Evaluation of expansion forces of five pure bone-borne maxillary expander designs anchored with orthodontic mini-implants: An in vitro study

Aim: This in vitro study investigates the limit of expansion forces and torque wrench forces developed by five skeletal bone expander designs (MICRO 2/4 expanders™) for clinical application.

Material and methods: A total of 30 skeletal expanders were placed in artificial bone blocks and mechanically tested, simulating maxillary expansion. Differences in jackscrew (Dentaurum™ [D], Superscrew™ [S] and Powerscrew™ [P]), number of orthodontic mini-implants (OMIs; two or four) and their placement inclinations (parallel 0° or 10° inclination) form five designs (D4/10°, S4/0°, S4/10°, P4/10° and P2/10°). Expansion forces and torque wrench values were registered, and radiographs were made initially and after 4 mm of expansion. Stress-strain curves were obtained after successive activations and the statistical analysis was performed as appropriate.

Results: Plastic deformations in the OMIs and jackscrew occurred around the activation numbers 11-13, with torque wrench values in the range of 500-700 cN. The maximum expansion forces in expanders with four OMIs varied from 93.0 (D4/10°) to 166.6 N (P4/10°) whereas two OMI expanders (P2/10°) registered forces of 79.4 N. Radiographs revealed during loads bending forces (S4/00°, S4/10°) with jackscrew and OMIs deformation in a convex shape, and shear forces (P4/10°, P2/10°) demonstrated only OMIs deformation in a concave shape, providing 15% more expansive force. The jackscrew D4/10° did not have any deformation, but its wire key did not allow reliable activations from activation number 10 and compared to S4/10° and P4/10°, these expanders provided greater expansion forces (P = 0.000 and P = 0.032, respectively).

Conclusion: The different results obtained in stability and expansion forces indicate that if the activations are carried out under extreme conditions, they may have clinical importance with deformations and non-working expansion mechanics. Jackscrew designs play an important role in expansive forces and expander stability. Torque wrench values can be used clinically as a tool to asses the expansion forces and to avoid deformations.