The Flexor Digitorum Longus (FDL) is a powerful muscle located deep within the posterior compartment of the lower leg. It is one of the three deep muscles that control the intricate movements of the ankle and foot. The FDL serves as a primary mover for the toes, contributing significantly to stability and balance during movement. Its long tendon pathway makes it susceptible to a range of orthopedic issues.
Anatomy and Location
The FDL muscle belly originates high up on the posterior surface of the tibia, the larger of the two lower leg bones, just below the soleal line. The muscle fibers run vertically down the leg, tapering into a long, strong tendon.
This lengthy tendon descends toward the ankle, passing behind the prominent medial malleolus. The FDL tendon travels through the tarsal tunnel, a narrow fibro-osseous passageway on the inside of the ankle. Within the tunnel, the tendon is encased in a protective sheath, lying alongside the posterior tibial nerve and artery.
After exiting the tarsal tunnel and entering the sole of the foot, the tendon separates into four distinct slips. These slips insert onto the plantar surface of the bases of the distal phalanges. This attachment allows the muscle to exert force to the tips of the second, third, fourth, and fifth toes.
Primary Function and Role in Gait
The primary mechanical action of the FDL is the flexion, or curling, of the four lesser toes. This action allows the toes to grip the ground, which is important when walking barefoot or on uneven terrain. The FDL also contributes to the plantar flexion of the foot, pointing the foot downward at the ankle joint.
Beyond its direct action on the toes, the FDL plays a significant role in maintaining the structural integrity of the foot. It assists in supporting the medial longitudinal arch. During the push-off phase of walking (toe-off), the muscle contracts to help propel the body forward.
The FDL’s coordinated contraction stabilizes the foot as weight is transferred from the heel to the ball of the foot. This stabilizing force is essential for efficient and smooth locomotion.
Common Conditions and Injuries
The FDL tendon’s long and winding course makes it prone to several overuse and compressive injuries. FDL tendonitis (tendinopathy) is a common condition caused by repetitive strain, presenting as pain and tenderness along the tendon’s path, particularly behind the ankle. This inflammation often results from activities involving excessive toe gripping, such as running or dancing.
Acute rupture of the FDL tendon is less common but can occur following a sudden, forceful contraction, especially in older individuals or athletes. A rupture leads to immediate pain and weakness, making it difficult to actively curl the lesser toes.
The FDL tendon sheath can become inflamed and swollen, contributing to Tarsal Tunnel Syndrome (TTS). In this condition, the posterior tibial nerve is compressed, causing symptoms like burning, numbness, or tingling along the sole of the foot. Chronic FDL irritation or an accessory muscle belly within the tunnel can lead to nerve impingement.
Medical Interventions for FDL Issues
Initial management for FDL tendon irritation or mild tendinopathy involves conservative strategies to reduce inflammation and pain. These non-operative treatments include rest, applying ice, and taking non-steroidal anti-inflammatory drugs. Physical therapy focuses on stretching the calf muscles and strengthening the small muscles within the foot.
Custom-made orthotics may be prescribed to correct underlying biomechanical issues, such as excessive foot pronation, which places undue stress on the FDL tendon. For Tarsal Tunnel Syndrome (TTS), the goal is to decrease pressure on the compressed nerve, often through rest and immobilization. Failure of conservative management for TTS may lead to surgery to release the flexor retinaculum, decompressing the nerve.
Surgical Repair and Transfers
Surgical options become necessary for acute ruptures or chronic tendon dysfunction. An acute rupture requires surgical repair to reattach the two ends of the tendon.
The FDL is frequently used in tendon transfer procedures. The tendon is detached from its insertion and rerouted to replace a damaged or dysfunctional tendon elsewhere.
For instance, the FDL tendon is commonly transferred to the navicular bone to supplement a failing posterior tibial tendon, a procedure often used to correct flexible flatfoot deformity. It may also be transferred to the calcaneus (heel bone) to augment the repair of a severe Achilles tendon tear.