The topological changes in vertebral cancellous bone were estimated in vertebrae from calcium-restricted ovariectomized Sinclair S-1 minipigs, a recently described animal model of cancellous osteopenia. Connectivity was estimated using unbiased stereological principles in disector pairs of sections from the first lumbar vertebrae. Connectivity density was increased approximately twofold when compared with sham-operated minipigs fed a standard diet. These alterations in topology occurred coincident with a 25% increase in final resorption depth and a 150% increase in vertebral marrow star volume. Taken together, these changes suggest that in calcium-restricted ovariectomized minipigs, trabecular plates are transformed into rods by perforation. These changes in topology appear to be due, at least in part, to excessive resorptive cell function at the level of the bone remodeling unit. Conventional two-dimensional estimators of structural parameters of cancellous bone were not only less sensitive to these changes in topology but, in some cases, the estimates were directionally reversed.