Arthroscopy: The Journal of Arthroscopic & Related Surgery
Original ArticleThe Tibial Tubercle-Trochlear Groove Distance/Trochlear Dysplasia Index Quotient Is the Most Accurate Indicator for Determining Patellofemoral Instability Risk
Introduction
Young and physically active individuals represent the majority of patients suffering an initial episode of patellofemoral instability (PFI).1 The clinical importance of this condition is reflected by the fact that nearly half of this population will sustain further dislocations after conservative management.2 Anatomical variables are often considered in guiding plans of care for PFI.3, 4, 5 The tibial tubercle-trochlear groove (TT-TG) distance represents a common measurement examined in these cases.5, 6, 7, 8, 9 The measure is associated with high degrees of intrarater10,11 and inter-rater11, 12, 13, 14 reliability, as well as being accurate for determining PFI risk.11,13,15 However, such indices are dependent upon various factors. These include imaging method16 and related parameters,17 landmarks used for measurement,18 and statistical analyses underpinning inference.
As an absolute measure, utility of the TT-TG distance is limited, considering it is affected by demographic and anthropometric factors like age,17 height,19 and trochlear dysplasia.20 Therefore, alternative approaches for bolstering its clinical application have been suggested. One of these entail using the TT-TG distance to generate relative measures that determine PFI risk.21,22 Recently, a quotient that included trochlear width (TW) was identified to be a greater indicator of PFI risk compared with TT-TG distance alone.23 The TT-TG distance/TW quotient represents an individualized measurement that reflects anatomical considerations of the patient.23 Although notable, the results of this previous study23 were limited by not controlling for magnetic resonance imaging (MRI) methods, which are known to impact measurements.17 Furthermore, previous articles22,23 reporting related quotients have not accounted for trochlear depth that is indicative of dysplasia. The trochlear dysplasia index (TDI), used to quantify trochlear depth, is known to be significantly less in PFI patients compared with controls.24 Therefore, comparative analysis that controls for MRI technique and includes a TT-TG distance/TDI quotient would address these limitations and gaps in the literature.
The primary aim of our study was to evaluate diagnostic accuracy of the TT-TG distance relative to associated quotients produced from TT-TG distance/TW and TT-TG distance/TDI for detecting PFI. Secondary aims included identifying thresholds for risk and comparing differences between cases and controls. We hypothesized that quotients would yield more accurate indicators of PFI risk than TT-TG distance alone and that a quotient, including TDI, would produce the most accurate criterion. We also hypothesized that PFI patients would display greater values for all measurements than controls.
Section snippets
Methods
A retrospective comparative observatory trial design was conducted at our University Bone and Joint Institute. An a priori power calculation was based on the primary aim of our study. Using the following criteria—noninferiority method; significance level (α) of .05; power (1-β) of 80%; area under the receiver operating characteristic (ROC) curve of .700 for the standard test (i.e., TT-TG distance); area under the ROC curve of .800 for the new test (i.e. TT-TG distance/TW and TT-TG
Results
Electronic health records from November 2013 to November 2017 were surveyed. The AUC values were .69 (95% Confidence Interval [CI] = .60, .79) for TT-TG distance, .81 (95% CI = .73, .88) for TT-TG distance/TW, and .85 (95% CI = .78, .91) for TT-TG distance/TDI. A plot of the ROC curve is found in Fig 2. Optimal thresholds were calculated as 14.7 mm (sensitivity = .63, 1-specificity = .28) for TT-TG distance, .36 (sensitivity = .71, 1-specificity = .29) for TT-TG distance/TW, and 1.88
Discussion
Our results confirmed the hypothesis that TT-TG distance/TW and TT-TG distance/TDI were more accurate for determining PFI risk than TT-TG distance alone. Our outcomes also supported the hypothesis that TT-TG distance/TDI was the most accurate indicator of risk. These findings account for limitations in the preceding literature. Accordingly, we generated a power calculation for ROC curve analysis, considering using an odds ratio has limited application with gauging diagnostic accuracy,
Acknowledgement
We thank the Department of Radiology for their assistance with data collection.
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Joaquin Moya-Angeler, M.D., and Giampietro L Vairo, Ph.D., A.T.C. are co-first authors. Full ICMJE author disclosure forms are available for this article online, as supplementary material.