24 Flexor Digitorum Longus Transfer for Posterior Tibial Tendon Dysfunction Abstract The flexor digitorum longus (FDL) transfer is an integral part of the adult flatfoot reconstruction for posterior tibial tendon dysfunction (PTTD). The posterior tibialis tendon (PTT) acts as an arch supporter main invertor for push off during gait. As the PTT fails due to tearing and degeneration, the foot falls into a more planovalgus configuration. It is crucial to reestablish the equilibrium of the foot to maintain proper force distribution. The FDL tendon is the most viable substitute for the torn PTT given it is isometric to the PTT. The operation for PTTD has evolved from a simple transfer of the FDL for the torn posterior tendon (which often failed because the flatfoot was not addressed) to one step of a major hindfoot procedure. The practice now is to reconstruct the arch with a series of osteotomies (medial slide calcaneal osteotomy, lateral column lengthening, and cotton osteotomy) in conjunction with an FDL transfer on the medial side for the torn tendon. The rationale for this approach is twofold. First, the FDL tendon is much weaker than the PTT (relative power is 1.8 for FDL and 6.4 for PTT), and as a stand-alone procedure it cannot support a valgus hindfoot. Second, the disease is often not limited to just the tendon, but falls on a spectrum of peritalar subluxation (spring ligament, PTT, and deltoid ligament) and correction of the hindfoot valgus and forefoot abduction is crucial for the tendon transfer to work. The only time a tendon transfer should be considered a stand-alone procedure is for an acute PTT laceration and in the rare case of a torn tendon in a cavus foot. Keywords: FDL transfer, posterior tibial tendon dysfunction, adult-acquired flatfoot • Posterior tibial tendon dysfunction (PTTD) has been well documented as a distinct pathological entity. The tendon is particularly susceptible to attritional tears in the medial malleolar, because of attritional blood supply and with the watershed area being 2 to 3 cm proximal to the medial malleolus and is often the site of rupture (Fig. 24.1). • As the disease progresses, the peritalar subluxation occurs with progressive deterioration of the medial hindfoot structures (spring ligament, deltoid) and the foot falls into abduction and hindfoot valgus. • Four stages of PTTD have been described; most cases requiring a tendon transfer are stage 2 (Table 24.1). • A standard exam of the foot and ankle is performed with the patient seated as well as standing. Palpation of the medial ankle will reveal tenderness and swelling along the course of the PTT (Fig. 24.2). The ankle and hindfoot should be put through a full range of motion to determine if there are any fixed deformities. The patient is then examined as they stand. The amount of forefoot abduction and hindfoot valgus are determined and compared to the opposite side. The classic finding when viewing the heel from the rear is “too many toes sign” where you can see the fourth and fifth toes due to forefoot abduction (Fig. 24.3). The patient is asked to perform a single heel rise—inability to do so is pathognomonic for a ruptured PTT. Fig. 24.1 Multiple longitudinal splits are seen in the inframalleolar region of this torn posterior tibial tendon. Table 24.1 Stages of posterior tibial tendon dysfunction
24.1 Indications and Pathology
24.1.1 Clinical Evaluation
Stage | Posterior tibial tendinopathy | Planovalgus foot description |
1 | Peritendinitis and/or degeneration | Minimal/No deformity |
2 | Tendon elongation | Minimal/No deformity |
2b | Tendon elongation | Correctable deformity |
3 | Tendon elongation | Fixed planovalgus |
4 | Tendon elongation | Valgus tilt of talus within ankle with mortise with degenerative changes |
24.1.2 Radiographic Evaluation
• Preoperative magnetic resonance imaging (MRI) and standing X-rays of the foot and ankle are obtained for all patients undergoing flexor digitorum longus (FDL) transfer. A Saltzman rearfoot view is also obtained.
• X-rays are used to determine concomitant procedures, while the MRI is used to determine the status of the PTT and extent of the medial soft disease (spring ligament and deltoid).
Fig. 24.2 Posterior view showing left medial ankle swelling due to posterior tibial tendon dysfunction.
Fig. 24.3 Patient showing classic “too many toes sign” on left side due to posterior tibial tendon dysfunction.
• Weight-bearing cone computed tomography (CT) exams are becoming more commonly used to determine hindfoot position as well as lateral subluxation of the subtalar joint.
24.1.3 Nonoperative Options
• Rest, ice, and nonsteroidal anti-inflammatory medication are a first line.
• A cam walker boot is used for acute cases of tendinitis.
• An Arizona ankle–foot orthosis (AFO) or Richie brace is prescribed to control the hindfoot valgus.
• Steroid injections are almost never used given that they can cause rapid deterioration of the tendon. Platelet-rich plasma (PRP) injections will be used as an anti-inflammatory and possible regenerative therapy in conjunction with an Arizona type AFO for flexible cases early in the course of the disease.
