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Influence of residual alveolar bone height on implant stability in the maxilla: an experimental animal study.

AIMS/BACKGROUND
Empirically, for implant placement associated with sinus floor augmentation, a minimum of five mm of residual crestal bone height has been recommended in order to achieve sufficient initial implant stability. It has been the aim of the study to test this assumption in an experimental animal trial.


MATERIAL AND METHODS
In eight mini pigs, three premolars and two molars were removed on one side of the maxilla. Three months later the animals were assigned to four groups of two animals each. A cavity was created at the base of the alveolar process so that the residual bone height was reduced to 2, 4, 6 and 8mm, respectively. The coronal part of the alveolar crest remained unchanged. An inlay augmentation procedure was carried out using a particulated autogenous bone graft from the iliac crest, and six implants (Xive, diameter 3.8mm, length 13mm) were placed. Implant stability was assessed by resonance frequency analysis at the time of implant placement (T0), after 6 months of unloaded healing (T1) and after 6 months of functional loading (T2).


RESULTS
During follow-up, two implants were lost in sites with a residual alveolar bone height of 2mm. At the time of implant placement, resonance frequencies were 6754.4 +/- 268, 6500.3 +/- 281.5, 6890.3 +/- 255.4 and 7877.9 +/- 233.7 Hz for residual bone heights of 2, 4, 6 and 8mm, respectively. At stage-two surgery and after 6 months of functional loading, resonance frequencies were 6431.7 +/- 290.8, 6351.8 +/- 437.6, 6213.4 +/- 376.2 and 6826.8 +/- 458.9 Hz vs. 6171 +/- 437.4, 6047 +/- 572.4, 6156.7 +/- 272.6 and 6412.8 +/- 283.5 Hz. Statistical analysis revealed an association of residual alveolar height and implant stability at T0 and T1 only (P<0.01), while bone height was not found to influence implant survival.


CONCLUSION
The results of the present trial demonstrate an association of alveolar bone height and implant stability at the time of implant placement and stage-two surgery. Yet the assumption that 5mm of residual crestal bone height is a relevant threshold for simultaneous implant placement and sinus floor augmentation is not supported from an experimental point of view.