The fibularis longus (also known as the peroneus longus) is a muscle in the outer compartment of the lower leg. Its tendon runs behind the bony prominence on the outside of the ankle. Its primary functions are to tilt the sole of the foot outward (eversion) and assist in pointing the foot downward (plantarflexion), while also stabilizing the foot’s arch. Pain in this region is common, especially among athletes, often signaling an issue with the tendon or surrounding structures.
Chronic Pain from Overuse
The most frequent cause of prolonged discomfort is fibularis longus tendinopathy, an overuse injury. This condition involves micro-trauma within the tendon structure resulting from repetitive stress, not a sudden, acute injury. Strain often occurs due to a sudden increase in activity, such as starting a new running program or increasing mileage too quickly.
Poor footwear or a change in running surfaces can also contribute to this chronic irritation. Individuals with higher foot arches are sometimes more susceptible because the heel’s inward tilt places mechanical stress on the tendons. The pain develops gradually, presenting as a dull ache on the outside of the ankle that worsens during activity.
A symptom of tendinopathy is tenderness when pressing along the tendon sheath, located just behind the outer ankle bone. Pain is noticeable during movements that engage the muscle, particularly when pushing off the ground or rolling the ankle outward against resistance. This issue reflects the tendon’s inability to adapt to the workload, leading to structural changes.
Sudden Traumatic Injuries
Pain that begins immediately and sharply is likely the result of an acute traumatic event, such as a partial or complete tear of the fibularis longus tendon. Tears usually stem from a single, forceful incident, most commonly a severe ankle sprain where the foot is twisted awkwardly inward. This rapid movement strains the contracted peroneal muscles, causing the tendon fibers to exceed their capacity.
An acute tear presents with immediate, sharp pain on the outer edge of the ankle and foot, often accompanied by rapid swelling and bruising. Following the injury, a person may experience weakness or inability to bear weight, depending on the tear’s severity. Symptoms of acute trauma necessitate immediate attention.
Tendon Slipping Out of Place
A distinct mechanical problem causing pain and instability is peroneal tendon subluxation, where the tendon slips out of its normal position. The fibularis longus tendon normally glides within a groove behind the ankle bone, held in place by the superior peroneal retinaculum. Subluxation occurs when this retinaculum is torn or stretched, permitting the tendon to shift.
The defining symptom is a palpable and often audible snapping, popping, or clicking sensation felt on the outside of the ankle, particularly when moving the foot. This mechanical instability is painful and is frequently associated with an acute twisting injury. The tendon slides back and forth across the bone, leading to friction and irritation.
Diagnosis and Initial Management
Any severe pain, visible deformity, inability to bear weight, or persistent clicking requires prompt evaluation by a healthcare provider. If pain and swelling continue for more than a few days despite self-care, a professional diagnosis is necessary. Diagnosis begins with a physical examination, where the doctor checks for tenderness and assesses the ankle’s range of motion and stability.
Imaging studies are used to confirm the issue and rule out other problems, such as a fracture. X-rays assess bone structure, while magnetic resonance imaging (MRI) or ultrasound provides detailed images of the soft tissues. For initial self-care of acute pain and swelling, the R.I.C.E. protocol—Rest, Ice, Compression, and Elevation—is recommended.
Rest involves avoiding activities that exacerbate the pain. Ice should be applied for 15 to 20 minutes every two to three hours during the first 24 to 48 hours to minimize inflammation. Applying a compression bandage helps reduce swelling, and elevating the injured foot above the heart assists in fluid drainage. This initial management helps control symptoms while the underlying cause is determined.