Optimization of the dimension of computer numerical control–milled polyetheretherketone clasps: An in vitro evaluation of accuracy

Abstract

Statement of problem

The accuracy and optimal dimensions of computer numerical control (CNC)-milled polyetheretherketone (PEEK) removable partial denture (RPD) clasps are unclear.

Purpose

The purpose of this in vitro study was to investigate the trueness and precision of CNC-milled PEEK clasps with different thicknesses and lengths.

Material and methods

Ladder-shaped specimens of 2 thicknesses with 5 lengths of clasps were designed and milled with PEEK and commercially pure titanium (CP Ti) (n=6). All milled specimens were scanned and superimposed onto the design data. Three-dimensional and 2-dimensional deviation analyses were carried out to evaluate the trueness of milled PEEK clasps. The scanning data of each group were superimposed pairwise, and the 3-dimensional deviations were analyzed to evaluate the precision. Nonparametric tests, ANOVA, the Pearson correlation, and univariate linear regression were used for statistical analysis (α=.05).

Results

The deviation of trueness of the PEEK clasps (0.047 to 0.164 mm) was higher than that of the CP Ti clasps (0.037 to 0.060 mm) (P<.001). Increasing the length of the clasps increased the deviations (P<.001). Deviation in the 2 thicknesses was not significantly different (P=.210). The correlation coefficients of 1.0-mm-thick and 1.5-mm-thick PEEK and CP Ti clasps were 0.843, 0.794, 0.638, and 0.405. The positive correlation coefficients of PEEK were higher than those of CP Ti and those of 1.0-mm-thick clasps was higher than those of 1.5-mm-thick clasps. The deviations were evenly distributed in the 9-mm length of the clasp for CP Ti and in the 6-mm length of the clasp for PEEK. Beyond these lengths, deviations increased with increased length. The increasing amplitude of CP Ti was smaller than that of the PEEK group, and that of the 1.5-mm-thick clasp was smaller than that of the1.0-mm-thick clasp. The measured range of precision of PEEK clasps was 0.079 to 0.152 mm, while that of CP Ti clasps was 0.036 to 0.096 mm. CP Ti clasps tended to have better precision than PEEK clasps, except for the 1.0-mm-thick clasps with a length greater than 9 mm and the 1.5-mm-thick clasp with a 12-mm length. The correlation of the clasp length with precision showed that the lengths of 1.0-mm-thick clasps strongly influenced precision (PEEK, P=.020; CP Ti, P<.001); this correlation decreased sharply when the thickness of clasps was 1.5 mm (PEEK, P=.199; CP Ti, P=.107).

Conclusions

Greater elasticity increased the deviations of milled clasps. The increased thickness helped the clasp remain stable during the milling process. The 1.5-mm-thick PEEK clasps in the 3-mm and 6-mm lengths were the optimal design tested.