We examined the hypothesis that peak magnitude strain gradients are spatially correlated with sites of bone formation. Ten adult male turkeys underwent functional isolation of the right radius and a subsequent 4-week exogenous loading regimen. Full field solutions of the engendered strains were obtained for each animal using animal-specific, orthotropic finite element models. Circumferential, radial, and longitudinal gradients of normal strain were calculated from these solutions. Site-specific bone formation within 24 equal angle pie sectors was determined by automated image analysis of microradiographs taken from the mid-diaphysis of the experimental radii. The loading regimen increased mean cortical area (؎SE) by 32.3 ؎ 10.5% ( p ‫؍‬ 0.01). Across animals, some periosteal bone formation was observed in every sector. The amount of periosteal new bone area contained within each sector was not uniform. Circumferential strain gradients (r 2 ‫؍‬ 0.36) were most strongly correlated with the observed periosteal bone formation. SED (a scalar measure of stress/strain magnitude with minimal relation to fluid flow) was poorly correlated with periosteal bone formation (r 2 ‫؍‬ 0.01). The combination of circumferential, radial, and longitudinal strain gradients accounted for over 60% of the periosteal new bone area (r 2 ‫؍‬ 0.63). These data indicate that strain gradients, which are readily determined given a knowledge of the bone's strain environment and geometry, may be used to predict specific locations of new bone formation stimulated by mechanical loading. (J Bone Miner Res 1997;12:982-988)

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increased rate of high-grade cancer was observed in the finasteride group (1). The recently published analysis by Redman et al. (2) suggests that this observation was the result of detection bias rather than a true increase. However, the marked increase in the estimated rate of high-grade cancer in the placebo group calls into question the validity of the model. If the true rate of high-grade cancer in the placebo group was indeed 8.2%, then ∼40% of all high-grade cancers were missed despite annual screening. Because untreated, highgrade prostate cancer is associated with a 45% rate of metastatic disease after 5 years (3), one may conservatively estimate that 100 cases of metastatic prostate cancer should have developed in the placebo group in the 5 years since publication of the Prostate Cancer Prevention Trial. Do the authors have follow-up data to confirm this expectation? Disclosure of Potential Conflicts of InterestNo potential conflicts of interest were disclosed.