A comparative study of the determinants of bonestrength and the propensity to falls in black andwhite South African women

[摘要] The comparative study presented in this dissertation specifically aimed to assess fracture riskin black (Xhosa) and white South African women by evaluating known determinants of bonestrength as well as the propensity to falls. We thus compared the prevalence of clinical(historic) risk factors for osteoporosis, measured and compared vertebral and femoral bonemineral density (BMD) employing dual energy X-ray absorptiometry (DEXA), ultrasoundvariables using the Sahara sonometer, serum parathyroid hormone (PTH) and 25-OH VitaminD, mineral homeostasis and modern biochemical markers of bone turnover, bone geometryand the propensity to falls. Finally, we determined the prevalence of vertebral fractures inthese black and white South African females.1. Significant ethnic differences were noted in the presence and frequency of historicalclinical and lifestyle risk factors for osteoporosis. Blacks were heavier and shorter, theyconsumed less calcium, were more inactive, preferred depot-medroxyprogesteroneacetate as contraceptive agent and were of higher parity. Whites smoked more,preferred oral oestrogen containing contraceptive tablets and were more likely to havea positive family history of osteoporosis. Hormone therapy was used almost exclusivelyby postmenopausal whites. Inter-ethnic differences in weight, physical activity and highparity was most marked in the older subjects.2. We found that peak spinal BMD was lower, but peak femoral BMD similar or higher(depending on the specific proximal femoral site measured) in black South-Africanfemales compared with whites. The lower peak spinal BMD was mainly attributed tolower BMD's in the subgroup of black females with normal to low body weight,indicating that obesity either protected black females against a low spinal BMD orenhanced optimal attainment of bone mineral. An apparent slower rate of decline inboth spinal- and femoral BMD with ageing was noted in the black females comparedwith whites in this cross-sectional study – an observation which will requireconfirmation in longitudinal, follow-up studies. This resulted in similar spinal BMDvalues in postmenopausal blacks and whites, but significantly higher femoral BMDmeasurements in blacks. The volumetric calculation of bone mineral apparent density(BMAD) at the lumbar spine and femoral neck yielded similar results to that of BMD.Spinal BMAD was similar in blacks and whites and femoral neck BMAD was consistentlyhigher in all the menopausal subgroups studied. Weight significantly correlated withpeak- and postmenopausal BMD at all sites in the black and white female cohorts.Greater and better maintained body weight may be partially responsible for slowerrates of bone loss observed in black postmenopausal females. Most of the observedethnic difference in BMD was, in fact, explained by differences in body weight betweenthe two cohorts and not by ethnicity per se.3. A low body weight and advanced age was identified as by far the most informativeindividual clinical risk factors for osteopenia in our black and white females, whereasphysical inactivity was also identified as an important individual risk factor in blacksonly. Risk assessment tools, developed and validated in Asian and Europeanpopulations, demonstrated poor sensitivity for identification of South African women atincreased risk of osteopenia. The osteoporosis risk assessment instrument (ORAI)showed the best results, with sensitivities to identify osteopenic whites at most skeletalsites approaching 80% (78% - 81%). The risk assessment tool scores appear to beinappropriate for our larger sized study cohort, especially our black subjects, thusresulting in incorrect risk stratification and poor test sensitivity. General discriminantanalysis identified certain risk factor subsets for combined prediction of osteopenia inblacks and whites. These risk factor subsets were more sensitive to identify osteopeniain blacks at all skeletal sites, compared with the risk assessment tools described in theliterature.4. Higher ultrasonographically measured broadband ultrasound attenuation (BUA) andspeed of sound (SOS) values were documented in our elderly blacks compared withwhites, even after correction for differences in DEXA determined BMD at the spine andproximal femoral sites. BUA and SOS showed no decline with ageing in blacks, incontrast to an apparent significant deterioration in both parameters in ageing whites. Ifthese quantitative ultrasound (QUS) parameters do measure qualitative properties ofbone in our black population, independent of BMD as has been suggested in previouswork in Caucasian populations, the higher values documented in elderly blacks implybetter preservation of bone quality in ageing blacks compared with whites. Thecorrelation between QUS calcaneal BMD and DEXA measured BMD at the hip and spinewas modest at best. QUS calcaneal BMD was therefore unable to predict DEXAmeasured BMD at clinically