Precision of In Vivo Quantitative Tooth Wear Measurement Using Intra-Oral Scans

This is the first method that measures real tooth wear in real tooth wear patients, which is a huge step in the management of tooth wear. It provides quantitative and visual information to both patients and dentists, using regular intra-oral scanners. Quantifying the rate of tooth wear is vital in providing the right treatment at the right time.

The scanning part is something many dentists are already familiar with. This sequence software analysis takes some time but it's not necessarily difficult To begin, isolate the dentition using lip retractors dry pads, and curly saliva ejectors. Lightly powder the dentition before scanning if required, for the intra-oral scanner used and scan the dentition.

To proceed with super imposition and quantitative wear measurement, open the 3D software program and import the old and new scans files of the upper and lower jaws. Select a scan and then use lasso selection and select through to select a tooth. Save the separated tooth using tools, new object and from selection.

Select, copy, and paste, and give the object a name. Repeat this procedure for each tooth of the upper and lower jaws in the old and new scans. Before selecting a new tooth, deselect the previously selected tooth.

Select a specific tooth and model manager and change select through with select visible. Use the lasso tool to select soft tissue and contact areas, and delete these parts. Repeat this for all individual teeth.

Calculate the best fit per tooth by selecting model manager in the old scan, and set this as set reference. Similarly, set the new scan as set test. Select the reference scan and in the alignment tab, choose best fit alignment.

Set deviator elimination to one, press apply, and then okay. To check the quality of the best fit. Go to analysis, then selection through object and create an intersection perpendicular to the surface from the buccal to the palatal side.

Press on compute. A cross section of both scans in red and black lines will become visible. Check if the best fit is correct, and that the new scan is not superpositioned as compared to the old scan.

Press okay to go back to the scan, deselect areas on the old and new scans with severe material loss using the lasso tool. If possible, select the areas on at least three surfaces, and repeat the steps. Select analysis and 3D compare to create a colorized model of the wear.

To obtain wear facets as negative values in the results, change the spectrum by selecting color segment to 21, maximum critical at 0.2 millimeters, maximum nominal at 0.02 millimeters, minimum nominal at minus 0.02 millimeters, minimum critical at minus 0.2 millimeters, and decimal places to three. Click on apply, and then okay. The result of the 3D compare is presented in the model manager.

The blue areas indicate wear, green areas indicate no change, and gray areas indicate wear so high that the software cannot connect the surfaces. Measure the vertical height loss by clicking on set result on the fitted and compared tooth. Go to create annotations in the analysis tab.

Change the deviation radius to 0.1 millimeters. Select the area with the largest amount of wear or the darkest blue point, and click on okay to go back to the scans. Use edit spectrum to raise or decrease the value of max critical when the darkest blue area is too large to determine the point of highest wear.

Export the value from the annotation on the point of highest wear to the data system. To determine the vertical material loss on 2D images with 2D dimensions, set the old scan as the reference scan and the new scan as the test scan. Make multiple cross sections on the locations with the largest loss of material, by using the 3D compare result to determine the location.

Click on test scan, then select 2D dimensions under the analysis tab. In view control, select the cross section showing the highest height difference in the to be measured area. Select parallels for dimension type.

Next, in the pick methods, click on test. Make a mark on the test, scan at the location of the largest wear, then click on ref in the pick methods and make a mark on the reference scan. Then click on one selected point with the highest amount of wear to obtain the result, and export this to the data system and click okay.

Trim the teeth by selecting the tooth to be compared. Right click on the scan and then click on duplicate to make copies of the old and new scans of the to be measured tooth. Remove the automation from the copy by selecting automations, right clicking it and clicking on delete.

Select both copies of the old and new scans of the tooth. Go to polygons and select trim with plane. Trim the interdental and cervical areas by creating intersections, leaving only a closed occlusal surface.

Trim by drawing the intersection, resulting in a red selected area and a blue unselected area, divided by the intersection. Click on intersect plane, delete selection, and then close intersection, and okay, to cut off the selected area on the interdental and cervical surfaces and create a closed off volume. If necessary, first reverse the selected area if the software selects the occlusal side to be deleted.

Close small holes in the scans by selecting fill all. Selected holes are marked with a green border and after filling, those will become red. If there are too many or too large holes preventing measurement of the volume, exclude the tooth.

To measure the volumes of both objects, go to analysis, then compute volume. If the volume is zero a hole is still present in the object. Export the values of the old and new volumes to the preferred data system.

The data for protocol precision was visualized in violin plots. The intra and inter-rater precision data was visualized in Bland-Altman plots. For height, a statistically significant difference was found between R1 and R3, which is clinically not significant, as can be seen from the entire confidence interval being close to zero.

The precision of height was determined mainly by droplets of saliva and scanning powder creating tiny spikes, causing a large height difference when measuring the highest point on the surface. When analyzing the height data on wear progression comparing rater one to rater two, it became clear that for height, a group of outliers could be attributed to two factors. Measurements on teeth with severe wear were made with the 2D compare method, and a set of measurements was wrongly made on pooled saliva, which was mistaken for wear by rater two.

The data was therefore split into three groups and analyzed separately as saliva, normal, and 2D compare. Rater three made no measurements on pooled saliva proving that training was successful in that regard. This method is a first step for using scanners and scanning software to aid dentists and patients in making choices based on data, not instincts.

The best fit is the crucial step. Without superimposing the scans as perfectly as possible, tiny differences in height can be missed or misinterpreted. We would still like to find a slightly adjusted way to measure volume precisely, which can be very insightful in measuring tooth wear progression.