Elbow snapping is known to be caused by various conditions, including intra-articular loose bodies, instability, synovial plicae [1], ulnar nerve subluxation, aberrant tendon of triceps brachii [2], annular ligament [3-5], and meniscus-like tissue [6]. Elbow snapping by annular ligament causes lateral sided elbow snapping and pain, which can be misdiagnosed as lateral epicondylitis, because many clinicians are not familiar with this pathologic condition.

In the most of the literatures describing elbow snapping by annular ligament, the diagnosis was made by arthroscopy. The first radiologic diagnosis of elbow snapping by annular ligament was made by magnetic resonance imaging (MRI), which was reported by Huang et al. [5]. Here, we report a case of elbow snapping by annular ligament, diagnosed by dynamic ultrasonography.

A 20-year-old man visited our hospital with palpable snapping at his right elbow joint anterolateral side. He exercised intensively 2 months ago. He is currently serving in the military. Musculoskeletal ultrasonography was performed with the 17-MHz linear array transducer of an iU 22 scanner (Philips Healthcare, Bothell, WA, USA). The ultrasonography at the anterolateral elbow showed proximal border of thickened annular ligament interposed between radial head and capitellum in extended elbow. And the annular ligament slipped out of the radiocapitellar joint, relocated in front of the radial head during elbow flexion (Fig. 1, Electronic Supplementary Video 1).

The patient underwent arthroscopic exploration to excise the excessive soft tissue causing the snapping. Arthroscopy revealed hypertrophied leading edge of annular ligament, which is proximally migrated between radial head and capitellum in elbow extension (Fig. 2A) and repositioned anterior to radial head in elbow flexion (Fig. 2B). No more snapping was observed after arthroscopic debridement of hypertrophied proximal edge of annular ligament.

In 1963, Wightman et al. [3] reported a case in which a torn annular ligament displaced in the radiohumeral joint caused a noticeable click in the lateral elbow. The author explained that tightened anterior capsule pulled the annular ligament proximally, which caused the torn proximal band of the annular ligament slip over the radial head in elbow extension showing bucket-handle appearance.

The other reports [4,5] described degenerated annular ligaments as the cause of snapping elbow. Aoki et al. [4] reported two cases of snapping annular ligaments of the elbow in the throwing arms of young brothers. Pathology of removed tissue was ligament with degenerated fibrocartilage. In 2005, Huang et al. [5] reported a case of snapping annular ligament, diagnosed by MRI. T2-weighted MRI examination showed a meniscus-like annular ligament interposed between radial head and capitellum in the periphery of anterolateral aspect of radiocapitellar joint. Histology of removed tissue was ligament with focal myxoid change. The authors of two reports commonly described the degeneration of ligament as a result of inflammation and fibrosis of chronic torn or loose annular ligament with repetitive mechanical stress.

In our case, dynamic ultrasonography showed only thickening of annular ligament without visible separation of proximal edge. The result of arthroscopy was consistent with the ultrasonographic finding. The patient’s history of intensive exercise and occupation may have role in the degeneration of annular ligament.

In summary, we report a case of snapping annular ligament, which can be properly diagnosed by dynamic ultrasonography, which has its own strength in high resolution and feasibility of dynamic evaluation.