Poster 138: Intra-articular Femorotibial Rotation is Associated with Tibial Tubercle-Trochlear Groove Distance

OBJECTIVES: A tibial tubercle-trochlear groove (TT-TG) distance of 20 mm is typically used when determining whether tibial tubercle anteromedialization (AMZ) is needed for patellar instability. TT-TG distance, however, may depend on the internal/external (IE) rotation of the tibia in relation to the femur, which is typically not controlled for. Without knowledge of the variability in an individual’s TT-TG influenced by femorotibial rotation, the use of a specific TT-TG distance during pre-operative planning for patellar instability may lead to incorrect decisions on the use of tibial tubercle AMZ. We hypothesized that knee joint IE rotation is related to the TT-TG distance. METHODS: A total of 8 independent human cadaveric knee specimens (age: 32±6; 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 internal/external (IE) rotation under applied moments of ±5Nm at full extension. Two independent reviewers selected the trochlear groove and tibial tuberosity points on CT images of each specimen to define TT-TG (ICC=0.969). To determine the influence of knee joint IE rotation on TT-TG, 3D models generated from CT scans were registered to tibiofemoral kinematics. Subsequently, linear regression was performed to determine the relationship between knee joint IE rotation and TT-TG. Regression coefficient and standard error of measurement (α=0.05), and coefficient of determination (r2) were reported. RESULTS: Knee joint IE rotation averaged 23.0 ± 4.2°. TT-TG changed by 12.1 ± 2.8 mm over this range. For every degree of knee joint IE rotation, TT-TG changed by 0.52 ± 0.07 mm (p<0.001 and r2>0.987). At neutral rotation, the TT-TG averaged 14.21 ± 5.0 mm. CONCLUSIONS: TT-TG was highly dependent on knee joint IE rotation changing by 0.52 mm for every degree of knee joint IE rotation. Thus, an offset in IE rotation of 10° would lead to a change in TT-TG of 5.2 mm, enough to alter surgical decision making for or against tibial tubercle AMZ. Physicians should pay close attention to knee joint IE rotation when measuring TT-TG in their patients, specifically in patients found to have a TT-TG near 20 mm, a key indication for surgical treatment of patellar instability.