The 2-Visit CAD-CAM Implant-Retained Overdenture: A Clinical Report

The acronym computer aided design-computer aided machining (CAD-CAM) is commonly used in prosthodontics to denote computer-aided design-computer-aided manufacturer (or computer assisted machining).¹  It is a subtractive method of manufacturing that uses images from a digital file for creation of an object by machining (cutting/milling) to physically remove material to achieve the desired geometry.²  This is in contrast to the additive method of manufacturing (such as rapid prototyping), where images from a digital file are used for creation of an object by laying down successive layers of a chosen material. The subtractive method of manufacturing is widely popular in fixed prosthodontics for fabricating inlays, onlays, and single crown restorations in a single clinical visit using a chairside CAD-CAM machine.³  It is also popular for fabricating abutments and substructures in the dental laboratory in the field of implant prosthodontics. In contrast, removable prosthodontics has been slower in adopting this technology. There are a few reports in the literature that describe clinical use of CAD-CAM technology for fabricating metal frameworks for removable partial dentures.^4–6  However, hardly any reports exist in the literature that describe clinical use of this technology for complete dentures. This is probably due to the inherent nature of fabricating complete dentures, which includes multiple steps of recording, transferring, testing, and then creating artificial substitutes of teeth and gingiva, all of which are required to be in harmony with the patient's mouth and face.

Historically, computer-aided technology for complete dentures has encompassed additive manufacturing (such as rapid prototyping) as well as subtractive manufacturing (such as CAD-CAM). Maeda et al⁷  are credited with the first published report in English language scientific literature on the concept of using computer-aided technology for complete dentures. They used rapid prototyping technology for fabricating a complete denture from light polymerized composite resin material. The authors acknowledged that although much improvement was needed before clinical applicability, their method provided proof of concept that complete dentures could be fabricated using computer-aided technology. Since this report, multiple researchers have contributed to advancement of this field either using rapid prototyping technology or CAD-CAM technology.^8–14  However, none described clinical use of these complete dentures on patients.^8–14  Therefore, currently there are no clinical reports or trials on complete dentures fabricated from computer-aided technology.^7,8–14  However, recently, 2 manufacturers in the United States have started offering CAD-CAM complete dentures for clinical use (Dentca-CAD/CAM denture, Dentca Inc, http://www.dentca.com; AvaDent Digital Dentures, Global Dental Science LLC, http://www.avadent.com). The clinical and laboratory protocols offered by these 2 manufacturers are exclusive and require proprietary materials and techniques. These protocols are designed to allow fabrication of a complete denture in 2 clinical visits: the first for collection of data and the second for insertion of the denture. The purpose of this clinical report is to describe the use of CAD-CAM technology for fabrication of a mandibular implant-retained overdenture and maxillary complete denture in 2 clinical appointments.

An 83-year-old male presented to the prosthodontist for fabrication of an implant-retained mandibular overdenture. The patient had a medical history of controlled hypertension and gait disturbances and required assistance for travel. He had been edentulous for several years and had 2 implants (Straumann USA, Andover, Mass) placed by the author in the anterior mandibular region 6 weeks before, with the intent of fabricating a mandibular overdenture and a maxillary complete denture. The patient had been advised to continue wearing his existing complete dentures during the healing phase. On clinical exam, the soft tissues in the mandible showed excellent healing after implant surgery, and the maxillary tissues were unremarkable. The implants exhibited no clinical signs of mobility, tissue loss, or infection and were deemed to be successful (Figure 1). Due to the patient's age, treatment expectations and his challenges with travel, he was offered the option of definitive prosthodontic treatment with CAD-CAM maxillary and mandibular dentures using materials and protocols provided by the manufacturer (Avadent Digital Dentures; Global Dental Sciences, Scottsdale, Ariz). The patient consented to this treatment plan, which required only 2 clinical visits for denture fabrication.

During the first clinical appointment, the patient's existing dentures were used to create a “remount cast” using silicone putty material (Avadent). Appropriately sized thermoplastic proprietary trays were then selected and tried in the mouth as well as on the putty casts. The handle of the mandibular tray interfered with the mandibular lip and appeared to obviate appropriate border molding procedures. Therefore, the handle was removed and a customized handle was prepared using acrylic resin material (Figure 2). Thereafter, using a hot water bath at a temperature of 165°F and a butane torch, both maxillary and mandibular trays were adapted and trimmed on the remount casts to generate custom trays. Subsequently, border-molding procedures were accomplished intraorally on maxillary and mandibular arches using the proprietary silicone impression material. Additional material was added as necessary to deficient areas to accurately register anatomic boundaries of the planned dentures. Maxillary and mandibular final impressions were then made using light body silicone impression materials provided by the manufacturer (Figures 3 and 4).

