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Tibial Tubercle-Trochlear Groove Distance Is a Reliable and Accurate Indicator of Patellofemoral Instability

Vairo, Giampietro L., PhD, ATC; Moya-Angeler, Joaquin, MD; Siorta, Michael A., MD; Anderson, Ashley H., MD; Sherbondy, Paul S., MD

Clinical Orthopaedics and Related Research®: June 2019 - Volume 477 - Issue 6 - p 1450–1458
doi: 10.1097/CORR.0000000000000711

Background Tibial tubercle-trochlear groove (TT-TG) distance is a measurement generally made on CT scans that is commonly used to quantify the risk of patellofemoral instability (PFI); however, its interrater reliability and accuracy as an indicator of PFI in patients is poorly characterized.

Questions/purposes The purposes of our study were (1) to primarily analyze interrater reliability of the TT-TG distance among orthopaedists with varied experience as measured by MRI, (2) to secondarily compare TT-TG distances between PFI and control groups, and (3) to determine an accurate TT-TG distance threshold indicative of PFI.

Methods The electronic medical records of a senior fellowship-trained orthopaedic sports medicine surgeon were surveyed between 2012 and 2016 for patients who had experienced at least one episode of patellar subluxation or dislocation, who reported no other knee-related history, and who underwent MRI due to persistent PFI signs and symptoms. The records of 48 PFI patients (23 males, 25 females; 19 ± 4 years of age) were compared with 83 controls (60 males, 23 females; 31 ± 8 years of age) having no history of PFI, presenting with an isolated meniscal lesion as determined from MRI and treated by the same orthopaedist during this time. All records meeting study criteria were consecutively included to offset selection bias of the retrospective analysis. Two sports medicine fellows, one who had just completed orthopaedic residency training, and another with a year of experience after residency, and a sports medicine subspecialist with more than 15 years of experience in practice independently recorded TT-TG distance, indicative of tibial tubercle lateralization relative to the femoral trochlea, to the nearest millimeter (mm) in a blinded and randomized fashion. Intraclass correlation coefficient computed interrater reliability accompanied by standard error of measurement (SEM); a one-tailed, two-sample t-test analyzed group differences with accompanying effect size per Cohen’s d; receiver operating characteristic (ROC) curve determined accuracy and threshold for PFI risk. A p value < 0.05 denoted statistical significance.

Results Interrater reliability was excellent, at 0.93 (95% confidence interval [CI], 0.84–0.97; SEM = 0.6 mm) for PFI patients and 0.95 (95% CI, 0.91–0.97; SEM = 0.4 mm) for controls. Distance was greater (95% CI, 2–5; p < 0.001) in PFI patients (14 ± 4 mm; range = 7–24 mm) than controls (10 ± 3 mm; range = 3–19 mm) with an effect size of 1 (95% CI, 0.3–2). Area under the ROC curve was 0.75 (95% CI, 0.66–0.83) and threshold was 13 mm (sensitivity = 0.52, 1-specificity = 0.25), suggesting the measure is a fairly accurate indicator of risk and values of 13 mm or greater are better suited to rule in PFI. Respective positive and negative likelihood ratios of 2 and 0.6 at this threshold confirm that this distance yields a small increase in probability for PFI and a minimal decrease in probability for risk; specifically, a 13-mm TT-TG distance is two times more likely to be found in patients with PFI. Furthermore, this threshold is estimated to increase a correct PFI diagnosis by approximately 15%.

Conclusions The MRI-specific TT-TG distance, based on a single measurement using cartilaginous-tendon landmarks within a standardized trochlear range, is reliable as performed by orthopaedists of varied experience. Patients with PFI display a 4-mm greater distance than controls, which may represent a difference large enough for clinicians to discern in practice using MRI. A 13-mm TT-TG distance is two times more likely seen in patients with PFI. However, this threshold increases a correct PFI diagnosis by only about 15%; therefore, clinical decision-making should not be influenced by this criterion alone and instead used in conjunction with other relevant variables.

Level of Evidence Level IV, diagnostic study,

G. L. Vairo, Departments of Kinesiology and Orthopaedics & Rehabilitation, Colleges of Health & Human Development, and Medicine, The Pennsylvania State University, University Park, PA, USA

J. Moya-Angeler, M. A. Siorta, A. H. Anderson, P. S. Sherbondy, Department of Orthopaedics & Rehabilitation, College of Medicine, The Pennsylvania State University, University Park, PA, USA

G. L. Vairo, 146 Recreation Building, Departments of Kinesiology, and Orthopaedics & Rehabilitation, Colleges of Health & Human Development, and Medicine, The Pennsylvania State University, 146 Recreation Building, University Park, PA 16802, USA, Email:

Each author certifies that neither he or she, nor any member of his or her immediate family, has funding or commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.

Clinical Orthopaedics and Related Research® neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDA-approval status, of any drug or device prior to clinical use.

Each author certifies that his or her institution approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.

Received August 20, 2018

Accepted February 14, 2019

© 2019 Lippincott Williams & Wilkins LWW
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