Tibialis Posterior Tendon Transfer for Foot Drop

Surgical Indications

Absolute Indications

• Established common peroneal (lateral popliteal) nerve palsy with no clinical or electromyographic recovery after at least 12-18 months of observation — confirmed irreversible loss of tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus brevis function
• Leprosy (Hansen disease) with foot drop secondary to common peroneal nerve involvement — the TP is spared because the tibial nerve is less commonly affected in leprosy, making this a classic indication
• Irreversible anterior compartment muscle loss from trauma (compartment syndrome, crush injury), ischaemia, or Volkmann-type contracture where the dorsiflexors are non-functional but the tibialis posterior is intact

Relative Indications

• Post-traumatic foot drop from deep peroneal nerve injury where surgical repair has failed or was not possible, and at least 12 months has elapsed without recovery
• Cerebral palsy or static encephalopathy with isolated foot drop and spasticity managed, where a tendon transfer may improve gait efficiency (selected cases)
• Post-polio residuals with isolated anterior compartment weakness and adequate TP power

Contraindications

Absolute:

• Tibialis posterior power less than MRC grade 4 — the transferred tendon cannot generate sufficient force for functional dorsiflexion
• Active infection in the foot, ankle, or leg
• Non-reconstructable vascular insufficiency of the limb

Relative:

• Fixed hindfoot deformity (planovalgus or equinus) — the transfer must not be performed until the hindfoot is corrected and plantigrade; address deformity first
• Stiff subtalar joint — limits the conversion of TP pull from inversion to dorsiflexion
• Severe ankle arthritis — the tendon transfer will not address underlying arthritic pain
• Poor patient compliance or inadequate rehabilitation support — the postoperative cast and physiotherapy protocol is essential to outcome
• Obesity or body mass index substantially above normal range — the transferred tendon may be overloaded and fail

Prerequisites for Transfer

Motor Assessment

• Tibialis posterior: MRC grade 4 or 5 (active heel inversion against full resistance). Test by having the patient invert the heel with the foot plantigrade while the examiner applies resistance to the medial forefoot. The tibialis posterior is the dominant invertor.
• Anterior compartment muscles: Confirmed absent or non-functional (MRC grade 0-2) — tibialis anterior, extensor hallucis longus, extensor digitorum longus.
• Peroneal muscles: Peroneus longus function is less critical but weakness is expected in common peroneal nerve palsy. Peroneus brevis is also typically involved.

Hindfoot and Foot Assessment

• Hindfoot alignment: Must be plantigrade with a neutral or correctable valgus heel. Fixed planovalgus is a contraindication until corrected.
• Subtalar joint: Must be mobile. Stiffness or arthritis at the subtalar joint impairs the transfer function.
• Midfoot: No fixed deformity or significant midfoot collapse. Assess for supple versus rigid flatfoot.
• Ankle joint: Dorsiflexion to at least neutral from the starting position. Fixed equinus contracture must be addressed (Achilles lengthening or gastrocnemius recession) before or during the transfer procedure.

Timing of Transfer

• Nerve palsy: Wait at least 12-18 months after injury before declaring the palsy irreversible and proceeding to tendon transfer. Electromyography showing complete denervation at 12 months supports the decision for transfer; clinical assessment of no recovery is equally important.
• Traumatic loss: If the anterior compartment muscles are destroyed (crush, ischaemia), transfer can be considered earlier once the soft tissues have healed and any infection is resolved.

Evidence for Surgery

Interosseous Membrane Route vs Circumtibial Route

Interosseous membrane route (Watkins, classic):

• Direct line of pull from posterior to anterior compartment, producing a more efficient dorsiflexion moment arm
• The tendon passes through a single membrane window at the proximal third of the leg
• Requires careful protection of the anterior tibial vessels and deep peroneal nerve at the membrane window
• Theoretical disadvantage: adhesion formation at the membrane window can restrict tendon excursion

Circumtibial route (around the lateral aspect of the fibula):

