Variations in Tibial Tuberosity to Trochlear Groove and Posterior Cruciate Ligament Distances due to Tibial External and Valgus Rotations.

Tibial rotations with respect to the femur influence measurements used to assess the lateral position of the tibial tuberosity. This study utilized computational modeling to quantify how the tibial tuberosity to trochlear groove (TT-TG) and tibial tuberosity to posterior cruciate ligament attachment (TT-PCL) distances vary with tibial internal/external and varus/valgus rotations. Computational models were developed from magnetic resonance imaging data to represent eight knees with patellar instability. TT-TG and TT-PCL distances were quantified from the computational models for a neutral orientation and with the tibia rotated internally and externally and into varus and valgus in 1-degree increments to 5 degrees. Regression analyses related tibial rotations to TT-TG and TT-PCL distances. TT-TG distance increased with tibial external rotation, and both TT-TG and TT-PCL distances increased with valgus orientation ( > 0.94 for all regressions). The average increase in TT-TG distance for each 1 degree of tibial external rotation was 0.55 mm (range: 0.50-0.62 mm), compared with 0.00 mm (range: -0.05 to 0.04 mm) for TT-PCL distance. The average increase in TT-TG distance for each 1 degree of valgus was 1.01 mm (range: 0.91-1.14 mm), compared with 0.46 mm (range: 0.32-0.60 mm) for TT-PCL distance. TT-TG distance varies more with tibial rotations than TT-PCL distance due to both points being on the tibia and a smaller proximal-distal distance between the points for TT-PCL distance.