Measurement of tibial tuberosity-trochlear groove distance: evaluation of inter- and intraobserver correlation dependent on the severity of trochlear dysplasia

Excessive tibial tuberosity-trochlear groove distance (TT-TG) is considered as one of the major risk factors in patellofemoral instability (PFI). TT-TG characterises the lateralisation of the tibial tuberosity and the medialisation of the trochlear groove in the case of trochlear dysplasia. The aim of this study was to assess the inter- and intraobserver reliability of the measurement of TT-TG dependent on the grade of trochlear dysplasia. Magnetic resonance imaging (MRI) scans of 99 consecutive knee joints were analysed retrospectively. Hereof, 61 knee joints presented with a history of PFI and 38 had no symptoms of PFI. After synopsis of the axial MRI scans with true lateral radiographs of the knee, the 61 knees presenting with PFI were assessed in terms of trochlear dysplasia. The knees were distributed according to the four-type classification system described by Dejour. Regarding interobserver correlation for the measurements of TT-TG in trochlear dysplasia, we found r = 0.89 (type A), r = 0.90 (type B), r = 0.74 (type C) and 0.62 (type D) for Pearson's correlation coefficient. Regarding intraobserver correlation, we calculated r = 0.89 (type A), r = 0.91 (type B), r = 0.77 (type C) and r = 0.71 (type D), respectively. Pearson's correlation coefficient for the measurement of TT-TG in normal knees resulted in r = 0.87 for interobserver correlation and r = 0.90 for intraobserver correlation. Decreasing inter- and intraobserver correlation for the measurement of TT-TG with increasing severity of trochlear dysplasia was detected. In our opinion, the measurement of TT-TG is of significance in low-grade trochlear dysplasia. The final decision to perform a distal realignment procedure based on a pathological TT-TG in the presence of high-grade trochlear dysplasia should be reassessed properly.