Optical impressions in odontology
Introduction to optical impressions
Optical impressions first appeared in the 1980s. They have now replaced conventional impressions, which were affected by various clear and recognised limitations, including stretching of impression materials, tearing, air bubbles that compromised the quality of these impressions and timeframes, as well as all processing phases required from the moulding stage, which considerably increased the number of adjustments in the mouth, and the risk of the final prosthesis failing.
Since their invention, intra-oral scanners and cameras have undergone numerous developments and are now an integral part of dental surgeries, their use indicated in all practices as well as in various odontology fields.
Definition
Optical impressions are defined as being a projected light signal onto dental surfaces. The reflected signal will be recaptured by the camera, allowing it to create the 3D model.
Optical impressions are the first link in the dental digital chain
Using optical impressions is an initial stage in data acquisition, followed by a phase in which this data is processed.
This can be done either by:
Scanning the model in the laboratory using an extraoral scanner or tabletop scanner (with or without contact), which scans a plaster model from an impression or straight from a traditional physical impression.
An intra-oral camera placed directly in the mouth.
Various types of optical impressions and acquisition techniques
1-Extraoral acquisition
This is an automated indirect technique used to control the positioning, distance, speed and accessibility of undercuts.
2-Intraoral acquisition
This is a direct impression technique. It involves manual operation, so is operator-dependent, which makes it impossible to accurately control the distance, speed and accessibility of undercuts.
3D imaging using the intraoral scanner can be done either by recording successive shots, the main disadvantage of which is the acquisition speed, or by using dynamic mode (film type). In this case, acquisition will be much quicker (around 5 mins to scan both arches and occlusion).
Digital processing of the optical impression:
The file produced by dental surface acquisitions constitutes the entry point for the dental CAD/CAM chain.This comprises a representation of the mould or the patient’s arch in the form of a point cloud. This point cloud is not actually useable, and requires processing by dedicated software to optimise it by resetting, repositioning and merging the overlaid points, cleaning and filtering of outliers, and standardising densities, so as to reduce the file size and accelerate the processing of virtual data.
Files obtained when exiting the acquisition link
These files can be in a proprietary format, which can only be read by design software of the same brand or a universal one. But the trend is currently towards opening up systems; universal files can be read by all design software.
Universal files are STL (Standard Tesselation Language or Standard Triangle Language) and PLY (Polygonal File Format).
The STL universal file defines a triangular mesh that will create a 3D object in black and white and which does not include colour or texture.
The PLY file consists of a collection of polygons that can transfer information such as transparency and colour. But not all software is yet able to support this file format.
Characteristics of intraoral cameras
There are several parameters to consider when choosing an intraoral camera. The technology selected determines access to certain settings (e.g. no powder, colour, continuous recording). The main characteristics of intra-oral cameras are listed below.
Ergonomics of the handpiece
Ergonomics include the size and shape of the camera, its weight, its handling, the necessary height at the tip, and the size of the head.
It also includes how well the camera balances, which is key in ensuring optimal stability and handling during the acquisition process.
A wide head means a larger acquisition field, but this can make it harder to access certain areas, especially those at the back.
The focal length is defined as the distance between the camera tip and the surfaces to record. Some brands require contact with the teeth, while others recommend a distance of over 10mm, which makes optical impression tricky when it comes to the posterior aspect, especially for disto-vestibular areas.
The most common and familiar way to operate it is as a “pen”, but there are also “gun” type cameras available.
Wireless camera versions have recently been developed that obviate the need for a cable, with all its constraints, and deliver greater freedom of movement.
Powder
Ergonomics include the size and shape of the camera, its weight, its handling, the necessary height at the tip, and the size of the head.
It also includes how well the camera balances, which is key in ensuring optimal stability and handling during the acquisition process.
A wide head means a larger acquisition field, but this can make it harder to access certain areas, especially those at the back.
The focal length is defined as the distance between the camera tip and the surfaces to record. Some brands require contact with the teeth, while others recommend a distance of over 10mm, which makes optical impression tricky when it comes to the posterior aspect, especially for disto-vestibular areas.
The most common and familiar way to operate it is as a “pen”, but there are also “gun” type cameras available.
Wireless camera versions have recently been developed that obviate the need for a cable, with all its constraints, and deliver greater freedom of movement.