Summary

TT-TG has excellent inter and intra-observer reliability. Also, in high grade dysplasia it is always possible to find an accurate measure of the TT-TG simply by taking a more distal trochlear groove reference

Abstract

Introduction

The etiology of objective patellar instability (OPI) is multifactorial.
According to the literature there is strong evidence that TT-TG is a risk factor for developing first‑time and recurrent patella dislocation. In high-grade dysplasia with completely flat or even convex trochlea, it is not always easy to get this measure according to the currently accepted method.

Hypothesis

The aim of this study is to investigate the absence of differences between TT-TG measured at different trochlea levels, assuming that this measurement is also measurable in high-grade dysplasias.

Materials And Methods

A retrospective analysis of magnetic resonance imaging (MRI) images was conducted from patients who presented to our institution between 2019 and 2023. A total of 136 knees and 125 patients were included for the measurement of TT-TG and TT-PCL.
For each patient, TT-TG was measured at three levels:
- TT-TG high (hTT-TG): the first cranial axial image presenting a complete cartilaginous trochlea
- TT-TG middle (mTT-TG): Axial image at mid-height (with reference to the sagittal plane) of the medial condyle.
- TT-TG low (l TT-TG): last image in cranial caudal direction where the two femoral condyles appear still joined before splitting.
The intraclass correlation coefficient (ICC) for TT-TG and TT-PCL was evaluated for measurements conducted by the two different observers. ICC values range from 0 to 1, where a value higher than 0.75 denotes excellent agreement. For the intra-observer correlation coefficient, observer 1 read the MRI results blindly on two separate occasions (with a 4-week interval). For the inter-observer correlation coefficient, the two observers were blinded to the results obtained by the other. A p-value <0.05 was considered statistically significant.

Results

The mean and the standard deviation (SD) were determined for each TT-TG (low lTT-GT, middle mTT-GT and high hTT-TG). On 136 knees it was always possible to measure the lTT-TG, however, regarding the hTT-GT, it was only measurable in 115 out of 136 MRIs. Among 21 hTT-TG missing measures, 16 were knee with dysplasia type D, 3 with dysplasia type B, 1 dysplasia type A and 1 with dysplasia type C.

The intraclass correlation coefficients for interobserver reliability was considered excellent for l TT-TG, mTT-TG, and hTT-TG (respectively 0.877; 0.929; 0.892).
Poorer results were obtained for TT-PCL ICC interobserver (0.606).
The intraclass correlation coefficients for intraobserver reliability was considered excellent for mTT-TG and TT-PCL (0.989 and 0.922).
No statistically significant differences were found between TT-TG measured at the 3 established trochlea levels.

Conclusion

TT-TG has excellent inter and intra-observer reliability. Also, in high grade dysplasia it is always possible to find an accurate measure of the TT-TG simply by taking a more distal trochlear groove reference.
These findings are interesting because no other measures are needed to assess the lateralisation of the tibial tuberosity, even in high-grade dysplasia, to plan surgical procedure for patients requiring tibial osteotomy in OPI.