Objectives: The aim of this study was to investigate the effect of surface variations of unloaded and loaded step-cylinder implants (Frialit-2, Dentsply Friadent, Mannheim, Germany) on histology of the peri-implant bone, and on implant stability. Methods: 72 implants (each 36 TPS and 36 HA coated) were placed in the mandibles of 18 sheep dogs. After uncovering 3 months following placement, half of the implants were loaded (LO) at random with fixed prostheses, the second half remained unloaded (UL). 3, 6 and 9 months after placement, the torque strength in bone of each 6 implants of any group was determined (Wera electronic, Wuppertal, Germany). After the last measurement, the mandibles were sectioned and specimens were obtained using the sectioning and grinding technique, and the tissues around the non torqued implants were examined histologically. Results: Histology yielded titanium particles which came off the TPS-implants as well as signs of dissolution of the coating of HA-implants. Particularly, bone remodelled the resorbed areas and loose particles were removed. In addition, the bone structure appeared irregularly compared to areas without surface variations. ANOVA revealed that torque strength of the HA-implants decreased not significantly from 28±2 dNm to 25±3 dNm for LO and to 20±4 dNm for UL between 6 and 9 months after placement. The torque strength of unloaded TPS-implants did not change during the whole observation period (17±5 dNm) but increased significantly (p<0.05) to 24±2 dNm along with functional loading. It was assumed that the resorption of the HA-coating caused the reduction of the torsional strength. The yielded titanium particles of the TPS-surface had no influence on the torque strengths of these implants. Conclusion: The resorption of the HA layer of implants had a detrimental effect on their long-term stability in bone. In comparison yielded titanium particles of the TPS-surface had less negative effects.

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