Summary

This study assesses how knee flexion affects the sagittal tibial tuberosity to trochlear groove (sTT-TG) distance and the concordance between CT and MRI measurements. Flexion significantly changes sTT-TG, with an average increase of 7mm. CT measurements at different angles were consistent, but CT and MRI measurements at 20 degrees showed poor concordance, suggesting they are not interchangeable.

Abstract

Purpose

The sagittal tibial tuberosity to trochlear groove (sTT-TG) distance is a newly introduced method for estimating the anteroposterior force vector across the patellofemoral (PF) joint. Research has linked a posteriorly positioned tibial tuberosity to increased pressure on the PF joint, potentially leading to pain and chondral wear. While sTT-TG has been validated as a highly reliable measurement in both CT scans and MRIs, the concordance between these imaging modalities has not yet been reported. Additionally, recent cadaveric studies suggest that knee flexion could affect the sTT-TG measurement, though these studies had small sample sizes, and in vivo biomechanics may yield different results. Thus, this study has two primary aims: 1) to determine the influence of knee flexion on sTT-TG measurements, and 2) to assess the concordance between CT and MRI measurements.

Methods

We conducted a cross-sectional study involving individuals with either PF pain or PF instability, who underwent both MRI and CT scans of the knee. The CT scans were performed at 0 and 20 degrees of knee flexion, while MRI was conducted only at 20 degrees of flexion. Sagittal TT-TG was measured according to previously validated methods. Based on the literature, a minimum sample size of 75 knees was estimated. Mean differences were analyzed using one-way analysis of variance (ANOVA) with Holm-Bonferroni post-hoc correction. The correlation between CT scan measurements was assessed via Pearson correlation coefficient, while the concordance between CT and MRI was evaluated using Lin’s concordance correlation coefficient (CCC). Statistical significance was defined as p < 0.05.

Results

The study included 75 knees, with a mean age of 27.7 years (± 9.1), and 68.8% of the participants were female. The mean sTT-TG measured was 2.48 mm (± 4.22) in CT at 0 degrees, 9.72 mm (± 5.08) in CT at 20 degrees, and 4.99 mm (± 5.55) in MRI at 20 degrees. Significant differences were found between sTT-TG measurements at the three different positions (p = 0.02, <0.001 and <0.001). As expected, there was a strong linear correlation between sTT-TG in CT at 0 and 20 degrees (Pearson’s coefficient = 0.74). In contrast, the concordance between MRI and CT measurements at 20 degrees was poor, with a CCC = 0.4.

Conclusion

Knee flexion significantly affects the sTT-TG distance, with the tibial tuberosity positioned approximately 7 mm more posterior at 20 degrees of flexion. The strong linear relationship between sTT-TG measurements at 0 and 20 degrees in CT scans suggests that these values represent the same phenomenon, and adjusting the metric by 7 mm when switching between these positions could approximate the true value with high confidence. Conversely, sTT-TG measurements between CT and MRI at 20 degrees were different and poorly concordant, indicating that these measurements are not interchangeable.