Digital Workflows in Implant Dentistry

2

Surface Scans

C. Evans

2.1

Introduction

Fig 1 The mobility of the peri-implant tissues, vestibular mucosa, and frenal attachments may complicate the exact duplication of implants and related structures.

When undertaking dental implant procedures, an accurate duplication of the teeth/implants and surrounding tissues is required for both treatment planning and to enable fabrication of the prosthesis. Historically, such duplicates have taken the form of a physical stone model or working cast, which is produced from an impression of the oral cavity. Exact duplication of the structures in the oral cavity is complicated by factors such as multiple undercut surfaces due to variations in tooth morphology and axial inclination, the presence of fixed and movable soft tissues, frenal attachments, and the underlying muscles (Fig 1). The mouth is also an inherently moist environment due to the presence of saliva and crevicular fluid, which can compromise the accurate capture of shapes and contours without distortion. Inaccuracies can also arise from different properties of impression materials and issues relating to tray construction and rigidity, or patient compliance and movements.

Conventional impression materials are often hydrophilic to accommodate moisture and elastomeric to allow reversible deformation on removal from the mouth. The desired extent of the surface to be captured is determined by the type of prosthesis planned. For removable prostheses, a full-border extension of the impression will be necessary to avoid overextension of the prosthesis into the moveable tissues.

Impressions are then poured in type 3 or type 4 dental stone to provide a physical model. Inaccuracies can also occur in model production. When prosthetic reconstructions are made on immobile structures such as dental implants, inaccuracies in the dental cast as a consequence of the above-mentioned factors can result in an incorrect fit of the prosthetic framework. This in turn will result in delays, additional costs, frustration for the dentist, and patient dissatisfaction.

The introduction of the computer-aided design/computer-aided manufacture (CAD/CAM) concept to replace conventional impression/model techniques was first presented by François Duret in his thesis presented at the Université Claude Bernard, Faculté d'Odontologie, in Lyon, France in 1973, entitled "Empreinte Optique" (Optical Impression). Duret was able to complete intraoral scans using two cameras, two lasers, and a fiberoptic feed to enable the information to be transmitted to a large dental laboratory who could then manufacture a CAD/CAM restoration. This technology was subsequently refined by Werner Mörmann and Marco Brandestini in the 1980s at the University of Zürich for use in restorative dentistry and became commercially available as a CAD/CAM system for dental restorations in 1987 (Cerec; Dentsply Sirona, Bensheim, Germany). This was the first optical non-contact direct intraoral scanning system.

With the introduction of CAD/CAM in dental prosthetics, the first step in the workflow is acquiring a digital representation of the oral cavity. Digitization of the important structures by means of surface scans is considered a more straightforward technique than conventional impressions and may show less variability (Figs 2 and 3).

Fig 2 Clinical case with advanced gingival recession.

Fig 3 Surface scan of the clinical case in Fig 2.

2.2

Analog Impressions

Conventional impressions have been used for many years to capture the position of dental implants. They require an impression post to be placed onto the dental implant and a viscous impression material to set in the patient's mouth. A very high degree of dimensional accuracy is required for these materials to accurately duplicate the positions of the implants (Fig 4), and such materials have not been without limitations (Hamalian and coworkers 2011).

2.2.1Material Accuracy

Traditionally, different types of impression material may be selected depending on the required level of accuracy for the intended dental procedure (Hamalian and coworkers 2011). The accuracy of impression materials may be affected by:

Storage conditions

Temperature

Errors in mixing dosage and time

Tray rigidity and positioning in the mouth

Clinical technique

Patient movement

Setting time

Continued chemical reaction after initial setting

Accurate surface detail is essential to avoid occlusal inaccuracies when positioning the antagonist model. Injectable low-viscosity materials are first flowed over surfaces to reduce the risk of air voids, and a heavier viscosity material is then placed in an impression tray, which supports and slightly displaces the material completely around the target structure. The time for setting will vary depending on the nature of the material. Voids or air bubbles within the impression may further reduce the accuracy of the impression.

2.2.2Patient Comfort

Impression materials frequently require setting times in excess of four minutes. While many patients can tolerate conventional impression techniques, some patients find the procedure unpleasant, reporting a gagging feeling, excess saliva production, TMJ pain from prolonged opening, restricted access for appropriately fitting trays sizes, breathing difficulties, or an unpleasant taste.

Fig 4 Conventional impression material being flowed around impression posts.

Fig 5 Impression.

2.2.3Cast Production

When removed from the mouth, the impression material produces a "negative" of the relevant anatomy and requires a suitable dental stone to be poured in order to form a replica of the oral structures. Following removal from the patients' mouth, an appropriately matched laboratory analog is connected to the impression coping within the dental impression (Fig 5). Usually, a removable silicone material will first be placed around the implant analog to replicate the peri-implant soft tissue, and the gypsum stone is subsequently poured (Figs 6 and 7). There is a delay involved in releasing the model, as dental stone requires time for setting. The model itself is prone to dimensional errors caused by factors including:

The mixing ratio of the dental stone

Handling by the dental technician

Surface abrasion and damage such as chipping and cracking

Additionally, bubble formation can result in poor contact-point accuracy and occlusal errors (Fig 8)

(Buzayan and coworkers 2013; Holst and coworkers 2007).

Fig 6 Stone cast with implant analogs in position, gingival mask in place.

Fig 7 Stone cast with implant analogs in position, gingival mask removed.

Fig 8 Stone cast showing bubbles and dragging, abrasion of the contact points, and residual plaster from articulation, all of which degrade the quality of the model.

2.3

Digital "Impressions"-Digitization of the Oral Cavity

Conventional impression techniques capture the impression coping connected to the dental implant. The production of a CAD/CAM dental prosthesis first requires the digitization of the relevant intraoral structures. A digital or "virtual" working model can then be used for the computer-aided processes.

When scanning dental implants, a geometric object of known dimensions called a scanbody (Fig 10) is connected to the dental implant instead of the conventional impression coping. The scanbody is usually constructed from PEEK material and has dimensions that can be recognized by the CAD software. A surface scan of the clinical situation is then obtained with specialized hardware, producing a digital file that can be imported into software packages for CAD/CAM.

2.3.1File Formats

The standard file format of intraoral scanners is the STL file (Surface Tessellated Language). This file describes the surface geometry of three-dimensional objects by triangulation in binary code. The STL file format was created in 1987 by 3D Systems (Rock Hill, SC, USA) when they first developed the process of stereolithography (Wong and Hernandez 2012; Joda and coworkers 2017).

Fig 9 Following translation of the XYZ cloud points to a mesh of 3D triangles, the final contour is represented. Note the discrete appearance of meshed triangular geometry.

Digitization of the oral cavity creates a "point cloud." This is a set of data points in a three-dimensional coordinate system, usually X- Y-, and Z-coordinates, intended to represent the external surface of an object. Point clouds are usually polygon or triangle mesh models converted through a process commonly referred to as surface reconstruction to form the STL file (Fig 9).

The STL file creation links the continuous geometry of small triangles together to form the intended shape. This process can be inaccurate if the size of the mesh triangles is too large to fit the contour of the desired shape; in this case, information will be lost. Smaller...