Reversal of vertebral deformities in osteoporosis: measurement error or "rebound"?

Digitized morphometry of vertebral bodies on lateral spine films is used to identify and quantify vertebral deformities or fractures. One problem associated with this method is the phenomenon of "disappearing fractures," which results from the apparent increase in vertebral body heights of previously deformed vertebrae on subsequent radiographs. These have been considered biologically implausible and therefore a result of measurement error. Measurement error is unlikely to be unidirectional, so that a proportion of fractures identified by morphometry is also the result of measurement error. Since some vertebral deformities are real events, some disappearances of deformities detected by morphometry may be real events. In this report, we examine the data from our clinical trial of sodium fluoride in spinal osteoporosis to assess critically the plausibility of two hypotheses: (1) The "rebound" phenomenon results from measurement error. If this is the case, then some fractures of the same magnitude as the rebound must also represent measurement error. (2) Some deformed vertebrae in fact rebound toward their original shape and size, displaying an elastic response to deformation. If this occurs, then some vertebral deformities are transient events, not true fractures. We conclude that the variability inherent in morphometric data obtained from serial spine x-rays results in both disappearing fractures and a high false positive fracture rate. The use of more stringent criteria for defining significant deformities, or true fractures, will minimize these problems. We cannot exclude the second hypothesis, that some vertebral deformities may be transient events, but this needs further study.