Summary

The application of TT-TG as an index in relation to joint size gives larger differences between patients with PFI and a control group, compared to absolute TT-TG distance and can help surgeons when establishing the indication for surgical treatment with tibial tubercle medialization, with the highest sensitivity having been found for the quotient of TT-TG divided by the width of the patella.

Abstract

Introduction

A tibial tuberosity-trochlear groove distance (TT-TG) greater than 18 mm is used as cut-off value for directing surgical treatment with medial transfer of the tibial tuberosity in patients with patellofemoral instability (PFI). This absolute value, however, does not respect differences in individual knee joint size. Indices, based on the assumption that the magnitude of TT-TG is associated with the size of the knee joint, have recently been proposed as a means of taking patient size into account when indicating surgery. The aim of this study was to evaluate which index is the most reliable to detect a pathologic TT-TG distance in patients with PFI

Methods

From a total of 560 CT scans in our PACS 378 cases with previous operations or fractures in the region of interest were excluded from this study. Of the remaining 182 cases 36 had a diagnosis of patellofemoral instability when referred to the CT and were taken as PFI-group. For the control group 30 cases without a history of patellofemoral instability were picked randomly.
The TT-TG distance was evaluated in axial CT slices. TT-TG indices were given as the quotient of the TT-TG divided by five measures of knee joint size: medio-lateral femur width (mlF), antero-posterior lateral condylar height (apF), medio-lateral width of the tibia (T), width of the patella (P) and the proximal-distal joint size (TT-TE).
Differences between the groups were evaluated using the T-Test. Results are given as (mean PFI-group vs. mean control-group; p-value). To calculate the sensitivity of each index, the upper bound of the 95% confidence interval in the control group was used as the cut-off value between “normal” and “pathologic” and the number of patients that could be allocated correctly to the PFI group by using this cut-off value was evaluated.

Results

The TT-TG distance (15.7 vs 12.0; p= 0.001738) as well as the TT-TG indices calculated were significantly larger in the PFI group than in the control group. The most significant difference between both groups was found when calculating the TT-TG/P quotient [0.40 vs 0.28; p= 0.000045], followed by TT-TG/mlF [0.21 vs 0.15; p= 0.000125], TT-TG/apF [0.25 vs 0.19; p= 0.000198], TT-TG/T [0.22 vs 0.16; p= 0.000199] and TT-TG/TT-TE [0.32 vs 0.23; p= 0.000246].
The highest sensitivity (69.4%) to diagnose PFI based on a TT-TG index was found with the TT-TG/P index.

Conclusion

The application of TT-TG as an index in relation to joint size gives larger differences between patients with PFI and a control group, compared to absolute TT-TG and can help surgeons when establishing the indication for surgical treatment with tibial tubercle medialization. The TT-TG/P index showed the highest sensitivity.