Radiographic distortion of bones: a marker study.

The osseous shadows observed on radiographs are distorted by parallax and magnification error. Parallax may be minimized by proper centering of the subject; magnification error is governed by the subject-to-cassette and exposure distances. The usual assumption that radiographic magnification is 15% is, therefore, erroneous and misleading. A triangular marker was used to assess radiographic distortion of bones on films exposed at "standard" distances of 40 and 72 inches, varying the subject-to-cassette distance to simulate the effect of positioning problems posed by body habitus or joint contracture. Osseous magnification ranged from 6% to 36% on the 40-in exposures and from 3% to 17.5% at 72 in. Using a mid-sagittal marker, true bone size can be more closely determined; once the magnification is known, linear measurements can be normalized for accuracy, permitting valid comparison of quantitative data. Preoperative planning of reconstructive procedures, including implant sizing with templates, is greatly facilitated.