Interrater reliability of TT-TG distance on MRI is not dependent on preselection of slices
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Preselection of MRI slices compared with independently selected MRI slices did not significantly improve the interrater reliability of the tibial tubercle-trochlear groove distance measurement in patients with patellar instability, according to the results of this study.
"The key findings of this study are that inter-rater reliability of [tibial tubercle-trochlear groove] TT-TG distance measurement on MRI is generally good and not highly dependent on the specific MRI slices selected for the measurements,” Robert Magnussen, MD, told Healio Orthopedics. “In contrast, inter-rater reliability of trochlear morphology measures on axial MRI slices (specifically trochlear depth and sulcus angle) is only fair. Interrater reliability of sulcus angle measurements can be improved pre-selecting slices for measurements. The high dependence of sulcus angle and trochlear depth on slice selection suggest that trochlear morphology cannot be completely and accurately described by measurements made on a single axial MRI slice."
Researchers identified 26 patients with at least one recorded episode of patellar instability. There were six raters who used patients’ MRIs to measure and record the TT-TG distance, trochlear morphology, trochlear depth, sulcus angle and patellotrochlear index. The measurements were repeated using preselected slices. Intraclass correlations were used to calculate interrater reliability for slice selection and for TT-TG distance, trochlear morphology measures and patellofemoral instability with selected slices and preselected slices.
Investigators found interrater reliability was excellent for tibial axial slice selection and sagittal slice selection. They noted the interrater reliability was good for femoral axial slice selection. The interrater reliability was good for TT-TG distance when independently selected slices were used. Also, when independently selected slices were used, the interrater reliability was fair for the trochlear depth, sulcus angle and patellotrochlear index. Investigators noted the interrater reliability was good for the TT-TG distance, sulcus angle and patellotrochlear index when reselected slices were used and fair for the trochlear depth. When preselected slices were used, there was a significant improvement in the interrater reliability for the sulcus angle only. – by Monica Jaramillo
Disclosure: The study was funded by the Ralph C. Wilson Jr. Foundation.
Perspective
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Vicente Sanchis-Alfonso, MD, PhD
Lateral patellar instability is a frequent cause of knee dysfunction in the young active patient. MPFL is the cornerstone of surgical treatment. However, there are certain anatomical factors that may predispose to failure of an isolated MPFL reconstruction. According to the medical literature, one of these factors is the lateral position of the tibial tubercle relative to the trochlear groove that is evaluated measuring the TT-TG distance. The objective of Beals and colleagues is to evaluate the effect of slice selection on measured values of TT-TG distance. They conclude that inter-rater reliability of TT-TG distance on MRI is good and does not vary based on preselected vs. independent slice selection on MRI.
TT-TG distance is used in clinical practice to evaluate the indication or lack of it of the tibial tubercle medialization or anteromedialization in patients with lateral patellar instability. From my point of view, the use of TT-TG distance to justify a tibial tubercle osteotomy surgery is more than debatable. In 2015, Keiji Tensho and colleagues demonstrated that the TT-TG distance is affected by knee rotation more than by tibial tubercle malposition. Therefore, its use as an indicator for tibial tubercle transfer may not be appropriate. Even more, these authors have demonstrated that there is no difference in the tibial tubercle lateralization and in the trochlear groove medialization between patients with patellar instability and the control group. That is why, from my standpoint, the key question is not how to measure the TT-TG distance but to confirm that the points used for measurements are the correct ones. Going even further, is measuring the TT-TG distance worth it?
Hospital Amau de Vilanova,
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