Peroneal tendinopathy usually causes load-related pain along the tendons behind the lateral ankle without major mechanical instability. A tendon tear is more likely when pain is persistent, strength is reduced or imaging shows fiber disruption. Subluxation is different again: the tendon moves abnormally during ankle motion and may produce a reproducible snap or click. Because static imaging can miss dynamic instability, ultrasound can be particularly useful when subluxation is suspected.
The anatomy behind the outer ankle
The peroneus brevis and peroneus longus (also called the fibularis brevis and longus) run down the outside of the lower leg and pass together behind the lateral malleolus, the bony bump on the outside of the ankle. They sit in a shallow retromalleolar groove on the back of the fibula and are held there by a band of tissue called the superior peroneal retinaculum.
Below the ankle the tendons separate. The brevis attaches to the base of the fifth metatarsal on the outer edge of the foot. The longus turns under the foot near the cuboid, where some people have a small accessory bone called the os peroneum, and crosses the sole toward the inner side of the foot. Together they evert the foot, assist push-off and help control the ankle on uneven ground.
Three different problems can affect these tendons: tendinopathy, tears and instability. They can feel similar and can occur together, but they are managed differently.
Peroneal tendinopathy
Peroneal tendinopathy is a load-related tendon problem. Pain is usually felt behind or just below the outer ankle bone and rises with activities that load the tendons: running, especially on uneven or cambered surfaces, hiking, cutting and repeated push-off. It is often worse at the start of activity or the next day after a heavy session. The tendons may be tender to press along their course, and there may be mild swelling.
The key feature is that the tendons stay where they belong. There is no reproducible snapping, and strength is usually preserved, even if resisted eversion is painful.
Peroneal tendon tears
A chronic peroneal tendon tear can mimic tendinopathy. Split tears of the peroneus brevis are particularly important because they may produce persistent posterolateral ankle pain without an obvious acute rupture event. These longitudinal splits are thought to develop where the brevis is pressed between the longus tendon and the back of the fibula. Peroneus longus tears are less common but clinically important, and can cause pain further along the tendon under the outer foot.
A tear becomes more likely when:
- Pain is persistent and focal despite appropriate loading
- Eversion strength is reduced
- Swelling along the tendon sheath persists
- There are mechanical symptoms
- Symptoms began or worsened after an inversion injury
- Long-standing tendon symptoms fail to progress
None of these signs proves a tear on its own, and many people with long-standing tendon pain do not have one. They are reasons to consider imaging.
Peroneal tendon subluxation and instability
Subluxation means the tendons move out of their normal position during ankle motion. People describe a snap, click or shifting sensation behind the outer ankle, often reproduced when they circle the ankle or push the foot up and out.
According to a 2023 review, the typical injury mechanism is sudden forced dorsiflexion, which rapidly tightens the peroneal tendons, combined with forced eversion of the hindfoot. In the classic form, the superior peroneal retinaculum is injured and the tendons slip forward over the fibula.
Peroneal instability does not always mean the tendons visibly jump over the fibula. Intrasheath subluxation can occur within the retromalleolar groove, and dynamic ultrasound has directly demonstrated abnormal tendon switching in patients with painful snapping.
In a series of 57 patients with painful snapping behind the fibula, 43 had tendons that could be reproducibly pushed out of the groove. The other 14 could not, yet dynamic ultrasound showed the tendons switching their relative positions within the sheath during dorsiflexion and eversion. All 14 had an intact retinaculum at surgery. The authors described two types:
- Type A (10 patients): intact tendons switching positions within the groove
- Type B (4 patients): the peroneus longus slipping through a longitudinal split in the peroneus brevis
Intraoperative confirmation of the ultrasound findings was 100%. The practical lesson is that a normal-looking ankle during a static exam does not automatically exclude instability, and not every snapping ankle involves a torn retinaculum.
A side-by-side comparison
| Feature | Tendinopathy | Tear | Subluxation |
|---|---|---|---|
| Main symptom | Load-related pain behind the outer ankle | Persistent focal pain, sometimes swelling | Snap, click or shifting sensation |
| Strength | Usually preserved, may be painful | May be reduced | Often preserved |
| Tendon position | Normal | Normal, unless combined with instability | Abnormal during motion, over the fibula or within the sheath |
| Most useful imaging | Often none initially | Ultrasound or MRI | Dynamic ultrasound |
These are tendencies, not rules. Tears and subluxation can coexist, as the type B intrasheath pattern shows.
Other causes of lateral ankle pain
- Lateral ankle ligament injury: a history of inversion sprains, giving way and pain in front of and below the ankle bone over the ATFL and CFL. Chronic ligament instability and peroneal problems can occur together.
- Sural nerve irritation: burning, tingling or altered sensation along the outer heel and edge of the foot, rather than load-related tendon pain.
- Os peroneum and cuboid region pain: more distal pain under the outer foot that behaves like a peroneus longus problem.
- Fifth metatarsal base: focal bony tenderness after an injury raises the possibility of a fracture or avulsion.
- Space-occupying structures: a low-lying peroneal muscle belly or an extra muscle called the peroneus quartus can crowd the groove.
