The TT-TG Index: a new knee size adjusted measure method to determine the TT-TG distance

Medial transfer of the tibial tubercle has become a standard procedure in cases of patella instability caused by an increased tuberositas tibae-trochlear groove (TT-TG) distance. However, the TT-TG distance has always been assessed as an absolute value without taking individual joint size into consideration. It was assumed that the pathological influence of the TT-TG distance correlates with individual joint size. Aim of the current study therefore was to develop a method to express TT-TG distance in relation to these joint variables. Two hundred knee MRI scans of healthy individuals (69 females and 131 males) were evaluated retrospectively in a control group. First, the TT-TG distance was measured as described by Schoettle et al. To determine joint size, the proximal-distal distance between the entrance of the chondral trochlear groove (TE) and the onset of the patella tendon at the tibial tubercle (TT) was selected. Subsequently, the TT-TG/TT-TE ratio expresses the relationship between the TT-TG distance and the proximal-distal distance from the entrance of the chondral trochlear groove to the height of the tibial tubercle. The TT-TG Index can also be expressed as an angle (TT-TG angle). Likewise, in another patient group, 54 knee MRTs of patients with patellofemoral instability were evaluated. The average TT-TG distance of the control group was 7.5 +/- A 3.5 mm (range 0-17.4 mm) with no significant differences between genders. The mean TT-TE distance was 63.9 mm (range 49-79 mm) with there being significant differences between genders. The resulting mean TT-TG Index was 0.12 +/- A 0.05 (range 0-0.25). In the patient group, the average TT-TG distance was 13.5 +/- A 4.1 mm and the average TT-TE distance was 61.3 +/- A 6.8 mm. The resulting average TT-TG Index of 0.22 +/- A 0.07 in the patient group (PFI) approximates the threshold determined by the 95 % confidence interval in the healthy control group. A direct comparison between the control group and the patient group revealed a significant difference in the TT-TG distance (p = 0.0001), in the TT-TE distance (p < 0.0042) and in the resulting TT-TG Index (p < 0.0001). The measurement of the TT-TG Index is a reliable and differentiated approach for determining the lateral displacement of the tibial tubercle in relation to the proximal trochlear groove. The pathological influence of the TT-TG distance in case of patella instability depends on individual joint size, confirming the initial hypothesis. We currently consider a TT-TG Index > 0.23 to be pathological based on our findings. Particularly, in case of a marginal TT-TG distance, the additional relative TT-TG Index facilitates a decision concerning an indication for a operative medial transfer of the tibial tubercle.