2.5. Anatomic Measurements

We measured patellar and tibial tracking relative to the femur using an anatomic coordinate system (Figure 1) constructed for each bone (Seisler and Sheehan, 2007). For the current study a knee angle of 10° is equivalent to full knee extension measured clinically using the hip, knee, and ankle centers of rotation (Freedman et al., 2014). Medial, superior, and anterior shift defined positive displacements. Three-dimensional mechanics convention (Mitiguy, 2011) established the rotation angles, with the order of positive rotation being flexion, medial tilt, and lateral spin (i.e., lateral spin is a varus rotation, which causes the superior pole of the patella to move laterally). We quantified the maltracking parameters for each participant in a PF pain cohort as the difference in each displacement and rotation variable relative to the mean from the age-matched control cohort. Thus, when visualizing maltracking graphically, the control mean is always zero. Axial plane tracking (Figure 1) consisted of medial-lateral shift and tilt of the patella relative to the femur.

A) Knee angle (Φ: the angle between a vector bisecting the distal femoral shaft and a vector parallel to the proximal anterior edge of the tibia) was measured in the full extension anatomical, sagittal, cine phase contrast image. A 10° knee angle measured on an MR image corresponded to a 0° clinical knee angle, measured using the hip, knee, and ankle (Freedman and Sheehan, 2013). Axial cine images for full extension at the B) level of the femoral epicondyle and C) mid-patellar level. Medial-lateral patellar shift (Δ) was defined as the distance from the patellar origin (most posterior patellar point on this image) to the femoral origin (deepest point in the sulcus at the level of the femoral epicondyle [B]), in the direction parallel to the posterior edge of the femur (Fx at the level of the femoral epicondyle [B]). Patellar tilt (θ) was defined as the angle between Fx and the lateral-posterior patellar edge. These three parameters (along with all patellofemoral superior, posterior displacement, flexion, and lateral spin) were then tracked throughout the motion cycle using the kinematic data derived from cine phase contrast MR imaging.