important fracture sites in our study population.5. Bone turnover, as assessed biochemically, was similar in the total pre- andpostmenopausal black and white cohorts, but bone turnover rates appeared to differwith ageing between the two racial groups. A lower bone turnover rate was noted inblacks at the time of the menopausal transition and is consistent with the finding of alower percentage bone loss at femoral sites at this time in blacks compared withwhites. Bone turnover only increased in ageing postmenopausal blacks, and this couldbe ascribed, at least in part, to the observed negative calcium balance and the morepronounced secondary hyperparathyroidism noted in blacks. Deleterious effects ofsecondary hyperparathyroidism on bone mineral density at the proximal femoral siteswere demonstrated in our postmenopausal blacks and contest the idea of an absoluteskeletal resistance to the action of PTH in blacks. The increase in bone turnover andthe presence of secondary hyperparathyroidism due to a negative calcium balance maythus potentially aggravate bone loss in ageing blacks, especially at proximal femoralsites.6. Shorter, adult black women have a significantly shorter hip axis length (HAL) thanwhites. This geometric feature has been documented to protect against hip fracture.The approximately one standard deviation (SD) difference in HAL between our blacksand whites may therefore significantly contribute to the lower hip fracture ratepreviously reported in South African black females compared with whites. Averagevertebral size was, however, smaller in black females and fail to explain the apparentlower vertebral fracture risk previously reported in this population. Racial differences invertebral dimensions (height, width) and/or other qualitative bone properties assuggested by our QUS data may, however, account for different vertebral fracture ratesin white and black women – that is, if such a difference in fact exists.7. The number of women with a history of falls was similar in our black and white cohorts,and in both ethnic groups the risk of falling increased with age. There is a suggestionthat the nature of falls in our black and white postmenopausal females may differ, butthis will have to be confirmed in a larger study. Fallers in our postmenopausal studypopulation were more likely to have osteoporosis than non-fallers. Postmenopausalblacks in our study demonstrated poorer outcomes regarding neuromuscular function,Vitamin D status and visual contrast testing and were shown to be more inactive withageing compared with whites. An increased fall tendency amongst the black femalescould not however be documented in this small study. Quadriceps weakness and slowerreaction time indicated an increased fall risk amongst whites, but were unable todistinguish black female fallers from non-fallers.8. Vertebral fractures occurred in a similar percentage of postmenopausal blacks (11.5%)and whites (8.1%) in our study. Proximal femoral BMD best identified black and whitevertebral fracture cases in this study. Quite a number of other risk factors i.e. physicalinactivity, alcohol-intake, poorer physical performance test results and a longer HALwere more frequent in the white fracture cases and could therefore serve as markers ofincreased fracture risk, although not necessarily implicated in the pathophysiology ofOP or falls. However, in blacks, only femoral BMD served as risk factor. Similar riskfactors for blacks and whites cannot therefore be assumed and is deserving of furtherstudy. White fracture cases did not fall more despite lower 25-OH-Vitamin D, poorerphysical performance and lower activity levels than non-fracture cases. Calcanealultrasonography and biochemical parameters of bone turnover were similar in fractureand non-fracture cases in both ethnic groups. Our study data on vertebral fractures inthis cohort of urbanized blacks thus cautions against the belief that blacks are not atrisk of sustaining vertebral compression fractures and emphasize the need for furtherstudies to better define fracture prevalence in the different ethnic populations of SouthAfrica.9. In our study, hormone therapy in postmenopausal white women improved bonestrength parameters and reduced fall risk. In hormone treated whites compared withnon-hormone users, a higher BMD at the spine and proximal femur as determined byDEXA were documented and all QUS measurements were also significantly higher. Thebiochemically determined bone turnover rate, as reflected by serum osteocalcin levels,was lower in hormone users. Fall frequency was lower in the older hormone treatedwomen (≥ 60yrs) and greater quadriceps strength and reduced lateral sway was noted.Only one patient amongst the hormone users (2%) had radiological evidence ofvertebral fractures compared with four patients (6%) amongst the never-users. Ashormone therapy was used almost exclusively by whites in this study population, theimpact of hormone therapy on postmenopausal black study subjects could not be assessed.

[发布机构] Stellenbosch University