CAD/CAM technology for fabricating complete dentures

Dr. Maha Ghotmi
Dr. Danielle El Hakim
Dr. Najib Abou Hamra
Dr. Rita Eid

Abstract

The process for fabricating complete dentures has three major steps: the impression procedures, the denture design and the denture fabrication.

This paper is about the fabrication of a removable complete denture using CAD/CAM technology. It has the potential to simplify the traditional process and resolve the associated problems. CAD/CAM technique is a two-step appointment process. Impressions, jaw relation records, occlusal plane orientation, tooth mold, shade selection and maxillary anterior tooth positioning record are registered in the first appointment. The second appointment is for insertion.

Introduction

Computer Aided Design – Computer Aided Manufacturing (CAD/CAM) technology has already been used significantly in dentistry. CAD software defines the geometry of an object while the CAM software directs the fabrication process^1,2.

In the early 1980s, CAD/CAM was used to produce clinical dental restorations when Andersson envisioned the use of titanium for fabrication of crowns¹. Another important dental application of CAD/CAM technology also occurred in the 1980s. Mörmann developed an interest in tooth-colored restorations. In September of that same year, he placed the first chairside fabricated ceramic restoration with equipment introduced and marked as the CEREC 1 system³. In the recent 20 years, dozens of dental CAD/CAM systems have been presented for inlay, crown, veneer and fixed partial denture^4-6.

However, the development on CAD/CAM for removable complete denture has been very slow⁴. Advanced Manufacturing Technology (AMT) has not been successfully applied in this field. Goodacre et al wrote, “When the CAD/CAM technology for fabricating complete dentures becomes commercially available, it will be possible to scan the denture base morphology and tooth positions recorded with this technique and import those data into a virtual tooth arrangement program where teeth can be articulated and then export the data to a milling device for the fabrication of the complete dentures”. Over years, different methods for duplicating⁷ and fabricating complete dentures using CAD/CAM system have been tried. With the introduction of commercially available CAD/CAM denture systems like AvaDent and Dentca, Inc., the era of digital complete dentures has arrived^2,4,8.

The purpose of this article is to describe the clinical procedure required to fabricate a complete denture using CAD/CAM technology.

Conventional fabrication technique of a removable complete denture

Removable complete denture is a dental prosthesis, which replaces the entire dentition and associated structures of the maxilla and mandible⁹. A complete denture functions to restore aesthetics, mastication and speech. It has various parts and surfaces such as denture base, flange of denture, denture teeth and denture border.

The conventional complete denture technique is a five-step appointment process^2,10

      -    Making preliminary impressions

      -    Making final impressions

      -    Creating jaw relationship records

      -    Arranging prosthetic teeth and try-in

      -    Placement/insertion of complete dentures

Figure1: Dentca and AvaDent starting kits (Courtesy of Dr Tony Daher)

CAD/CAM fabrication technique of a removable complete denture

The advanced fabrication technique is a two-step appointment process²

     -    Impressions, jaw relation records, occlusal plane orientation, tooth mold and   shade selection, and maxillary anterior tooth positioning record

      -    Insertion of dentures

The process for fabricating complete dentures has three major steps: the impression procedures, the denture design and the denture fabrication¹¹.

The impression procedure

The procedure involves the fabrication of a putty cast formed by pressing mixed polyvinyl siloxane putty into the intaglio surface of the patient’s existing dentures. If these dentures are unacceptable then diagnostic casts can be generated from a preliminary impression. And if the denture is not adapted to the mucosa, a refitting or relining can be made^2,10.

It is important to check the adaptation of the thermoplastic trays on the cast and  try them in the patient’s mouth to make the necessary adjustments. It is also important that the maxillary tray extends posteriorly to cover the area of the vibrating line and the pterygomaxillary fissures while the mandibular tray covers the retromolar pads².

After applying an appropriate adhesive and adding tissue stops, a border molding impression material or a medium body polyvinyl siloxane impression material is then used to border mold the maxillary and mandibular trays employing the method used with conventional custom trays. Final impressions of the maxillary and mandibular arches are made using either with a specific impression material or with a light-body polyvinyl siloxane impression material². (Goodacre et Al use speech method to identify muscular and phonetic locations for the prosthetic teeth and to establish palatal morphology; this requires impressions that record the shape of both the intaglio and cameo surfaces of complete denture bases.)¹

Figure 2: Maxillary and mandibular final impressions (Courtesy of Dr Tony Daher)

