The correlation among gripping volume, insertion torque, and pullout strength of micro-implant

The fixation stability is the key factor for orthodontic micro-implant to succeed. This study evaluated the mechanical properties of three types of micro-implants by analyzing their structural configurations. Thirty micro-implants of three types (diameter 1.5 mm, Types A, B, C) were assessed. All micro-implants were manually driven into artificial bones at an 8-mm depth. The insertion torque (IT), pullout strength (PS), and gripping volume (GV) of each type were measured. The indexes of mechanical properties denoted as the PS/IT, GV/IT and PS/GV ratios. Intergroup comparisons and intragroup correlation were examined using statistical analysis. Type B had the greatest inner–outer diameter ratio (0.67), and Type A had the smallest (0.53). The IT of Type A (5.26 Ncm) was significantly (p = 0.038) lower than that of Type C (8.8 Ncm). There was no significant difference in the pullout strength. The GV of Type A (9.7 mm3) was significantly greater than Type C (8.4 mm3). Type C was significantly greater than Type B (7.2 mm3). The ratios of mechanical properties (PS/IT, PS/GV, and GV/IT) were found significant in intergroup comparison. The PS/GV ratio was in order: Type B (26.5) > Type A (23.0) > Type C (20.2). Spearman's rho rank correlation test showed that PS of Type B was correlated significantly with GV. The design of thread and gripping volume were the important factors that contributes to the mechanical strengths of micro-implant.