Effects of newly designed mini implant ring apparatus on miniscrew stability and force resistance against forces in different directions

The aim of this in-vitro study was to compare primary stability of normal miniscrews with primary stability of miniscrews supplemented by mini implant ring apparatus (MIR) in two different insertion angles against forces in differentdirections.For this purpose, a total of 80 conical TM (Trimed, Ankara Turkey) miniscrews (1,6 mm diameter and 8 mm length) and 40 MIR apparatus were used in our study. All of the miniscrews and MIR apparatus were applied to the specimens prepared from the ilium bone of bovine. Before application, in order to quantify the cortical bone thickness of the bone specimens, computed tomography (CT) images of bone specimens were obtained. In accordance with the cortical bone thickness values, bone specimens which contain cortical bone thickness ranging from 1 mm to 1.5 mm, were statistically so grouped that homogeneous distribution of miniscrews to bone specimens occurred.In the study miniscrews were divided into four groups and applied into bone specimens with the use of an angled miniscrew placement apparatus which was developed in our clinic. Miniscrews were placed into bone specimens at two different angles (70° and 90°) and MIR apparatus were applied to miniscrews in two groups. In the study first group consisted of 30 normal miniscrews placed at an angle of 70°, second group consisted of 30 miniscrews supplemented by MİR apparatus and were placed at an angle of 70°, third group consisted of 10 normal miniscrews placed at an angle of 90° and fourth group consisted of 10 miniscrews supplemented by MİR apparatus and placed at an angle of 90°.During placements, maximum insertion torque (MIT) values of all miniscrews were measured with the use of a torque screw driver (Checkline TSD 50, USA). After that, miniscrews placed at an angle of 70° were subjected to force applications in three different directions, miniscrews placed at an angle of 90° were subjected to force applications in one direction through a lateral displacement of 0.6 mm of the miniscrew with the use of the Instron test machine and force resistance (FR) values were measured and recorded. Maximum removal torque (MRT) values of tested miniscrews were measured with the use of the torque screw driver while removal from the bone.When the results were statistically evaluated, it was seen that MIT, FR and MRT values of miniscrews placed at an angle of 70° were significantly higher than MIT, FR and MRT values of miniscrews placed at an angle of 90° (p<0.05). MIT, FR and MRT values of miniscrews supplemented by MIR apparatus were significantly higher than MIT, FR and MRT values of normal miniscrews without MIR apparatus (p<0.05). The FR values of the normal miniscrews placed at an angle of 70° were significantly higher against forces in lateral directions (p<0.05). The FR values of the miniscrews placed at an angle of 70° and supplemented by MIR apparatus were significantly higher in broad angle group, in which the miniscrews were oriented in the same direction as the applied force vector (p<0.05).In conclusion, miniscrew insertion at an angle of 70° andsupplementation of MIR apparatus to miniscrews increased primary stability of miniscrews significantly and different results were observed in groups with normal miniscrews and in groups with miniscrews supplemented by MIR apparatus related to force application directions. In accordance with the obtained results, insertion of miniscrews at an angle of 70° to bone surface and MIR apparatus application (especially in bone regions with low cortical bone thickness) are advised.Key Words: Miniscrew, mini implant ring (MIR) apparatus, placement angle, primary stability.