An experimental study on the effects of the cortical thickness and bone density on initial mechanical anchorage of different Straumann® implant designs

Implant stability quotient (ISQ) has been quantified by using resonance frequency analysis (RFA) via Osstell device (Osstell Mentor, Integration Diagnostics Ltd., Göteborg, Sweden). For each implant design, a suitable transducer was inserted into the implant body (SmartPeg, Osstell, Göteborg, Sweden). Measurements were taken in four different directions perpendicular to the SmartPeg, according to the manufacturer’s guidelines. The mean value of four measurements has been calculated to determine the final ISQ value of each implant. All measurements were conducted in triplicate.

The tissue samples were histologically evaluated according to the technique described by Donath et al. [16] and later developed by Acil et al. [17, 18]. Briefly, tissue samples were placed into 10% neutral buffered formalin (NBF) for fixation for 4 days and embedded in methacrylate prior to sawing and grinding. Sawing and grinding were performed, and the samples were placed in glass vessels filled with monometric resin solution and incubated at 37–40 °C for 2 to 4 days for resin impregnation.

The samples were longitudinally cut with a band saw (Exakt, Norderstedt-Germany), and sections of about 100 μm were obtained via an oscillating diamond saw (Exakt, Norderstedt-Germany), grounded with the Sapphire 360 E grinder (ATM, Altenkirchen-Germany), and highly polished with silicon carbide paper (grades 500, 1200, 2400, and 4000). Staining was performed by using toluidine blue. The ground surface was decalcified with 0.1% formic acid and 20% methanol was applied for better cell and soft tissue staining. The samples were rinsed in distilled water and stained in a toluidine blue solution for 2 min. The height of the cortical bone was measured after micro-radiography of the specimens (Fig. 5). The preparations were digitally photographed with the Nikon photomicrography with a magnification of 509 and assessed manually with the Leica Q-Win Imaging program (McBain Systems, CA, USA). Relative amounts of bone area vs total tissue of the spongious bone were measured as described by Takahashi et al. [19] (% bone area/total tissue (% BA/TA)).

The samples were taken from the slide and put on high-resolution micro-radiography plates in which the resolution is 2000 lines per millimeter (high-resolution plates, Kodak®, Rochester, NY 14650, USA). Subsequently, the exposure was performed in Faxitron 53855A (Hewlett-Packard, McMinnville, OR 97128, USA) at a focus distance of 16 cm. The voltage was set to 25 kV at a power of 3 mAs, exposure time of 6 min and 30 s for 70-μ samples, and up to 10 min and 30 s for 110-μm film thickness. The exposure time was increased by increments of 1 min per 10-μm thickness of the specimen. The plates were embedded for 5 min in Kodak® HRP developer at 20 °C under constant movement swung (HRP developers/distilled water, 1/3). In a 1% acetic acid bath, the development was stopped after 1 min. For fixation, Kodak® fixer 3000A was used for 10 min with agitation (Kodak® fixer 3000A/distilled water, 1/3). This was followed for 15 min by washing with water and a final rinse with Agepon® (400 mL of distilled water and 2 mL Agepon®) for 1 min. The plates were air dried and then covered with cover glasses (4 × 4.5 cm) using 1–2 drops of n-butyl acetate (xylene substitute) and a drop of Eukitt-air for 24 h. After a drying period of 7 days under an air extractor, the preparations were digitally photographed under a light microscope at a magnification of 1:18. The evaluation was performed using an image editing program (Adobe Photoshop 7.0 for Windows) (Fig. 2).

Statistical software SPSS (IBM) was used for statistical analysis. Power analysis was performed to determine the smallest sample size that is suitable to detect the effect of a given test at the desired level of significance. The distribution of the variables was assessed by using the Shapiro-Wilk test. The comparison of two variables was performed via independent-T and paired-T tests. One-way analysis of variance (ANOVA) was conducted for comparative analysis of the variables. p < 0.05 was considered as statistically significant.

The average cortical bone height in the control group ranged between 2.34 and 3.21 mm (2.58 ± 0.615 mm). The histological examination revealed that, totally, n 6 (15.0%) of the bone structures correlated with type III bone (SP [n 1], TE [n 3], BLX [n 2]), whereas 34 (85.0%) of the specimens were identical to type IV bone (SP [n 9], BLT [n 10], TE [n 7], BLX [n 8]) [20]. The evaluation of the mean primary stability values of the implants inserted only in type IV bone in both groups (regardless of the presence of cortical bone) revealed that the BLX design had significantly higher mechanical anchorage compared to other implant systems (p 0.043) (Fig. 3).

In the study group, the BLX implant showed higher ISQ values compared to other groups. The significant difference was found when compared to SP and BLT groups (p 0.001 and p 0.0016, respectively). Besides that, TE implants showed statistically significant superiority compared to SP (p 0.001). The difference between BLT and SP groups was also remarkable (p 0.003) (Fig. 4a).

In the control group, TE implants showed slightly higher values compared to BLX; however, the difference was statistically insignificant. BLX and TE implants showed significantly higher ISQ values compared to BLT and SP groups (BLX-BLT p 0.004, BLX-SP p 0.001 and TE-BLT p 0.001, TE-SP p 0.001, respectively) (Fig. 4b).

All implant systems showed a significant decrease of mechanical anchorage in the study group (BLX p 0.000*, TE p 0.001*, BLT p 0.000*, and SP p 0.002*, respectively). The decrease in ISQ values was assessed in percentages (Fig. 5). According to this, the decrease was lowest in the BLX group, followed by BLT. The difference was significant compared to the loss of mechanical anchorage obtained in the TE group (p 0.033 and p 0.008, respectively).

The results of the study revealed that the TE and BLX implants showed higher ISQ values compared to BLT and SP implants in both experimental and control groups.