Summary

This study introduces a novel measurement, the MCL radiographic shadow, for diagnosing high-grade MCL injuries using plain knee X-rays. Compared to healthy controls, significant differences were found in measurements for proximal and distal injuries. The method showed excellent diagnostic performance and reliability, suggesting it as a valuable tool for initial injury assessment.

Abstract

Purpose

Plain AP and lateral view knee X-rays are typically used during the initial evaluation of suspected acute knee medial collateral ligament (MCL) injuries. However, except for avulsion fractures, there are no specific radiographic indicators for high-grade MCL injuries. While MRI is the imaging gold standard for diagnosing these injuries, it is expensive, time-consuming, and may not be available in a timely manner, particularly in cases requiring acute surgical treatment. The ability to diagnose high-grade MCL injuries in plain radiographs would, therefore, be highly valuable. This study aims to: 1) introduce a novel MCL measurement on X-rays, 2) compare this measurement between healthy individuals and those with high-grade MCL injuries, and 3) assess the diagnostic performance and reliability of this new measurement.

Methods

We conducted a cross-sectional study involving healthy participants and patients with acute high-grade proximal and distal MCL injuries. All participants underwent both knee X-rays and MRIs within 10 days. Based on a pilot study, a minimum sample size of 30 healthy participants, 15 proximal injuries, and 7 distal injuries was estimated. We proposed a novel measurement: the MCL radiographic shadow, defined as the horizontal distance between the medial bony border and the first change in radio-opacity of the medial soft tissues, measured at three points: 15 mm proximal to the articular line (femoral MCL or fMCL); at the tibial articular surface (articular MCL or aMCL); and at the tibial physeal scar (tibial MCL or tMCL). Four expert observers, blinded to clinical status, evaluated the anteroposterior knee x-rays in two sessions separated by a 30-day interval.
Medians were compared using the Mann-Whitney test. The diagnostic performance of each measurement for proximal and distal injuries was evaluated using ROC curves and their corresponding area under the curve (AUC). Reliability was assessed with the intraclass correlation coefficient (ICC). Statistical significance was set at p < 0.05.

Results

The study included 30 healthy participants, 22 with proximal high-grade MCL injuries, and 9 with distal high-grade injuries. The fMCL, aMCL, and tMCL measurements showed significant differences between the healthy and injured groups.
Among the ROC curves, the fMCL measurement was the most effective for diagnosing proximal injuries at a threshold of 7.5 mm, with an AUC of 0.92, 95% sensitivity, and 77% specificity. The tMCL measurement was the best for diagnosing distal injuries at 4.7 mm, with an AUC of 0.93, 100% sensitivity, and 83% specificity. Inter-observer agreement for fMCL, aMCL, and tMCL measurements ranged from moderate to good, with ICC values of 0.774, 0.722, and 0.712, respectively. Intra-observer agreement was similarly strong across all three measurements.

Conclusion

The MCL radiographic shadow is a novel and effective tool for diagnosing acute high-grade MCL tears on plain radiographs. It demonstrated excellent diagnostic performance, with near 100% sensitivity, making it an outstanding screening test. The measurement also showed adequate reliability, with moderate to good inter- and intra-observer agreement. Future prospective studies are needed to further validate this promising new tool.