Background
The tibial tubercle-trochlear groove (TT-TG) distance is a common measurement employed in the evaluation of patella instability patients. The purpose of this study was to evaluate the diagnostic accuracy of the TT-TG distance in differentiating between subjects with recurrent patella dislocation and controls in comparison to a three-dimensional (3D) measurement of trochlear groove curvilinearity, the entry point (EP)-transition point (TP) angle.
Methods
3D prints were generated from patients with a history of recurrent patella dislocation treated by the senior author between January 2020 and November 2021. 24 recurrent patella dislocation patients (18 female, six male) were included. TT-TG distances and trochlear widths were measured on axial CT images. 3D prints were generated from 10 control subjects, all female, with no history of patella dislocation whose whole-body CT scans were included in the New Mexico Decedent Image Database. To evaluate 3D trochlear groove curvilinearity, the entry point (EP)-transition point (TP) angle was used. All EP-TP measurements were obtained with the models resting on their posterior femoral condyles. All statistical analyses were performed in Prism 9.0 using two-way, unpaired student’s t-tests and receiver operating characteristic curves. P-values of <0.05 were considered statistically significant.
Results
No significant differences were observed in TT-TG distance or TT-TG/TW between the recurrent dislocation and control cohorts. Conversely, a significant difference was observed in EP-TP angles between the control and instability cohorts, p<0.0001. More obtuse, larger EP-TP angles were observed in the control subjects. TT-TG and TT-TG/TW area under the curve (AUC) analyses failed to attain statistical significance. An AUC of 0.9250 was observed for EP-TP angle measurements, p<0.0001. The highest likelihood ratio was observed at an EP-TP angle cutoff <144.6 degrees, such that EP-TP angles <144.6 degrees were more 16 times more likely to be observed in control patients than recurrent patella instability patients. This threshold value demonstrated a sensitivity of 80% and specificity of 95% for recurrent patella instability.
Discussion
The EP-TP angle demonstrated superior discriminative accuracy between a cohort of patients with recurrent patella instability and age-matched controls over the absolute TT-TG distance and normalized TT-TG/TW ratio. This three-dimensional measurement of trochlear groove curvilinearity may be incorporated in clinical decision-making for patients with recurrent patella instability.