Association between patella alta and the prevalence and worsening of structural features of patellofemoral joint osteoarthritis: the multicenter osteoarthritis study.

Symptomatic knee osteoarthritis (OA) is present in 12% of individuals over the age of 60 and is a leading cause of disability in the United States (1, 2). Research has focused on risk factors for tibiofemoral joint OA, but both symptoms and disease often occur in the patellofemoral joint (PFJ) (3-6). Despite growing awareness of PFJ OA as a common source of pain and functional limitation among older adults, there continues to be paucity of literature on its risk factors. Those studies that have investigated risk factors have focused on patellar tilt and displacement, femoral trochlea morphology (7-9), and frontal plane tibiofemoral alignment (10, 11). Patella alta, or a “high riding” patella, is a distinct feature of PFJ alignment that has been proposed to be a risk factor for patellofemoral pain and OA (12-14). Patella alta is measured by the Insall-Salvati ratio (ISR), a ratio between the length of the patellar tendon and the length of the patella (15), with a ratio ≥1.2 indicating patella alta. Ward et al. (14, 16, 17) found subjects with patella alta to have increased PFJ malalignment, decreased PFJ contact area, and increased PFJ stress. These findings suggest that persons with patella alta may be at increased risk for damage of the PFJ cartilage and underlying bone. Similarly, in studies using dynamic knee simulators and cadaveric knees, PFJ contact forces were higher in knees with patella alta compared to knees with normal patellar position (13, 18). Magnetic resonance imaging (MRI) directly visualizes structural features of OA including focal cartilage damage, bone marrow lesions (BMLs), and subchondral bone attrition (SBA). These features can be graded specifically for the medial and lateral patella and trochlea facets yielding a comprehensive picture of compartment specific PFJ structural change. While BMLs and SBA have been shown to be associated with pain, occur in the same subregions as cartilage damage, and alignment factors affecting loading at the tibiofemoral joint (19-24), few studies have investigated their relationship with alignment of the PFJ. The small number of cross-sectional studies that have explored the relationship of patella alta to MRI features of PFJ OA have yielded conflicting results. Ali et al. found no differences in the ISR among groups with no, mild, or severe patellofemoral articular cartilage damage (25). In contrast, Kalichman (8) used a more detailed method of defining cartilage damage and BMLs and found an association between the modified ISR (ModISR) (26) and cartilage damage in the lateral and medial PFJ and BMLs in the lateral PFJ. To our knowledge there have been no longitudinal studies investigating this question. The purpose of this study was to determine the association between patella alta and the prevalence and worsening of PFJ cartilage damage, BMLs, and SBA in middle aged and older persons with or at risk for knee OA. We hypothesized that knees with patella alta would have a greater baseline prevalence and greater worsening at follow-up of cartilage damage, BMLs, and SBA in the PFJ.