## 4 Discussion  
Considerable numbers of crack formation and plastic deformation of bone have been shown as a consequence of implant site preparation and implant insertion. While exact thresholds for bone damage are unknown, bone resorption as a consequence of mechanical trauma may originate from such regions. Consequently, marginal bone loss of up to 1 mm in the first year after implant placement is still widely accepted as success criterion [11; 12]. Based on biomechanical parameters, Straumann implants showed a trend towards greater primary stability as compared to Astra implants. In agreement with this finding,  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline  & **Torque** & **Strain** & **ISQ** & **BMDc** & **BMDt** & **BICc** & **BICt** & **microC** & **microT** & **macroC** & **macroT** & **defC** & **defT** \\ \hline  
icroC** & \(0.615\) & \(0.703\) & \(0.266 greater values of bone to implant contact, both in cortical and trabecular areas of bone, were seen in Straumann implants. While the insertion torque of Straumann implants correlated with bone to implant contact in the cortical area, for Astra implants, a correlation for insertion torque with bone to implant contact in trabecular bone was observed. Macro cracks in trabecular bone correlated with bone to implant contact in Straumann implants but with implant stability in Astra implants. Furthermore, Straumann implants caused less microcracks in cortical bone as compared to Astra implants, for which bone to implant contact in cortical bone correlated with the number of macroracks detected. Given that bone mineral density did not significantly differ between Straumann and Astra specimens, it appears that the implants tested here derive primary stability by engaging different areas of bone [2; 5]. Differences in thread design in the cervical area in combination with the drill protocols used here obviously result in less cortical bone damage in Straumann implants as compared to Astra implants. Only a few and weak correlations among measurement parameters were observed in this study, which has previously also been experienced by other authors [5]. This may be due to the fact that clinical measurements, such as insertion torque and implant stability are rather global in nature, while histologic analysis focusses on very specific parts of the implant bone interface. The study at hand used two dental implant systems differing in length, diameter, thread pitch and drill protocol. Instead of standardizing, e.g., implant site preparation with respect to undersizing the osteotomy [6; 7], the manufacturers' guidelines were applied in order to best reflect clinical practice. While not reported here, this also resulted in different vertical start positions for implant insertion and varying amounts of time required for the insertion process despite a standardized insertion velocity. As with every in vitro investigation, several limiting factors have to be considered when interpreting the results. For analyzing bone damage, no staining was carried out here based on preliminary tests with toluidine blue staining [17] and fluorescent dye [18], as native bone resulted in the best visual appearance of cracks and deformed bone areas. However, originally basic fuchsin staining has been described as being necessary for analyzing bone damage [19]. Bone to implant contact is normally defined as intimate contact between dental implant and bone during or after healing. In this study, BIC was defined as an area in which surgical trauma caused by implant site preparation or implant insertion may have caused bone damage. Based on an initial screening of the specimens, a radius of 500 \(\upmu\)m surrounding a specific point of an implant was chosen while a radius of 300 \(\upmu\)m centered at the bony wall of an implant socket was chosen as area of interest for determining bone damage.