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The Tibial Tubercle–Trochlear Groove Distance on Axial CT and MRI: Letter to the Editor

Dear Editor: I have read with the utmost attention a very informative and clearly written article by Camp et al entitled ‘‘CT and MRI Measurements of Tibial Tubercle–Trochlear Groove Distances Are Not Equivalent in Patients With Patellar Instability.’’ In this work, the authors conclude that the tibial tubercle–trochlear groove (TT-TG) distance can be measured with excellent interrater reliability on both computed tomography (CT) and magnetic resonance imaging (MRI), although the values obtained should not be interchangeably used because MRI underestimates the value of the TT-TG distance. I hereby sincerely commend the authors for their original as well as practical contribution to the world of musculoskeletal imaging. The thing that caught my eye here was the excellent interrater reliability. As I gather, the measurements were performed using bony landmarks. If so, the shape of the femoral trochlea on the axial reference cut could have some bearing on the interrater reliability score. If the shape is regular, it resembles a groove. In the course of the measurement, one can visually approximate the deepest point of the groove with ease to draw a vertical TG line passing through it. Yet, if the shape is irregular, as in trochlear dysplasia, it can be very difficult to visually approximate the exact location of the point, more so if the femoral trochlea is flat. This obvious increase in subjectivity while deciding where to draw the TG line may potentially result in a decrease in the interrater reliability score. In fact, the meta-analysis of Smith et al, probably the only one published on the radiological assessment of patellar instability until now, showed acceptable(!) intrarater and interrater reliability of the TT-TG distance measurement on axial CT scans, whereas several other authors reported the low interrater reliability of the method. Moreover, as well as unfortunately, trochlear dysplasia is the most common instability factor in patients with patellofemoral instability, thus posing a very frequent hindrance for measurement of the TT-TG distance in everyday practice. The problem is well known, but there seems to be only 1 published attempt to circumvent it, which is rather outdated and available to connoisseurs of French only. Alternatively, one could draw a pericondylar rectangle on the axial reference cut and regard its vertical midline as the TG line. There happens to be no statistically significant difference between the positions of the visually approximated TG line (as is usually drawn) and the geometrically approximated TG line (vertical midline of the pericondylar rectangle) in knees without trochlear dysplasia. Hopefully, in this way, in knees actually exhibiting trochlear dysplasia, the shape of the femoral trochlea on the axial reference cut would become irrelevant for measurement of the TT-TG distance because the TG line would be geometrically approximated where it would have been visually approximated in the first place had the shape of the femoral trochlea been regular in that particular knee. However, this is only the first and modest step on a long and toilsome path toward finding a valid solution. To begin with, one could, for example, compare measurements of the TT-TG distance for interrater reliability using a visually and geometrically approximated TG line. Be that as it may, and with all of the aforementioned in mind, it would be interesting to know how many patients with trochlear dysplasia were included in the study, perhaps even classified according to Dejour’s classification, and how this fact influenced the interrater reliability of both modalities. Although this was not explicitly stated in the text of the article, and judging from Figure 1 (page 1837), I suppose that the authors were using the single-image technique in the measurement of the TT-TG distance. It is a detail worth mentioning because it has been shown that the interrater reliability of the single-image technique outmatches that of the classic (and still widely utilized) double-image technique. Also, Saudan and Fritschy write, ‘‘Nos résultats mettent ainsi en doute la précision supposée de la mesure de la TA-GT par des radiologues non spécialistes de l’appareil moteur’’ (‘‘Our results thus question the assumed accuracy of the TA-GT measurement by the radiologists that are not specialists for the locomotor system’’). In the Camp et al article, 2 fellowship-trained musculoskeletal radiologists performed the measurements. In that regard, it could have been useful to analyze the difference in the interrater reliability between the TT-TG distance measurements performed by the musculoskeletal radiologists and radiologists without any specific training in musculoskeletal imaging. The American Journal of Sports Medicine, Vol. 41, No. 12 DOI: 10.1177/0363546513512785 2013 The Author(s)