A split tear of the peroneus brevis tendon is a longitudinal crack that runs along the length of the tendon on the outer side of your ankle, dividing it into two halves rather than snapping it in two. The peroneus brevis is one of two tendons running behind the bony bump on the outside of your ankle (the lateral malleolus), and its job is to help you turn your foot outward and stabilize the ankle during movement. Unlike a full rupture, a split tear leaves the tendon in place but progressively weakens it, often causing chronic pain that gets misdiagnosed for months or years as a simple ankle sprain that never heals.
How the Tear Develops
The peroneus brevis tendon passes through a shallow groove on the back of the fibula before curving under the foot. That groove is a tight space, and the tendon shares it with the larger peroneus longus tendon sitting right behind it. A band of tissue called the superior peroneal retinaculum holds both tendons in place, like a strap over a cable channel. When that retinaculum stretches out or tears, the peroneus brevis can start sliding over the sharp posterior edge of the fibula. That repeated contact essentially saws the tendon open from the inside out, producing one or more longitudinal splits.
The process is usually gradual. Repeated ankle sprains or chronic ankle looseness let the tendon shift slightly out of position each time, and the fibular ridge digs in a little more with every episode. Surgeons who have studied the mechanism describe it as subclinical or overt subluxation of the tendon over the posterolateral edge of the fibula, producing multiple longitudinal splits over time.1PubMed. Peroneus brevis tendon tears: pathophysiology, surgical reconstruction, and clinical results Tears in the retinaculum allow the front portion of the tendon to ride over that sharp edge, which is where the split begins.2PubMed Central. Longitudinal Split of the Peroneus Brevis Tendon and Lateral Ankle Instability: Treatment of Concomitant Lesions
Anatomical Quirks That Raise Your Risk
Not everyone’s ankle anatomy is identical, and some variations make split tears much more likely. Two stand out in the research: a low-lying muscle belly and the presence of an extra tendon called the peroneus quartus.
Most muscles taper into tendon well above the ankle joint, but in some people the peroneus brevis muscle extends further down into the groove behind the fibula. That extra bulk crowds the groove and increases pressure on the tendon. One study found that among patients with symptomatic peroneal tendon problems, roughly 88% had muscle tissue extending below the fibular groove, compared to about 54% of people without symptoms.3PubMed. Magnetic Resonance Imaging Analysis of Peroneal Tendon Pathology Associated With Low-Lying Peroneus Brevis Muscle Belly: A Case-Control Study A separate surgical series confirmed the association: a low-lying muscle belly appeared in over 60% of patients with chronic lateral ankle pain and was present in about 81% of those found to have a peroneus brevis tendon tear during surgery.4The Journal of Foot and Ankle Surgery. A peroneus brevis low-lying muscle belly (LLMB) is a rare anomaly
The peroneus quartus is an accessory muscle-tendon unit that not everyone has. When it is present, it adds yet another structure competing for space in the already-tight retromalleolar groove. An MRI study of over 1,100 ankles found that having a peroneus quartus roughly tripled the odds of a peroneus brevis tear compared to ankles without the extra tendon.5PubMed Central. Prevalence of the Peroneus Quartus Muscle and Its Association with Peroneal Tendon Pathologies: An MRI Study of 1160 Ankles Neither of these anatomical variants is something you can control. They are just the way your ankle happens to be built. But if you keep having lateral ankle problems that do not respond to typical sprain treatment, they may help explain why.
What It Feels Like
The hallmark symptom is pain along the outer back of the ankle, right behind the bony bump of the fibula. It is not the typical “front and outer” ankle sprain pain most people recognize. Instead, it sits further back and often comes with swelling and tenderness when you press the area behind the lateral malleolus.6PubMed Central. Split Peroneus Brevis: An Overlooked Cause of Ankle Dysfunction Many people also notice a feeling of the ankle giving way, as if the joint is unstable, because a split tear often coexists with stretched-out ankle ligaments.2PubMed Central. Longitudinal Split of the Peroneus Brevis Tendon and Lateral Ankle Instability: Treatment of Concomitant Lesions
The sneaky part is how much this overlaps with other ankle injuries. Chronic lateral ankle sprains, peroneal tenosynovitis, peroneus longus problems, and even stress fractures of nearby bones can all produce similar pain patterns. A key clinical clue is that pain gets worse when you try to turn your foot outward against resistance, and swelling tends to follow the line of the peroneal tendon sheath rather than spreading diffusely around the joint.7Clinics in Podiatric Medicine and Surgery. Peroneus Brevis Tendon Tears If you have had repeated ankle sprains and still have pain months later despite standard rehab, a split tear is worth considering.