• The tendon is passed around the lateral border of the fibula (distal to the common peroneal nerve neck) and then subcutaneously to the dorsum
• Avoids the interosseous membrane entirely — lower risk to the anterior tibial vessels
• Longer tendon path with a less direct line of pull — potentially less efficient force transfer
• Useful when the interosseous membrane route is not possible (previous surgery, scarring, very proximal TP harvest)

Bridle Procedure (Combined Transfer)

In some cases, the TP transfer is combined with transfer of the flexor digitorum longus (FDL) and the peroneus longus through the interosseous membrane in a single sling or bridle to the dorsum. This variant is described as the Bridle procedure and may provide stronger dorsiflexion force than TP transfer alone. However, the Bridle procedure sacrifices additional tendons (FDL, peroneus longus) and is a more complex reconstruction. It is generally reserved for cases where a single TP transfer may not produce sufficient dorsiflexion power.

Key Evidence

Tibialis Posterior Tendon

Course and Insertion

• The tibialis posterior muscle originates from the posterior surface of the tibia, fibula, and interosseous membrane in the proximal third of the leg
• The tendon passes behind the medial malleolus in the tarsal tunnel (groove on the medial malleolus, covered by the flexor retinaculum)
• At the medial malleolus the TP tendon turns anteriorly and inserts primarily on the navicular tuberosity (main insertion), with slips to the sustentaculum tali, all three cuneiforms, the cuboid, and the bases of the second, third, and fourth metatarsals
• The tendon is the dominant dynamic invertor of the foot and the primary dynamic support of the medial longitudinal arch
• Total tendon length from musculotendinous junction to navicular insertion is approximately 14-18 cm, giving adequate length for transfer when harvested as proximally as possible

Neurovascular Supply

• Motor innervation: tibial nerve (L4, L5, S1) — supplied proximally in the deep posterior compartment
• Blood supply: the posterior tibial artery gives segmental branches to the tendon sheath. The tendon has a vascularised zone at the navicular insertion and a relatively hypovascular zone approximately 1-2 cm proximal to the insertion (the degenerative zone in tibialis posterior tendinopathy)
• For transfer, the tendon is harvested distally at the navicular insertion with as much length as possible, maximising the tendon available for routing to the dorsum

Interosseous Membrane

Structure and Attachments

• The interosseous membrane spans between the tibia and fibula, filling the space between the bones in the middle and proximal thirds of the leg
• Fibres run obliquely from tibia (proximal-medial) to fibula (distal-lateral) — important when orienting the window
• The membrane is strongest in its middle third and provides the attachment for the deep muscles of the anterior and posterior compartments

Neurovascular Structures Anterior to the Membrane

• Anterior tibial artery: runs on the anterior surface of the interosseous membrane in the proximal third of the leg, immediately deep to the extensor hallucis longus and tibialis anterior muscles
• Deep peroneal nerve: runs with the anterior tibial artery on the membrane surface. The deep peroneal nerve provides motor supply to tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius — all of which are paralysed in the indication for this procedure
• Superficial peroneal nerve: runs in the lateral compartment, outside the interosseous membrane — less relevant for the membrane window but at risk during lateral incisions

Safe Zone for Window Creation

• The safest zone for creating the interosseous membrane window is the proximal third of the leg, approximately 4-6 cm distal to the fibular head
• At this level the anterior tibial vessels and deep peroneal nerve are more mobile and can be retracted with less risk than at the distal third, where they run in a tighter space closer to the tibia
• The window should be proximal to the distal tibiofibular syndesmosis to avoid destabilising the ankle mortise
• Window size: approximately 3 cm in length, centred in the mid-substance of the membrane

Dorsum of the Foot — Fixation Sites

Lateral Cuneiform

• The lateral cuneiform is on the lateral side of the midfoot, articulating with the navicular proximally and the third metatarsal distally
• It provides a strong cancellous bone surface for tendon fixation with an interference screw or suture anchor
• The dorsal surface is easily accessible through an anterolateral incision on the dorsum of the foot
• Fixation into the lateral cuneiform gives an appropriate line of pull for dorsiflexion

Third Metatarsal Base

• An alternative fixation site, deep to the extensor digitorum longus tendons
• Offers a slightly different line of pull compared to the lateral cuneiform
• May be preferred if the lateral cuneiform bone quality is poor or the surgeon prefers a more lateral attachment

