Characterizing orthodontic mini-screws in the hard palate of pigs: An experimental and finite-element study.

Objective: This paper analyses the mechanical response of mini-implants under pull-out, push-in, and shear forces. Materials and methods: The authors have devised a specialized testing apparatus, using a universal testing machine, for the mechanical characterisation of orthodontic mini-implants (OMI) installed in pig hard palate. The experimental investigation encompasses seven screw types, each subjected to pull-out, push-in, and shear forces. Each test was conducted three times to assess the inherent uncertainties, with three torque insertions, with a total of 189 tests. The resulting load-displacement curves provide information on the secant stiffness and ultimate capacity of the tested screws in all three loading scenarios.These findings were used to develop a 3D finite element model to assess the OMI mechanical performance. Results: Torque is particularly influential in push-in tests for both dependent variables. At the same time, the length appears to be more critical in pull-out tests for the capacity. Diameter's influence is consistent across all tests but is especially significant for the secant stiffness in shear tests. Conclusions: Pull-out tests displayed linear behaviour until failure, whereas push-in tests showed increasing stiffness with more significant deformation. Higher torque led to higher capacity with a minor effect on the stiffness. Clinical significance: This paper provides insights into the mechanical behaviour of orthodontic mini-implants, offering guidance to orthodontic practitioners and researchers. Understanding how torque affects stability and performance under different loads informs the selection and design of mini-implants, potentially improving orthodontic treatments and patient outcomes.