Why It Gets Missed So Often
Split tears of the peroneus brevis are frequently misdiagnosed or diagnosed late. One reason is that the symptoms mimic a chronic sprain, so many patients (and clinicians) assume the ankle just needs more time. Another is that the standard imaging tool, MRI, is not as reliable for this particular injury as many people expect.
A systematic review and meta-analysis comparing MRI and ultrasound for detecting peroneal tendon tears found that MRI picked up peroneus brevis tears with a pooled sensitivity of about 73%, meaning it missed roughly one in four tears. Ultrasound actually performed better at about 93% sensitivity, though both had comparable specificity (around 85–88%).8PubMed. Diagnostic accuracy of MRI and US for peroneal tendon tears: a systematic review and meta-analysis Individual MRI studies have reported even lower accuracy. In one surgical series, one radiologist’s sensitivity for peroneus brevis tears on MRI was just 52%, while another’s was 76%, highlighting how much reader interpretation matters.9Indian Journal of Musculoskeletal Radiology. Improving pre-operative MRI diagnosis of peroneal tendon tears with a new objective sign and assessing the value of peroneus brevis fatty atrophy
Another study that compared MRI findings to what surgeons actually found during operations reported a sensitivity of about 84% and specificity of roughly 75% for peroneal tendon problems overall. False positives were common (13 out of 82 cases), and the study confirmed that MRI sometimes identifies tears that turn out not to be present at surgery, and vice versa.10PubMed Central. Reliability of MRI findings of peroneal tendinopathy in patients with lateral chronic ankle instability Ultrasound has a particular advantage for suspected tendon subluxation because it allows real-time, dynamic assessment. You can watch the tendon move in and out of the groove while the patient moves their foot, which a static MRI scan cannot show.11PubMed Central. Dynamic ultrasound of peroneal tendon instability
The practical takeaway: a normal MRI does not necessarily rule out a split tear. If clinical suspicion is high, an experienced ultrasound evaluation or even direct surgical exploration may still be warranted.
The Clinical Examination
Before any imaging, a careful physical exam gives important information. Clinicians should check the shape of the foot, because a high-arched (cavus) foot places more stress on the peroneal tendons, while a flat foot changes the mechanics differently. Palpation of the tendons in the groove behind the fibula while the patient pushes their foot outward and upward against resistance is one of the most useful maneuvers. Testing the lateral ankle ligaments for looseness is also standard, since the two problems so often travel together.12PubMed Central. Peroneal tendon disorders
The differential diagnosis includes ligament tears, tenosynovitis (inflammation of the tendon sheath without a structural split), peroneus longus injuries, fractures of the os peroneum (a small bone embedded in the longus tendon), and even anomalous tendons or ganglion cysts pushing on the peroneal structures.13European Journal of Radiology Open. Peroneus brevis split tear – A challenging diagnosis: A pictorial review of magnetic resonance and ultrasound imaging – Part 2: Imaging with magnetic resonance and ultrasound Sorting through these possibilities is why imaging alone is not enough; the full picture comes from the history, the exam, and the scan together.
The Link to Chronic Ankle Instability
Chronic lateral ankle instability and peroneus brevis split tears feed off each other. Loose ankle ligaments allow the ankle to roll more easily, which repeatedly forces the peroneal tendons to compensate. That extra workload, combined with the mechanical irritation from the fibular edge, sets the stage for a split. And once the tendon is split, the ankle loses one of its key dynamic stabilizers, making instability worse. It becomes a vicious cycle.
Surgeons have recognized this overlap for decades. In one series, all 18 patients undergoing surgery for split tears of the peroneus brevis also had chronic ankle instability.14PubMed. Split lesions of the peroneus brevis tendon in chronic ankle laxity The association matters for treatment planning: if you fix the tendon but leave the ankle ligaments loose, the tear is more likely to recur. Many surgeons address both problems during the same operation.
Treatment Options
Small split tears identified early can sometimes be managed without surgery. Immobilization in a walking boot, activity modification, and a physical therapy program focused on peroneal strengthening and ankle proprioception are typical first-line approaches. But because most split tears are diagnosed after they have been present for months, and because the underlying mechanical cause (subluxation, crowding, instability) does not resolve on its own, surgery is common.
The two main surgical strategies are repair (tubularization) and partial resection. In tubularization, the surgeon sews the split tendon back together into a tube shape, restoring its cross-section. If the tear is too extensive and too much of the tendon is damaged, the degenerated portion is removed. Biomechanical testing shows why surgeons prefer repair when possible: suturing the split back together reduced the elongation caused by the tear to near-normal levels. By contrast, removing half of the tendon’s width more than doubled the elongation and significantly weakened the tendon’s stiffness.15PubMed. A Biomechanical Analysis of Peroneus Brevis Split Lesions, Repair, and Partial Resection In short, the less tendon you remove, the better the tendon performs afterward.