A specially designed proprietary device called the “anatomic measuring device” (AMD) of appropriate size was selected and relined intraorally using silicone occlusal registration material. The relining procedure was done one arch at a time to permit an accurate registration for better stability of the bases and allow future superimposition of the final impressions and the AMD registrations on the computer. The AMD also consisted of a central bearing-tracing device with a plastic pin on the palate and a central plate on the mandibular base. Thus, the AMD served as a surrogate for record bases and occlusal rims to allow registration of occlusal vertical dimension (OVD), centric relation, dental midline, maxillary incisal edge position, maxillary cervical edge position, and the horizontal plane of occlusion. A hex screwdriver provided by the manufacturer was inserted into prefabricated slots to alter OVD and lip support. Using Niswonger's technique,¹⁵  the OVD was determined by measuring 2 points marked on the nose and chin, with the mandible at rest and in occlusion. Additionally, the patient's existing dentures were used a guide to determine the OVD. The centric relation was then obtained at the determined OVD, using an intraoral gothic arch tracing method. A round acrylic bur was used to create a dimple on the lower AMD plate to allow exact seating of the maxillary pin for stability (Figure 5). The centric relation record was not recorded at this stage.

A special device called the “bite plane reader” was inserted into anterior prefabricated slots on the lip support component of the AMD. The bite plane reader had a bilateral protractor to record the horizontal angulation of the occlusal plane. Subsequently, incisal edge position, midline, and the highest smile line were marked on the lip support component using a red lead pencil. A clear plastic template provided by the manufacturer contained the silhouettes of maxillary anterior teeth and was selected to determine the size of the anterior teeth. The maxillary incisal edge position of the template was then aligned with the incisal edge and midline position marked previously on the lip support element. It was then attached to the lip support element, using a flowable form of light polymerized composite resin. The cervical margins of the teeth in relation to the patient's highest smile line were registered. At this stage, silicone bite registration material was injected into the space between the upper and lower AMD plates and centric relation was finally recorded (Figure 6). The mold and shade of the teeth were then selected, and all records were shipped to the manufacturer. A lingualized occlusion scheme was prescribed for this patient.

Once the manufacturer received the information, the impressions and AMD were digitized by use of light-scanning technology to convert physical information obtained clinically, to digital information and facilitate subsequent steps of manufacturing (http://www.avadent.com). The artificial teeth were digitally arranged in lingualized occlusion scheme, using complex algorithms on the manufacturer's proprietary software. A digital preview was then provided by the manufacturer using web-conferencing technology. During the web-conference, the digital teeth arrangement was carefully examined by the author in all three dimensions, and required modifications were discussed with the laboratory technician. All teeth modifications were immediately corrected on the computer, and the occlusion was digitally adjusted and approved by the clinician (Figures 7–9).

Thereafter, the CAD files were uploaded to a CAM device with fine milling tools, and denture bases were precision milled using prepolymerized milling pucks prepared by a patented technology from commercially available denture base materials. The milled denture base had recesses into which the chosen denture teeth were manually bonded using proprietary materials to complete the finished denture. The dentures were then finished, polished, and returned to the clinician.

During the second clinical appointment, the CAD-CAM dentures were inserted in the patient's mouth individually and an excellent fit was confirmed using pressure-indicating paste. The maxillary denture displayed palatal contact and intimate fit at the first insertion, and only minimal adjustments were necessary (Figure 10). The mandibular denture was first relieved over the implant sites, and the intimate adaptation of the base was confirmed using pressure-indicating paste (Figure 11). Thereafter, centric relation was recorded again using silicone material, and a clinical remount procedure was performed to refine the occlusion to the planned lingualized occlusal scheme. Additionally, the maxillary anterior teeth were adjusted and characterized to optimize the esthetics.

