Biomechanical simulation of segmented intrusion of a mandibular canine using Robot Orthodontic Measurement & Simulation System (ROSS)

Objective: Aim of this study was to investigate the forces and moments during segmented intrusion of a mandibular canine using Cantilever-Intrusion-Springs (CIS). Methods: Three different CIS modifications were investigated using a robotic biomechanical simulation system: unmodified CIS (#1, control), CIS with a lingual directed 6 degrees toe-in bend (#2), and CIS with an additional 20 degrees twist bend (#3). Tooth movement was simulated by the apparative robotic stand, controlled by a force-control algorithm, recording the acting forces and moments with a force-torque sensor. Statistical analysis was performed using Shapiro-Wilk, Kolmogorov-Smirnov, Kruskal-Wallis ANOVA and post hoc tests with Bonferroni correction (alpha = 0.05). Results: The initial intrusive force, which was uniformly generated by a 35 degrees Tip-Back bend, decreased significantly (p < 0.05) from 0.31 N in group (#1) to 0.28 N in group (#3). Vestibular crown tipping reduced significantly (p < 0.05) from 2.11 degrees in group (#1) and 1.72 degrees in group (#2) to 0.05 degrees in group (#3). Matching to that the direction of orovestibular force significantly (p <0.05) shifted from 0.15 N to vestibular in group (#1) to 0.51 N to oral in group (#3) and the orovestibular tipping moment decreased also significantly (p <0.05) from 4.63 Nmm to vestibular in group (#1) to 3.56 Nmm in group (#2) and reversed to 1.20 Nmm to oral in group (#3). Apart from that the orovestibular displacement changed significantly (p < 0.05) from 0.66 mm in buccal direction in group (#1) to 0.29 mm orally in group (#2) and 1.49 mm in oral direction as well in group (#3). Significance: None of the modifications studied achieved pure mandibular canine intrusion without collateral effects. The significant lingual displacement caused by modification (#3) is, not least from an aesthetic perspective, considered much more severe than a slight tipping of the canine. Consequently, modification (#2) can be recommended for clinical application based on the biomechanical findings.