Extensor Retinaculum and Dorsal Anatomy

• The superior extensor retinaculum binds the extensor tendons (tibialis anterior, extensor hallucis longus, extensor digitorum longus) to the anterior aspect of the ankle
• The deep peroneal nerve and dorsalis pedis artery run between extensor hallucis longus and extensor digitorum longus on the dorsum — identify and protect at the fixation site
• The transferred TP tendon is passed subcutaneously or beneath the extensor retinaculum to the fixation site

Positioning and Preparation

Patient position: Supine on the operating table with a sandbag under the ipsilateral buttock to internally rotate the leg, bringing the medial ankle and leg to a more accessible position. The foot should be at the end of the table or over a foot bump to allow unrestricted access to the medial and lateral aspects of the ankle and leg.

Tourniquet: A thigh tourniquet is recommended (250 mmHg for adults). The procedure involves two incision sites (medial leg and anterolateral leg) and may require up to 90 minutes — adequate exsanguination improves visualisation, especially at the interosseous membrane window.

Anaesthesia: General anaesthesia or spinal anaesthesia. A popliteal block is an acceptable adjunct but not a substitute for general or spinal when a thigh tourniquet is used.

Antibiotic prophylaxis: Standard cefazolin 2 g IV at induction (or appropriate alternative for penicillin allergy).

Preparation: Prep the entire lower limb from tourniquet to toes. Ensure the medial border of the foot (navicular access) and the lateral leg (interosseous membrane access) are both in the sterile field. Mark the navicular tuberosity, the course of the posterior tibial tendon, and the proposed incisions before inflation of the tourniquet.

Consent: Specifically counsel regarding common peroneal and deep peroneal nerve injury, anterior tibial vessel injury at the membrane window, failure of transfer (persistent foot drop), over-correction (calcaneus gait), progression of planovalgus deformity, wound infection, deep vein thrombosis, and the possibility of revision surgery.

Step-by-Step Operative Technique (Interosseous Membrane Route)

Step 1: Medial Incision and TP Tendon Exposure

Make a longitudinal incision along the medial border of the foot, centred on the navicular tuberosity. Extend the incision proximally along the course of the posterior tibial tendon for approximately 6-8 cm to allow exposure of the tendon sheath and adequate length for harvest.

Open the flexor retinaculum distal to the medial malleolus. Identify the tibialis posterior tendon sheath posterior to the medial malleolus. Open the sheath and identify the TP tendon running towards its navicular insertion.

Step 2: TP Tendon Harvest from Navicular

Detach the TP tendon from the navicular tuberosity with a sharp knife or scalpel. Take a small periosteal sleeve with the tendon to improve the pull-off strength. Extend the harvest distally by continuing the tenotomy along the tendon slips to the adjacent cuneiforms if additional length is needed — however, the main insertion on the navicular provides the bulk of the tendon.

Pull the free end of the tendon proximally through the sheath using a tendon passer. Deliver the tendon as far proximally as possible into the posterior incision. The musculotendinous junction of the TP typically lies in the proximal third of the leg — deliver the tendon to this level to maximise the length available for routing through the interosseous membrane and across the dorsum.

Step 3: Anterolateral Incision and Interosseous Membrane Exposure

Make a second longitudinal incision on the anterolateral aspect of the leg, approximately 4-6 cm distal to the fibular head, centred over the proximal third of the interosseous membrane. The incision should be 8-10 cm in length.

Develop the interval between the tibialis anterior and the extensor hallucis longus. Identify the anterior tibial artery and deep peroneal nerve on the anterior surface of the interosseous membrane. Gently retract these structures laterally and protect them.

Step 4: Interosseous Membrane Window

Create a generous elliptical window in the interosseous membrane, approximately 3 cm in length, centred in the proximal third of the membrane and proximal to the distal tibiofibular syndesmosis. Excise the membrane fragment to create a clean defect.

Under direct vision, use a blunt haemostat or tendon tunneller to pass from the posterior compartment through the membrane window to confirm the pathway. The posterior tibial tendon (already harvested and delivered proximally) is now accessible from the anterior compartment side of the membrane.

