Introduction: Clinically measured Quadriceps angle (Q-angle) has low reliability. Measurement of angle between femoral shaft and patellar tendon (FSPT angle) on routine knee MRI was described in this study to represent the lateral vector forces of quadriceps mechanism. The cross-sectional study was designed to compare this angle between subjects with objective patellofemoral instability (PFI) versus those without PFI, to assess its reliability, and to assess its validity in terms of its ability to differentiate between PFI and non-PFI subjects using the "Receiver Operating Characteristic" (ROC) curve.
Materials and methods: MRI scans of 20 subjects with PFI and 20 without PFI were obtained. FSPT angle was measured in each MRI by three different raters. In addition, the clinical Q-angle was also measured in the control group.
Results: The FSPT angle was significantly higher in PFI group as compared with the non-PFI group (p < 0.001). It had substantial inter-rater reliability of 0.82 (95% CI = 0.67-0.92) in the non-PFI group and 0.89 (95% CI = 0.78-0.95) in the PFI group. Test-retest reliability was more than 0.90. The AUC for the ROC curve was 0.86 (95% CI = 0.74-0.97). The clinical Q-angle measured in non-PFI subjects had inter-rater reliability of only 0.48 (95% CI = 0.21-0.72), and showed a fair correlation of 0.58 with the MRI angle.
Conclusion: Measurement of FSPT angle was described on MRI with substantial intra-rater and inter-rater reliability. The angle was significantly higher in PFI versus non-PFI subjects and also showed a good ability to differentiate between these two groups in the ROC curve.
Keywords: MRI; Patellar dislocation; Patellofemoral instability; Quadriceps angle; Reliability.