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.

A comparative surface evaluation of orthodontic mini-implants before and after en masse retraction-A SEM study

OBJECTIVE

To evaluate the changes in surface morphology of two different types of mini-implants after clinical en masse retraction using scanning electron microscopy.

MATERIAL AND METHODS

Fifty mini-implants of Dentos (Korea, Absoanchor, BH-1817-08) and Orlus (Korea, Yesanchor, C-1817) were inserted in patients in a split-mouth design who required en masse anterior retraction and absolute anchorage. Surface characteristics of mini-implants such as pitch (distance between consecutive threads), flank width (distance between root and crest), and taper were studied using scanning electron microscope (FEI nanosem450) before and after clinical use.

RESULTS

Statistically significant difference (p value = 0.003) was found in a mean reduction of pitch dimension among the two groups with a mean difference of 25.000 μm. Also, a statistically significant difference was noted (p value = 0.001) in a mean reduction of flank width among Dentos implants as compared to Orlus implants. A statistically significant difference (p = 0.001) was seen in the mean reduction of taper dimension among Dentos group (0.0140 ± 0.02271) as compared to the Orlus group (0.0810 ± 0.05152).

CONCLUSION

A marked reduction in surface morphology such as the pitch, flank width, and taper of both mini-implants after retrieval was observed. Dentos group of mini-implants displayed better dimensional stability post-retrieval as compared to the Orlus group of mini-implants. All the mini-screws showed milling defects in form of scratches on observation under scanning electron microscopy despite a smooth appearance to the naked eye.