When the retromalleolar groove itself is too shallow and contributes to ongoing subluxation, surgeons may deepen it during the same procedure. This involves reshaping the bone behind the fibula to create a deeper channel for the tendons. Combined with retinaculum repair, this approach aims to eliminate the mechanical irritation that caused the split in the first place.16International Journal of Surgery Case Reports. Peroneal groove deepening as the treatment of peroneal tendon subluxation: A case report
Recovery After Surgery
Recovery from peroneus brevis split tear surgery is not fast, and people tend to underestimate the timeline. A commonly used protocol starts with a splint for the first two days, then a non-weight-bearing cast for about 12 days until stitches are removed. After that, weight-bearing in a walking boot or cast begins for four weeks. Formal physical therapy starts around six weeks after surgery, focusing on restoring range of motion and rebuilding strength.17PubMed Central. Ankle Rehabilitation after surgical treatment of peroneal tendon tears and ruptures
For athletes, the functional rehabilitation that follows is staged. One detailed rehab program for athletes started with balance exercises on a wobble board, progressed through walking and jogging in a straight line, then added jumping and cutting. Players in that series returned to sport in an average of about 12 weeks, ranging from 8 to 16 weeks.18Journal of Korean Foot and Ankle Society. Peroneal Tendon Repair in Sports Injury Those numbers sound encouraging, but they do not tell the whole story about how fully people return to their previous activity levels.
Return to Sport and Long-Term Outcomes
The research on long-term results after peroneus brevis repair paints a mixed picture. A larger study found that about 83% of patients reported returning to regular exercise and sports at final follow-up. At one year, roughly 77% had returned to the same preinjury activities, though only about 62% felt they were back at the same level of performance.19PubMed. Patient-Reported Outcomes and Return to Activity After Peroneus Brevis Repair That gap between “doing the activity” and “doing it as well as before” is a real distinction patients should be aware of going in.
Another study reported a lower return-to-sport rate of 46%, along with a high rate of residual symptoms: about 58% had ongoing scar tenderness and 54% had persistent lateral ankle swelling, even though nine out of ten working patients returned to their previous employment level.20PubMed. Peroneal tendon tears: return to sports after operative treatment The variation between studies likely reflects differences in surgical technique, severity of tears, and how strictly “return to sport” was defined. But the general pattern is consistent: surgery reliably improves pain and function for daily life, while full return to competitive athletics is less predictable.
Split Tears in Younger Patients
Split tears are overwhelmingly discussed as an adult problem, typically linked to years of repeated ankle sprains or chronic instability. But they can appear on imaging in younger patients too, and interpreting those findings requires caution. A study examining MRI results in pediatric and adolescent patients found that the majority of positive MRI scans (70%) showed a peroneus brevis split tear pattern. However, the positive predictive value of MRI with a matching clinical exam was only about 35%, meaning that most of the time the MRI showed a tear, the patient’s clinical presentation did not match.21PubMed. MRI findings of peroneal tendon tears do not necessarily correlate to clinical findings in paediatric and adolescent patients
Thirty-one patients in that study had positive MRI findings but a negative clinical exam. This suggests that what looks like a split tear on MRI in a younger person may often be an imaging artifact or an anatomical variant that is not actually causing symptoms. Operating based on MRI alone in this age group would risk unnecessary surgery. The lesson applies to adults too, but it is especially important in younger patients where the natural history of these findings is even less well understood.
When the Diagnosis Is Not a Split Tear
Because lateral ankle pain has so many possible sources, it is worth knowing what else might be going on if a split tear is suspected but not confirmed. The main conditions in the differential include lateral ligament tears (the anterior talofibular ligament is the most commonly sprained), peroneal tenosynovitis (where the sheath around the tendon is inflamed but the tendon itself is intact), injuries to the peroneus longus tendon, fractures of the os peroneum, and even ganglion cysts near the tendons that can mimic tendon pathology on imaging.2PubMed Central. Longitudinal Split of the Peroneus Brevis Tendon and Lateral Ankle Instability: Treatment of Concomitant Lesions MRI plays an important role in sorting these out, even though it has limitations for the split tear itself. It is particularly good at identifying bone edema, ligament damage, and fluid collections that point toward alternative diagnoses.22PubMed. The MR imaging appearance of longitudinal split tears of the peroneus brevis tendon
The fact that so many conditions share this neighborhood of pain is exactly why chronic outer-ankle symptoms deserve a thorough workup rather than just being labeled as “ankle sprains” indefinitely. If you have been told your ankle should have healed by now but it has not, asking specifically about the peroneal tendons is a reasonable next step.