Introduction

Flexor hallucis longus (FHL) tenosynovitis is a well recognized cause of posteromedial ankle pain. In ballet dancers this entity has been described as ‘'dancer's tendinitis''. Athletes performing repetitive forceful pushoffs are at risk to develop flexor hallucis longus tendinopathy.

Isolated flexor hallucis longus tendon pathology is almost exclusively located behind the medial malleolus at the level of the fibro-osseous tunnel. 
 
Hypertrophy, a nodule, or a low riding muscle belly can cause the musculotendineous junction to be pulled inside the narrow tunnel, causing stenosing tenosynovitis also referred to as triggering of the great toe or hallux saltans. Especially maximal tendon tension, i.e. hyperdorsiflexion of the ankle and great toe, could induce tendinopathy of the FHL. Furthermore, tendon degeneration and ruptures most frequently occur in the region of this fibro-osseous tunnel. This could possibly be explained through a relative incongruity between the FHL and its tunnel in fully plantarflexed or dorsiflexed position. 
 
Another hypothesis is an avascular zone at this level of the tendon, as described by Petersen and co-workers. The posterior ankle impingement, based upon a soft tissue or bony impediment, is frequently associated with flexor hallucis longus tenosynovitis at the level of the fibro-osseous tunnel. 
Displacement of an os trigonum, a Cedell fracture, or a hypertrophic posterior talar process are examples of posterior bony ankle impingement can cause associated tenosynovitis. Scar tissue around the tendon can provide local irritation. 
 
Posteromedial located talar osteochondral defects have been mentioned as a cause of FHL tendinopathy. During the stance phase of walking the ankle joint is in dorsiflexion while the talar dome is in closest contact with the FHL tendon. In the push-off phase the toes are actively flexed, moving the tendon in an opposite direction, as compared to the talar movement. The tendon hereby shreds against the osteochondral defect and becomes irritated and inflamed.  FHL tendinitis and posterior ankle impingement based on the os trigonum syndrome are distinct entities, nevertheless frequently coexist because of their close anatomical orientation.

Anatomy

The flexor hallucis longus is the most laterally located bipennate muscle of the human calf. At the level of the ankle - subtalar joint complex the FHL tendon runs distally in a fibro-osseous gliding channel located in between the posteromedial and lateral talar process. At this level the tendon is kept in place by the flexor retinaculum. The FHL tendon passes distally and medially underneath the sustentaculum tali to eventually insert in the distal phalanx of the hallux.

History & Physical Examination

Patients typically complain of pain located at the posteromedial aspect of the ankle which exacerbates with ankle motion and hallux dorsiflexion and diminishes at rest. The tendon can be palpated behind the medial malleolus at the level of the subtalar joint. Asking the patient to repetitively flex the big toe with the ankle in 10 to 20 degrees of plantarflexion, will increase the ability to palpate the tendon in its gliding channel. This manoeuvre will also differentiate between FHL and posterior tibial tendon pathology. The FHL tendon glides up and down under the palpating finger of the examiner. In case of stenosing tendinitis or chronic inflammation, crepitus and recognizable tenderness can be provoked. In some patients a nodule can be palpated moving up and down with active movement of the great toe. 
In patients with associated posterolateral ankle pain a posterior impingement syndrome must be ruled out by means of a hyper plantarflexion test (fig). The forced passive hyper plantar flexion test is positive when the patient experiences recognizable posterior ankle pain. A negative test rules out a posterior ankle impingement syndrome. A positive test is followed by a diagnostic infiltration with lidocaine in the posterior ankle compartment. Disappearance of pain following infiltration confirms the diagnosis.

Diagnostic Imaging

After history taking and physical examination the diagnosis can be confirmed or rejected based upon different available imaging techniques. In case history taking and physical examination do not reveal abnormalities, additional diagnostics can be used to search for a clue or to rule out pathology, i.e. medico-legal reasons. Close consultation between the orthopaedic surgeon and the radiologist is necessary to decide upon optimal radiographic diagnostics. 
In patients without a history of trauma but with isolated recognizable posteromedial ankle pain during flexion of the great toe while palpating the tendon at the level of the gliding channel, no additional diagnostics are needed. In case conservative treatment options fail, the intervention will be a release regardless of the pathology. A MRI scan can be valuable to rule out tendon ruptures.
In patients with posteromedial ankle pain associated with a positive hyperplanterflexion test, standard weight-bearing radiographs in the antero-posterior and lateral direction must be made. In case of doubt for the differentiation between hypertrophy of the posterior talar process or an os trigonum, a lateral hindfoot radiograph view with the foot in 25 degrees exorotation in relation to the standard lateral ankle view is advised. In ballet dancers a lateral radiograph with the foot in maximal plantarflexion can be useful to determine whether a bony posterior ankle impingement is present. Especially in posttraumatic cases, a spiral CT scan can be important to ascertain the extent of the injury / location of the osteochondral defect and/or the exact location of calcifications or fragments. 

1).   2).

1). Axial MRI of a 37- year old female patient with an FHL tendinitis. Fluid is visible around the tendon, colouring white on T2.

2). Sagittal image of the same patient.

Surgical treatment

Conservative treatment is always the first treatment of choice. If it fails, surgical intervention involves tendon debridement and a release of the flexor retinaculum and tendon sheath at the level of the posterior talar process.

Post-operative rehabilitation

The patient can be discharged the same day of surgery and weightbearing is allowed as tolerated. The patient is instructed to perform active dorsiflexion of the ankle to the neutral position (knee slightly bent). This exercise should be performed 2 or 3 days times per hour for the first days after surgery. The patient is instructed to elevate the foot when not walking to prevent edema. The dressing is removed 3 days post-operative. Normal walking without crutches at 4 - 5 days post surgery. Patients with limited range of motion are directed to a physiotherapist. Running can be started when range of motion is normal and in absence of swelling. This can on avarage be expected at 3 weeks post surgery. Sport resumption can on average be expected at 6 weeks post surgery. 

Recommended literature

Van Dijk CN, Fievez AWFM, Heijboer MP, et al. Arthroscopy of the ankle. Acta Orthop Scand 1993;64(suppl. 253):9.

Van Dijk CN, Scholte D. Arthoscopy of the Ankle Joint. Arthroscopy: The journal of Arthroscopic and Related Surgery, 1997;13(1):90-96.

Van Dijk, Verhagen RAW, Tol JL. Arthroscopy for problems after ankle fracture. J Bone Joint Surg (Br) 1997;79B(2):280-284.