A standardized fluoroscopic sequence to reveal residual MCL instability after repair of the LUCL in elbow injury

Indications for stabilization of the medial collateral ligament (MCL) after repair of the lateral ulnar collateral ligament (LUCL) remain controversial. Here, we propose a standardized fluoroscopic sequence to reveal residual medial elbow instability to facilitate intraoperative decision-making.

Eight matched cadaveric upper extremity pairs (N = 16) were mounted to simulate intraoperative positioning. Fluoroscopic images were acquired using the following: full extension, 45-degree flexion, 90-degree flexion, and full flexion with the forearm in neutral/pronation/supination. These were acquired at "baseline" and following destabilization of the LUCL/MCL. The proposed fluoroscopic sequence was then repeated following surgical fixation of the LUCL ("post-LUCL repair") followed by MCL repair ("post-LUCL & MCL repair). Blinded images were fitted using a best-fit circle to compute ulnohumeral distance (UHD, millimeters) and determine residual lateral (supination) and medial (pronation) instability defined by the presence of a drop sign (UHD>4 mm). Radiocapitellar ratio (RCR) was computed to determine radiocapitellar instability (RCR>10 %). Blinded images were also qualitatively evaluated against the contralateral baseline to simulate intraoperative assessment.

Apparent instability in supination status-post destabilization resolved following LUCL repair with evident residual medial-sided instability showed in pronation, which resolved after MCL fixation. Evaluation of the drop sign at 45 and 90 degrees of flexion showed comparable quantitative sensitivity at 97 % and 98 %, unlike in full extension or full flexion (sensitivity <35 %). Quantitative sensitivity was 88 % for RCR in mid-flexion. Qualitative evaluation for the drop sign and RCR resulted in sensitivity of 93 and 75 %, respectively.

The proposed fluoroscopic sequence provides reliable intraoperative assessment to evaluate for residual medial-sided instability in the setting of multi-ligamentous elbow injuries. After repair of the LUCL, medial residual instability due to MCL rupture is best revealed with the presence of a drop sign in full pronation and midflexion.

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