Osteoporosis in rheumatoid arthritis
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University of Cape Town
The literature is replete with reports of osteoporosis in rheumatoid arthritis, but the mechanism of bone loss remains obscure. This is probably due to the overlap with bone loss of aging and the menopause, whose exact mechanisms are also poorly understood. Against this background, a study was designed to evaluate generalised bone loss in young, premenopausal (if female), patients with rheumatoid arthritis. The protocol was designed to record demographic data, as well as information pertaining to the disease. Cortical bone mass was measured at the metacarpals and left femur, using an automated, computer-controlled technique. Trabecular bone was evaluated at the left femur (Singh index) as well as at the 3rd lumbar vertebra (Saville index). Bone kinetics were studied by the measurement of urinary excretion of calcium, phosphate and hydroxy-praline (resorption) and serum alkaline phosphatase (formation). Disease activity was measured clinically and with laboratory indices. Physical activity was indirectly measured by quantitating the disability, using the Keitel function test as well as a modified health assessment questionnaire (HAQ). The radiograph of the right wrist was scored by the Larsen index. The carpometacarpal ratio was also calculated from the radiograph. Numerous statistical techniques were applied in the analysis of the data. Healthy volunteers were used as controls. Patients with SLE were also studied, in order to compare the 2 inflammatory diseases. Patients with RA had generalised cortical bone loss (metacarpal and femur) (p < 0.001). Trabecular bone measurements were not significantly different from normals, using the crude radiographic techniques. Duration of disease was the most important clinical determinant of this bone loss. The relative contributions of disease activity and lack of physical activity to the loss of bone could not be adequately separated using conventional statistical techniques. Corticosteroid therapy did not promote metacarpal bone loss in these subjects, but may have contributed to thinning of the femoral cortex. Nonsteroidal anti-inflammatory drugs and disease modifying agents did not seem to influence the extent of the bone loss. Nutritional status and skinfold thickness did not correlate with bone mass. Dietary factors played no role in the genesis of bone loss, but may have had some effect on disease activity. Metacarpal measurements showed a sensitivity of 80% and specificity of 85% in discriminating between osteopaenic and normopaenic groups with RA. Osteopaenia could not be adequately predicted in the absence of metacarpal measurements. Metacarpal bone loss in RA was due to endosteal resorption, while in SLE it was due to periosteal resorption. The semi-automatic technique for measurement of metacarpal bone mass showed good reproducibility among 5 observers and at 2 different centres. The pathogenesis of bone loss in RA was multifactorial, the largest contribution probably coming from a humoral factor in the circulation, closely related to disease activity. Ionised calcium was elevated in 55% of RA patients, but only 5% of SLE patients. Serum PTH levels were normal in 99% of the RA subjects. Elevations in alkaline phosphatase. (25%) probably reflected disease activity rather than increased bone formation. Factor analysis of 27 variables showed that disease activity was central to the development of OP in RA. CS therapy tended to be used in the presence of active disease. Disability was not an important determinant of bone loss in RA, but may be a useful measure of activity of the disease. This study did not evaluate the relationships with sex hormonal status or vitamin D metabolism. Future research should aim at cohort analysis at 2 different periods, in order to improve our understanding of the pathogenesis of bone loss in RA.
Kalla, A. 1989. Osteoporosis in rheumatoid arthritis. University of Cape Town.