Background

To evaluate the intra/inter reader and diagnostic reliability of the sagittal plane adjusted patellar instability ratios (PIRs) compared to tibial tubercle-trochlear groove (TT-TG) distance alone while employing matched case-control analysis for age and sex to minimize potential confounding effect.

Methods

A retrospective case-control study was performed of all MRI studies of patients diagnosed with patellar instability, between 2005-2020 at regional tertiary medical centre.
Using 1:1 case-control matching for categories of sex and age at the time of the diagnosis, one control subject was assigned to each case of patellar instability.
Measurements of TT-TG distance, sagittal patellar length (PL), sagittal patellar tendon length (PTL), TT-TG/PL Ratio and TT-TG/PTL ratio were conducted.
Two orthopaedics residents were assigned to assess the intra/inter-reader reliability, and inter class coefficients were calculated (ICC). The receiver operating characteristic (ROC) curve for each parameter was conducted, and area under curve (AUC) was compared to evaluate for diagnostic reliability. Odds ratios (OR) and their 95% confidence intervals (CI) were calculated and multivariable logistic regression model was conducted to controlled for possible confounders.

Results

The study included 340 individuals (170 case-control matched pairs). In terms of intra-reader and inter-reader reliability, TT-TG/PL and TT-TG/PTL ratios show an excellent correlation within and between readers (TT-TG/PL; intra-reader ICC 0.95 and inter-reader ICC 0.94, TT-TG/PTL; intra-reader ICC 0.93 and inter-reader ICC 0.9). The ROC curve showed a slightly higher AUC of TT-TG/PL ratio compared to TT-TG distance alone (0.75 vs 0.73, p<0.001).
When applying the pathologic cut-off of TT-TG>20mm and TT-TG/PL>0.5; the calculated odds ratios for above cut-off were as follows; TT-TG distance alone OR of 7.9 (95% CI 1.8-35.47, p=0.006) and TT-TG/PL ratio OR of 22.12 (95% CI 2.93-166.81, p=0.003). In the multivariable analysis, adjusted for height and weight, the association between TT-TG distance alone and patellar dislocation remained statistically significant with adjusted OR of 5.2 (CI 1.04-26.00, p=0.045) compared to TT-TG/PL ratio adjusted OR of 21.11 (CI 2.46 -181.38, p=0.005).

Conclusion

PIRs are significantly more reliable indicators, in terms of diagnostic reliability, sensitivity and specificity, and intra\inter reader reliability, when evaluating for patellar instability.