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Background
The primary goal is to review the anatomy and pathology of common tendon subluxations. The peroneal, bicep, and extensor carpi ulnaris tendons are common sites for subluxation and should be considered in patients with chronic joint pain. MRI provides detailed visualization of the anatomy, but dynamic ultrasound may be helpful for transient subluxations. Understanding the anatomy and pathogenesis allows for appropriate planning and treatment of tendon subluxations.

Educational Goals / Teaching Points
Describe the normal anatomy of the biceps tendon, peroneal tendon, and extensor carpi ulnaris tendon. Recognize that tendon subluxation is often associated with traumatic injury, repetitive microtrauma, and chronic disease. Transient subluxations may not be seen on routine imaging, and dynamic ultrasound may be the modality of choice. Chronic instability may lead to other pathologic conditions including tendinosis, tenosynovitis, partial tears, and tendon rupture.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The peroneal longus and brevis muscles and tendons originate from the fibula and travel within the lateral compartment. The origin of the peroneus brevis begins at the distal 2/3 of the lateral fibula and inserts on the lateral aspect of the proximal 5th metatarsal. The origin of the peroneus longus arises at the lateral condyle of the tibia, head of the proximal 2/3 of the lateral fibula, and travels posterior to the peroneus brevis and inferiorly in the foot. The peroneus longus crosses the calcaneocuboid joint through the plantar aspect and inserts on the medial cuneiform and plantar 1st and 2nd metatarsals. These tendon positions are maintained by the superior and inferior peroneal retinaculum. The origin of the long head of the biceps tendon begins at the supraglenoid tubercle and partly from the superior glenoid labrum. The tendon is primarily intra-articular and is stabilized by the superior glenohumeral ligament (SGHL) and coracohumeral ligament (CHL) which fuse along the rotator interval with fibers from the subscapularis and supraspinatus to form the biceps pulley. The long head biceps tendon turns as it passes through the bicipital groove, transforming into a rounded structure while traveling deep to the transverse humeral ligament. The extensor carpi ulnaris tendon is formed from two heads that attach to the lateral epicondyle and the middle 1/3 of the posterior ulna. The tendon inserts on the posterior aspect of the base of the 5th metacarpal. The tendon travels through a tunnel described as the extensor carpi ulnaris subsheath at the level of the distal ulna, lying deep to the extensor retinaculum. Disturbances of these tendons through incompetent retinaculum can be recognized on MRI.

Conclusion
Diagnosis of persistent pain within the lateral ankle, shoulder, and lateral wrist may be clinically difficult. In these cases, recognizing tendon instability and retinacular injuries may serve to assist with the correct diagnosis. MRI can help with detailed depictions of the anatomy to allow for future treatment planning.