Femorotibial rotation is linearly associated with tibial tubercle-trochlear groove distance: A cadaveric study

Objectives: A tibial tubercle-trochlear groove (TT-TG) distance of 20 millimeters (mm) is typically used when determining whether tibial tubercle medialization is performed for the surgical treatment of patellar instability. Without knowledge of how the variability of an individual's TT-TG distance is influenced by through-the-knee femorotibial rotation, the use of a specific TT-TG distance during preoperative planning for patellar instability may lead to incorrect decisions on the use of tibial tubercle medialization. We hypothesized that knee joint internal/external (IE) rotation is related to the TT-TG distance. Methods: Eight independent human cadaveric knee specimens (age: 32 f 6 years; 4 males, 4 females) were utilized. A robotic manipulator (ZX165U, Kawasaki Robotics, Wixom, MI, USA) instrumented with a universal force/moment sensor was used to determine knee joint IE rotation under applied moments of f5 newton-meters (Nm) at full extension. Two independent reviewers selected the trochlear groove and tibial tuberosity points on computerized tomography (CT) images of each specimen to define TT-TG. To determine the influence of knee joint IE rotation on TTTG distance, three-dimensional (3D) models generated from CT scans were registered to tibiofemoral kinematics. Linear regression was performed to determine the relationship between knee joint IE rotation and TT-TG distance. The regression coefficient, standard error of measurement (alpha = 0.05), and coefficient of determination (r2) were reported. Results: At 0 degrees of rotation, the mean TT-TG distance was 14.2 f 5.0 mm. Knee joint IE rotation averaged 23.0 f 4.2 degrees. For every degree of knee joint IE rotation, TT-TG distance changed by 0.52 mm. Conclusion: TT-TG distance was linearly dependent on knee joint IE rotation, changing by 0.52 mm for every degree of knee joint IE rotation. Thus, an offset of IE rotation of 10 degrees would lead to a change in TT-TG distance of 5.2 mm, enough to alter the surgical decision-making for/or against tibial tubercle medialization. Level of Evidence: IV: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.