Flexor Hallucis Longus Tendon-Related Pathology: Keys To Arthroscopic Evaluation And Treatment

When evaluating posterior ankle pain, considering flexor hallucis longus tendon pathology is important. Due to the anatomical location of the tendon, posteromedial pain and even posterior ankle and subtalar joint pain can be directly related to pathology of this tendon. Pathology of the flexor hallucis longus tendon may include direct causes such as tenosynovitis, tendon injury, nodular tendinosis, a hypertrophic retinaculum as well as indirect causes including functional hallux limitus.¹

Sports in which there is forced plantarflexion of the ankle can have a direct effect on the flexor hallucis longus tendon, resulting in pathological changes in the posteromedial aspect of the ankle. The athletes most commonly affected are ballet dancers, javelin throwers, football players, soccer players and runners who go up and mainly down hills.^2–4

The flexor hallucis longus tendon originates from the posterior distal two-thirds of the fibula. It runs distally and crosses the ankle through a flexor retinaculum at the level of and between the posterior talar processes. This small posterior channel created by the posterior processes can impinge the flexor hallucis longus tendon if it is thickened due to tenosynovitis, if there is a hypertrophic muscle belly or if a constrictive retinaculum exists.⁵

Forced dorsiflexion of the first metatarsophalangeal joint (MPJ) can result in traumatic damage of the flexor hallucis longus tendon in the posterior ankle as it passes through the small posterior channel. The contrary motion can lead to injury as well with forced repetitive plantarflexion resulting in overuse injuries in ballet dancers and other athletes.⁶ Due to the flexor hallucis longus tendon’s avascular zone and the pressure imposed by the length of the retinaculum, injury to the flexor hallucis longus tendon can also originate at this level.⁷

Essential Diagnostic Insights

During clinical evaluation, symptoms will present in the posteromedial ankle. Palpate the tendon posterior to the medial malleolus. During examination, with passive range of motion of the first MPJ, occasionally the clinician can palpate the thickened flexor hallucis longus tendon or elicit crepitus of the tendon.  

One potential underlying etiology of functional hallux limitus can be related to flexor hallucis longus tendon thickening, which subsequently can lead to impingement in the posterior ankle tunnel. In patients with first MPJ discomfort during strenuous exercise and without signs of bone impingement, one should evaluate for a functional hallux limitus due to flexor hallucis longus pathology.

The clinician can test for this condition by holding the ankle slightly plantarflexed and performing first MPJ range of motion (ROM). While loading the second through fifth metatarsal heads with the ankle in a neutral position, repeat first MPJ ROM testing. If first MPJ ROM is restricted with this maneuver, impingement of the flexor hallucis longus tendon in the retinaculum of the posteromedial ankle may be the underlying etiology of limited hallux dorsiflexion.  

Magnetic resonance imaging (MRI) plays a key role in confirming the clinical diagnosis due to its sensitivity for both soft tissue hypertrophy and inflammatory signal changes. Imaging the ankle in neutral position aids in visualization of the flexor hallucis longus tendon as it passes through the retinaculum. A good quality MRI allows you to visualize tenosynovitis, edema, an enlarged muscle belly, impingement in the retinaculum and tendon tears. Obtaining an MRI post-exertion can be very useful in truly visualizing the impingement and post-exertional edema. Make sure patients reproduce their symptoms via activity just prior to entering the MRI scanner. This should help enhance the appearance of any pathology.  

Pertinent Pearls On Arthroscopic Treatment

Surgeons can perform posterior ankle arthroscopy and inspection of the flexor hallucis longus tendon with a 4.0 mm 30-degree arthroscope. Ensure the use of general anesthesia and prone positioning of the patient with his or her foot hanging off the end of the bed.

Draw a line from the most distal tip of the fibula to the Achilles tendon parallel to the sole of the foot. The first incision typically is just above this line and anterior to the Achilles tendon. Use a hemostat to spread the tissues through Kager’s triangle in the same direction as the second metatarsal. Once you have come into contact with the posterior tibia, remove the hemostat and insert the blunt trocar and cannula.

Make the medial incision at the same level as the lateral incision just anterior to the Achilles tendon. Perform blunt dissection to the tip of the scope and the hemostat should be visible. Then replace the hemostat by the shaver.

Entering the posteromedial portal, one should aim lateral. Identify the flexor hallucis longus tendon and commence debridement. Pay careful attention to stay lateral to the medial edge of the flexor hallucis longus tendon in order to avoid the neurovascular structures. Release the flexor hallucis longus tendon via resection of the flexor retinaculum and debridement of the tendon.8 Debride both the retinaculum and flexor hallucis longus tendon of synovitis and one can debulk an enlarged muscle belly. At this point, the tendon can pass freely through the canal. Check the tendon position visually through the scope with passive dorsiflexion and plantarflexion of the first MPJ.

Postoperatively, patients can bear weight as tolerated in a controlled ankle motion (CAM) boot and early ankle and first MPJ ROM exercises begin on postoperative day one. If weakness or ROM issues at either the ankle or the first MPJ present, consider physical therapy. 

In Conclusion

In their study of arthroscopic treatment of flexor hallucis longus tenosynovitis in 60 feet, Ogut and colleagues reported a low complication rate of 3.4 percent and an improvement of AOFAS scores by 56.7 points to a final score of 85.9 points after a mean follow-up of 26.7 months.⁹ Current evidence shows that posterior arthroscopic debridement is a safe and effective method for treating a variety of flexor hallucis longus tendon pathologic conditions and functional hallux limitus caused by flexor hallucis longus tendon impingement.

Dr. Jennings is affiliated with the Department of Orthopedics and Podiatry at the Palo Alto Medical Foundation in Mountain View, Calif. She is a Fellow of the American College of Foot and Ankle Surgeons.

References

1. Vallotton J, Echeverri S, Dobbelaere-Nicolas V. Functional hallux limitus or rigidus caused by a tenodesis effect at the retrotalar pulley. J Am Podiatr Med Assoc. 2010; 100(3):220-229.
2. Kudas S, Donmez, G, Isik C, Celebi M, Cay N, Bozurt M. Posterior ankle impingement syndrome in football players: Case series in 26 elite athletes. Acta Orthop Traumatol Turc. 2016; 50(6):649-654.
3. Hamilton W, Geppert M, Thompson F. Pain in the posterior aspect of the ankle in dancers. Differential diagnosis and operative treatment. J Bone Joint Surg. 1996; 78(10):1491-1500.
4. Hedrick M, McBryde A. Posterior ankle impingement. Foot Ankle Int. 1994; 15(1):152-158.
5. Smyth N, Zwiers R, Wiegerinck J. Posterior hindfoot arthroscopy: a review. Am J Sports Med. 2014; 42(1):225-234.
6. VanDijk C. Posterior ankle impingement. In: VanDijk C, ed. Ankle Arthroscopy. Springer, Berlin, 2014, pp. 231-258.
7. Peterson W, Pufe T, Zantop T, Paulson F. Blood supply of the flexor hallucis longus tendon with regard to dancers tendinitis: injection and immunohistochemical studies of cadaver tendons. Foot Ankle Int. 2003; 24(8):591-596.
8. De Leeuw P, Van Sterkenburg M, Van Dijk C. Arthroscopy and endoscopy of the ankle and hindfoot. Sports Med Arthrosc. 2009; 17(3):175-184.
9. Ogut T, Ayan E, Irgit K, Sarikaya A. Endoscopic treatment of the posterior ankle pain. Knee Surg Sports Traumatol Arthrosc. 2011; 19(8):355-1361.