Differences in patellofemoral contact mechanics associated with patellofemoral pain syndrome.

Patellofemoral pain syndrome (PFPS) is a disorder of the patellofemoral (PF) joint in which abnormal tracking is often cited as a factor in pain development. PF tracking is partially dependent on passive stabilizers (ex: PF geometry). Relations amongst PFPS, PF tracking, and contact mechanics are poorly understood. In-vivo investigation of passive PF joint stabilizers including PF tracking, contact mechanics, cartilage thickness, and patellar shape will allow structural characterization of the PF joint and may highlight differences associated with PFPS. This study examined the role that passive stabilizers play in PFPS (n=10) versus healthy subjects (n=10). PF tracking (contact area centroid migration), cartilage thickness, shape, congruence, and contact patterns were quantified using magnetic resonance imaging during isometric loading at 15 degrees , 30 degrees , and 45 degrees of knee flexion. Distinct relationships were identified between patellar shape and tracking and contact, particularly at low flexion (15-30 degrees ). Healthy subjects exhibited distinct PF tracking and contact patterns related to Type I patella shape (80%) with increasing total contact area (p<0.001) and proximal centroid migration (15-30 degrees p=0.012; 30-45 degrees p<0.001) for increasing knee angles. PFPS subjects deviated from these patterns at low flexion, demonstrating higher total contact area than healthy subjects (p=0.046 at 15 degrees ), lack of proximal centroid migration (15-30 degrees ), and more Type II (30%) and III (20%) patella shapes. This study highlights a new finding that patellar shape combined with low degrees of flexion (15-30 degrees ) may be important to consider, as this is where PFPS tracking and contact patterns deviate from healthy.