Response to "Concerns regarding the use of 3D-DXA".

We thank the Editor for the opportunity to respond to the Letter to the Editor: “Concerns regarding the use of 3D-DXA” in relation to our recent publication, Harding et al., Bone, 136, 2020, 115,362. The Letter author writes “It is my understanding that it is not possible to measure the cortical parameters from a two-dimensional dual-energy X-ray absorptiometry (DXA) image.” This is correct. Technically, all so-called ‘measures’ from indirect densitometry are estimates, but more to the writer's point, it is typically not possible to parse out cortical from trabecular bone from a standard areal BMD exam. The 3D-DXA modelling software was built from a database of Quantitative Computed Tomographic scans of the proximal femur, to develop a statistical shape and density model; technical details of the modelling algorithm have been published previously.