Summary

Accurately predicting the reduction in the tibial-tubercle to trochlear-groove (TTTG) distance following supratubercle tibial derotational osteotomies is important for pre-operative planning in patients with rotational malalignment.

Abstract

Supratubercle tibial rotational osteotomies are a useful adjunct in patellar stabilizing procedures in patients with a high tibial-tuberosity to trochlear-groove (TTTG) distance caused by excessive external tibial torsion. An investigation published by Jud et al. (2020) determined that one degree of internal tibial rotation results in a 0.68 mm reduction of TTTG distance. However, this linear relationship does not account for anatomical variations and has yet to be externally validated. This study derives a formula which could more accurately predict the change in TTTG and validates it radiographically.
Bilateral pre-operative CT imaging for 37 patients who were worked-up for patellar instability in the context of torsional deformities were reviewed. The mean change in TTTG was calculated using our novel four-variable equation and the empirical relationship established by Jud et al. (2020), following simulated derotations of 5°, 10° and 15°. These were then compared to the true change in TTTG measured directly from superimposed axial CT cuts to obtain the true change in TTTG.

The difference between the true change in TTTG and that obtained using our novel four-variable formula was statistically insignificant (p > 0.01 for all derotations). Conversely, the true values were statistically different from those found using Jud et al.’s empirical relationship (p < 0.0001 for all). Both inter-and intra-rater analyses demonstrated insignificant statistical variations (p > 0.05 for all).

Our novel four-variable equation accurately predicted the true change in TTTG following simulated supratubercle tibial osteotomy. TTTG change was grossly overestimated using the linear relationship proposed by Jud et al., which may result in less correction than predicted pre-operatively. This new equation accounts for individual patient anatomy, and it presents a more accurate relationship between internal rotation of the distal segment of the tibia and the subsequent decrease in the TTTG. This process will help to predict the change in TTTG more accurately for patients undergoing supratubercle osteotomy for excessive tibial external rotation and avoid under correction.