Does tibial tuberosity-trochlear groove distance (TT-TG) correlate with knee size or body height?

Since excessive tibial tuberosity-trochlear groove distance (TT-TG) is one of the major risk factors for patellofemoral instability, TT-TG is an often-used parameter in knee realignment surgery. Up to date, TT-TG is measured and interpreted using absolute values, disregarding the knee size of the individual. It was hypothesized that there is a relation between TT-TG and knee size and body height, respectively.

Consecutive MRI scans of 120 knee joints were analysed retrospectively. Of these, 60 MRI scans were obtained from patients with trochlear dysplasia and another 60 MRI scans were acquired from patients presenting with a different pathology of the knee joint. TT-TG was measured and TD was classified into low and high grade. Interepicondylar distance as an expression of knee size was measured on transverse MRI slices presenting the maximal distance from the medial to the lateral epicondylus. TT-TG was correlated with interepicondylar distance and body height.

Interepicondylar distance as an expression of knee size correlated highly with body height in the control group with normal trochlea (r = 0.78) as well as in the TD group (r = 0.69). Correlation of TT-TG with interepicondylar distance or body height in the control group as well as in the TD group showed poor values with r < 0.30 (range r = 0.072-0.28).

TT-TG seems associated neither with the size of the individual knee, nor with body height. For this reason, TT-TG has to be considered as very individual parameter in knee realignment surgery.