-D- ARTICULATORS: FROM MECHANICAL TO VIRTUAL
Fig. D22: A CAD/CAM system consists of three parts. A first unit realizes the 3D optical impression. A second unit performs digital processing of the resulting impressions. It is associated with a virtual articulator and a prosthetic design software. A third unit is responsible for carrying prosthetic.
Fig. D23a: The objective of the mechanical articulator was to be a simulator of mandibular movements. to achieve this goal, its mechanical characteristics have been exactly copied on the characteristics of the components of the masticatory tract. The recording and the postponement of a number of clinical reference parameters was supposed to help to set the articulator so that it becomes an external clone of the masticatory apparatus and working identically. But the reality is quite different.
Fig. D23b : The imagination of articulator designers, has been almost without limits, but without changing the functioning principle.
Summary: Regarding static occlusion, mechanical articulators are reliable and often indispensable. In this context, mounting on articulator may be useful or necessary for occlusal analysis, to choice the vertical dimension, to determine the intermaxillary relationship, the MIO setting and prosthetic achievements.
But as regards the dynamic movements, the conventional articulators, settled in the gnathological model are unable to simulate the closing of the posterior teeth up to touch, during mastication and especially during dental cycles-in. The adaptable articulators tend to improve this simulation, but the final balancing should always be checked and finalized in the patient’s mouth.
The optical impressions associated to computer-aided design and computer-aided manufacturing (CAD/CAM), can they do better?
The Mechanical articulators
The recording and reproduction of the chewing cycles outside to the patient’s mouth is a challenge that has not been satisfactorily resolved by conventional techniques. The mechanical articulators are the most used means for the external simulation mandibular kinetics. They are broadly used for occlusal analysis and for extended or medium prosthetic implementation.
But under the same name of articulator are grouped miscellanous devices with very varied design, ranging from the hinge occludator, to the fully adjustable articulator. During the analysis and static reconstruction of the occlusion, mechanical articulators are reliable and accurate instruments (Inter-Maxillary relationship, Vertical Dimension and MIO). The simplest occludators are able to reproduce the static occlusal relationships and show vestibular occlusal and lingual reports when occlusal analysis.
But during dynamic movements, their design limits their kinetic possibilities only to laterality movements and protrusion. Their ability to simulate, optimally, the mandibular kinetics of mastication is very uneven. Moreover, the way they are programmed and used effectively, can enhance or distort the reproductive capacity. Indeed, these articulators are typically pre-set or set to arbitrary or average values. And in this case, their ability to reproduce the kinetics of a particular patient can be only a coincidence, given the significant variability affecting the dental anatomical shapes, and values of TMJ settings.
Fig. D24: The articulator joint cases are not compressible, contrarily the human TMJ during chewing cycles. they are therefore unable to to simulate the gradual closing of the posterior teeth up to touch, which don’t allows the balancing of their chewing guidances.
When the semi-adaptable and fully adaptable articulators, are actually programmed by axiography, or the use of “chekbite” intraoral, it’s by the recording of particular positions on the displacement of lateral and protrusive motions, requested to patients The articulators programmed according to these traditional techniques are able to reproduce the movements of protrusion and laterality (but are of opposite direction of mastication, with a different muscle recruitment). What makes them unable to simulate all the finesse of dental phases of mastication cycles because the cusps sectors are not in contact (Fig. 24). Therefore, it is impossible to make sliding the opposite posterior teeth on their functional facets. These functional guides and rails, on posterior teeth, lead the chewing cycles, and are readily observable on models. The posterior occlusal analysis becomes dubious, like the balancing of the prosthetic restorations ,with an increasing risk of forgetting under-guidances or over-guidances on the posterior restorations, because they can’t be checked on the articulator (the prosthetists frequently dismantle the models from articulators, in order to manipulate them, to try to compensate for this deficiency).
