Effects of the cortical bone thickness and location in primary stability, a simulation study

Objectives: Although, cortical-bone contributes to dental implants primary stability (PS), the extent of such contribution has not been clearly understood. It is also not clear how PS is affected if cortical-bone contacts the cervical (conventional implant) or the apical areas (implants penetrating maxillary sinus). Therefore the aim of the present study was to measure and compare PS of dental implants in bone blocks when 1,2mm-thick simulated-cortical-bone were in contact with different areas of the implant.
Methods: Polyurethane-bone-blocks (Sawbones) with densities of cancelous-bone were laminated with 1,or 2mm polyurethane sheets of cortical bone density. Six Straumann® (10mm,ø3.3mm) implants were placed in each experiment so that either cervical or apical areas of implants contacted with 1, or2mm of cortical sheets. Implants were also placed in cortical bone sheets alone to measure the contribution of cancelous-bone to PS. Implant stability quotients were measured by two testers with both Periotest and Osstell. PS was further measured using pullout test in an Instron unit. One-way ANOVA and Bonferoni post-hoc analyses were used to compare significant differences of PS in each condition (p≤0.05).
Results: Both Osstell and Periotest indicated significant increase in PS when cortical-bone sheets were in contact with implants. However, there was significantly lower PS (p<0.05) when cortical bone sheets contacted apical area of implants. When cancelous-bone was omitted, there was a significant drop in PS (p<0.05) only when cortical bone was in contact with the apical area of the implant. Instron tests; however, did not find any difference in PS in either apical or cervical contact cases.
Conclusions: Cortical-bone increases PS regardless of where it contacts the implant surface. However, Osstell and Periotest cannot reliably measure PS if cortical-bone contacts apical areas of an implant. Dental implants are increasingly placed in contact with cortical wall of maxillary sinus; therefore, the accuracy of such measurements should be interpreted with cautions.