Abstract
BACKGROUND: Computed tomography (CT) scans are useful for objectively measuring bone alignment because they show bone detail particularly well, and these scans have been used extensively to assess patellar orientation. The tibial tubercle-trochlear groove (TT-TG) offset distance has been shown to be influenced by knee flexion and weightbearing, yet conventional CT scans are obtained with the subject relaxed, supine, and with the knee in full extension. A new cone-beam CT scanner has been designed to allow for weightbearing images, potentially providing a more physiologically relevant assessment of patellofemoral alignment. PURPOSE/HYPOTHESIS: The purpose of this study was to measure the TT-TG offset in healthy individuals without any history of knee complaints when CT scans were obtained while fully weightbearing on a flexed knee. Our hypothesis was that the TT-TG offset measurement in these healthy knees would be reproducible and less than the historically reported normal range. STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: Twenty healthy volunteers without any history of knee complaint were recruited to undergo a weightbearing cone-beam CT scan of the knee flexed at 30°. The scans were reviewed by a radiologist and an orthopaedic surgeon, and TT-TG offset was measured using the digital tools of a picture archiving and communication system. Paired t tests were used to compare TT-TG offset on 2 separate occasions for both raters. Inter- and intrarater reliability were assessed using a 2-way mixed-effects model intraclass correlation coefficient with corresponding 95% confidence intervals for TT-TG offset. RESULTS: The mean TT-TG offset was 2.7 mm. There were no statistically significant differences in TT-TG offset between raters (P(rater1) = .70; P(rater2) = .49) and time of read (P(time1) = .83; P(time2) = .19). Good to moderate interrater reliability was found at the time of both reads, and good intrarater reliability was found for both raters. CONCLUSION: When measured by CT scan and obtained from a subject while weightbearing on a flexed knee, the TT-TG offset is reproducible and the distance is less than that obtained via a conventional CT scan.