Effects of Q-angle, tibial insert type, and the insert rotational alignment on patellofemoral kinematics

Previous studies have demonstrated the effects of quadriceps angle (Q-angles), the angle between the resultant quadriceps load vector and the patellar tendon load vector, on natural knee kinematics. In vitro knee simulations were performed to investigate the effects of varying Q-angles on the kinematics of a total knee replacement with a rotating platform (RP) and two orientations of fixed tibial bearings; auto-rotation (AR) and medial-third (M3). Seven fresh cadavers were tested with simulated walking at neutral Q-angles and 2.5° medial and 2.5°, 5.0° and 7.5° lateral from the neutral. Surgery was performed on these knees and post-surgery testing was done at varying Q-angles. In all four knee conditions, the patella increasingly tilted medially and shifted laterally at increasing lateral Q-angles throughout the gait cycle. Other patellar motions were not sensitive to the changing Q-angle. The average increase in medial patellar tilt and lateral shift during stance phase at the 7.5° lateral Q-angles was 4.8° (+/-1.8°) and 4.0 mm (+/-0.6 mm) at natural knee, 9.9° (+/-3.0°) and 6.2 mm (+/-2.0 mm) for RP, 11.1° (+/-2.9°) and 6.9 mm (+/-1.4 mm) for AR and 10.7° (+/-1.8°) and 5.8 mm (+/-1.6 mm) for M3 insert respectively. This increase in patellar tilt and shift was significantly larger than its respective neutral Q-angle. Medialized Q-angle increased lateral tilt and medial shift in all knee conditions. Tibiofemoral kinematics was not affected by Q-angles but the insert type changed the tibial internal-external rotation. The results demonstrated that Q-angle has a greater effect on patellofemoral kinematics than does the tibial insert type. Although the increases in tilt and shift for RP and fixed insert were similar, results showed lesser magnitudes for RP than AR and M3.