Tibial Tubercle‐Trochlear Groove Distance Has Better Diagnostic Reliability Than Tubercle‐Posterior Cruciate Ligament Distance For Predicting Patellar Instability: A Systematic Review

OBJECTIVE: The tibial tubercle‐trochlear groove (TT‐TG) distance is now routinely utilized to help determine whether a realignment procedure is necessary for patients with patellar instability. The tibial tubercle‐posterior cruciate ligament (TT‐PCL) distance has been explored as an alternative measurement. The aim of this study is to compare the reliability of TT‐TG and TT‐PCL; to explore whether there is a relationship between the TT‐PCL and the TT‐TG distance; to determine whether there is a relationship between the TT‐TG and TT‐PCL distances and knee rotation; and to compare the abilities of the TT‐PCL distance and the TT‐TG distance with predicted patellar instability. METHOD: This systematic review was performed in accordance with PRISMA guidelines. Three databases, PubMed, EMBASE, and Cochrane Central Register of Controlled Trials, were searched from inception to September 2021 to identify clinical studies comparing TT‐TG and TT‐PCL distances to patellar instability. Data on patient baseline characteristics, TT‐TG and TT‐PCL distances, inter‐observer reliability, and area under the receiver‐operating characteristic curve (AUC) were recorded. The methodological quality of the studies was assessed using the quality assessment form recommended by the Agency for Healthcare Research and Quality (AHRQ). RESULT: Twenty studies were included in the final analysis, comprising 2330 knees from 2260 patients. The current study showed that TT‐TG and TT‐PCL have similar observer reliability. The inter‐ and intra‐observer reliability of TT‐TG ranged from 0.807 to 0.98 and 0.553 to 0.99, respectively. The inter‐ and intra‐observer reliability of TT‐PCL ranged from 0.553 to 0.99 and 0.88 to 0.981, respectively. Six studies compared the AUC for predicting patellar instability and showed that TT‐TG had better predictive performance than TT‐PCL. Three studies reported a correlation between TT‐TG and knee rotation, but no such relationship was found for TT‐PCL. Eight studies reported a weak or moderate correlation between TT‐TG and TT‐PCL. CONCLUSION: TT‐TG and TT‐PCL have similar inter‐ and intra‐rater reliability (as measured by ICC), but TT‐TG has greater discriminatory power to predict patellar instability than TT‐PCL (as measured by AUC values and odds ratio). However, considering trochlear dysplasia and individual variations, future studies need to find more accurate and individualized methods to predict patellar instability.