FDL surgery is a tendon transfer procedure where the flexor digitorum longus, a tendon that curls the lesser toes, is rerouted to replace a damaged posterior tibial tendon in the foot. The posterior tibial tendon is the primary support structure for your foot’s arch, and when it fails, the arch gradually collapses into what’s commonly called flatfoot. FDL transfer aims to relieve pain and restore arch height when nonsurgical treatments like orthotics and physical therapy haven’t worked.
Why the FDL Tendon Is Used
The posterior tibial tendon runs along the inner side of your ankle and attaches to the navicular bone in the midfoot. It does the heavy lifting of holding your arch up every time you take a step. When this tendon becomes diseased or torn, a condition called posterior tibial tendon dysfunction (PTTD), the arch slowly falls and the heel shifts outward. Walking becomes painful, and the foot’s mechanics change in ways that affect everything from your ankle to your knee.
Surgeons choose the FDL tendon as a replacement because it sits right next to the posterior tibial tendon and runs a similar path. It’s the closest match in terms of location and pulling direction. While the FDL isn’t as strong as the original tendon, it provides enough support to restore meaningful arch function when combined with other corrective procedures.
When Surgery Becomes Necessary
FDL transfer is typically reserved for stage II PTTD, where the flatfoot deformity is still flexible, meaning a doctor can manually push your foot back into a more normal arch position. Before reaching this point, conservative treatment is recommended for three to four months. That usually includes anti-inflammatory medications, activity changes, arch supports, and sometimes a walking boot or cast for several weeks followed by physical therapy with targeted strengthening exercises.
If your pain persists and your arch continues to collapse despite these measures, surgery enters the conversation. The key requirement is that the deformity remains flexible. Once the foot becomes rigid or arthritis develops in the joints, FDL transfer alone won’t work, and more extensive procedures like joint fusion become necessary.
What Happens During the Procedure
The surgeon makes an incision along the inner side of the foot and ankle to access the damaged posterior tibial tendon. The diseased tendon is cleaned up or partially removed. The FDL tendon, which is the first tendon visible after opening the deeper tissue layers, is then detached from its normal attachment point at the lesser toes and rerouted to the navicular bone, essentially taking over the job of supporting the arch.
In most cases, FDL transfer is not performed alone. It’s commonly paired with a medial displacement calcaneal osteotomy, a procedure where the heel bone is cut and shifted inward. This realignment corrects the outward drift of the heel and reduces the pulling force that calf muscles exert on the already-compromised hindfoot. Depending on the severity of your deformity, your surgeon may also lengthen the Achilles tendon or reconstruct the spring ligament, a structure on the bottom of the foot that further supports the arch.
Recovery Timeline
The standard recovery protocol begins with four to six weeks in a non-weight-bearing cast, though newer approaches are pushing toward earlier mobility. Some protocols now transition patients from a cast to a walking boot at two weeks, assuming the surgical wound is healing well. Gentle toe-pointing exercises can start at that point, but pulling the foot upward is avoided until the six-week mark to protect the transferred tendon.
If a calcaneal osteotomy was performed alongside the transfer, protected weight-bearing in a boot may begin between two and four weeks. Full weight-bearing is generally considered safe within four weeks of surgery. The prolonged immobilization that older protocols required carried its own risks, including muscle loss, joint stiffness, and blood clots, so the trend has moved toward getting patients moving sooner when healing allows it.
Most people spend roughly three months transitioning back to regular shoes and normal walking. Full recovery, including return to more demanding activities, often takes six months to a year.
What Happens to Your Toes
Since the FDL tendon normally controls the lesser toes (the four smaller toes), harvesting it does have consequences. Research on 51 feet that underwent FDL transfer with calcaneal osteotomy found that lesser-toe function during walking was measurably impaired. Patients showed significantly decreased pressure under the smaller toes compared to their unoperated foot.
The good news is that the great toe appears largely unaffected. Maximum force at the big toe measured nearly identically between operated and unoperated feet (about 124 newtons on each side). This matters because the great toe handles the majority of push-off force during walking. So while you may notice reduced grip strength in your smaller toes, the functional impact on everyday movement tends to be modest for most people.
Risks and Complications
Tendon transfer surgery carries the standard surgical risks of infection, wound problems, and nerve irritation. Data from tendon transfer procedures in the foot show wound breakdown occurring in roughly 8% of cases, infection in about 2%, and persistent pain in around 10%. Weakness in pushing the foot downward occurs in a small number of patients, reported at about 1.5% overall.
The transferred tendon can also stretch over time, gradually losing some of its corrective effect. This is one reason surgeons pair the transfer with bony procedures like the calcaneal osteotomy: the bone realignment shares the mechanical load so the tendon isn’t working alone.
Outcomes and Satisfaction
Patient satisfaction after foot and ankle tendon transfer procedures is consistently high. Studies on similar tendon transfers report satisfaction rates above 90%, with patients across multiple studies indicating they would choose to have the surgery again. For FDL transfer specifically, success depends heavily on selecting the right candidates (flexible deformity, failed conservative care) and combining the transfer with appropriate bony corrections.
Functional testing after FDL transfer with calcaneal osteotomy shows meaningful improvements in how the foot distributes pressure during walking, particularly through the midfoot and arch region. The foot doesn’t return to completely normal mechanics, but it shifts significantly closer to a healthy pattern. For most patients, the combination of pain relief and improved stability makes a substantial difference in daily activity levels, with many no longer needing braces or orthotics after full recovery.