24.1.4 Contraindications
• Stage 3 (fixed deformity) where the deformity is fixed and the foot requires a hindfoot arthrodesis or stage 4 as a stand-alone procedure.
• Relative conditions prohibiting the patient from undergoing FDL transfer are advanced age, obesity, and diabetes.
• Absolute contraindications prohibiting the patient from undergoing FDL transfer are Charcot’s foot or vascular insufficiency.
24.2 Goals of Surgical Procedure
• Stable and pain-free plantigrade foot with inversion power. Additional goal are to produce a mobile (nonfused) hindfoot to lessen stress on the ankle.
24.3 Advantages of Surgical Procedure
• FDL is readily accessible in approaching the PTT as it lies directly behind the tendon in its own sheath.
• There is little function loss of toe flexion because of cross-connections between the FDL and flexor hallucis longus (FHL) in the midfoot.
24.3.1 Alternative Tendon Transfer Procedures
• FHL transfer: this tendon is much harder to harvest and results in weakness of the hallux and the tendon must be passed from posterior to anterior around the neurovascular bundle.
• Free tendon graft: allografts or autografts are not recommended because the disease is usually too extensive given the PTT with the segmental graft most likely will continue to degenerate.
24.4 Key Principles
• The FDL is fixed to the navicular through a drill hole from plantar to dorsal and fixed either by suturing back on itself or with interference screws or anchors, which should be available depending on surgeon’s preference.
• Tension the FDL tendon in 15 degrees of plantar flexion and slight inversion but still able to get the ankle to neutral without the FDL transfer subluxing out of the posterior tibialis sheath.
• Medial calcaneal osteotomy, lateral column lengthening, or cotton osteotomies are performed prior to the tendon transfer.
24.5 Preoperative Planning and Patient Positioning
24.5.1 Anatomy
• The origin of the tibialis posterior is the lateral part of the posterior surface of the tibia, medial two-thirds of the fibula, interosseous membrane, intermuscular septa, and deep fascia.1 The PTT runs in the deep posterior compartment, then crosses from lateral to medial beneath the tendon of the FDL and enters the foot immediately behind the medial malleolus.2 It changes direction from vertical to horizontal posterior to the medial malleolus.1 PTT splits before inserting on the navicular tuberosity. The anterior band inserts on the navicular, the medial band inserts on bones of the midfoot, and the posterior band inserts on calcaneus and cuboid.3
• The PTT is found just below the medial malleolus with the FDL tendon found behind the PTT and can easily be accessed through the PTT sheath (Fig. 24.4). Care must be taken to avoid the neurovascular bundle. The FDL is traced down past the master knot of Henry, where the FHL crosses, and is harvested deep in the midfoot to get enough length. The most difficult part of the harvest of the FDL is the vascular leash of vessels underneath the navicular, which must be cauterized to prevent excessive bleeding.
24.5.2 General
• Spinal or general anesthesia is used.
• A popliteal block will require heavy sedation because the popliteal block is not as effective along the medial ankle.
24.5.3 Positioning
• Patient is placed on a beanbag with the foot inverted for the lateral procedures (calcaneal osteotomy, lateral column) that are done first.
• The beanbag is deflated and removed (or hip rolled) to allow the foot to externally rotate to allow access to the medial foot.
24.6 Operative Technique
24.6.1 Surgical Approach
A medial incision extends along the course of the PTT. Depending on the degree of damage and amount of PTT that needs to be resected, the incision will begin either at the myotendinous junction 6 cm proximal to the medial malleolus or at the tip of the malleolus. The incision extends over the medial tubercle of the navicular and then curves plantarly and distally along the path of the FDL tendon toward the knot of Henry.
24.6.2 Surgical Procedure
The tendon sheath of the PTT is opened with iris scissors to inspect the tendon. Usually the tendon is too degenerated to incorporate into the repair, but some authors opt to include the PTT or muscle into the soft-tissue reconstruction. Options include repairing the PTT and supplementing with the FDL plantar to the PTT; weaving the FDL into the degenerated PTT; resecting and tenodesing the FDL to the PTT proximally at the myotendinous junction level; or resecting the PTT completely and transferring the FDL into navicular without any tenodesis. This decision is a combination of surgeon’s preference and intraoperative and preoperative (MRI) appearance of the PTT. The tendon is the main pain generator medially, so the author tends to completely resect the torn PTT (take off the navicular and resect in the lower calf) tendon and rarely performs any of the tenodesing procedures, given the torn PTT can remain a pain generator (Fig. 24.5). We have found that tenodesis proximally to the PTT often results in retromalleolar pain and weaving distally often leaves a bulky, painful area just inferior to the navicular.
