A Reproducible Method to Assess Aligner Thickness Effect on Force Output

Abstract

The relationship between applied force and aligner thickness is poorly understood. Therefore, the aims of the present study were to: 1) Evaluate the effect of initial material thickness on force output of clear aligners; and 2) Recommend a consistent, repeatable method for measuring aligner cross-section and distribution. A total of 40 clear aligners were fabricated over stereolithographic (SLA) models with ideal dentition, using proprietary gRxh material in four different thicknesses: (10) Rx15 [0.015h], (10) Rx20 [0.020h], (10) Rx30 [0.030h], (10) Rx40 [0.040h]. For measuring initial force output when tipping a tooth, a custom-made force measurement apparatus (FMA) was used with the upper right central incisor translated facially 0.25 mm. All six components of force and torque were measured (Fx, Fy, Fz, Tx, Ty, Tz) over a 16 second period of time upon seating each aligner. After gathering FMA data, cross-section and distribution of aligner material thickness was then measured using three methods for comparison: hall effect transduction, computed tomography (CT) scanning, and cross-sectional analysis under a stereomicroscope. All measurements were taken along the mid-sagittal plane of the facial surface of the upper right central incisor. Force output and variability both generally increased with an increase in material thickness. The range of measured forces were 3.5 to 28 times higher than ideal tipping forces reported by Proffit. The results for lingual force correlated well with those of total force (ã(Fx)2 + (Fy)2 + (Fz)2). As for the thickness measurement results, the hall effect transducer (HET) data correlated well with that of the CT scan. The stereomicroscope produced values that were relatively elevated. Mean measurements taken with both HET and CT consistently got smaller when moving from occlusal to middle to gingival points. The same trend was noted with the stereomicroscope, but with less consistency. The thinner Rx15 material appeared best suited for lingual tipping of a maxillary central incisor in vitro. The hall effect transducer produced simple measurements that were extremely similar to those from the CT scan, and appears to be a suitable stand-in for the more expensive and labor intensive CT scanning process.