New approaches for interpreting projected bone densitometry data

TLDR

Bone densitometry using DPA or DXA is a standard method for assessing bone mineral content, but the projected areal density (BMD) can be misleading when comparing bones of different sizes because it is biased by bone thickness differences. The study proposes new analysis methods that reduce bone size confounding by introducing bone mineral apparent density (BMAD) to better reflect bone apparent density. The authors compute BMAD and use it to derive an index of bone strength (IBS) for whole vertebral bodies. The analyses show that BMD is biased by bone size, and that applying BMAD and IBS to lumbar spine DXA data in women 17–40 years old offers advantages over conventional techniques.

Abstract

Abstract Bone densitometry using dual-photon absorptiometry (DPA) or dual-energy x-ray absorptiometry (DXA) has become a standard method for assessing bone mineral content in the spine and other skeletal regions. A projected areal density, referred to as bone mineral density (BMD,g/cm2), is normally calculated to assess regional bone density and strength. We demonstrate that this measure can be misleading when used to compare bones of different sizes due to inherent biases caused by bone thickness differences. For example, assuming that volumetric bone density remains constant and bony linear dimensions are proportional to height, a 20% increase in height would result in a 20% increase in both the thickness and the BMD of any bone. We describe new analysis methods to reduce the confounding effect of bone size, and we introduce a parameter, bone mineral apparent density (BMAD, g/cm3), that better reflects bone apparent density. Using this parameter, we calculate a quantity that serves as an index of bone strength (IBS, g2/cm4) for whole vertebral bodies. These analyses were applied to lumbar spine (L2–4) DXA measurements in a population of women 17–40 years old and appear to offer advantages to conventional techniques.