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 ± 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 ± 0.05 (range 0–0.25). In the patient group, the average TT-TG distance was 13.5 ± 4.1 mm and the average TT-TE distance was 61.3 ± 6.8 mm. The resulting average TT-TG Index of 0.22 ± 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.

II.