Fig. D25 a,b: Some adaptable articulators allow the closing of the posterior teeth up to touch, but not always in the right position., On the presented case, the mandible is too far back and prevents the guide rail, starting from the upper disto-buccal cusp, to work normally, because it is not in place between the central and disto-buccal cusp of the mandible M1.
Programming techniques or modifications in the design of articulators and/or TM boxes have been developed to allow a better simulation of chewing movements. These techniques, well known today developed in the book “The Occlusal function” (Ed. CDP, in French). But they are relatively difficult to implement and require meticulous protocol and excellent coordination with the laboratory.
In practice many errors are found during manipulations:
- When taking impressions: It is often observed secondarily on models: raffle zones, deformation, the presence of bubbles …
- if the impression trays are not well adapted or ill-prepared and if the impression technique approximative.
- If the impressios (alginate) are not poured quickly, the risk of shrinkage is high.
- When recording on the patient clinical parameters setting. Error records from the intermaxillary relationship is also significant.
- When mounting on articulator: the expansion of the plaster may slightly change the position of the models and thus the occlusion reports.
- When setting articulators in the laboratory … the list is not exhaustive.
These errors are the common lot of clinical practice and distort the sequence of successive steps accuracy. The errors recording and reproduction can only lead to approximate restorations.
So it is always necessary to do a check at the prosthetic insertion, first by adjusting the inter-maxilla and static occlusion relationship and then simulating the chewing on articulators with procedures, still unknown today. This has almost always resulted to compel to sometimes significant alterations, addition and / or subtraction, which is the opposite of the objective.
One of the decisive advantages of optical impressions and of 3D modeling is to remove much of these errors, to get faithful models that will must then allow the realization by CADCAM (machining or 3D printer) of much more accurate restorations. A provided that the virtual articulators would have been adapted to the reproduction of mastication cycles.
The present and the future belongs to these digital techniques. That is why they will be favored in the next chapter, at the expense of conventional articulators.
The optical impression, the virtual articulator and CAD/CAM (computer-aided design and computer-aided manufacturing)
General principles of 3D modeling
Fig. D26a, D26b: The recording a high number of points allow to define a virtual frame representing the anatomical envelope of the teeth, whose deformation precise and coordinated is easy to adapt to the opposing teeth and their functional kinetics..
The basic principle of 3D modeling applied to a tooth is to record the coordinates of a maximum of regularly spaced reference points on the surface of the volume to be recorded. This recording is generally performed by a digital video camera, recording in the visual spectrum or other. These points connected together, form a 3D frame supporting a sort of digital skin, masking its existence. More the number of registered points is large, the better the recorded volume is defined. The deformation of the frame, allows to modify the anatomy of the tooth, in a coordinated and progressive manner. This recording can be extended to its adjacent teeth, to the hemi-arcade, to the full arcade and the antagonists, and their general anatomical references, used to align and link dental units between them. These are:
– The lines of largest contour of the teeth, buccal and palatal (or lingual)
– The ridge lines, that delimit the occlusal table through the tips and furrows of the buccal cusps and palatal (or lingual)
– The mesiodistal line connecting the bottom of the grooves, of the occlusal surface.
– And the lines of crests and bottom of the grooves, of all of the cusps slopes (buccal, occlusal and palatal). All connected to the previous lines.
Simplifying, the operation of all these parameters allows to deform the teeth either individually or collectively, to fit the individual anatomy of a tooth to rebuild to those of neighboring teeth and antagonists, either individually or collectively to adapt the anatomy of more teeth to be reconstructed to the general shape of the patient’s arcades, and antagonists and neighboring teeth. Obviously this applies especially to occlusal anatomy, during the static step, and then dynamic.
The morphology of the reference teeth was recorded from a model of anatomy considered as an optimal one (the teeth of the Louvre), the variability of different types of tooth morphology is stored in a scalable and adaptive anatomical database, which is regularly enriched with new informations. Where possible, the comparison of the anatomy of the patient’s residual teeth with the memorized models, will allow the database to propose for the missing teeth rebuild, the stored form that best suits the type of teeth still present. In the absence of teeth, the patient’s morphologic type, could also be an interesting reference .
