Introduction:
Flexor hallucis longus belongs to the deep flexors of the calf, which also include the popliteus, tibialis posterior, and flexor digitorum longus muscles. FHL is a unipennate muscle that attaches to the tendon obliquely, and the muscle fibers converge to attach to the tendon on one side. The primary function of flexor hallucis longus is flexion of the great toe joints. It also has additional functions such as foot inversion and plantar flexion. Extensor hallucis longus performs an antagonistic function to the flexor hallucis longus.
The majority of flexor hallucis longus originates from the distal two-thirds of the posterior surface of the fibula, and the remaining fibers arise from the interosseous membrane, posterior intermuscular septum, and fascia of the tibialis posterior muscle. They run down the fibular side of the leg and then cross over the posterior part of the ankle and reach the sole, where it inserts into the hallux (great toe). The flexor hallucis longus is a tibial nerve branch with a cutaneous supply from root S2. They receive their blood supply from the peroneal branch of the posterior tibial artery.
How Is the Flexor Hallucis Longus Injury Examined Clinically?
Clinical examination is the initial evaluation performed on posteromedial ankle pain for flexor hallucis longus injury. First, the knee flexion performs this at 90 degrees, relaxing the gastrocnemius. Then the knee is extended by plantar flexion of the great toe and ankle with the physician palpating behind the medial malleolus. This helps differentiate tendinitis from the cause of hallux rigidus. Some triggering factors of hallux rigidus are the thickening of the tendon slips through the fibro-osseous tunnel and the hypertrophy of flexor hallucis longus at the musculotendinous junction.
What Are the Possible Injuries Associated With Flexor Hallucis Longus?
- Tendinopathy - It is the degeneration of the collagen protein of the tendon.
- Tenosynovitis - Swelling or infection of the peritendinous tissue.
- Complete or Partial Tendon Tear - Partial tear refers to the tear of only the part of the tendon. A complete tear is sometimes associated with tendon retraction.
- Tendon Dislocation - Complete or incomplete dislocation.
- Tendon Entrapment - Tethering the flexor hallucis longus tendon under the flexor retinaculum.
What Imaging Techniques Are Used to Interpret Flexor Hallucis Longus Injury?
Radiography:
- Conventional radiography can show the soft tissue swelling of the ankle at its posteromedial aspect. In addition, fractures of the distal medial malleolus, calcaneus, and os trigonum are seen in flexor hallucis longus tendinopathy, partial or complete tears, dislocation, or entrapment. Though radiographs do not directly show the flexor hallucis longus injury, they can show the ancillary findings such as fractures and swelling, which helps detect the damage.
- There might be a chance of false-positive results with soft tissue swelling, which can also occur with hematoma. Therefore, conventional radiography requires additional imaging techniques such as CT or MRI to confirm the injury.
Computed Tomography (CT):
- The computed tomography scan can locate the presence, size, location, and anatomy of flexor hallucis longus and the presence of fluid in the synovial sheath. Usually, the tendon appears round or slightly oval with intermediate density. In about 20 percent of healthy individuals, fluid may be found in the tendon sheath.
- Discontinuity or complete absence of the tendon indicates tendon tear with refraction. Thickening or attenuation is suggestive of tendinopathy or partial tear. Increased fluid in tendon sheath in CT scan highly indicates flexor hallucis longus entrapment.
- Fractures associated with the tendon injury, os trigonum, and foreign bodies are clearly visible in the CT scan.
Magnetic Resonance Imaging (MRI):
- MRI can visualize the intactness of the fibers, the size of the tendon, abnormal signal intensity in tendons, and the surrounding fluid collection. It can also visualize inflammation, contusion, bone marrow edema, and nondisplaced fractures. T2 weighted fat-saturated fast spin-echo and T1 weighted sequences of the ankle are used to evaluate the flexor hallucis longus tendon injury. In addition, it can identify tendinopathy due to direct irritation or anatomic variant. Direct irritation appears as cystic changes, irregular fraying, and nodular enlargement.
- Partial tendon tears appear as fusiform tendon thickening and hyperintense on T2 weighted fat-saturated MRI.
- A Complete tendon tear is shown as an area of fluid signal intensity between the torn ends. Soft tissue swelling and edema are also visible.
- MRI easily identifies tendon dislocation as the tendon slips to the posterior intertubercular groove. Sometimes MRI cannot differentiate the abscess, tendon masses, or flexor hallucis longus injury. There might be a chance of a false-negative report as the intermittent dislocation is not visible.
Ultrasonography:
- Ultrasonography is commonly used to evaluate Achilles tendon injury. However, the posterior medial location of flexor hallucis longus makes the imaging more challenging. Sometimes, the transverse and longitudinal views at the medial malleolus level visualize the tendon's size, location, and echogenicity.
- The fluid collection appears as a low echogenic area surrounding the ankle joint or tendon. A thickened tendon in ultrasonography is suggestive of tendinopathy.
- A Tendon tear is shown as the complete absence or retraction of the tendon and a fluid-filled space between the end of the tendon.
- Tendon entrapment usually occurs in three possible sites: proximally within the fibro-osseous tunnel, inferiorly into the posterior aspect of the sustentaculum tali, and distally between the two sesamoids at the base of the phalanx of the great toe.
Conclusion:
The radiographs cannot visualize the tendons, so they are not an effective diagnostic tool in flexor hallucis longus injury. Ultrasound helps determine the intermediate dislocation of the flexor hallucis longus tendon, which is not evident in magnetic resonance imaging. Fracture of the sustentaculum tali results in the trapping of flexor hallucis longus, which requires reconstructive surgery. In addition, MRI can visualize the partial tendon tear effectively, whereas a CT scan can show the complete tears with the ends of the torn retracted.
