Results
Characteristics of the Analytical Cohort
Baseline characteristics of the analytical cohort are shown in Table 1 . Tabulations of osteoporosis according to three different definitions ( Table 2 ; Supplement Tables A2, A3, Supplemental Digital Content 1, http://links.lww.com/MENO/A121) revealed that 51% (170 of 344) of osteoporosis diagnoses were determined by lumbar spine BMD measurement (lumbar spine T score ≤−2.50; femoral neck and total hip T scores >−2.50).
Estimated Time to Hip or Clinical Vertebral Fracture
The following proportions of women (including those with osteoporosis at baseline) sustained a hip or clinical vertebral fracture before initiating osteoporosis treatment: aged 50 to 54 years at baseline, 12 of 1,069 (1.1%); aged 55 to 59 years at baseline, 24 of 1,391 (1.7%); aged 60 to 64 years at baseline, 37 of 1,608 (2.3%); all ages, 73 of 4,068 (1.8%).
Unadjusted (Figure 2) and covariate-adjusted estimates of the cumulative incidence of first hip or clinical vertebral fracture as a function of testing interval length were similar. The adjusted estimated times for 1% of women without osteoporosis to transition to hip or clinical vertebral fracture decreased with increasing age ( Table 3 ), ranging from 12.8 years (95% CI, 8.0-20.4) for women aged 50 to 54 years to 11.7 years (95% CI, 6.9-20.0) for women aged 55 to 59 years to 7.6 years (95% CI, 4.8-12.1) for women aged 60 to 64 years. For those with osteoporosis, the age-adjusted time interval was 3.0 years (95% CI, 1.3-7.1) for women aged 50 to 64 years analyzed together.
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Figure 2.
Unadjusted cumulative incidence of hip or clinical vertebral fracture by baseline age range. The proportion of women who had transitioned to first hip or clinical vertebral fracture is plotted as a function of time. The cumulative incidence curves are estimated by parametric cumulative incidence models for known fracture dates. The dotted horizontal line shows the area where 1% of women have transitioned to first hip or clinical vertebral fracture; where this line intersects with each cumulative incidence curve, a vertical line to the x axis marks the estimated time interval.
The crude proportion (not accounting for competing risks) of participants who had at least one hip or clinical vertebral fracture by the end of follow-up was significantly higher for women with osteoporosis than for women without osteoporosis at baseline in all subgroups of age, body mass index, personal fracture history, parental history of hip fracture, smoking, and hormone therapy use ( Table 4 ). Tabulations of primary endpoint and competing risk events ( Table 2 ) showed that treatment was the first event for 673 women (16.5% of the analytical cohort), that is, 531 (13.1% of the analytical cohort) participants without osteoporosis at baseline were treated before they developed osteoporosis or before they had a hip or clinical vertebral fracture, and 142 (3.5% of the analytical cohort) women with baseline osteoporosis were treated before they had a hip or clinical vertebral fracture.
Estimated Time to Major Osteoporotic Fracture
These proportions of women (including those with osteoporosis at baseline) sustained a major osteoporotic fracture before initiating osteoporosis treatment: aged 50 to 54 years at baseline, 38 of 1,069 (3.6%); aged 55 to 59 years at baseline, 71 of 1,391 (5.1%); aged 60 to 64 years at baseline, 93 of 1,608 (5.8%); all ages, 202 of 4,068 (5.0%).
The estimated times for 3% of women without baseline osteoporosis to have a major osteoporotic fracture (hip, clinical vertebral, proximal humerus, or wrist) ranged from 11.5 years (95% CI, 8.0-16.7) for women aged 50 to 54 years to 8.6 years (95% CI, 5.8-12.8) for women aged 60 to 64 years at baseline ( Table 5 ).
For all women aged 50 to 64 years with baseline osteoporosis analyzed together, the age-adjusted time for 3% of women to have a major osteoporotic fracture was 2.5 years (95% CI, 1.2-5.3) (event, 31 of 344 [9.0%]). The proportions of women with baseline osteoporosis who had a major osteoporotic fracture before initiating treatment were as follows: 3 of 40 (7.5%) for women aged 50 to 54 years; 10 of 106 (9.43%) for women aged 55 to 59 years; and 18 of 198 (9.1%) for women aged 60 to 64 years. Within these groups, three, seven, and nine women respectively had a major osteoporotic fracture before age 65 years.