However depending on the case and customization of dental wear and of TMJ , the data proposed by the bank must be adapted. Considering that the functional relationship of guiding facets of mastication, must be in harmony with the TMJ kinetics to allow to the cycles to regain their optimal amplitude. This point, to my knowledge, has not yet been developed. What will be the share of the numerical simulation, in a first step, and the remaining share to do in the mouth? We do not know yet.
The optical impression
The optical impression is extremely accurate and reliable. It result in, a virtual three-dimensional model exact that can be manipulated and oriented in any position. This type of footprint eliminates virtually all the risks of error, associated with the itraditional impression, to the casting of models and the mounting on mechanical articulator.
Several optical cameras models are currently available on the European market.
Their presentation and their weights are different
The latests one’s not require any surface powdering processing before scanning
Some cameras are part of more or less closed complete systems, sold with their machining device, like the one shown Figures 29 and 30 (Sirona® Cerec®). Others systems are open. In this case, the cameras are sold only with their software and use “languages” industrial, as STL or PY to export their data, which can be read by all machining devices, not closed. The camera Condor® Professor Duret, or 3shape®, fall into this category. The levels of performance of these cameras are also quite different. Their development is extremely fast.
Note: During demonstrations or during the practical work, the optical impressions taken are often made on plaster casts. Given the high number of errors observed on the models from conventional impressions, when compared to direct optical impressions, this technique should not be applied to the production of restorations (semi-direct technical). The optical impressions must be done in the oral cavity.
Fig. D27: On left: camera Cerec Omnicam® and 3M True definition scanner®, followed by 2 pictures from the Condor® digital sensor, and then the 3shape®
Fig. D28a, Fig. D28b
Fig. D28a,b: On left, 3D picture obtained with the Cerec Omnicam®, on right, 3D picture obtained with Condor® (Image distortion is obviously not due to the Condor sensor).
Fig. D28c: Fig. D28c,d: Left sectoral optical impression, from 3shape sensor. Right, full maxillary arch (camera Condor). All these virtual models can be oriented on the screen, in all planes of space. Color reproduction by the models obtained from Condor® (28b, d) and 3shape® (28c) appears better than the Cerec® (28a).
Parameters of the Virtual Articulator. Incidence on Occlusal Faces
By cons, virtualization makes it easy to change their functioningt model. We will analyze the parameters to be considered to enable them to simulate chewing function and how to improve their operating model to get there.
The possibilities and settings of the virtual articulators, as I could see, are copied to those of conventional articulators. By cons, virtualization makes it easy to change their functioning model. We will analyze these parameters to investigate whether manufacturers have sought to improve and how the possibilities of their virtual articulator, to allow them to simulate chewing function between the posterior teeth.
Several parameters can be used:
– The full kinetic and shape of the cycle:
It seems that it is not yet possible. However even if it were possible, in a mouth to rebuild where, the occlusal anatomy is lost, the shape of the cycle also and these remaining cycles, of the patient, present an adaptive kinetic, often reduced to a simple shear. In these circumstances, would it be possible, from a pathological or adaptive cycle, to succeed in the rebuilding of an optimal functional occlusal anatomy? The answer is no. Currently, one must first rebuild the occlusal anatomy to residual teeth or temporary restorations so that the cycle can regain its optimal shape and become a reference.
– Occlusal anatomy: When the cycle is physiological and that the anatomy of the neighboring teeth is optimal, the solution is simple. This is the kinetic chewing of these teeth, which will allow to find the lost anatomy to reconstruct the virtual impressions, provided to simulate the kinetics and the approximation of the posterior teeth to the contact, while chewing. CADCAM is able to master this situation.
