Association of incident symptomatic hip osteoarthritis with differences in hip shape by active shape modeling: the Johnston County Osteoarthritis Project.

Hip osteoarthritis leads to the majority of total hip arthroplasty (THA) procedures in the United States and the rate of hospitalization for THA is rising, increasing 33% from 1997 to 2009 [1]. Estimates of the prevalence of radiographic hip OA (rHOA) range widely, from <1% to 27% of adults [2]. Symptomatic radiographic hip OA (srHOA) is less well-studied but clearly less frequent than rHOA, and estimates of both are dependent on the definitions used [3]. At the baseline visit (1991–1997) of the Johnston County Osteoarthritis Project, 27% of participants had rHOA (Kellgren-Lawrence [KL] grade of 2 or more) and nearly 10% had srHOA (KL≥2 with symptoms present) [4]. Alterations in hip morphology, such as acetabular dysplasia and femoroacetabular impingement, have gained attention recently as potential risk factors for the development of rHOA [5–9]. Early recognition of individuals at risk for hip osteoarthritis based on such morphologic characteristics could allow early preventative interventions, encourage enrollment or improve stratification in randomized clinical trials of therapeutic modalities. For the purpose of epidemiologic research in large cohorts, such morphologic alterations are typically assessed using visual and simple geometric measures on anteroposterior hip radiographs, which only include one aspect of hip shape at a time. Active shape modeling (ASM) is a method to model shape variation from a set of images, providing a way to model the shape of the proximal femoral head as a whole, and to compare mean shapes and variations in shape between groups using mode scores. Gregory, et al, the first to apply ASM in rHOA, used radiographs from the Rotterdam study [10]. They found differences in mode scores for the femoral head at baseline between those hips that developed rHOA after 6 years of follow up and those that did not. They also identified changes in hip morphology over time in hips developing rHOA, but not in control hips. Lynch, et al, used a similar but more comprehensive model on radiographs from the Study of Osteoporotic Fractures and identified several modes that were associated with incident rHOA after 8 years of follow up [11]. A more extensive ASM, including points along the proximal femur, acetabulum, and pelvis, has been applied to hip radiographs from the familial Genetics, Osteoarthritis, and Progression (GARP) study and the prospective Cohort Hip and Cohort Knee (CHECK) study [12, 13]. In the GARP study, 4 shape modes were associated with prevalent rHOA; mode scores were generally more highly correlated within-persons (right and left hip) than between sibling pairs [12]. Agricola, et al, using data from CHECK, found that 5 shape modes were associated with progression to THA in 5 years [13]. Studies of ASM to date have used white populations, either primarily or exclusively among women, and have focused on rHOA or THA. In the Johnston County Osteoarthritis Project (JoCo OA), African Americans compared with whites had a similar but slightly higher prevalence of rHOA (32 vs. 27%) and srHOA (12 vs. 9%) [4], were found to have individual radiographic features of rHOA that may predict progression [14], but were less likely to develop rHOA [15]. Racial differences in dysplasia and impingement have recently been reported between white and Chinese women [16]. The present analysis uses data from the JoCo OA, a longitudinal, community-based cohort that includes African American and white men and women. Using a nested case control study design, our aim was to use ASM (after the methods of Lynch, et al [11]) to describe baseline proximal femur shape by sex and race, and to determine the associations between femur shape and incident rHOA and srHOA.