Peroneal tendon pathology is a well-recognised cause of lateral foot and ankle pain, particularly in active individuals and those with chronic lateral ankle instability. ^,, Disorders of the peroneus longus and peroneus brevis include tendinosis, tendinitis, tenosynovitis, longitudinal split tears, subluxation, and complete rupture. The peroneus brevis is particularly vulnerable to longitudinal tearing at the level of the distal fibula within the retromalleolar groove, where it is mechanically compressed between the fibula and the peroneus longus tendon during ankle motion. ^, Consequently, the majority of published literature describes peroneus brevis tears within the retromalleolar region.

The peroneus brevis inserts distally onto the styloid process at the base of the fifth metatarsal. While the literature describing retromalleolar peroneus brevis tears is well established, isolated insertional tears have received comparatively little attention. ^,, This relative paucity of literature may contribute to diagnostic uncertainty when patients present with persistent pain along the lateral border of the foot.

Magnetic resonance imaging (MRI) is commonly utilised in the assessment of suspected peroneal tendon pathology and is generally considered effective in identifying longitudinal split tears. However, imaging findings may be subtle, particularly in partial-thickness or chronic degenerative lesions, and false-negative interpretations have been reported. In such cases, clinical suspicion, careful physical examination, and correlation with imaging findings remain important in guiding management decisions.

We present a case of an uncommon insertional longitudinal tear of the peroneus brevis tendon at the base of the fifth metatarsal in a patient without cavovarus deformity, lateral ankle instability, or preceding trauma. Although the formal MRI report described the peroneal tendons as normal, independent surgeon review identified subtle abnormalities at the tendon insertion that were considered inconsistent with a completely normal tendon appearance. Surgical exploration subsequently confirmed a longitudinal insertional tear of the peroneus brevis tendon. This case highlights the diagnostic challenges associated with distal peroneus brevis pathology and emphasises the importance of clinical-radiological correlation when symptoms persist despite apparently reassuring imaging findings.

A 60-year-old White British female presented to the Department of Foot and Ankle Surgery with persistent pain localized to the lateral aspect of the left foot. Symptoms had been present for more than 12 months and had progressively worsened over time. The patient denied any history of trauma, inversion injury, or recurrent lateral ankle sprains. The pain was described as an intense burning sensation that was constant in nature and exacerbated by weightbearing activity. Night pain was also reported. The patient rated the pain as 10/10 on the numerical rating scale at its worst.

Prior to presentation, extensive conservative management had been attempted including physiotherapy, orthotic therapy, activity modification, immobilisation, and extracorporeal shockwave therapy, without meaningful symptomatic improvement.

Clinical examination demonstrated focal tenderness on direct palpation of the styloid process of the fifth metatarsal and along the course of the peroneus brevis tendon as it coursed proximally toward the distal fibula. The fourth and fifth tarsometatarsal joints, calcaneocuboid joint, subtalar joint, and ankle joint were asymptomatic on range of motion testing. Talar tilt and anterior drawer testing were negative for both pain and instability. Manual muscle testing demonstrated reduced strength secondary to pain, most notably during resisted eversion of the foot. Due to the severity of symptoms, the patient was immobilised in an Aircast boot for a period of six weeks.

Weightbearing radiographs of the left foot and ankle demonstrated irregularity around the styloid process of the fifth metatarsal without evidence of acute osseous pathology. The fifth tarsometatarsal and calcaneocuboid joints appeared unremarkable. Magnetic resonance imaging (MRI) was subsequently requested to further evaluate the persistent lateral foot pain and suspected peroneal tendon pathology.

The formal MRI report described the peroneus longus and peroneus brevis tendons as normal, without evidence of tendon tear or tendinopathy. The surrounding ligamentous structures, including the lateral ankle ligament complex, were reported as intact. No abnormal marrow signal was identified. Minimal fluid was noted within the posterior subtalar joint with mild nonspecific soft tissue oedema around the ankle. Overall, the MRI study was interpreted by the reporting radiologist as demonstrating no significant structural tendon or ligament pathology.

Despite the reassuring radiology report, the treating foot and ankle surgeons remained clinically suspicious of insertional peroneus brevis pathology due to the patient’s persistent focal symptoms, examination findings, and failure of prolonged conservative management. Independent review of the MRI demonstrated subtle abnormalities involving the distal insertion of the peroneus brevis tendon which were felt to be inconsistent with a completely normal tendon appearance.

On retrospective review, sagittal T2-weighted imaging demonstrated mild irregularity and subtle increased intrasubstance signal involving the distal peroneus brevis tendon at its insertion on the fifth metatarsal base compared with the expected smooth low-signal appearance of a normal tendon ( Fig. 1 ). Axial T1-weighted imaging demonstrated flattening and irregularity of the distal tendon contour with loss of the normal homogeneous morphology at the insertional region ( Fig. 2 ). Although no complete tendon discontinuity was appreciated on MRI, the tendon morphology was considered abnormal when interpreted in conjunction with the clinical presentation.

Sagittal T2-weighted MRI demonstrating subtle increased signal at the peroneus brevis insertion on the fifth metatarsal base without a clearly defined tendon tear. The study was reported as normal, highlighting the potential for under-recognition of insertional peroneal tendon pathology.

Axial T1-weighted MRI demonstrating the peroneus brevis tendon at its insertion on the fifth metatarsal base. No discrete tendon tear is identified however, there is subtle contour irregularity with intermediate signal at the insertion, which may reflect underlying pathology.

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