Objective: A prospective study of sinus lift surgeries was initiated utilizing pre and post-op CAT Scans and vertical bone cores (2.6x10-13mm) taken directly from implant sites being prepared for 5-6mm diameter implants. Methods: Sinus lift procedures were performed on subjects utilizing frozen, irradiated, cancellous bone allograft. Cores were removed at time intervals from 6-15 months. Extraction sites were used as controls. Qualitative assessments and histomorphometric analysis were performed on step serial sections stained with hematoxylin and eosin. Measurements of the area occupied by bone trabeculae in a given field were used as a density measurement. Descriptive statistics were utilzed for the analysis. Measurements were done at the crest, just above the sinus floor (location calculated from the pre-op C-scan) and at the apical end of the specimen. Results: Elimination of graft particles seems to be greater at the sinus floor than at the apex. The average amount of new bone was similar at both locations suggesting a uniform rate of activity. Conversion of graft material to new bone appeared to progress by conductive deposition along the smooth surfaces of graft particles with no apparent preceding osteoclastic activity. A phenomenon of "bridging" appeared to be a prelude to the final conversion. Comparison of this pattern to extraction site cores showed that extraction sites healed with a pattern of diminishing density from coronal to apical. Highest bone density was observed at the crest areas while the apical area was denser than residual bone adjacent to the sinus floor. Conclusion: Proximity to the sinus floor seems to be associated with more rapid turnover of graft particles to new bone.

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