Endoscopic Flexor Hallucis Longus Tendon Transfer for the Treatment of Chronic Achilles Tendon Defects
Authors: Rust Andrew , Roebke Logan , Martin Kevin D. 
Published: January 2025

Background:
An all-inside endoscopic flexor hallucis longus (FHL) tendon transfer is indicated for the treatment of chronic, full-thickness Achilles tendon defects. The aim of this procedure is to restore function of the gastrocnemius-soleus complex while avoiding the wound complications associated with open procedures.

Description:
This procedure can be performed through 2 endoscopic portals, a posteromedial portal (the working portal) and a posterolateral portal (the visualization portal). The FHL tendon is identified, and the joint capsule is debrided to identify the subtalar joint. A shaver is utilized to circumferentially debride the FHL at the level of the subtalar joint, allowing for full visualization of the tendon. Care is taken to avoid the posteromedial neurovascular bundle by keeping the shaver against the tendon. An endoscopic suture-passing device is utilized to pierce the FHL tendon and shuttle a nonabsorbable suture through the tendon; this step is done 2 times. The tendon is then cut at its distal-most aspect (adjacent to the subtalar joint) with an endoscopic cutter. The tendon is then brought through the posteromedial portal and prepared for transosseous passage with nonabsorbable suture. Next, the anterior insertion of the Achilles tendon is endoscopically identified and debrided. With use of needle endoscopy-targeted pin placement, a Beath pin is placed at the anterior footprint of the Achilles via the posteromedial portal. The pin is advanced dorsal to plantar and out the bottom of the foot and is confirmed on fluoroscopy. With use of an appropriately sized reamer, the Beath pin is loaded with the 2 grasping sutures and shuttled plantarly. The needle endoscope is then placed in the posterolateral portal to visualize the FHL tendon, advancing into the tunnel with the foot held in 15° of plantar flexion. An appropriately sized interference screw is then placed in the tunnel, using direct endoscopic visualization to confirm placement and depth of the screw.

Alternatives:
Chronic Achilles tendon ruptures with symptomatic weakness often necessitate operative treatment; however, high-risk patients may be better managed nonoperatively with an ankle-foot orthosis. These patients often demonstrate improved gait and function with this orthosis
1
. The choice of operative technique for the treatment of chronic Achilles tendon defects is primarily based on tendon gap length; options include end-to-end repair, fascial advancement, and turn-down procedures with or without transferring the FHL, peroneus brevis, or flexor digitorum longus tendons. These techniques require substantial incisions and violation of the posterior compartments and Achilles paratenon, creating substantial postoperative scarring.

Rationale:
Chronic Achilles tendon ruptures with defects or gaps leave the patient with weakness and biomechanical loss of the gastrocnemius-soleus complex. The gold standard algorithm in which the gap length determines the type of fascial advancement requires lengthy incisions and violation of the posterior compartments and paratenon. These reconstruction procedures do restore gastrocnemius-soleus complex tension, but also result in diminished gliding and substantial scarring and thickening. These incisions are also prone to wound complications, sural nerve injury, and painful scarring. An all-inside endoscopic FHL tendon transfer has several advantages over the standard approach. The all-inside approach prevents violation of the compartments and the Achilles, avoiding painful scars and hypertrophic tissue changes. The use of an FHL tendon transfer is advantageous as it is an in-phase transfer that maximizes neuromuscular control. The anatomic position of the flexor hallucis longus muscle also creates optimal force vectors allowing for optimal gait propulsion. The FHL also has a robust muscle belly that can hypertrophy and strengthen over time. The endoscopic approach allows for immediate weight-bearing as part of an accelerated rehabilitation, which helps to reduce muscle wasting, deep vein thrombosis, and wound complications, and facilitates an earlier return to work
3
.

Expected Outcomes:
This procedure provides excellent clinical outcomes with decreased complication rates, as compared with open treatment. In a study of 22 patients with chronic Achilles tendon rupture with a large tendon gap who underwent endoscopic FHL tendon transfer, the mean American Orthopaedic Foot & Ankle Society score improved from 55 preoperatively to 91 at the time of final follow-up
4
. All patients in this cohort returned to daily activities. In another study, a total of 42 patients with chronic Achilles tendon rupture underwent either endoscopic (18 patients) or open treatment (24 patients). Patients in the endoscopic cohort demonstrated better functional outcomes and decreased complication rates compared with the open treatment cohort
3
. Patients undergoing the endoscopic procedure also had a significant increase in American Orthopaedic Foot & Ankle Society scores postoperatively and a lower rate of complications. One patient in the open treatment cohort had a wound dehiscence. There were no wound-healing complications in the endoscopic group.

Important Tips:

Utilize a low-flow straight-forward viewing endoscope.Utilize an endoscopic suture passer to avoid iatrogenic injury.Visualize the bone tunnel prior to passing the tendon in order to confirm that the wall is intact.Plantar flex the ankle and great toe when performing the tenotomy to allow for adequate tendon length for transfer.If a low-lying FHL muscle belly is present, it can be taken back to the level of the tibial talar joint. We have found that cutting the FHL at the level of the subtalar joint is optimal for transfer.Inadequate visualization of the tendon of the subtalar joint can prevent tenotomy from being distal enough for transfer.Avoid soft-tissue bridging by minimizing utilization of the posteromedial portal and by passing a looped grasper down the suture to confirm that no soft-tissue bridges are present.Failure to utilize both endoscopy and fluoroscopy can lead to inadequate tunnel placement.

Acronyms and Abbreviations:

FHL = flexor hallucis longusHPI = history of present illnessPMH = past medical historyNSAIDs = nonsteroidal anti-inflammatory drugsPT = physical therapySH = social historyPE = physical examinationMRI = magnetic resonance imagingCAM = controlled ankle motionAOFAS = American Orthopaedic Foot & Ankle Society