Proximal radioulnar joint, but not posterolateral, instability in patients with symptomatic minor instability of the lateral elbow

Background: The elbow joint is stabilized laterally by a complex of bony and ligamentous structures, such as the lateral ulnar collateral ligament and radial lateral collateral ligament (RLCL). Pathogenesis of many elbow conditions like symptomatic minor instability of the lateral elbow (SMILE) is based on ligamentous incompetency. The aim of this study is to measure ligament elongation in SMILE patients and determine the correlation with the development of this pathologic condition. Materials and Methods: Twenty-eight patients were included in this study: half of them were operated for suspected SMILE whereas the others were affected by other elbow diseases. All the patients underwent elbow arthroscopy. During this procedure, 4 parameters were analyzed: annular drive through (ADT), ulnar-humeral stressed distance (UHSD) at 45° and 30° of flexion, and loose collar sign (LCS). A dedicated arthroscopic mini-spreader introduced through an arthroscopic portal was used to take measurements. Results: Statistically significant differences were found for LCS (P = .0003) and ADT (P = .000002). UHSD at 30° and 45° elbow flexion was slightly higher in SMILE patients, but not significantly. Discussion and Conclusions: Progressive stretching of the RLCL and annular ligament may lead to their elongation, resulting in a pathologic anteroposterior shifting of the radial head and a lower congruence between the inner surface of the annular ligament and the radial head cartilage, resulting in proximal radioulnar instability. The study shows how RLCL and annular ligament are elongated in SMILE patients compared with controls, causing instability of the proximal radioulnar joint and confirming that ligament incompetency is involved in the SMILE condition.