Current Concepts on FHL Tendon Transfer for Achilles Tendon Ruptures

Achilles the Great

Achilles was one of the greatest and strongest warriors and heroes in Greek mythology.¹ He received his gift of strength when Thetis, his mother, immersed him in the Waters of Styx, and as a result, he became virtually invulnerable. As Thetis submerged him into the waters, she held him by the heels. In doing so, his heels were penetrable and ultimately became his undoing.¹ The Achilles tendon continues to hound patients and surgeons alike many centuries after this story began.

The Achilles tendon is the site of one of the most common overuse injuries of the lower extremity.² It usually occurs in the “zone of avascularity,” two-to-six cm proximal to the Achilles tendon insertion.³ One can successfully treat many of these injuries with initiation of therapeutic modalities and exercise, anti-inflammatory medication, and shoe modifications.² Some injuries, much like the one sustained by our warrior Achilles, are a fatal blow. In ruptures of the Achilles, the Achilles tendon often shortens and the gastrocnemius and soleus muscles atrophy, increasing the necessity for surgical remediation.^2,3 The purpose of this article is to summarize the use of flexor hallucis longus tendon transfer/augmentation for repair of the Achilles tendon.

Flexor Hallucis Longus: Anatomy and Advantages

The flexor hallucis longus (FHL) is a muscle in the deep posterior compartment of the lower leg.⁴ Originating from the posterior aspect of the lower fibula and the interosseous membrane, the FHL muscle courses distally alongside of the flexor digitorum longus and tibialis posterior through the tarsal tunnel.⁴ At the level of the medial malleolus, the FHL tendon begins and makes it descent through the groove between medial and lateral tubercles of the talus.⁴ The FHL tendon then curves around the sustentaculum tali, making its way to the Master Knot of Henry, where the flexor digitorum longus (FDL) crosses over the FHL.⁴ The journey of the FHL tendon ends by inserting on the plantar aspect of the distal phalanx of the great toe.⁴  Besides functional support as the main flexor of the hallux, the FHL supports foot flexion when the toe is fixed during terminal stance phase and push-off.²

The FHL is an in-phase muscle to the gastrocnemius and soleus muscles, in which the axis of contraction follows similarly with the Achilles Tendon.³ It is approximately twice as strong as FDL, in addition to 30 percent stronger than peroneus brevis.⁵ The FHL is in close proximity to the Achilles tendon, allowing its harvest through the same incision.³ After such harvest, the Master Knot of Henry allows for maintenance of some flexion strength even after tenotomy.³ The FHL may increase the vascularity of the Achilles tendon when the surgeon sutures its muscle belly to the tendon, a theoretical advantage being that the muscle belly will enhance the healing potential in the region.^3,6

Compared with other transfers, the FHL adds thickness to the anterior portion of the Achilles tendon, which can prevent late problems with incision.⁶The FHL provides additional mechanical support to protect the reattached Achilles tendon, and in the event of inadvertent early loading, it can assist in preventing failure.⁷ Due to its small lever arm, FHL has minute rotational effect on the ankle and therefore will not have significant influence on the biomechanics of the rearfoot.² Of note, there is also potential for the FHL to hypertrophy up to 52 percent after transfer, making for a robust addition to a weakened Achilles.⁸

Determining Viability for Transfer

Saad and colleagues recommend a preoperative MRI assessment of the FHL to establish suitability for transfer.⁴ They determined that a Goutallier classification Grade 0 is most preferrable for FHL augmentation, however Grades 1 and 2 are also usable.⁴ Grades 3 and 4 hypothetically provide a poor result.⁴ This is due to increased fat deposition into the muscle that decreases insulin sensitivity. Ultimately, this impairs normal protein synthesis, having a direct effect on muscle mass and strength.⁴

Goutallier classification Grade 0 – Normal muscle without fat

Goutallier classification Grade 1 – Few fatty streaks within the muscle

Goutallier classification Grade 2 – Less fat than muscle within the muscle

Goutallier classification Grade 3 – Same amount of fat and muscle within the muscle

Goutallier classification Grade 4 – More fat than muscle within the muscle

Additionally, they determined and highlighted multiple FHL pathologies that should undergo evaluation before determining viability for Achilles’ tendon augmentation.⁴

Posterior impingement of the FHL muscle or tendon can result from multiple scenarios, including os trigonium, synovitis, scar tissue, accessory peroneus quartus, or a low-lying FHL muscle belly.⁴ From this impingement, The FHL can develop stenosing tenosynovitis, which can lead to partial- or full-thickness tears.⁴ This could be problematic and deliver unsatisfactory results if a surgeon were to attempt to repair the Achilles tendon with a the FHL augmentation, while the FHL is in a pathological state.