What the exam looks for
We localize the pain along the brevis and longus, check for swelling, test resisted eversion and plantarflexion with eversion, compare strength side to side, and watch single-leg loading, balance and gait. We ask the patient to reproduce any snapping while watching and feeling behind the fibula, test lateral ankle ligament stability, assess hindfoot alignment, and check the fifth metatarsal, cuboid and os peroneum region and the sural nerve distribution. No single test settles the diagnosis; the pattern does.
Ultrasound or MRI?
Ultrasound and MRI answer somewhat different questions. Dynamic ultrasound is especially valuable when the complaint is snapping or positional instability, while MRI can be useful when a broader structural assessment is needed.
Dynamic ultrasound for subluxation. Because subluxation often occurs only with the foot dorsiflexed and everted, static images can look normal. In an early surgical-correlation study of 13 patients with suspected subluxation, 12 had positive dynamic ultrasound findings and went to surgery. All 12 patients with a positive dynamic-ultrasound diagnosis of peroneal tendon subluxation had the diagnosis confirmed at surgery, giving a positive predictive value of 100%. Ultrasound also identified five peroneus brevis tears and one peroneus longus tear, all confirmed surgically. Interestingly, 4 of 20 asymptomatic volunteer ankles also showed subluxation, a reminder that findings must match symptoms.
A direct comparison with surgery. In a surgical-comparison study, ultrasound was more sensitive than MRI for peroneal tendon subluxation, 100% versus 66%, while both modalities were highly specific. MRI was slightly more accurate for tendon tears in that cohort: ultrasound had 88% sensitivity and 100% specificity for tears, compared with 100% and 100% for MRI. Both detected tendinopathy with 100% sensitivity and specificity. This was a small, selected surgical cohort of 21 patients who had both tests before surgery, so the numbers should not be read as universal real-world performance.
The newest meta-analysis. The newest diagnostic meta-analysis suggests ultrasound may be more sensitive than MRI for peroneal tendon tears. For peroneus brevis tears, pooled ultrasound sensitivity was 93% compared with 73% for MRI. For peroneus longus tears, ultrasound sensitivity was 94% compared with 60% for MRI.
| Tear | Ultrasound sensitivity | Ultrasound specificity | MRI sensitivity | MRI specificity |
|---|---|---|---|---|
| Peroneus brevis | 93% (75–98) | 85% (55–96) | 73% (56–87) | 88% (70–95) |
| Peroneus longus | 94% (71–99) | 94% (79–98) | 60% (35–85) | 91% (76–97) |
However, the available studies were heterogeneous and at substantial risk of bias, so the authors could not confidently declare one modality universally superior. Eight of the included studies assessed MRI, three ultrasound and one both.
The surgical comparison and the meta-analysis point in slightly different directions for tears. The single study compared both tests in the same small group of surgical patients, while the meta-analysis pooled different studies, most of which assessed only one test. Both support the same practical conclusion: choose the test that fits the clinical question.
Why getting the diagnosis right matters
A recent systematic review of peroneal tendoscopy reinforces that peroneal disorders span tendinopathy, tenosynovitis, tendon tears, stenosis and intrasheath subluxation, several distinct problems that can produce similar lateral ankle symptoms. In that review, low-lying muscle bellies were frequently under-recognized before surgery.
Uncomplicated tendinopathy usually responds to progressive loading. A substantial tear or recurrent instability may need a different pathway, including orthopedic referral. Treating a subluxating tendon as simple overload, or a split tear as routine tendinitis, risks months of ineffective care.
How Novo approaches lateral ankle pain
We decide whether the peroneal tendons are the source, whether the picture fits tendinopathy, a tear or instability, and whether dynamic ultrasound or MRI would change management. See how we approach peroneal tendinopathy, read what the evidence shows about shockwave for peroneal tendinopathy, or book a new patient exam.
References
- Ghandour S, Mercer RW, Strahan JA, Bejarano-Pineda L, Stewart ZE. Diagnostic accuracy of MRI and US for peroneal tendon tears: a systematic review and meta-analysis. European Radiology. 2025;35(9):5360-5368. PMID 40035861. (link)
- Melville DM, Taljanovic MS, Gimber LH, et al. Comparison of ultrasound and MRI with intraoperative findings in the diagnosis of peroneal tendinopathy, tears, and subluxation. Journal of Clinical Medicine. 2024;13(3):740. PMID 38337434. (link)
- Neustadter J, Raikin SM, Nazarian LN. Dynamic sonographic evaluation of peroneal tendon subluxation. American Journal of Roentgenology. 2004;183(4):985-988. PMID 15385290. (link)
- Raikin SM, Elias I, Nazarian LN. Intrasheath subluxation of the peroneal tendons. Journal of Bone and Joint Surgery (American). 2008;90(5):992-999. PMID 18451390. (link)
- Hosack T, Perkins O, Bleibleh S, Singh R. Snapping ankles: peroneal tendon subluxation and dislocation. British Journal of Hospital Medicine. 2023;84(10):1-7. PMID 37906073. (link)
- Tham A, Rajivan R, Rubin J, et al. Peroneal tendoscopy for peroneal tendon disorders: a systematic review of indications, diagnostic utility, and clinical outcomes. Journal of Orthopaedics. 2026;74:24-32. PMID 41536605. (link)
