Sex and Height Impact Tibial Tuberosity to Trochlear Groove Distance Thresholds for Malalignment on Magnetic Resonance Imaging When Comparing Knees With Patellar Instability Versus Age- and Sex-Matched Controls.

PURPOSE: To identify magnetic resonance imaging (MRI)-based thresholds for measurements of malalignment and assess their ability to identify knees with patellar instability, while evaluating the impact of patient sex and height on these thresholds.

METHODS

A retrospective analysis was conducted to identify patients with symptomatic patellar instability and compare them with age- and sex- matched controls, defined as patients with meniscus tears without patellofemoral pathology. Tibial tuberosity to trochlear groove (TTTG) and tibial tuberosity-posterior cruciate ligament (TT-PCL) distances were measured on axial knee MRI. Receiver operating characteristic curves were calculated, and an area under the curve greater than 0.7 was considered a good diagnostic test. Youden's J statistic was used to establish diagnostic thresholds. Subgroup analysis was performed based on sex and height.

RESULTS

The study included 240 patients (64% female; age: 27 ± 9 years). Area under the curve was 0.75 (0.69, 0.81) for TTTG distance, but below 0.7 for TT-PCL distance. Cutoff values for TTTG distance identifying patellar instability were 13.7 mm and 14.7 mm in female patients < and >= 165.1 cm, respectively. In male patients, cutoff values were 16.1 mm and 20.1 mm for male patients < and >= 177.8 cm, respectively. TTTG distance had good diagnostic accuracy in both female patients (0.78 (0.7, 0.85)) and male patients (0.7 (0.57, 0.82)), with greater predictive ability in male patients below (0.79 (0.61, 0.94)) versus above (0.56 (0.39, 0.72)) the median height and similar efficacy among female patients below (0.76 (0.64, 0.86)) and above (0.77 (0.64, 0.88)) the median height.

CONCLUSIONS

TTTG distance measurements on MRI showed that threshold values for patellar instability were 16.1 mm vs 20.1 mm in male patients below and above the median height, and in female patients were 13.7 mm vs 14.7 mm for those below and above the median height, respectively. These differences highlight a need for incorporating patient-specific factors when using these thresholds in clinical decision-making.

LEVEL OF EVIDENCE: Level IV, retrospective comparative case series.