Socioeconomic status and modifiable risk factors for osteoporosis and knee osteoarthritis.

Musculoskeletal disease is a major public health burden in developed countries worldwide, with significant attributable morbidity. Two of the most commonly occurring musculoskeletal conditions are osteoporosis and osteoarthritis both leading causes of long-term pain and disability. The significant burden of impact of musculoskeletal diseases (osteoporosis and arthritis) on the Australian population was recognised in July 2002, when the National Health and Medical Research Council declared musculoskeletal diseases to be the 7th National Health Priority Area. Osteoporosis is a disease that is characterised by bone fragility with the common endpoint of fracture, which is associated with an increased likelihood of morbidity in the elderly and a reduction in mobility, ability to self-care and quality of life. Low bone mineral density (BMD) is used as a surrogate marker for osteoporosis. Osteoarthritis is a complex disease that most commonly affects the knee joint and is the most prevalent single cause of pain and disability in the elderly. Our understanding of the early stages of disease, prior to radiographic disease, has been limited due to the lack of a sensitive, non-invasive tool to assess disease severity and detect small changes in knee structure over time. The recent application of Magnetic Resonance Imaging (MRI) to examination of osteoarthritis enables an assessment of all joint structures, including direct measure of both bone and cartilage, and thus offers itself as a promising imaging modality to examine osteoarthritis onset and progression. It has been suggested that osteoporosis and osteoarthritis share common modifiable risk factors that are associated with socioeconomic status (SES). SES is a construct that considers the level of social disadvantage, and usually measured by income, education, occupation, marital status, or by area-based aggregate scores. Furthermore, individuals of lower SES are less likely to undergo screening for musculoskeletal disease. However to date, little is known of the relationship between SES and osteoporosis, and osteoarthritis. While these common diseases differ in their presentation, they share potential risk factors and tissue involvement, although how they are related remains contentious. This thesis aimed to examine current understandings of the association between SES and modifiable risk factors on osteoporosis. Further, this thesis aimed to examine the effect of modifiable and systemic risk factors for osteoarthritis on knee cartilage (defects and volume) and bone, and the significance of their change over time in both healthy/asymptomatic subjects and in those with knee osteoarthritis. Overall, fourteen publications are included within the body of this thesis. Socioeconomic status and risk factors for osteoporosis and osteoarthritis Section 3 presents Socioeconomic status and risk factors for obesity and metabolic disorders in a community based sample of adult females, and Socioeconomic status, obesity and lifestyle in men: the Geelong Osteoporosis Study, which identified a significant inverse association for both genders between SES and risk factors for osteoporosis and osteoarthritis. These include measures of obesity, associated with greater prevalence of osteoarthritis, and physical inactivity and smoking; associated with both osteoporosis and osteoarthritis. A letter to the editor Urban-rural comparison of weight status among women and children living in socioeconomically disadvantaged neighbourhoods suggested socioeconomic status as a stronger determinant of obesity than urban or rural locality. Osteoporosis Socioeconomic status and bone mineral density In Section 4.1, a systematic review of existing literature, Association between socioeconomic status and bone mineral density in adults: A systematic review identified consistent, yet limited, evidence for a positive association between educational attainment and BMD in women; however no evidence was found regarding an association between income or occupation and BMD in either gender, or education and BMD in men. Data from a randomly-selected cohort of adult women is presented in BMD in population-based women is associated with socioeconomic status, and showed that at the spine, the maximum difference of 7.5% was observed between SES quintiles. Observed differences in BMD across SES quintiles, consistent for both SES indexes, suggest that low BMD may be evident for both the most disadvantaged, and the most advantaged. Further analysis of BMD in men, presented in Socioeconomic status and bone mineral density in a population-based sample of men: The Geelong Osteoporosis Study identified that in younger men, BMD was highest at the spine in the mid quintiles of SES. In older men, the pattern of BMD across SES quintiles was reversed, and subjects from the mid quintiles had the lowest BMD. Differences in BMD at the spine across SES quintiles represent a potential 1.5- fold increase in fracture risk for those with the lowest BMD. Socioeconomic status and fracture In Section 4.2 The association between socioeconomic status and osteoporotic fracture in population based adults: A systematic review, presents strong evidence for an association between being married/living with someone and reduced risk of osteoporotic fractures. Limited evidence existed of the relationship between fracture and occupation type, employment status or for type of residence. Conflicting evidence existed for the relationship between osteoporotic fracture and level of income and education. A published letter to the editor Educational achievement and fracture risk: Response to Clark and Tobias responds to an alternate suggestion for the relationship between SES and fracture. A third systematic review, The association between urban or rural locality and hip fracture: A systematic review, identified moderate evidence for residents of rural regions to have lower risk of hip fracture compared to urban residents. Incident hip fracture and social disadvantage in an Australian population aged 50 years or greater presents an examination of hip fracture rates across SES in the Barwon Statistical Division (BSD) for the period of 2006-7 and showed an inverse association between SES and fracture risk in both genders. Osteoarthritis Socioeconomic related modifiable factors and knee structure Chapter 5 examines osteoarthritis and contains five publications that examine risk factors for knee structure, with a focus on SES related modifiable factors, and the relationship to osteoporosis. Firstly, Section 5.1 examines the association between BMI and knee structure in Does an increase in body mass index over 10 years affect knee structure in a population-based cohort study of adult women? After adjusting for age and bone area, current BMI was associated with cartilage defects, with a trend for increased BMI over 10 years associated with reduced cartilage volume. Bone marrow lesions were associated with baseline BMI, current BMI, and change in BMI over the 10 year study period. This study provides longitudinal evidence for the importance of avoiding weight gain in women during early to middle adulthood. Section 5.2 examines BMD and knee structure in Bone mineral density is cross-sectionally associated with knee cartilage volume in healthy, asymptomatic adult females: Geelong Osteoporosis Study, and showed a positive association between BMD and tibiofemoral cartilage volume. Whilst site-specific BMD is associated with cartilage volume, cartilage defects, thought to represent early OA changes, showed a trend of association with BMD. These data suggest that the association between cartialge volume and axial/lower limb BMD relates to common local, possibly biomechanical, rather than systemic factors. Section 5.3 examines change in knee structure in a population with prevalent osteoarthritis disease in Women lose patella cartilage at a faster rate than men: a 4.5year cohort study of subjects with knee osteoarthritis. Annual change in patella cartilage volume over 4.5 years in this population was lost at a higher rate in women compared to men, after accounting for age, BMI and bone volume at baseline. Socioeconomic status and endstage osteoarthritis Finally, the relationship between SES and endstage osteoarthritis is examined, whereby all knee and hip joint replacements performed for osteoarthritis in residents of the BSD during 2006-7 were examined for an association with SES. Section 5.4 presents an analysis of 691 total knee replacements, and showed that after adjusting for multiple comparisons, a trend for difference between SES quintiles was observed; however, this was not linear. In Section 5.5, Socioeconomic status and primary total hip joint replacements 2006-7 in the Barwon Statistical Division: Australian Orthopaedic Association National Joint Replacement Registry, presents a similar analysis of 642 total hip replacements across SES, and showed that there appeared a bimodal trend for total hip replacement incidence rates to be higher in the upper and lower SES quintiles for males, but not for females.