Peroneal tendonitis is an rare issue with the tendons on the outside of the rearfoot. The condition almost always happens in athletes where the stresses on these structures are therefore higher. There are two peroneal muscles on the lateral side of the leg whose tendons pass across the outside of the ankle joint with one tendon attaching on the outside of the foot at the bottom of the fifth metatarsal. The other tendon passes underneath the foot to connect to an spot close to the middle of the arch of the foot. The peroneal muscles have many different actions, but a key one is to counteract the ankle rolling laterally and winding up with a ankle sprain. Since they work hard during that task, the stress on the tendons might be too much for the tendon to take and they are prone to peroneal tendonitis.
Normally the tendonitis starts off with pain either over or just beneath the lateral ankle bone with or without some puffiness. In some the swelling develops later. With ongoing exercise the pains becomes more persistent and gradually worse. A common feature in those with peroneal tendinopathy is a lower supination resistance. Because of this it is easy for the rearfoot to supinate or roll outwards. This makes the peroneal muscles to be very active, so if you then combine it with higher level of athletic activity, then the tendon is at higher risk for an overuse injury.
The treatment of Peroneal Tendinopathy in most cases starts with reducing the strain by lessening activity levels and also the use of shoe wedging or foot orthotic to pronate or tip the feet inwards so the muscle does not have to work as hard. Ice and anti-inflammatory medicines will also help decrease the discomfort and swelling. Over the medium to longer term increasing stress by the way of exercise needs to be placed on the tendon in order that it can adapt to the strains placed on it. In some circumstances, surgery is often needed.