Radiopaque Tissue Surface-Based Digital Registration Technique for Completely Edentulous Ridge

In computer-guided implant surgery, an accurate 3-dimensional (3D) image matching of the hard and soft tissues obtained by cone-beam computerized tomography (CBCT) and optical surface scanners is a prerequisite for prosthetic treatment planning, implant positioning, and surgical guide fabrication.^1,2  Identical anatomic features that are clearly discernible in the optical and radiographic scan data are used as references to match the acquired images.^3,4  Natural teeth are commonly selected as fiducial landmarks for image matching in partially edentulous cases.³  In completely edentulous cases, because of the absence of natural teeth, special techniques are required to match the optical and radiographic data.^4,5 

The double scan protocol has been considered the most common method for image matching in completely edentulous cases.^6,7  The early developed technique consists of 2 radiographic scans, one of the patient wearing the scan prosthesis and the other of the prosthesis alone.⁸  The implant-planning software converts the radiographic scan files into 3D surface mesh images and merges the 2 scans by matching the radiopaque scan prosthesis so that the prosthesis is visible over the underlying bone structure.⁹  The implant positions are then determined, and the surgical guide template is designed in the computer software by modifying the mesh image of scan prosthesis.²  Recent developments in optical scan technology have allowed the process of image acquisition of the scan prosthesis by computerized tomography and image format conversion to be replaced by optical scanning of the existing or interim prosthesis.⁶  The direct digitization of the prosthesis increases the image quality and facilitates the image-matching process. Although the double scan protocol can be used to integrate the planned restoration with the radiographic anatomic data, the radiographic template covers the mucosa and makes it is impossible to visualize the soft-tissue contour of the edentulous ridge.² 

To integrate the surface scan image of the edentulous ridge into the radiographic scans, a triple scan technique has been proposed.^10,11  In this protocol, the first scan is a CBCT of the patient wearing a radiopaque appliance, and the second and third scans are optical scans of the edentulous soft-tissue ridges with and without the placement of the radiopaque appliance. The 3 scans are then matched to each other using the radiopaque appliance as the reference so that the planned restoration and anatomical information can be displayed simultaneously. The triple scan protocol has been reported to be an effective method for implant surgical guide fabrication¹¹ ; however, as the number of scans and the number of image-matching steps increase, the matching accuracy may be compromised.¹²  Moreover, there could be discrepancies between the acquired image of the edentulous soft-tissue ridge and the actual form encountered during application of the guide template because of the limitations in intraoral scanning or the possible errors in the conventional impression and stone cast fabrication processes.¹³ 

Image matching in the double and triple scan techniques can be achieved only with a fully adapted radiopaque scan appliance,¹⁴  which is commonly fabricated by duplicating existing or interim dentures of the patient using radiopaque materials,¹⁵  or the direct incorporation of radiopaque point-based markers, such as gutta-percha points, ceramic or metallic spheres, metal tubes, or metal strips into the patient's existing or interim dentures.^8,16,17  The fabrication process is labor intensive and requires considerable working time. Voids and inaccuracies may arise during the manual processing and result in inaccurate image matching, improper implant position, or inadequate adaptation of the surgical guide to the soft tissues.¹¹  Moreover, although the radiopaque point markers are widely used for image matching, factors such as the position, distribution, number, shapes, and scattering effects of the markers could negatively affect the accuracy of image matching.¹⁸ 

This article aims to introduce a radiopaque tissue surface-based digital registration technique that allows the appropriate visualization and registration of the edentulous soft-tissue ridge to the radiographic anatomic data. This workflow can streamline the image-matching process and improve the fit accuracy of the implant surgical guide by eliminating the use of extra scan prostheses or markers, as well as the need for additional image acquisition for the soft-tissue ridge.

A 55-year-old male edentulous patient presented with the chief complaint of an ill-fitting complete mandibular denture. Clinical examination and radiographic assessment revealed severe alveolar bone resorption in the posterior region on both sides (Figure 1). Several restorative treatment options, including a new complete denture, implant-retained overdenture, and an implant-supported fixed dental prosthesis, were discussed with the patient. Considering the available bone and the condition of the opposing maxillary complete denture, an implant overdenture retained with stud-type attachments in the anterior area was the preferred prosthesis to restore the edentulous mandible.

A computer-guided implant surgical template was fabricated using the following protocol for restoration-driven implant placement. First, a mandibular recording base and wax occlusal rim was made, and the mandible position in the centric relation was determined by using the bilateral manipulation technique described by Dawson.¹⁹  Simultaneously, the occlusal vertical occlusion was determined considering facial esthetic appearance, swallowing pattern, phonetics, and interocclusal clearance in a resting upright position.²⁰  Thereafter, a full-arch impression of the mandible was acquired using a radiopaque impression material (Permlastic regular, Kerr Corp, Romulus, Mich) and closed-mouth impression technique (Figure 2). The patient was instructed to move the lips, cheeks, and tongue as in the process of denture relining.²¹  A radiographic image of the patient with the radiopaque impression was obtained using a CBCT scanner (PaXFlex3D, Vatech Co, Hwasung, Korea) with a field of view of 120 × 85 mm, voxel size of 0.2 mm, and exposure conditions of 90 kVp, 10 mA, and 24-second pulsed scan. The CBCT data were saved in digital imaging and communications in medicine format. The impression body (Figure 3) and the maxillary denture were digitized using an intraoral scanner (CS 3600, Carestream, Rochester, NY), and the scan files were saved in standard tessellation language (STL) file format. The impression body scan was the negative imprint of the edentulous arch (Figure 4), which was reversed using the “flip normal” function of the computer software (Geomagic Design X, 3D Systems, Rock Hill, SC; Figure 5) to generate a digital model of the edentulous ridge. The generated edentulous model and maxillary denture scan files were exported to the computer design software (Ceramill Mind, AmannGirrbach AG, Koblach, Austria), where a virtual teeth arrangement was performed to visualize the prosthetic plan and determine the position of implant insertion (Figure 6).

