Background/Purpose

Osteoporotic fractures are a cause of morbidity in rheumatoid arthritis (RA) and low body mass index (BMI) is a risk factor for osteoporotic fractures in RA. Emerging evidence suggests that low lean mass is responsible for bone structural deficits among those with low BMI. Therefore we hypothesized that adjusting BMI for lean mass deficits commonly seen in RA would alter prediction of risk using the FRAX tool. We studied the effect of utilizing a lean mass-adjusted BMI as an independent variable in the FRAX tool in the determination of fracture risk.

Methods

Whole-body dual energy absorptiometry (DXA) was previously performed in 40 RA subjects and 500 controls. We have previously identified independent associations between the total fat mass index (FMI) and appendicular lean mass index (ALMI) among 500 healthy control subjects. Based on our published model, we determined the expected ALMI for RA subjects based on their age, sex, race, and FMI.  We then multiplied the actual BMI by the ratio of the actual to expected ALMI to determine a lean mass-adjusted BMI. We calculated fracture risk among RA subjects using the FRAX calculator, and evaluated the differences in fracture risk prediction using the lean mass-adjusted BMI compared to the standard BMI both with and without using bone density results from DXA.

Results

Using the lean mass-adjusted BMI in place of standard BMI, the calculated risk of a major osteoporotic fracture increased in 21 (52.5%), decreased in 8 (20%), and was unchanged in 11 (27.5%) subjects. The mean absolute change in calculated 10-year major osteoporotic fracture risk using lean mass-adjusted BMI was 0.28% ± 0.70 (p=0.02).  Similarly, the calculated risk of hip fracture using lean mass-adjusted BMI was increased in 23 (57.5%), decreased in 6 (15%), and unchanged in 11 (27.5%) subjects. The mean absolute change in calculated 10-year hip fracture risk was 0.25% ± 0.60 (p=0.01).  The calculated risk of hip fracture using lean mass-adjusted BMI was most increased among subjects with a BMI <25 kg/m². For subjects with a BMI<25 (n=11) the estimated fracture risk was significantly increased by 0.67% + 0.87 (p=0.03), as compared to -0.09% ±0.37 (p=0.2) for subjects with a BMI>25 (n=29) (p for comparison=0.005). When bone density at the hip was included into the FRAX calculation, the differences in calculated fracture risk using the lean mass-adjusted BMI and standard BMI were completely attenuated (Figure 1).

 Conclusion

Using lean mass-adjusted BMI in place of standard BMI in the FRAX equation results in an increase in calculated fracture risk for most patients with RA, particularly among those with a normal BMI (BMI<25). Incorporation of bone density measures significantly reduced these differences in risk prediction, which suggests that use of a lean mass-adjusted BMI would not affect risk prediction when bone density is available.