The Peroneal Tendons of the Ankle and Foot
The peroneals are two muscles and their tendons that lie along the outside of the lower leg bone (the fibula) and cross behind the lateral malleolus (the outer ankle bone). Major muscles that support the lateral part of the ankle are the peroneus longus and the peroneus brevis.
The tendons of these two muscles pass together in a groove behind the lateral malleolus. The tendons are kept within the groove by a sheath that forms a tunnel around the tendons. The surface of the tunnel is reinforced by a band of tissue called a retinaculum. Contracting the peroneal muscles makes the tendons glide in the groove like a pulley. The pulley action causes the foot to point downward (plantarflexion) and outward (eversion).
The peroneus brevis tendon connects to a bump on the base of the fifth metatarsal. This spot can be felt midway down the outer edge of the foot.
The peroneus longus tendon lies behind and below the peroneus brevis tendon. It wraps down and under the foot by way of the cuboid bone, the outer tarsal bone just in front of the heel bone (the calcaneus). The peroneus longus tendon angles forward under the sole of the foot and connects to the bottom of the main bone of the big toe. This tendon stabilises the arch of the foot when walking.
Lateral Ankle Pain
Tears of the peroneal tendons are not uncommon but remain an under appreciated source of chronic lateral ankle pain.
Tears of the Peroneus Longus Tendon as a Cause of Lateral Ankle Pain
The cause for acute tears is usually an acute inversion ankle sprain.
During the typical inversion ankle sprain, the foot rolls in. This type of injury sprains or tears the ligaments that support the lateral part of the ankle. The forceful stretch on the peroneals when the foot rolls in can also cause a lengthwise tear in the peroneal tendons.
In some patients, a peroneal tendon problem is caused by degenerative changes in the tendons themselves rather than by inflammation around the tendons. The tendon itself becomes abnormal. We call this condition tendonosis.
In tendonosis, the tendon becomes weakened. Tendons are made up of strands of a material called collagen. Degeneration in a tendon causes a loss of the normal arrangement of the collagen fibers that join together to form the tendon. Some of the individual strands of the tendon become abnormal due to the degeneration, some fibers break, and the tendon loses strength.
Over time, the tendon thickens as scar tissue tries to repair the damaged tendon. The area of tendonosis in the tendon is weaker than normal tendon. The weakened, degenerative tendon may tear. This usually causes a length-wise split in the peroneal tendons rather than a rupture. These splits or tears are most common in the peroneus brevis tendon, probably because it lies in front of the peroneus longus. It is more vulnerable to friction because it rubs against the groove in the fibula bone.
Patients with peroneal tendon problems usually describe pain in the outer part of the ankle or just behind the lateral malleolus. This pain commonly worsens with activity and eases with rest. Patients may have swelling behind or under the lateral malleolus. They may notice more pain when pressure is applied along the tendons.
MRI of the ankle will usually delineate the diagnosis accurately.
In the case illustrated in the MRI scan here, the peroneus longus tendon distal to the lateral malleolus showed a longitudinal tendon split.
Rest, immobilisation with a CAM walker, oral anti-inflammatory medications and use of crutches may help.
Surgical treatment may be needed if conservative treatment fails.
The surgery involves removing the inflamed tenosynovium of the injured tendons and to repair the torn tendon. A period of immobilisation with a plaster cast is usually needed post-surgery.