By cons when the occlusal anatomy of neighboring teeth is unbalanced or lost, the functional anatomy of these teeth must be first reestablished, to restore optimal cycles. Then they will become a model to reconstruct the occlusal surface of the tooth to replace.
When occlusal anatomy is lost, the form of the cycle is sometimes reduced to a simple shear. It is however easy to find again his optimal form:
– From joint shape memory on the same side, which is often the only parameter to recover and rebuild addition, the occlusal anatomy of the first molars and following, in coordination with joint kinetics. Recall that this kinetic imposed its shape to the joint during growth and the establishment of the chewing adult, on the pairs of first molars children.
– If the molar anatomy and contralateral are optimal cycle, digitally, it is possible to reverse, to apply them to deficient side. But this is subject to reservations, since the joint anatomy and the rings are generally not perfectly symmetrical.
How to record and reproduce?
In all cases, these parameters are specific to each patient and lead to individualized well occlusal surfaces. They can not be based on average values generally have no meaning for a particular patient.
If the recording of a full cycle for the occlusal reconstruction is not yet possible, the accurate recording of occlusal surfaces is wholly controlled and is the basis for their reproduction.
If the functional occlusal anatomy, of the tooth to be crowned, is restored on a temporary crown, an optical impression of this occlusal surface may be taken and quickly reproduced, by CADCAM (but not yet ceramic on a 3D printer)
The partial or total optical impression of the arcade and its antagonist will be done in the mouth for optimum accuracy.
The case illustrated a single crown. When the shape of the tooth to rebuild is enabled, the virtual models are brought into occlusion with the screen. The intermaxillary relationship is recorded by an optical impression of the buccal occlusion reports, using the camera.
The data is then transmitted either to the processing machine which will carry the crown on site or in the lab, internet transmission. The achievement can thus be placed in the same meeting that the impression when the machine tool is at the office.
The crown is machined from a ceramic block whose color is uniform, but there are blocks degraded hue. The actual machining time of a tooth is about 24mn. this machine is limited to 3 connected teeth. It is possible to machine much larger blocks on laboratory processing machines. The finish and the make-up must be carried out subsequently.
Fig. D29: During Workshops, the impressions are taken on models (but normally in mouth) with a camera Cerec Omnicam (a), the occlusion ratios are displayed (c) the preparation margin is drawn and validated (d). The shape of the crown is proposed. The location and intensity of the contact points is selected (e, f). Then the form is validated.
Fig. D29 bis: The software allows to distort and redirect the frame, to establish MIO and coordinate with neighboring guideways, which must be balanced in advance (Functional FGP)
Fig. D30: The 3shape® camera, that I could use, is a weight slightly above the Cerec® camera (it could be the main drawback of these two cameras). Imprecise areas that need to be “patched” are shown in green (c, d, e). The quality of the impression and color management obtained are very nice.
The optical impression is taken by slowly moving the camera on the occlusal and buccal and lingual or palatal sides. If the information transmitted to the computers are inadequate and leave appear unrecorded areas, simply board the camera on those areas that will be patched automatically by the software. Once completed the lateral sector model, it can be extended if necessary to the anterior and the contralateral side, that are then assembled to obtain the full arcade. The opposite sector is then carried away. Once digitized 3D models can be oriented in any position, including to observe the occlusal relationships palatally.
The software then analyzes the preparation indicates parallelism errors and any occlusal alterations, needed for the prosthetic volume. Then a shape for the crown is suggested by the software. The position and intensity of the points is selected on the screen and changed if necessary. Mandibular virtual model can be moved in opposition with the maxilla. The virtual articulator has been improved, compared to the conventional articulator, because closing up contacts between the posterior teeth are possible, during dynamic movements. It is therefore possible to simulate and solve the chewing cycle-out guidances, but with a reservation for cycle-in. We manipulated and observed that, during cycle-in, the limited adjustment movement Bennet does not allow a sufficient backward position to allow the mandibular first molar to enter the rail of its disto-maxilla buccal cusp. This software problem should be resolvable without difficulty, by the Cerec® engineers.