All acquired radiographic and mesh data were imported into the implant-planning software (R2GATE 2.0, MegaGen Implant, Daegu, Korea) for fabricating an implant surgical guide template. The digital model of the edentulous ridge was accurately registered to the 3D radiographic image using the radiopaque impression that was a replica of the soft-tissue surface of the edentulous ridge, as a reference (Figures 7 and 8). The whole surface image of the edentulous ridge was used as the fiducial area in the image registration process (Figure 9). The implant positions were then determined based on the anatomic and prosthetic aspects (Figure 10), and an acrylic resin guide template was fabricated using a 3D printing process (Objet Eden 260VS, Stratasys, Eden Prairie, Minn).

The polysulfide-based impression material has been widely used for taking impressions of edentulous jaws because of its high degree of flow and flexibility that allows precise recording of the finest details of the anatomical landmarks.²³  In this study, the polysulfide was used as a relining material during the fabrication of a computer-guided implant surgical template due to its radiopacity. The radiodensity of the polysulfide is similar to that of human enamel at 241.94 Hounsfield units (HU)²⁴  and is markedly different from those of the gingiva (50 HU), acrylic resin (70 HU), and cortical bone (1700 HU).²⁵  Accordingly, the polysulfide layer was discernible in the CBCT image and played the role of a fiducial marker in registering the optical scan of the appliance to the CBCT data. Especially in this study, the fiducial marker was presented in the form of a continuous layer on the edentulous dental arch. The surface-based image-matching method is probably more accurate than the point-based method because it allows the use of numerous fiducial points spread all over the surface of the jaw.^3–5  Moreover, in this study, the matching area was adjacent to the underlying bone that was the surgical site of implant surgery; thus, the substantive accuracy of image matching was expected to be higher than in cases that use additional external markers far from the surgical site.

Ill-fitting implant surgical guide templates lead to instability and unexpected positional deviations of the guide during the drilling process that could reduce the accuracy of the implant placement.^26,27  In this report, the misfit of the fabricated guide was measured at 0.072 mm, which was smaller than the previously reported mean gap value of the 3D-printed implant surgical guides for edentulous jaws ranging from 0.78 mm to 1.90 mm.²⁶  The favorable adaptation of the surgical guide template to the soft-tissue surface allows the direct conversion of a digitized dynamic impression of the soft-tissue surface into the digital edentulous model. In the presented technique, the impression material lining the tissue side of the recording base was directly scanned, and then the mesh image of the impression was spatially reversed to generate the digital model of the edentulous ridge. Because the impression scan image was a replica of the reverse image of the dental arch surface, the digital model of the edentulous ridge could be directly generated by inverting the mesh image of the impression, and possible errors associated with the stone cast fabrication and intraoral scanning could be eliminated. This direct digitization of the impression body has previously been reported to be reliable, and along with the image reversal process, it provided a close fit of the surgical template.^28,29 

With the recent advent of a wide variety of computer-assisted design/computer-assisted manufacturing (CAD-CAM) techniques, digitalization has become increasingly prevalent in the field of implant treatment.^30,31  The oral anatomic shape is registered virtually using an optical scanner and CBCT devices, and the scan data are then used for fabricating the implant surgical guides, milled models, customized abutments, and definitive prostheses.³²  The current optical scanners are reportedly reliable for taking an impression of the intraoral structures; moreover, newly developed 3D face scanners can be used in face analyses to increase the predictability of treatments and optimization of the esthetic results.³³  Once the scans were recorded as mesh data in STL format, the data can be used for diverse purposes in computer software programs. The simulation surgery can be performed in the virtual oral cavity environment of implant-planning software.³⁴  The guide templates and prosthetic components can be designed on the mesh data in the CAD software. The adaptation of the fabricated prostheses and guide templates can be evaluated using inspection software.³⁵  Moreover, the obtained scan data can be used for the 3D finite element analysis in simulation software, where the numerical model of anatomic structures, implant components, and prostheses are computerized to achieve a variety of static and dynamic simulation analysis of the mechanical properties.^36,37  Another benefit associated with the application of digital workflow is the versatility of data storage for scientific research or future clinical modifications.³⁴ 

Limitations of this method include the need of computer software for the image reversal process and the learning time for the digital technique in this workflow. Further in vitro and clinical studies are needed to compare the effects of this protocol with other image registration techniques in terms of accuracy and convenience.

In this article, a virtual edentulous model was digitally generated by inverting the radiopaque impression of the soft-tissue ridge. This radiopaque tissue replica also played as a surface-based fiducial marker that allowed the virtual edentulous model to be accurately registered to the radiographic data. The presented workflow could enhance the accuracy of computer-guided implant surgery by improving the image registration process and increasing the fit of the surgical template.

Abbreviations

3D:

three dimensional

CBCT:

cone-beam computerized tomography

HU:

Hounsfield units

STL:

standard tessellation language

The authors declare that they have no conflict of interest. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2020R1A2C4002518).