Loose bodies or osseocartilaginous bodies within the FHL tendon sheath may track down the tendon sheath to lie behind the ankle or between the talar tubercles (under the sustentaculum tali) up to the Master Knot of Henry.⁴ Although typically benign, studies show that if left untreated, loose bodies have the potential to cause damage and inflammation around the tendon, muscular atrophy, and joint degeneration.⁴

Considering Relevant Concerns for Morbidity

FHL transfer does come with some concerning associated morbidities. There is a loss of great toe plantarflexion strength, plantarflexion of the hallux interphalangeal (IP) joint, and cock-up deformity of the great toe.^3,7,9 This decrease in strength and diminished push-off strength could, in theory, affect balance, endurance, and gait.^7,9

Hunt and colleagues observed ankle and hallux plantarflexion using the MicroFET2® (Hoggan Scientific) isokinetic dynamometer and compared it to the nonoperative limb after FHL transfer with Achilles repair.⁷ They performed the isokinetic testing preoperatively, and three, seven, and 12 months postoperatively.They noticed that FHL ankle plantarflexion strength was greater compared to their control group at six and 12 months postoperatively and noticed no significant difference in hallux plantar flexion between groups at any point in time.⁷ Hunt and colleagues additionally found no difference in pain or function outcomes (measured by AOFAS scores) and patient satisfaction when comparing patients treated with Achilles debridement alone versus FHL tendon transfer.⁷ Although there was no difference in pain or function, they reported ankle plantarflexion strength improved with the FHL transfer without any loss of hallux plantar flexion strength.⁷

Schon and colleagues observed after 46 FHL transfer procedures that very few patients presented with weakness or decrease in function of the hallux as it relates to balance or problems with wearing sandals, despite patients having a mean decrease of 20 percent in first MTPJ passive arc of motion.¹⁰  Koh and colleagues compared the outcomes of 20 patients with FHL transfers versus 19 patients with turn-down flaps.⁸ While they did find no difference in outcome scores with the VAS score, AOFAS score, SF-36-PCS, or SF-36-MCS, they noticed more complications with the turn-down flaps, as the FHL transfer group had no reported complications.⁸

Schmidtberg and colleagues with 30 matched paired below the knee cadaveric specimens, divided the pairs of limbs into 3 groups which the Achilles was then transected to create 25, 50, and 75 percent defects.¹¹ To provide a control for each group, 1 cadaveric pair was selected to undergo FHL transfer with interference screw. They compared the tendons displacement during cyclic loading, peak elongation, stiffness, and mechanism of failure among the three groups. They concluded that FHL transfer is indicated when the necessary debridement of the Achilles tendon is greater or equal to 50 percent. ¹¹

Hahn and colleagues had 13 patients at mean of 46 months postoperative follow up undergo pedobarography and clinical evaluation of the foot.² They observed loss of active plantarflexion of the hallux, and, on pedobarography there was unloading of the first toe with an increase load placed to the metatarsal heads. Despite these findings, clinically they found no subjective or objective gait asymmetries with all patients being pain free and without restrictions during normal walking.² Coull and team assessed 16 patients with a FHL transfer with Achilles debridement to determine the morbidity of the FHL transfer.¹² They performed clinical evaluation of hallux function using AOFAS, SF-36, a clinical questionnaire to assess alteration in the function of the hallux, as well as forefoot pedobarography to determine changes in pedal pressures.¹² Clinically, all patients demonstrated absence of active plantarflexion of the hallux at the IPJ with normal active plantarflexion of the first MTPJ. Foregoing two patients with previous hallux pathologies (hallux valgus and hallux rigidus) the AOFAS hallux metatarsophalangeal-interphalangeal average was 100.¹² Not a single patient admitted to having any impairments of walking, running, ascending/descending steps, or squatting due to weakness of the hallux. Pedobarograph reading trended with a decreased peak pressure on the distal phalanx, however it was not significant for the numbers of patients studied and no significant increase in loading of the first or second MTPJs to suggest that transfer metatarsalgia may complicate FHL tendon transfer.¹²

