Continuous loss of bone during chronic immobilization: A monozygotic twin study

Acute immobilization is associated with rapid loss of bone. Prevailing opin ion, based on population cross-sectional data, assumes that bone mass stabi lizes thereafter. In order to address whole-body and regional skeletal mass in long-term immobilization, monozygotic twins were studied, one of each t win pair having chronic spinal cord injury (SCI) of a duration ranging from 3 to 26 years. The research design consisted of the co-twin control method using 8 pairs of identical male twins (mean +/- SD age, 40 +/- 10 years; r ange 25-58 years), one of each set with SCI. The twins were compared by pai red t-tests for total and regional bone mineral content (BMC) and bone mine ral density (BMD) measured by dual-energy X-ray absorptiometry. Linear regr ession analyses were performed to determine the associations of age or dura tion of injury with the differences between twin pairs for total and region al skeletal bone values. In the SCI twins, total-body BMC was significantly reduced (22% +/- 9%, p < 0.001), with the predominant sites of reduction f or BMC and BMD being the legs (42% +/- 14% 35% +/- 10%, p < 0.0001), and pe lvis (50% +/- 10% and 29% +/- 9%, p < 0.0001). Duration of SCI, not age, wa s found to be linearly related to the degree of leg bone loss in SCI twins (BMC: r(2) = 0.60, p < 0.05; BMD: r(2) = 0.70, p < 0.01). Our findings sugg est that pelvic and leg bone mass continues to decline throughout the chron ic phase of immobilization in the individual with SCI, and this bone loss a ppears to be independent of age.