Summary

This prospective study examined the sagittal tibial tuberosity to trochlear groove distance (sTT-TG) in relation to patellofemoral (PF) chondropathy. No significant differences were found between PF pain patients and controls, even after adjusting for knee size and flexion. These results question the utility of sTT-TG for predicting PF joint contact forces and chondral wear.

Abstract

Purpose

The sagittal tibial tuberosity to trochlear groove distance (sTT-TG) has recently been introduced to evaluate the anteroposterior force vector on the patellofemoral (PF) joint. This metric might be associated with increased contact forces, potentially leading to anterior knee pain and chondral injuries. Retrospective studies suggest that a more posterior tibial tubercle is common in patients with patellar cartilage lesions compared to those with intact cartilage. However, no study has prospectively examined the relationship between sTT-TG and PF pain or chondral wear. This study aims to: 1) prospectively assess the influence of sTT-TG on patients with symptomatic PF chondropathy versus healthy controls, and 2) analyze the role of knee size and flexion on sTT-TG measurement.

Methods

We conducted a prospective study with patients who had symptomatic atraumatic PF chondral lesions and a control group with isolated meniscal pathology, both assessed with knee MRIs. Exclusion criteria included knee instability and previous surgeries. Based on literature, we calculated a sample size of 24 patients per group. Knee flexion and size were measured to control for confounding factors. Spearman’s correlation coefficient was used to assess the relationship between sTT-TG and both knee flexion and size. When significant correlations were found, an adjusted sTT-TG was calculated using linear regression. Median raw and adjusted sTT-TG values were compared using the Mann-Whitney test, and performance was evaluated with ROC curves and area under the curve (AUC) metrics. Significance was set at p < 0.05.

Results

We evaluated 24 control patients and 24 patients with PF pain, with similar median ages. No difference in knee flexion was observed between groups with a median knee flexion of 27º in control group and 28º in PF pain group (p = 0.3). Control group had significantly less female patients (25%) compared to PF pain group (75%, p = 0.01). Knee flexion showed a moderate correlation with sTT-TG (Spearman’s rho = 0.61). Linear regression indicated a 0.36 mm increase in sTT-TG per degree of flexion. No correlation was found between knee size and sTT-TG, so adjustments were made only for knee flexion.
No significant difference in median sTT-TG was observed between the control and PF pain group (p = 0.33), with medians of 2.7 mm (IQR 0.5–6.9) and 4.5 mm (IQR 1.9–7.4), respectively. The adjusted median sTT-TG was 4.4 mm (IQR 1.4–5.2) in controls and 4.2 mm (IQR 3.0–5.8) in PF pain group, again with no significant difference (p = 0.3). The AUC for both the raw and adjusted sTT-TG was 0.58.

Conclusion

The sagittal TT-TG did not distinguish between patients with and without symptomatic PF joints, even after adjusting for confounders. Consistent with a recent study, sTT-TG correlates moderately with knee flexion, suggesting that adjustments should be considered when using this metric. This first prospective study on sTT-TG in PF chondral injuries challenges previous findings, questioning the measurement predictive value for assessing PF contact forces and chondral wear as AUC was slightly better than random guessing.