Correlation Between Implant Geometry, Implant Surface, Insertion Torque, and Primary Stability: In Vitro Biomechanical Analysis

Purpose: Primary implant stability represents the first step for successful osseointegration. The knowledge of the correlation between host bone density, insertion torque, and implant macrogeometry seems to be fundamental to achieve sufficient primary implant bone fixation in each clinical situation. The purpose of this study was to measure, in vitro, the impact of dental implant macrogeometry and insertion torque values on primary stability in relation to different bone densities, representing both the human mandible and maxilla. Materials and Methods: One hundred twenty 3.8 x 11-mm commercial dental implants were used. Forty implants had small threads with a machined neck, 40 implants had small threads with a microthreaded neck, and the last 40 implants had large threads with a reverse neck design. Fresh bovine ribs, representing a medium-dense bone density (D2-D3), and fresh ovine iliac crest, representing a soft bone density (D4), were used. Insertion torque and micromobility under lateral force data were recorded for each implant. Results: In the medium-dense bone type, the reverse neck implant design showed less primary implant stability than the conventional straight implant neck. In soft bone, both implants with the large thread design and microthreaded neck implants showed better implant stability than the implant with a small thread design with a straight machined neck. Implants with large and self-cutting threads showed significantly (P < .05) lower micromobility values than other implants in postextractive sites in low-density bone. Conclusion: Implant geometries and bone density are the main factors involved in the degree of primary implant stability. Large-thread implant designs are highly desirable in cases of poor bone quality. Each implant geometry generates an insertion torque value, which is correlated to the stability of that specific implant in a specific bone quality, but the insertion torque is not an objective value to compare primary stability between different implant types.