Falko Schmidt

Forces and moments applied during derotation of a maxillary central incisor with thinner aligners: An in-vitro study

Introduction: Recent studies have shown that therapeutic loads applied to individual teeth by aligners may substantially exceed recommended values. The primary purpose of this study was to quantify force and moment components during derotation of a maxillary central incisor when 0.3‐mm‐thick or 0.4‐mm‐thick polyethylene terephthalate glycol aligners were used instead of conventional polyethylene terephthalate glycol aligners with a minimum thickness of 0.5 mm. Methods: The test setup consisted of an acrylic model of a maxilla with a separated right central incisor mounted on a 3‐dimensional force and moment sensor. The force and moment components were recorded for aligners with thicknesses ranging from 0.3 to 0.75 mm during ±10° rotation and derotation of the separated incisor. Results: Moments exerted by the thinnest aligner currently available, 0.5 mm, were 73.57 Nmm for the 10° mesiorotation. In comparison, the corresponding moments with the 0.4‐mm and 0.3‐mm aligners were 41.08 and 17.84 Nmm, respectively. Moment values for derotation of the maxillary right central incisor into neutral position showed nonlinear return curves indicating viscoelastic material behavior. Conclusions: A significant load reduction can be achieved with the new thinner aligners. Because of the form instability of the 0.3‐mm aligner during handling, we suggest the novel sequence 0.4, 0.5, and 0.75 mm for aligner systems based on sequentially increased material thickness. This sequence combines sufficiently low initial aligner stiffness and steady load increases in single setup steps. The viscoelastic behavior of polyethylene terephthalate glycol aligners observed during incisor derotation should lead to a reduction of the high initial load exerted directly after intraoral aligner insertion. HighlightsForces and moments applied even by the thinnest aligners are relatively high.New, thin levelling aligner sequence is proposed to reduce risk of root resorption.Aligners made of 0.3‐mm PET‐G foils deformed irreversibly during handling.Aligners made of 0.4 mm PET‐G‐aligner were sufficiently stable.Proposed aligner sequence combines low stiffness and nearly equal load increases.Different collateral forces and moments occur during central incisor derotation.

An instrumented tooth

This paper presents a tool for orthodontic research and education, namely an instrumented tooth enabling all six force and moment components exerted on the tooth to be measured. The mechanical load values are inferred from mechanical stress values measured at the surface of a CMOS stress sensor system sandwiched between two pins dimensioned as a tooth root. The resulting sensor module measures 11.6 mm in height and 3.5 mm in diameter. It allows the measurement of forces up to 60 N and moments up to 10 Ncm and thus covers the range of forces and moments relevant for orthodontics.

Comparison of methods to determine the centre of resistance of teeth

In orthodontic treatment, the locations of the centre of resistance (CR) of individual teeth and the applied load system are the major determinants for the type of tooth movement achieved. Currently, CR locations have only been specified for a relatively small number of tooth specimen for research purposes. Analysing cone beam computed tomography data samples from three upper central incisors, this study explores whether the effort to establish accurate CR estimates can be reduced by (i) morphing a pre-existing simplified finite element (FE) mesh to fit to the segmented 3D tooth-bone model, and (ii) individualizing a mean CR location according to a small parameter set characterising the morphology of the tooth and its embedding. The FE morphing approach and the semi-analytical approach led to CR estimates that differ in average only 0.04 and 0.12 mm respectively from those determined by very time-consuming individual FE modelling (standard method). Both approaches may help to estimate the movement of individual teeth during orthodontic treatment and, thus, increase the therapeutic efficacy.

CMOS tactile sensor systems

Tactile sensor systems based on complementary metal-oxide-semiconductor (CMOS) technologies have found a wide variety of applications covering various types of man-machine interfaces as well as industrial applications. These sensor systems are realized using commercially available CMOS processes combined with appropriate assembly technologies for advanced system packages, and dedicated micromachining processes to realize membranes or beam structures to improve the sensor sensitivity. Piezoresistive CMOS-based tactile sensor systems make use of implanted resistors and field-effect transistors (FETs) exploiting the piezoresistive effect in silicon. The applied CMOS chips extract the mechanical stress distribution in the chip surface which is characteristic for the corresponding mechanical loading of the CMOS chip or its package. This paper describes a three-dimensional force sensor used in metrology to extract the 3D geometry of precision machined parts, and the Smart Tooth, an innovative tool for orthodontic research and education.