We sought to determine whether x-ray densitometry could be used to
provide quantitative information about the growth and resorption of bone in
a retrospective analysis of the radiographs of patients who had a
non-cemented hip prosthesis. We examined variables that were likely to be
uncontrolled in routine orthopaedic radiographs for their effect on
radiographic (film) density. Changes in film lots, exposure, target
distance, field variability (heel effect), and femoral orientation were
independently analyzed for radiographs of precisely oriented femora of
cadavera and a radiology step-wedge. Despite attempts to normalize these
variables, we found that retrospective comparison of serial radiographs of
patients for densitometric analysis of change in the bone was prone to
significant error. In uncontrolled radiographs, non-uniformity of exposure,
intensity of the radiation field, target distance, and deviation in femoral
rotation will be likely to result in substantial variations in the apparent
thickness or density of the femoral cortex and make quantitative
comparisons of serial radiographs unreliable.