When the shape of the crown is enabled on the screen, the virtual impressions are set occlusion and the intermaxillary relationship is recorded by an optical impression of the occlusal relationship with the camera which is progressively moved on the labial surface. The right choice of MIO is probably the most delicate phase according to Pr François Duret, which is the inventor of CADCAM, in 1976.
Fig. D31: The ceramic block of the retaining hue is fixed on the machine tool. The manufacture is then launched. It lasts just over 20 minutes.
Fig. D32: The camera 3shape® (a) can be connected to any industrial machine tool open (f), reading the “languages” industrial, as STL or PY, or others. (d) The occlusal adjustment shows consistent chewing guides, except perhaps the orientation of dental cycle-in, that does not seem to be enough posterior (as the CEREC®). (c)The impression of an implant abutment, before achieving its unitary prosthesis in a ceramic block (e), machine tools, Mes-Core (f).
-a- -b- -c- -d- -e-
D33: (a) 3M blocks to be machined are available in composite Lava®, (b) There is a choice of large blocks in CEREC®, (c) one 3shape® block. (d) Then a Coltene Brilliant Crios®. (e) A machined crown on a CEREC milling machine, the end of production.
The digital data are then transferred to the machine tool, whether it is on-site or sent by internet to the laboratory.
The crown is then milled in a ceramic block of uniform color, or degraded hue. On the fastest machine of the four we had, machining a tooth lasts approximately 24mn. This milling machine can make up to three contiguous elements (Fig. D33b).
Fig. D34: An additional feature of the 3shape® camera, is the possibility of taking the hue. The areas shown in blue, are those or shade-taking can not be done, such as having too bright reflections, lack of brightness, or hue of inconsistency…
Studies independent (of manufacturers), comparing the physical properties of materials available in CAD/CAM, begin to be published (Stawarczyk et al 2015-2016):
Fig. D35: CADCAM: QUALITIES OF THE OPTIMAL MATERIAL – High wear resistance and abrasion on antagonist, low and coordinated – Tooth-like modulus of elasticity, to allow a balanced dispersal of mechanical stresses – High flexural strength to improve the solidity of restorations – Possibility of oral modification and repair – Effortless oral polishing – No firing process to facilitate the process and reparations. – High Sustainability of aesthetics and translucency, no discoloration – Reliable Luting System
Fig. D36: Data for some manufacturers may agree or different, independent studies.
Fig. D37: Clinical protocols of CAD-CAM, during a workshop done on models
When the addition of ceramic will be controlled on 3D printers (Fig D35), which is not yet the case, another step will be taken, unless the solution does come 3D printers already mastered the addition of metal, and that could add ceramic composites where composite of type Lava® 3M or Coltene Brilliant®.
On the other hand, the fact that the design is made on articulators unfit for the reproduction of chewing poses a real problem. Independently masticatory movement recording devices such as Modjaw (Fenlec and Jaisson) represent a much more coherent solution. (see the online book “Canine 60 years after Amico myth or reality” direct download link: https://univoak.eu/islandora/object/islandora:71859/datastream/PDF/download/citation.pdf)
- Bohner et al. IADS 2015
- Stawarczyk B 1, Liebermann A, Eichberger M, Güth JF. Evaluation of mechanical and optical behavior of current esthetic dental restorative CAD/CAM composites J Mech Behav Biomed Mater 55, 1-11 (2015- 2016) 1Department of Prosthodontics, Dental School, Ludwig-Maximilians-University Munich, Goethestrasse 70, 80336 Munich, Germany. Electronic address: firstname.lastname@example.org.
- Fenlec S. et Jaisson M. “Comprendre la CFAO 4D“ Info Dent. Fr. 2018 n°3, 24 Janv. 2018: p 3-8