Flexor Digitorum Longus Transfer for Tibialis Posterior Dysfunction

The indication. The flexor digitorum longus (FDL) transfer is the soft-tissue component of a joint-sparing reconstruction for a flexible stage II adult-acquired flatfoot caused by posterior tibial tendon dysfunction (PTTD), after an adequate non-operative programme of three to six months has failed. It is never performed in isolation, and never for a fixed deformity. The non-negotiable prerequisite is flexibility. The hindfoot valgus MUST passively correct to neutral - confirmed in the clinic and re-checked under anaesthesia before any incision. Operative strategy is dictated entirely by the Johnson-Strom stage:

Three requirements for the operation. A flexible (stage II) deformity that passively corrects; failure of a compliant three-to-six month non-operative programme; and a correctible, symptomatic flatfoot in a patient fit and willing to undergo reconstruction and a structured rehabilitation programme. Absolute contraindications. A rigid stage III deformity; stage IV disease with ankle valgus and deltoid insufficiency; and active infection around the surgical field. The transfer cannot correct a fixed hindfoot, and a stage IV foot needs an extended fusion. Relative contraindications. Established subtalar or talonavicular arthritis even if the deformity is "flexible" (prefer realignment and fusion); morbid obesity, poorly controlled diabetes or active smoking (markedly elevated failure and wound-complication rates - optimise first); a neuropathic or neuromuscular flatfoot in which the FDL itself may be weak or protective sensation is absent; and advanced age with low functional demand, where a moulded orthotic or ankle-foot orthosis may serve the patient better. Why FDL transfer alone fails - the central principle. Biomechanical and clinical evidence converge on one point: a transferred FDL cannot hold a corrected arch against the powerful, lateralised valgus moment of the gastroc-soleus complex, which now pulls lateral to the hindfoot axis. Two bony manoeuvres unload and protect the transfer. A medial displacement calcaneal osteotomy (Koutsogiannis) translates the calcaneal tuberosity, and with it the Achilles insertion, medially by about one centimetre, recentring the calcaneus under the tibia and reducing the deforming lever arm. A lateral column lengthening (Evans) restores the forefoot's coverage of the talar head, corrects abduction and re-establishes arch height. The FDL transfer then provides DYNAMIC medial support and restores active inversion during gait - it augments the bony correction, it does not substitute for it. When the bony correction is omitted or inadequate, the transferred tendon stretches out and the deformity recurs. Non-operative treatment - the prerequisite. Non-operative management is first-line for stage I and is trialled for early stage II before reconstruction is offered: a medial arch support with a medial heel wedge or post, or a University of California Biomechanics Laboratory (UCBL) orthotic for moderate deformity, with an articulated or solid ankle-foot orthosis for severe or more rigid deformity; activity and weight modification, weight loss where appropriate, and a physiotherapy programme of eccentric and arch-strengthening exercises; and a short-leg cast or walking boot for an acute, painful exacerbation of tenosynovitis. Consent and setup. Counsel the patient honestly that this is a major reconstruction with a year-long recovery, that six weeks of strict non-weight-bearing is mandatory, and that some persistent single-heel-rise weakness is expected because the FDL is weaker than the original PTT. Position the patient supine with the ipsilateral hip bumped and the leg externally rotated so the medial foot faces the surgeon, apply a thigh tourniquet, and have image intensification available throughout. Most patients are admitted overnight given the magnitude of the reconstruction and the strict non-weight-bearing requirement.

The goal is to expose the diseased posterior tibial tendon through a medial approach, debride it and harvest the FDL, realign the hindfoot with a medial displacement calcaneal osteotomy (the bony component that protects the transfer), then route and fix the FDL through the navicular under tension in maximal inversion and equinus. The exposure is laid out in full as the first steps below.

