Measuring the Complete-arch Distortion of an Optical Dental Impression

Summary

Here, we present a protocol to measure the degree of distortion at each part of the compete-arch digital impression acquired from an intraoral scanner with 3D-printed metal phantom with standard geometries.

Abstract

Digital workflows have actively been used to produce dental restorations or oral appliances since dentists started to make digital impressions by acquiring 3D images with an intraoral scanner. Because of the nature of scanning the oral cavity in the patient's mouth, the intraoral scanner is a handheld device with a small optical window, stitching together small data to complete the entire image. During the complete-arch impression procedure, a deformation of the impression body can occur and affect the fit of the restoration or appliance. In order to measure these distortions, a master specimen was designed and produced with a metal 3D printer. Designed reference geometries allow setting independent coordinate systems for each impression and measure x, y, and z displacements of the cylinder top circle center where the distortion of the impression can be evaluated. In order to evaluate the reliability of this method, the coordinate values of the cylinder are calculated and compared between the original computer-aided design (CAD) data and the reference data acquired with the industrial scanner. The coordinate differences between the two groups were mostly less than 50 µm, but the deviations were high due to the tolerance of 3D printing in the z coordinates of the obliquely designed cylinder on the molar. However, since the printed model sets a new standard, it does not affect the results of the test evaluation. The reproducibility of the reference scanner is 11.0 ± 1.8 µm. This test method can be used to identify and improve upon the intrinsic problems of an intraoral scanner or to establish a scanning strategy by measuring the degree of distortion at each part of the complete-arch digital impression.

Introduction

In the traditional dental treatment process, a fixed restoration or a removable denture is made on a model made of gypsum and impregnated with a silicone or irreversible hydrocolloid material. Because an indirectly made prosthesis is delivered in the oral cavity, a lot of research has been done to overcome the errors caused by a series of such manufacturing processes^1,2. Recently, a digital method is used to fabricate a prosthesis through the CAD process by manipulating models in the virtual space after acquiring 3D images instead of making impressions³. In the early days, such an optic....

Protocol

1. Master specimen preparation

1. Model preparation
   1. Remove the artificial teeth (left and right canines, second premolar, and the second molar) on the mandibular complete-arch model with only 1/5 of the cervical portion left.
2. CAD design
   1. Acquire the data of the master specimen with a reference scanner.
   2. Design the cylinders (with a top diameter of 2 mm and a cylinder height of 7 mm) on top of the trimmed six teeth with the reverse engineering software.
   3. Add three reference spheres (3.5 mm in diameter) posterior to the left second molar for the purpose of defin....

Results

The coordinates of each cylinder calculated from the originally designed CAD data and the reference scan image of the 3D-printed metal master specimen scanned by the industrial-level model scanner are shown in Table 1. The difference between the two showed a value of lower than 50 µm, but the z coordinate value of the right second molar cylinder from the 3D-printed master specimen was low. Although the metal phantom was produced from a high-end industrial 3D prin.......

Discussion

Among the studies evaluating the accuracy of the intraoral scanner by evaluating the resultant digital impression body, the most common method is to superimpose the digital impression data on the reference image and calculate the shell-to-shell deviation^{12,13,14,15,20,23}. However, this method is limited to calculating the dev.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
EOS CobaltChrome SP2	Electro Oprical Systems	H051601	Powder type metal alloy for 3D printing
Geomagic Verify	3D Systems	2015.2.0	3D inspection software
Prosthetic Restoration Jaw Model	Nissin Dental Products Inc.		Mandibular complete-arch model
Rapidform	Inus technology	RF90600-10004-010000	Reverse engineering software
stereoSCAN R8	AICON 3D Systems GmbH		Industrial-level model scanner