Background In the clinical evaluation of the anterior cruciate ligament—deficient knee, anterolateral rotatory instability is assessed by manual tests such as the pivot-shift test, which is subjective and not quantitative. Hypothesis The anterolateral rotatory instability in an anterior cruciate ligament—deficient knee can be quantified by our newly developed method using open magnetic resonance imaging. Study Design Controlled laboratory study. Methods Eighteen subjects with anterior cruciate ligament—deficient knees and 18 with normal knees were recruited. We administered the Slocum anterolateral rotatory instability test in the open magnetic resonance imaging scanner and scanned the sagittal view of the knee. The anterior displacements of the tibia at the medial and lateral compartments were measured. Furthermore, we examined 14 anterior cruciate ligament—deficient knees twice to assess intraobserver and interobserver reproducibility and evaluated the difference and interclass correlation coefficient of 2 measures. Results In the anterior cruciate ligament—deficient knee, displacement was 14.4 ± 5.5 mm at the lateral compartment and 1.6 ± 2.3 mm at the medial compartment; in the normal knee, displacement was 0.7 ± 1.9 mm and —1.1 ± 1.2 mm, respectively. The difference and interclass correlation coefficient between 2 repeated measures at the lateral compartment were 1.0 ± 0.7 mm and .98 for intraobserver reproducibility and 1.1 ± 0.7 mm and .91 for interobserver reproducibility. Conclusion This method is useful to assess the anterolateral rotatory instability of the anterior cruciate ligament—deficient knee. Clinical Relevance This method can be used in the clinical assessment of anterior cruciate ligament stability, such as comparing studies of graft positions or 2-bundle anatomic reconstruction and the conventional 1-bundle technique.
