The patient (a woman of 28 years of age) was referred to our department as an outpatient for MRI of her right ankle with the indication of unspecific lateral ankle pain.
The MR examination included T1W, T2W, T2W/TSE with fat suppression and STIR sequences in three planes. The muscle belly of the soleus muscle was extending lower than usual [fig.1, 2]. There was a small amount of fluid in the tibiotalar joint mainly laterally at the level of the lateral malleolus [fig.2]. Behind the peroneal tendons, beneath the superior peroneal retinaculum, an additional mass was revealed. The signal intensity of this mass equalled that of muscle on all pulse sequences [fig.1, 2]. A linear low signal intensity structure, resembling a tendon, was also seen within the aforementioned mass. This low signal structure was inserting into the peroneus brevis tendon just caudally to the fibular tip [fig.1, 2, 3]. The findings were interpreted as a peroneus quartus muscle because of its location and tendon insertion.
The lateral compartment of the leg includes: The peroneus longus muscle which arises from the proximal fibula and the peroneus brevis muscle which arises from the lower two-thirds of the fibula [1]. They contribute the peroneus longus tendon (PLT) and the peroneus brevis tendon (PBT) respectively [1]. At the level of lateral malleolus the PBT lies anteriorly, in the peroneal groove of the distal fibula [1]. The superior peroneal retinaculum encircles both tendons at the level of the lateral malleolus [1]. PLT inserts onto the base of the 1st metatarsal and 1st cuneiform, and PBT onto the tuberosity of the 5th metatarsal [1]. In our case MRI identified both muscles and tendons.
In 83%–95% of cases a third peroneal muscle is found, the peroneus tertius [2], which lies in the anterior compartment of the leg. Its tendon passes through the inferior extensor retinaculum and inserts onto the base and the dorsal surface of the shaft of the 5th metatarsal [2]. In our case MRI did not disclose this muscle.
Besides these muscles, there are numerous accessory muscles at the distal lateral aspect of the leg. They include peroneus quartus, peroneus accessorius, peroneocalcaneus externum, and peroneus digiti minimi muscles [3]. It was suggested that they all should be referred with the term peroneus quartus muscle (PQM) [3]. They have a prevalence ranging from 10% (MRI studies) [4], to 26% (cadaveric studies) [3]. In our case MRI disclosed a muscle belly, lying behind PBT and PLT. This finding could represent a low situated belly of the peroneus brevis muscle, but a separate tendon within this muscle tissue was recognized. This tendon was inserting into the PBT just caudally to the fibular tip, so it represented a PQM.
It is difficult for the clinical examination to define the exact cause of unspecific lateral ankle pain, so MRI is needed. The PQM may be responsible for unspecific pain in the lateral aspect of the ankle [5], as in our case. It may also provoke a feeling of fullness at the level of the lateral malleolus, or it may be found incidentally [5]. The patient in our case was treated conservatively and the pain subsided within three weeks.
Peroneus quartus muscle.
1. MRI shows that the lateral tendons and muscle structures of the right ankle joint are clearly visible. The peroneus longus tendon (PLT) and the peroneus brevis tendon (PBT) can be identified, both running within the fibular groove of the lateral malleolus.
2. A supplementary muscle belly image can be observed posterior to both PBT and PLT. Near the PBT, a separate tendon structure can be seen, inserting into the PBT tendon tissue near the distal fibula, consistent in shape with the peroneus quartus muscle (PQM).
3. No obvious tenosynovial effusion, tendon rupture, or significant bony destruction is observed around the lateral malleolus. The articular surfaces and ligaments appear relatively intact as a whole, with no significant traumatic changes.
Based on the above imaging findings and the patient's clinical symptoms (unexplained lateral right ankle pain), the possible diagnoses include:
Taking into account the patient’s age (28 years), symptoms (persistent pain in the lateral ankle), physical examination findings (no clear evidence of significant ankle ligament damage), and MRI showing a marked variation of the peroneal muscle group (an additional tendon is visible), the most likely diagnosis is:
Lateral ankle discomfort caused by the Peroneus Quartus Muscle (PQM).
Because this anomalous muscle can create a sense of fullness in the lateral malleolus region or cause friction with surrounding tendons, it may induce pain. Combined with the imaging features and clinical symptoms in this case, the above diagnosis can be established.
This report is based on current imaging data and clinical history for reference only and cannot replace an in-person consultation or professional physician’s face-to-face evaluation and diagnosis. If there are any further changes in symptoms or questions, it is recommended to seek medical advice promptly and undergo related examinations or treatments.
Peroneus quartus muscle.
