Comparison of External and Internal Hex Implants' Rotational Freedom: A Pilot Study

Purpose: Loosening or fracturing of abutment or retaining screws is the main mechanical complication associated with single-tooth implants. The bearing surface of an implant and of its opposing abutment form a screw joint that must remain stable throughout the restoration's life. To reduce the incidence of screw loosening, biomechanical parameters of movement at the interface of the implant-abutment complex must be studied. Binon reports that optimal joint stability requires rotational freedom of less than 5 degrees. The present study sought to evaluate and compare the rotational freedom of external and internal hex implants and their abutment counterparts to verify if a particular measuring device is suitable and if all components are in accordance with manufacturing specifications. Materials and Methods: Five regular-diameter external hex implants and five regular-diameter internal hex implants with abutments (Conexao Sistemas de Proteses) were evaluated. To measure the rotational freedom, a device in which the implants were secured by a chuck to a table base with a degree scale was developed (Fig 1). The rotational freedom measuring device table was calibrated through geometric analysis of its scale and a calibrated protractor. A handle and needle pointer were fixed to the abutments through two opposing screw holes. Abutments were screwed to the implants using a manual torque wrench (20 Ncm). With a dynamometer, a 40-Ncm torque was applied to the abutment, and the difference between the clockwise and counterclockwise movement was recorded as rotational freedom with 0.5-degree accuracy. Measurements were taken in every possible position (three different positions, three times) and combination of implants and abutments. An optical measurement system (RAM Optical Instrumentation) with Auto MAP X-Y-Z software (Measurement Analysis Program) was used to measure both implant and abutment hexagons with 0.0001-mm accuracy. Three measurements were taken per hexagon, corresponding to the distance of its opposing boundary lines. Data were subjected to analysis of variance (P , .05) to determine statistical significance between internal and external hex implants. Mean values and standard deviations were recorded. Results: Mean rotational freedom was 5.5 ± 1.9 degrees for internal hex implants, which presented rotational freedom of between 3 and 5 degrees in 58.67% of the combinations; 41.33% showed rotational freedom higher than 5 degrees (recorded range 3.5 to 11.0 degrees). Mean rotational freedom was 2.9 ± 0.3 degrees for external hex implants. All external hex implant configurations presented rotational freedom lower than 5 degrees; 93.33% showed rotational freedom lower than 3 degrees (recorded range 2.5 to 3.5 degrees). Statistically significant differences were found between groups (P , .05). Conclusion: In this pilot study, internal hex implants presented more rotational freedom than external hex implants. All samples were in accordance with the manufacturing specifications. The reported measuring device appears suitable to analyze rotational freedom and will be used in further studies with other implant systems. The number of implants and abutments used in this pilot study is not large enough to draw more precise conclusions.