Step 5: Route the Tendon Through the Membrane to the Dorsum

Grasp the distal end of the harvested TP tendon using a tendon passer introduced from the anterior side of the interosseous membrane window. Pull the tendon from the posterior compartment through the window to the anterior compartment.

Create a subcutaneous tunnel on the anterolateral aspect of the leg and ankle, passing distally towards the dorsum of the foot. The tunnel should lie deep to the subcutaneous tissue but superficial to the extensor tendons and deep peroneal nerve. Alternatively, the tendon may be routed beneath the superior extensor retinaculum for a more anatomical position.

Deliver the tendon end to the planned fixation site on the lateral cuneiform or third metatarsal base.

Step 6: Dorsal Fixation — Lateral Cuneiform or Third Metatarsal

Make a small longitudinal or transverse incision over the lateral cuneiform on the dorsum of the foot. Identify and protect the deep peroneal nerve and dorsalis pedis artery.

Prepare a bony tunnel or socket in the lateral cuneiform using a drill and curette. The tunnel should be oriented to receive the tendon with a direct line of pull for dorsiflexion.

Tensioning: Hold the ankle in neutral dorsiflexion (0 degrees) and the hindfoot in neutral inversion. Pull the TP tendon taut through the bony tunnel. The tendon should hold the ankle in approximately 10-15 degrees of dorsiflexion when the foot is released from the neutral position. Confirm that passive plantarflexion to at least neutral is still possible — if the foot is fixed in dorsiflexion, the tendon is over-tensioned.

Fixation methods:

• Interference screw: A bioabsorbable interference screw in the bone socket over the tendon — the most common and reliable method
• Suture anchor: A metal or bioabsorbable anchor in the cuneiform, with the tendon sutured to the anchor sutures
• Transosseous sutures: Sutures passed through drill holes in the cuneiform and tied over a bone bridge — traditional and reliable but more technically demanding

Step 7: Wound Closure and Immobilisation

Check haemostasis at both incision sites. Close the wound layers in the standard fashion (deep fascia, subcutaneous, skin). Apply a below-knee plaster backslab or cast with the ankle in neutral dorsiflexion and the hindfoot in neutral.

Position of immobilisation: The cast must hold the ankle in neutral dorsiflexion. A mild degree of dorsiflexion (5-10 degrees) in the cast is acceptable and protects the transfer from stretching into a flexed position during early healing. The foot should not be forced into excessive dorsiflexion in the cast, which can over-stretch the transfer.

Circumtibial Route — Alternative Technique

Indications for the Circumtibial Route

• Previous surgery or scarring at the interosseous membrane
• Very proximal TP tendon harvest with insufficient length to reach the membrane window
• Surgeon preference in selected cases

Technique

After TP tendon harvest from the navicular (Steps 1-2 identical), the tendon is passed subcutaneously around the lateral aspect of the fibula, distal to the common peroneal nerve neck. The tendon is then routed subcutaneously across the anterior ankle to the dorsum for fixation in the same manner as the interosseous membrane route.

Post-operative Protocol

Immobilisation Phase (Weeks 0-6)

• Below-knee cast: Non-weight-bearing in a below-knee plaster cast with the ankle in neutral dorsiflexion (0-5 degrees). The cast protects the tendon transfer from excessive stretch during early healing and allows the tendon to adhere to its new bony fixation site.
• Cast care: Keep cast dry; inspect for pressure areas at the incision sites; monitor neurovascular status of the foot.
• DVT prophylaxis: Continue low molecular weight heparin (or institutional protocol) while non-weight-bearing.
• Suture removal: At 2 weeks (or when wounds are healed) through a window in the cast.

Transition Phase (Weeks 6-12)

• Cast removal and orthosis: At 6 weeks, remove the cast and fit an ankle-foot orthosis (AFO) or hinged ankle brace. The AFO provides dorsiflexion support during gait re-education while the transferred tendon progressively strengthens.
• Weight-bearing: Begin progressive weight-bearing in the AFO, advancing from touch-down weight-bearing to full weight-bearing as tolerated.
• Range of motion: Gentle active dorsiflexion exercises within the AFO. Avoid passive forced dorsiflexion or aggressive stretching of the transfer.
• Physiotherapy: Structured rehabilitation focusing on dorsiflexor strengthening, proprioception, and gait retraining. The transferred TP must be trained to fire as a dorsiflexor rather than as an invertor — this is a motor re-learning process.

