The flexor hallucis longus (FHL) is a muscle and tendon unit in the lower leg and foot. It is responsible for the powerful flexion of the big toe, an action fundamental for balance and propulsion during movement.
Anatomy and Function of the FHL Tendon
The FHL muscle originates from the posterior surface of the fibula, the smaller bone in the lower leg. Its tendon travels down the back of the leg, passing through a groove behind the talus, a key ankle bone. This tendon is held in place by a fibrous tunnel, ensuring it follows a specific path toward the foot.
The tendon wraps underneath a bony shelf on the heel bone called the sustentaculum tali. It then crosses deep to another tendon at a location known as the “Knot of Henry” before running along the sole of the foot. The FHL tendon finally inserts at the base of the last bone of the big toe, the distal phalanx.
This intricate pathway allows the FHL to flex the joints of the big toe, pointing it downward. This movement provides the final push-off from the ground during walking or running. It also helps to stabilize the foot’s arch and is active during activities requiring balance on the toes, such as in ballet or gymnastics.
Common FHL Tendon Injuries
Injuries to the FHL tendon often arise from its repetitive gliding motion within its sheath. One condition is tendonitis, which is inflammation of the tendon itself, often caused by friction. Another related issue is tenosynovitis, where the synovial sheath that encases the tendon becomes inflamed, leading to pain and restricted movement.
Another injury is FHL impingement, which occurs when the tendon becomes pinched or compressed along its path, most often in the narrow tunnel behind the ankle. In chronic cases of irritation, a nodule can form on the tendon, causing a clicking or locking sensation known as hallux saltans. If left untreated, this can progress to a stiff and immobile big toe, a condition called hallux rigidus.
In more severe cases, the FHL tendon can sustain partial or complete tears, also known as ruptures. These can result from a sudden, traumatic event or from the gradual wear and tear of chronic overuse. Ruptures can occur at different zones along the tendon’s path, from its course near the ankle to its insertion point at the toe.
Symptoms and Causes
Symptoms associated with FHL tendon problems manifest as pain and dysfunction. Individuals experience pain along the posteromedial aspect of the ankle, which is the inner-back portion. Pain may also be felt under the foot or at the base of the big toe, especially during activities that involve pushing off.
Overuse is a primary cause of most FHL injuries. Repetitive and forceful plantar flexion, the motion of pointing the foot and toes downward, places significant strain on the tendon. This is why ballet dancers performing “en pointe,” gymnasts, and runners are particularly susceptible. Anatomical factors, such as the presence of an extra bone called an os trigonum behind the ankle, can also contribute to impingement.
Acute injuries, such as a sudden forceful stretch, can also cause strains or tears in the FHL muscle or tendon. Biomechanical issues, like overpronation where the foot rolls inward excessively, can alter the line of pull on the tendon and increase stress over time.
Treatment Approaches
Most FHL tendon injuries are first managed with conservative, non-surgical methods. The initial approach involves rest from aggravating activities to allow the inflammation to subside. Applying ice to the painful area, using compression, and keeping the foot elevated can also help manage swelling. A healthcare provider may recommend nonsteroidal anti-inflammatory drugs (NSAIDs) to reduce pain and inflammation.
Physical therapy is a main component of recovery, focusing on rehabilitating the muscle and tendon. A therapist may guide a patient through gentle stretching exercises for the calf and ankle to improve flexibility and reduce strain on the FHL. Strengthening exercises are also introduced, targeting the FHL and surrounding muscles to enhance foot stability.
If conservative measures do not provide sufficient relief, other options may be considered. Corticosteroid injections can be used to target persistent inflammation, though they are used with caution. For chronic conditions like hallux rigidus or significant tears that do not heal, surgery may be an option. Surgical procedures can range from repairing the tendon to releasing it from an area of impingement.