expectations regarding donor morbidity and rehabilitation. I would discuss the 20-30 percent reduction in great-toe push-off strength and the 8-12 week protected weight-bearing protocol. **Surgical plan**: Posteromedial approach with identification and protection of the sural nerve. Debride the gap to healthy tendon. Harvest the FHL via the posteromedial wound, extending to a plantar incision at the knot of Henry if additional length is required. Protect the posterior tibial neurovascular bundle and both plantar nerves. Prepare a 7-8 mm calcaneal tunnel and secure the FHL with an interference screw tensioned to rest the ankle in 10-15 degrees plantarflexion. If the gap remains large after debridement, perform a V-Y turndown to reduce the defect before FHL augmentation. **Post-operative**: Non-weight-bearing cast for 2 weeks, then protected weight-bearing in a boot with gradual dorsiflexion stretching from week 4. Structured physiotherapy with return to running at 5-6 months. **Evidence**: Multiple Level III-IV series report greater than 80 percent patient satisfaction, significant improvement in AOFAS scores, and re-rupture rates of 5-10 percent with FHL transfer for chronic defects.

Viva scenarioAdvanced

Clinical prompt

“During an FHL transfer for a 5 cm chronic Achilles gap you have harvested the FHL via a posteromedial approach and prepared the calcaneal tunnel. You are about to tension and fix the tendon. What are the key technical points for tensioning and fixation, and what are the consequences of getting them wrong?”

Practical approach

Tensioning and fixation are the two most critical steps that determine the functional outcome of an FHL transfer. **Tensioning principles**: With the knee flexed 90 degrees (gastrocnemius relaxed) and the ankle in neutral to slight plantarflexion, I set the resting tension so the ankle comes to rest in 10-15 degrees of plantarflexion. This matches the contralateral side and prevents both a calcaneus gait (under-tensioned) and a stiff plantarflexed ankle (over-tensioned). I confirm the position by comparing to the contralateral limb before final fixation. **Fixation technique**: I use a 7-8 mm interference screw in a matching calcaneal tunnel drilled from posterior to plantar. The screw must achieve line-to-line purchase without fracturing the tunnel wall. I confirm solid fixation by attempting to pull the tendon out of the tunnel before final seating. Fluoroscopy confirms screw position and length. **Consequences of error**: Over-tensioning produces a stiff ankle with weak push-off and difficulty with stairs or rising from a chair. Under-tensioning produces a calcaneus gait with persistent weakness and increased risk of re-rupture. Tunnel blow-out from oversized reaming or eccentric drilling compromises fixation and may require alternative fixation methods (anchors, button) or bone graft augmentation. **Verification**: I always perform a final manual test of fixation strength and resting posture before wound closure.

Viva scenarioAdvanced

Clinical prompt

“A 48-year-old woman underwent FHL transfer 9 months ago for a chronic Achilles rupture. She has good pain relief and can walk unlimited distances but complains of weakness when rising onto her toes and difficulty with stairs. Single-leg heel rise is possible but weak compared with the contralateral side. How do you assess and manage her?”

Practical approach

This patient has a functional deficit in plantarflexion strength after FHL transfer — a recognised outcome that requires systematic assessment. **Expected outcomes**: After FHL transfer, single-leg heel rise strength is typically 60-80 percent of the contralateral side. Great-toe push-off strength is reduced by 20-30 percent. Most patients adapt and report good functional outcomes, but some notice weakness with high-demand activities. **Assessment**: I would perform a formal single-leg heel rise test (repetitions to fatigue) and measure calf circumference. I would assess great-toe IP flexion strength and compare to the contralateral side. I would examine for compensatory patterns (excessive use of FDL, peroneals) and check for contralateral Achilles tendinopathy (increased risk after unilateral rupture). **Differential diagnosis for persistent weakness**: (1) Under-tensioned transfer (calcaneus gait), (2) Incomplete rehabilitation or poor compliance, (3) Contralateral Achilles pathology limiting comparison, (4) Donor morbidity from FHL harvest (usually well tolerated), (5) Re-rupture or failure of fixation (unlikely at 9 months if pain-free). **Management**: If the transfer is well tensioned and the patient has completed structured rehabilitation, I would reassure her that 60-80 percent strength is an expected outcome and focus on activity modification and maintenance strengthening. If there is clinical suspicion of under-tensioning or failure, I would obtain MRI to assess tendon continuity and muscle quality. Revision surgery is rarely indicated for isolated weakness in a pain-free patient.

Exam day cheat sheet

Chronic Achilles Reconstruction with FHL Transfer — Exam Day Summary

References

Evidence

Level III

Wegrzyn J et al. Chronic Achilles tendon rupture reconstruction using a modified flexor hallucis longus transfer

Clinical implication: FHL transfer provides reliable functional restoration for chronic Achilles defects greater than 3 cm with low re-rupture rates and acceptable donor morbidity.

Source: Int Orthop 2010;34(8):1187-92

Verify on PubMed (PMID 19697026)

Evidence

Level III

Den Hartog BD. Flexor hallucis longus transfer for chronic Achilles tendonosis

Clinical implication: FHL transfer is effective for chronic Achilles reconstruction with durable results and minimal donor morbidity.

Source: Foot Ankle Int 2003;24(3):233-7

Verify on PubMed (PMID 12793486)

Evidence

Level IV

Rahm S et al. Operative treatment of chronic irreparable Achilles tendon ruptures with large flexor hallucis longus tendon transfers

Clinical implication: Large FHL transfers are effective for irreparable chronic Achilles ruptures with substantial gaps.

Source: Foot Ankle Int 2013;34(8):1100-10

Verify on PubMed (PMID 23624909)

Evidence

Level IV

Will RE et al. Outcome of single incision flexor hallucis longus transfer for chronic achilles tendinopathy

Clinical implication: FHL transfer via single incision is a reliable option for chronic Achilles pathology.

Source: Foot Ankle Int 2009;30(4):315-7

Verify on PubMed (PMID 19356355)