It is likely that the strength necessary for daily pedal function is accommodated for by the flexor hallucis brevis or from the preserved interconnection between FDL and FHL in the midfoot at the Master Knot.^7,9 The loss of hallux push-off strength is most often well-tolerated.⁸

In Conclusion

Flexor hallucis longus tendon transfer augmentation of the Achilles tendon for repair of chronic ruptures and recalcitrant Achilles tendinosis is well-described, with good clinical results.⁽³⁾ A majority of studies show minimal-to-no functional morbidity to the hallux after transfer at long-term follow-up.  Generally, in the studies mentioned above and the senior author’s 20+ years of experience, the patient population who sustain such injuries are older, have elevated BMI, and maintain a sedentary lifestyle. Thus, caution is wise if extrapolating these results to an athletic population, although there are reports of good results in active patients.^3,6

Dr. Miller is a Fellow of the American College of Foot and Ankle Surgeons. He is the Director of the Pennsylvania Intensive Lower Extremity Fellowship in Malvern, PA, and the Podiatric Residency Program at Phoenixville Hospital in Phoenixville, PA.

Dr. Capuzzi is a current Fellow of the Pennsylvania Intensive Lower Extremity Fellowship in Malvern, PA.

Dr. Tonucci is a first-year resident with the Phoenixville Hospital/Tower Health residency program in Phoenixville, Pa.

1. Achilles: the Trojan war hero. GreekMythology.com. Available at: https://www.greekmythology.com/Myths/Heroes/Achilles/achilles.html. Accessed January 25, 2022.

2. Hahn F, Maiwald C, Horstmann Th, Vienne P. Changes in plantar pressure distribution after Achilles tendon augmentation with flexor hallucis longus transfer. Clinical Biomech. 2008;23(1):109-116.

3. Richardson DR, Willers J, Cohen BE, Davis WH, Jones CP, Anderson RB. Evaluation of the hallux morbidity of single-incision flexor hallucis longus tendon transfer. Foot Ankle Int. 2009;30(7):627-630.

4. Saad A, McLoughlin E, Hanif U, Iqbal A, James S, Botchu R. Flexor hallucis longus augmentation for Achilles tendon–is pre-operative evaluation of flexor hallucis longus by MRI required? Indian J Med Sci. 2021;72(2):197-202.

5. Wapner KL, Pavlock GS, Hecht PJ, Naselli F, Walther R. Repair of chronic Achilles tendon rupture with flexor hallucis longus tendon transfer. Foot Ankle. 1993;14(8):443-449.

6. Monroe MT, Dixon DJ, Beals TC, Pomeroy G, Crowley DL, Manoli A. Plantarflexion torque following reconstruction of Achilles tendinosis or rupture with flexor hallucis longus augmentation. Foot Ankle Int. 2000;21(4):324-329.

7. Hunt KJ, Cohen BE, Davis WH, Anderson RB, Jones CP. Surgical treatment of insertional Achilles tendinopathy with or without flexor hallucis longus tendon transfer: a prospective, randomized study. Foot Ankle Int. 2015;36(9):998-1005.

8. Koh D, Lim J, Chen JY, Singh IR, Koo K. Flexor hallucis longus transfer versus turndown flaps augmented with flexor hallucis longus transfer in the repair of chronic Achilles tendon rupture. Foot Ankle Surg. 2019;25(2):221-225.

9. de Cesar Netto C, Chinanuvathana A, Furtado da Fonseca L, Dein EJ, Tan EW, Schon LC. Outcomes of flexor digitorum longus (FDL) tendon transfer in the treatment of Achilles tendon disorders. Foot Ankle Surg. 2019;25(3):303-309.

10. Schon LC, Shores JL, Faro FD, Vora AM, Camire LM, Guyton GP. Flexor hallucis longus tendon transfer in treatment of Achilles tendinosis. J Bone Joint Surg. 2013;95(1):54-60.

11. Schmidtberg B, Johnson JD, Kia C, et al. Flexor hallucis longus transfer improves Achilles tendon load to failure in surgery for non-insertional tendinopathy: a biomechanical study. J Bone Joint Surg. 2019;101(16):1505-1512.

12. Coull R, Flavin R, Stephens MM. Flexor hallucis longus tendon transfer: evaluation of postoperative morbidity. Foot Ankle Int. 2003;24(12):931-934.