Summary

TT-TG Ratio Provides a More Accurate Estimation of the Amount of Tibial Tubercle Medialization Needed in Patellofemoral Instability

Abstract

Introduction

The Tibial Tubercle Trochlear Groove (TT-TG) distance is one of the most commonly used measurement methods to quantify the lateralized patellar tendon insertion. However, this measurement is based on an absolute value that does not consider the patient's size and could over or underestimate the need for a medialization of the tibial tubercle. The aim of this study is to propose a ratio combining the TT-TG distance and the posterior bicondylar width (PBCW) to more precisely differentiate which patients have an altered TT-TG.

Methods

A retrospective cohort of objective patellofemoral instability (OPI) patients was compared to a control cohort. TT-TG distance and PBCW were measured on MRI, and a TT-TG ratio was defined using both measurements. Receiver operating characteristic (ROC) curves and the area under the ROC curve were used to assess diagnostic accuracy. Pearson’s correlation analysis was performed to examine the relationship between TT-TG and continuous variables. Reliability was tested using the intraclass correlation coefficient (ICC).

Results

A total of 138 knees were included in the OPI group and 107 knees in the control group. A significant difference in TT-TG distance was found between the OPI group (mean 15mm ± 5.2) and the control group (mean 8.6mm ± 3.6) (p < .001). A significant difference in TT-TG ratio was found between the study group (22.3% ± 3) and the control group (11.7% ± 4.6) (p < .001). The TT-TG distance had an AUC of 0.848, and the TT-TG ratio had an AUC of 0.892. TT-TG distance (p = 0.02) and PBCW (p < .001) were significantly different according to gender within the OPI cohort, while the TT-TG ratio did not show significant differences (p = 0.84). The ICCs for intra and inter-rater reliability were excellent for both TT-TG absolute and TT-TG ratio.

Conclusion

The TT-TG ratio demonstrated enhanced diagnostic accuracy compared to TT-TG distance without presenting differences in terms of sex or patient size. This personalized approach allows a more precise evaluation of the necessity for tibial tubercle transfer osteotomy, mitigating the risk of either overestimating or underestimating the indication for this procedure.