The AvaDent denture technique uses an Anatomical Measuring Device (AMD) that can be adjusted to the desired occlusal vertical dimension. This AMD maintains this  dimension while centric relation is recorded using the incorporated gothic arch tracing plate and stylus. The AMD is also used to determine the correct amount of upper lip support, the position of the maxillary six anterior teeth, and the desired mediolateral orientation of the occlusal plane. The AvaDent orientation ruler is attached to the maxillary AMD for determining the appropriate occlusal plane. The angle is noted and recorded on the laboratory work authorization form. The midline on the lip support flange as well as the smile line are marked. The size of the maxillary anterior teeth is selected from the three available tooth size templates that matches the patient’s desired tooth size. To serve as a guide during denture fabrication, flowable composite resin is applied to the inside of the selected tooth mold template, then positioned carefully in place and light polymerized to affix the template in position. AvaDent registration material is injected into the space between the maxillary and mandibular arches, with the jaw stabilized in centric relation. (Dentca uses another specific device for these important registrations.)^2,8

Figure 3: Interocclusal records (Courtesy of Dr Tony Daher)

The denture design 

After disinfection, the final impressions and all the registrations are mailed to the company producer of the digital dentures, along with any special instructions. At this stage, the impressions and interocclusal records are scanned, virtual casts are created and articulated, teeth are arranged and bases are virtually formed^1,2.

Figure 4: Virtual casts and teeth arrangement (Courtesy of Dr Tony Daher)

The denture fabrication

The denture base is milled from a block of pink denture base resin with recesses that accurately fit each denture tooth, and the teeth are bonded with a proprietary bonding mechanism. The denture base can be fabricated from different choices of base material, and different options are available for the denture teeth^1,2,10.

Figure 5: Milling and gluing stages (Courtesy of Dr Tony Daher)

Another technique for the denture fabrication exists: the 3-D laser lithography. This rapid prototyping (RP) technology was originally developed to fabricate prototypes for industrial purposes. This method automatically constructs physical models from computerized three-dimensional (3D) data. The RP systems join liquid, powder, or sheet materials to form physical objects. Through Layer by layer technique, RP machines process plastic, paper, ceramic, metal, and composites from thin, horizontal cross sections of a computer model. RP has recently seen successful application in various medical fields, such as in the fabrication of implant surgical guides^12,13, maxillofacial prosthetics^14-16 and frameworks for removable partial dentures^17,18.

After finishing the dentures, it’s time for the second appointment where the insertion of the complete dentures is made. The placement and post-placement adjustments of CAD/CAM complete dentures are similar to the placement of conventional dentures. The patient is seen as needed for routine follow-up and maintenance appointments^1,2.

Advantages and disadvantages of CAD/CAM technology

Several advantages to the patient and the dental practitionar are offered²

    - The clinical chair time is reduced considerably; all clinical data needed are recorded in the same appointment.

     -  The digital data for each case is saved; a spare denture or a radiographic or surgical template can be made rapidly.

      -    Because the digital data are associated with a specific practitioner, patients will return to the same dentist when future treatment is needed.

     -  Because the denture base is fabricated by machining, polymerization shrinkage of the resin is eliminated, and the fit of the denture base is superior to that of conventionally denture bases.

        -    Due to the method of processing the acrylic resin for denture bases at fifty times the conventional processing pressure, there is less porosity, and denture base material may have less C. Albicans adherence.

      

      Fewer disadvantages are present

         -    The artificial teeth and denture base are equipped with different colors and properties. The artificial teeth need high abrasion resistance and an aesthetic appearance. It is difficult to cut the artificial teeth from a single property block. Thus, only the denture base is fabricated by cutting then commercially available artificial teeth are adhered to the denture base. Special adhesives with higher adhesive properties are being developed¹⁰.

         -    Another disadvantage is the missed trial insertion appointment. This step allows making judgments of esthetics and pronunciation and verification of jaw relationship records, including orientation of the occlusal plane, vertical dimension, tongue space, tooth positioning, palatal seal and soft tissue support for proper external form. This is why a third appointment in the advanced fabrication technique could be added¹⁹. AvaDent Advanced Try-In (ATI) uses the final base with teeth waxed on. This technique provides teeth adjustment, relining, VDO modification, full adjustment capabilities. The only problem is the additive cost. Dentca Try-In is a Stereo Lithographic Analog (SLA) of the digitally designed denture that fits like the final denture with a close final contours and the possibility of checking the midline, the insical plane and the lip support. The problem is the frosty clear appearance of teeth that cause difficulties to evaluate shade and esthetics, plus the fact that we cannot move the teeth⁸.

Conclusions

It is now possible to fabricate a complete denture with CAD/CAM technology.

This fabrication has positive benefits for both the patient and the practitioner.

However the final result depends on the skill and knowledge of materials, anatomy, occlusion, function, making excellent impressions, registering the interocclusal record with a special device and determining the proper esthetic parameters.

References

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