Strengthening Phase (Weeks 12-24)

• Wean from AFO: Gradually reduce reliance on the AFO as dorsiflexor strength improves. Most patients can walk without the orthosis by 12-16 weeks, but some will continue to use an AFO for demanding activities or uneven terrain.
• Progressive resistance exercises: Ankle dorsiflexion against resistance (theraband, weight machines). Gradually increase load.
• Gait training: Focus on normal heel-strike, mid-stance, and push-off pattern. The patient must learn to use the transferred tendon to lift the foot during swing phase.
• Functional activities: Stairs, slopes, uneven ground. Progress as tolerated.

Long-term Follow-up

• 6 months: Assess dorsiflexion power, gait pattern, hindfoot alignment. Check for early planovalgus progression.
• 12 months: Final functional assessment. Expect near-normal dorsiflexion in most well-selected patients. Long-term ankle-foot orthosis use is common for prolonged walking or uneven terrain.
• Yearly review: Monitor hindfoot alignment for progressive planovalgus deformity. Advise the patient to report any medial foot or hindfoot pain, progressive arch collapse, or difficulty with shoewear.

Special Case: Foot Drop from Leprosy (Hansen Disease)

Why TP Transfer is the Gold Standard for Leprosy

• In leprosy, the common peroneal nerve is commonly affected (thickened, infiltrated by Mycobacterium leprae), producing foot drop from anterior compartment paralysis
• The tibial nerve (supplying the TP) is much less commonly affected in leprosy, so the TP typically retains normal or near-normal power
• This anatomical selectivity makes TP transfer the ideal reconstruction for leprosy-related foot drop — a strong, functioning donor tendon is available to replace the paralysed dorsiflexors
• Large worldwide series (Carayon et al., and subsequent work in endemic regions) confirm good functional outcomes in the majority of treated patients

Special Considerations in Leprosy

• Screen for concurrent tibial nerve involvement — if the TP is also weakened, the transfer will fail. Confirm TP power MRC grade 4+ before proceeding
• Assess for trophic changes, insensate feet, and ulceration risk — protective sensation is critical for postoperative mobilisation
• Multidrug therapy for leprosy must be completed or well advanced before elective surgery
• Physiotherapy and sensory re-education of the foot are essential components of the overall management plan

Special Case: Concurrent Hindfoot Deformity

Planning the Procedure

• If the patient has a fixed planovalgus hindfoot deformity, the TP transfer must not be performed in isolation — removing the TP will accelerate the deformity
• Stage 1: Correct the hindfoot deformity first (Dwyer calcaneal osteotomy, medialising calcaneal osteotomy, or triple arthrodesis if arthritic)
• Stage 2: Once the hindfoot is plantigrade and healed, proceed with the TP transfer

Concurrent Procedure

• In selected cases, hindfoot stabilisation and TP transfer may be performed in a single stage (e.g. medialising calcaneal osteotomy at the same sitting as the TP transfer)
• This approach requires careful planning of incisions and patient positioning but reduces the total number of operative episodes

Special Case: Failed Transfer

Causes of Failure

• Inadequate TP power (grade 3 or less — should not have been transferred)
• Tendon adhesion at the interosseous membrane window restricting excursion
• Over-tensioning producing calcaneus gait (different failure mechanism — active dorsiflexion but no push-off)
• Under-tensioning producing persistent foot drop despite an intact transfer
• Fixation failure at the cuneiform (anchor or screw pull-out)
• Progression of underlying neurological disease

Management of Persistent Foot Drop after Transfer

• If the TP was grade 4+ and the transfer was technically sound, investigate for adhesions or fixation failure
• If the TP was marginally grade 4 and the transfer is weak, consider a supplementary transfer (Bridle procedure with FDL and peroneus longus) or an AFO
• If the hindfoot has developed planovalgus, address the hindfoot alignment before considering revision transfer