MRI-Based Morphologic Patterns of the Peroneus Quartus in a Korean Clinical Cohort: An Anatomical Study with an Illustrative Surgical Case.

Background/Objectives: The peroneus quartus (PQ) is an accessory musculotendinous structure of the lateral leg with variable origin and distal attachment. Although its anatomy has been described in cadaveric and imaging studies, the morphologic patterns encountered on routine ankle MRI remain clinically relevant. This study aimed to characterize MRI-visible PQ morphology in a Korean clinical cohort and to compare the observed patterns with previously reported anatomical variants. An illustrative surgically treated case was also reviewed. Methods: Three hundred ankle MRI examinations obtained between February 2020 and February 2024 were reviewed. After exclusion of 33 examinations in which the peroneal compartment could not be reliably assessed, 267 unique patients were included. Two readers independently evaluated PQ presence and distal attachment using predefined MRI criteria. Interobserver agreement was assessed with Cohen's kappa. The surgically treated case was reviewed separately as an illustrative clinical correlation. Results: A PQ was identified in 41 of 267 ankles (15.4%; 95% CI, 11.5-20.2%). The proximal origin was directly visualized in 32 PQ-positive examinations, arising from the peroneus brevis in 19 and the distal fibula in 13; in nine examinations, the origin was outside the MRI field of view. Thirty-five PQs (85.4%) inserted into the retrotrochlear calcaneal region, whereas six (14.6%) joined the peroneus longus tendon distally. Interobserver agreement was complete for both PQ detection and distal attachment classification (κ = 1.00). In the illustrative case, a symptomatic ganglion was associated with an operatively confirmed degenerative PQ tendon tear. Conclusions: The PQ was identified in 15.4% of ankle MRIs in this selected Korean clinical cohort and showed two predominant MRI-visible distal attachment patterns. Most inserted into the retrotrochlear calcaneal region, whereas a smaller proportion joined the peroneus longus tendon. These findings describe MRI-visible anatomy rather than the full anatomical spectrum. The surgical case further illustrates that symptomatic pathology may involve the PQ tendon and its adjacent sheath.