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Rehabilitation The rehabilitation protects both the calcaneal osteotomy and the tendon-to-bone healing of the transfer. | Phase | Timing | Protection | Milestones | |-------|--------|------------|------------| | 1 | 0 to 2 weeks | Well-padded short-leg posterior splint; strict non-weight-bearing; elevation | Wound inspection at first review | | 2 | 2 to 6 weeks | Non-articulated short-leg cast or controlled-ankle-motion boot; continue non-weight-bearing; sutures out | Radiographs confirm maintenance of displacement and hardware | | 3 | 6 to 8 weeks | Protected weight-bearing in the boot as the osteotomy heals | Gentle active ankle and subtalar range of motion; wean the boot | | 4 | 8 to 12 weeks | Full weight-bearing in a supportive shoe with a medial arch orthotic | Progressive inversion/eversion strengthening, proprioception, start single-heel-rise training | | 5 | 3 to 6 months | Return to most daily activities | Functional and sport-specific rehabilitation | | 6 | 6 to 12 months | Maturation of the reconstruction | Functional plateau at six to nine months; full recovery judged at about twelve months | A medial arch support orthotic is generally used long-term to protect the reconstruction and support the corrected alignment. Supportive lace-up footwear is recommended, and an ankle-foot orthosis is reserved for the elderly or low-demand patient with residual weakness or imbalance. Complications

The posterior tibial tendon and the disease cascade. The posterior tibial tendon is the primary dynamic supporter of the medial longitudinal arch and the prime inverter of the subtalar joint in stance. It arises from the posterior tibia, interosseous membrane and fibula, passes behind the medial malleolus in its own fibro-osseous tunnel, and inserts principally on the navicular tuberosity with an extensive plantar expansion to the cuneiforms, cuboid and the bases of the lesser metatarsals. It is roughly twice the cross-sectional area of the FDL and is the anteriormost structure in the retromalleolar groove. Degenerative attrition of the tendon — driven by chronic overload, a hypovascular watershed zone just behind the malleolus, age, female sex, obesity, hypertension and diabetes — produces elongation and then rupture. Once the tendon fails, the uncoupled gastroc-soleus, now pulling lateral to the hindfoot axis, drives the calcaneus into valgus; the talonavicular joint uncovers medially (forefoot abduction); the medial arch collapses; and the spring ligament and then the deltoid ligament may fail in sequence. The end state is a rigid, arthritic valgus flatfoot, and ultimately ankle involvement (stage IV). The flexor digitorum longus. The FDL originates from the posterior tibia below the soleal line and runs in the deep posterior compartment immediately posterior (and slightly lateral) to the PTT. In the retromalleolar groove the order from anterior to posterior is tibialis posterior, flexor digitorum longus, the posterior tibial artery and vein, the tibial nerve, and flexor hallucis longus — remembered as Tom, Dick, and a very nervous Harry. Distally the FDL crosses the FHL at the knot of Henry on the medial midfoot before dividing into four slips to the lesser toes. Why the FDL is the ideal substitute. It lies immediately adjacent to the PTT, so it can be harvested and rerouted through a short medial transfer; it is in phase, firing through midstance and push-off much like the PTT; it is already a weak inverter and plantarflexor, which is precisely the function to be restored; and it is expendable, its loss well compensated by flexor hallucis longus and quadratus plantae, so most patients have no clinically important clawing of the lesser toes after harvest.

Key evidence. Johnson and Strom (1989) introduced the three-stage clinical classification that still governs operative decision-making and defined stage II as the flexible, passively correctible deformity. Koutsogiannis (1971) described the medial displacement osteotomy of the calcaneal tuberosity for the mobile flatfoot — the bony procedure now routinely combined with FDL transfer. Myerson (1997) and the multiauthor consensus symposium (Haddad, Myerson and colleagues, 2011) codified the contemporary stage-based algorithm, emphasising that the tendon transfer augments rather than replaces the bony realignment, and that the adequacy of the bony correction determines outcome more than the tendon itself. Wacker, Hennessy and Saxby (2002) reported durable symptomatic and functional improvement with high satisfaction after the combined calcaneal osteotomy and FDL transfer at medium-to-long-term follow-up.