Sonographic appearance of the peroneus quartus muscle: Correlation with MR imaging appearance in seven patients

PURPOSE: To evaluate the sonographic appearance of the peroneus quartus muscle. MATERIALS AND METHODS: Thirty-two consecutive patients who underwent both sonography and magnetic resonance (MR) imaging of the lateral ankle were retrospectively identified during 42 months. Seven of these patients demonstrated a peroneus quartus muscle, with MR imaging as the standard of reference. The peroneus quartus muscle was characterized at retrospective review of sonographic images with regard to origin, insertion, echogenicity, and location. RESULTS: Of the seven peroneus quartus muscles, six originated from the peroneus brevis muscle, and all seven inserted onto the calcaneus. The muscle portion of the peroneus quartus muscle was hypoechoic, while the tendon portion was hyperechoic and fibrillar. The location of the peroneus quartus musculotendinous junction was variable, Hence, the appearance of the peroneus quartus muscle in the region of the distal fibula ranged from 100% muscle to 100% tendon. CONCLUSION: Variability in the appearance of the peroneus quartus muscle is common with sonography due to variation in the location of the musculotendinous junction. Recognition of these variations will allow correct diagnosis of a peroneus quartus muscle, and more important, it will allow differentiation from adjacent tendon abnormality.