A novel approach to determining augmented bone volume in intraoral bone block augmentation using an intraoral scanner: an in vitro study

Abstract

Introduction: Bone augmentation procedures are established tools for reshaping the alveolar ridge and increasing bone volume. Different approaches are being used to measure postoperative bone volume gain. This study aimed to develop an objective and automated volume measurement tool equally as precise as manual slice-by-slice annotation.

Materials and methods: To evaluate the proposed workflow, we performed an in vitro study with 20 pig mandibles that were grafted using three different grafting techniques-autogenous full block, split block bone and shell augmentation. The pig jaws were scanned pre- and postoperatively using an intraoral scanner. The resulting surface files (baseline, full block, split block, shell) were processed using the new volume-measuring workflow as well as using manual slice-by-slice annotation at baseline (t0) and at 6 months (t1) using the same population. Two TOSTs (Test of One-Sided Significance) and NHSTs (Null Hypothesis Significance Test) were used to compare the two workflows. The intra-rater reliability between t0 and t1 was determined using intraclass correlation coefficients.

Results: The mean difference for the full block augmentation technique was - 0.015 cm³ (p < 0.001); for the split block technique, it was - 0.034 cm³ p = 0.01, and for the shell technique, it was - 0.042 cm³. All results were statistically not different from zero and statistically equivalent to zero. The results also showed an excellent absolute intra-rater agreement.

Conclusions: The semiautomatic volume measurement established in this article achieves comparable results to manual slice-by-slice measuring in determining volumes on STL files generated by intraoral scanners and shows an excellent intra-rater reliability.

Keywords: Autogenous bone; Automated volume measurement; Bone augmentation; Bone blocks, intraoral bone graft; Bone regeneration.

Fig. 1

Intraoral scan. The process of scanning the pig mandible is shown. On the screen, the already scanned 3D object is visualized with colour information. In the foreground, the surgeon moves the scan head across the pig mandible to capture all necessary aspects

Fig. 2

Weight map of the ICP-algorithm. To achieve the best alignment between the different surface models (one for each augmentation technique), stable region surface models were highlighted. These highlighted areas were then used for the ICP-algorithm. The colours correspond to the relative weight in the ICP-calculation from red (most important) to blue (not used for alignment)

Fig. 3

Aligned scan of full block augmentation. After the ICP alignment, the surface models were overlapping each other. The augmented volume of the full block augmentation (blue transparent) protruded beyond the base scan (green)

Fig. 4

Postprocessing of the intraoral scan. Important steps in postprocessing of the intraoral scan are shown. a On the right side of the object, the full block augmented area can be seen (black). On the left, a loop-select tool (dotted line) is used to select the area of the scan which is not necessary for volume measurement (orange) so it can be deleted. b–d Process of giving the surface model a volume by extruding the border of the 3D model and finally closing the open face is shown

Fig. 5

Selection of the region of interest. a Aligned scan of full-block augmentation as a closed object (blue transparent) and ground truth (green). b Region of interest is defined using a cube object (grey transparent) which includes the augmentation with a safety margin. c Intersection of the region of interest cube from that shown in b and the augmentation surface scan is calculated

Fig. 6

3D model of the augmented bone volume. After subtraction of the base scan the resulting 3D model comprised the augmented bone volume shown in frontal (a) and lateral (b) view

Fig. 7

Absolute values of augmented bone volume measured using each technique. Comparison of the measured volumes of the proposed semi-automatic approach with the manual slice-by-slice technique. Shown are boxplots of the values of the augmented bone volume for the different augmentation techniques (B: block augmentation, S1: split-block augmentation, S2: shell)

Fig. 8

Time needed to measure the volume. Time needed to measure the volume for different augmentation techniques (B: block augmentation, S1: split-block augmentation, S2: shell) and comparison of the semi-automatic approach and the manual slice-by-slice annotation