The healing abutments on the 2 mandibular implants were removed and Locator abutments (Zest Anchors, Escondido, Calif) of 2 mm cuff height were chosen and torqued onto the implants at 35 Ncm. The corresponding metal housings were then placed on the Locator abutments (Figure 12). Relief holes were then drilled into the mandibular denture at the abutment sites to have a passive path of insertion (Figure 13). Light polymerized composite resin material (Light Cure Material, Zest Anchors) was injected into the relief holes, and the mandibular denture was connected to the overdenture attachment using a direct technique.¹⁶  Thereafter, the nylon patrices were inserted into the metal housings at an appropriate level of retention determined by the patient (Figure 14). The patient was given postoperative instructions for insertion-removal procedures, maintenance of the prosthesis, implants, and surrounding tissues. At future follow-up visits, the occlusion and retention of both dentures remained excellent, requiring minimal adjustments, and the patient remained satisfied with the esthetics, function, and comfort of the CAD-CAM dentures (Figure 15). The patient was extremely appreciative of the minimal number of clinical visits required for completion of his treatment, as well as minimal follow-up visits for post-insertion adjustments (Figure 16).

The conventional method of fabricating complete dentures has a long track record and has proven to be predictable. Furthermore, the materials and protocols have been adopted with improvements dating for almost a century. One of the biggest advantages of the conventional method is the ability to confirm all preceding steps at the esthetic try-in stage (wax trial stage) and customize the artificial teeth arrangement. Additionally, it allows the patient to test the trial denture and offer input. However, some of the disadvantages of the conventional method of fabricating complete dentures are: (1) the need for a minimum of 4–5 patient visits and additional post-insertion visits; (2) decreased margin of profit for clinicians due to increased patient visits, laboratory expenses, and reduced reimbursement; (3) inability to obtain an intimate fit of the denture bases with tissues due to polymerization shrinkage; and (4) inability to easily create an optimal duplicate denture. Some of these disadvantages are significant enough to cause many dentists to evade treating patients requiring complete dentures in the United States.¹⁷ 

Use of CAD-CAM technology for fabricating complete dentures may overcome many of the above disadvantages. Indications for CAD-CAM dentures include individual patient care, education, research, and public health due to reasons such as: (1) continued growth of edentulous patients and increased demand for complete dentures in the United States;¹⁸  (2) increased numbers of aging population in the United States;¹⁸  (3) access to dental care issues around the world; (4) easier implementation in public health programs; (5) shortage of dental laboratory technicians in the United States;^19,20  and (6) shortage of dentists and dental faculty in the United States.²¹  Some of the expected advantages of CAD-CAM dentures are: (1) reduced number of patient visits, which is especially beneficial to elderly patients; (2) superior strength and fit of dentures due to use of prepolymerized acrylic resin blocks for milling; (3) reduced potential for dentures to harbor microorganisms and minimize resultant infections; (4) reduced cost for the patient and the clinician in the long run; (5) easier reproducibility (creation of duplicate dentures) due to stored digital data; (6) improved potential for standardization in clinical research on complete dentures as well as implant-retained overdentures; and (7) better quality control.

In the patient presented in this clinical report, CAD-CAM dentures provided many of the above advantages. However, some of the disadvantages of using CAD-CAM dentures in this case report were: (1) lack of a clinical esthetic try-in procedure; (2) learning curve required to adhere to newer protocols to record OVD, lip support, and maxillary incisal edge position; (3) lack of ability to determine the mandibular occlusal plane; and (4) minimal opportunity for patient input. Though this patient was extremely satisfied with esthetics, occlusion, and comfort of the dentures, further improvements are needed to address the above issues. Additionally, the artistic components of fabricating complete dentures—such as individual tooth rotations, characterizations, and festooning the gingiva to eliminate acrylic resin between denture teeth—are necessary to optimize the clinical outcome. Until further improvements occur, these considerations should be taken into account when selecting a patient for CAD-CAM dentures. Appropriate case selection can thus preclude expensive remakes and a dissatisfied patient.

Alternative methods considered to utilize CAD-CAM overdentures for this patient were: (1) immediate loading of the implants by having the CAD CAM dentures made before the implant surgery and (2) torquing the Locator abutments before making the final impression and simply attaching the CAD-CAM overdenture at the second clinical visit. Though each of these alternative treatments could provide unique advantages and disadvantages, the author did not utilize them in order to improve treatment predictability utilizing a new technology and minimize the potential for errors and remake of the prostheses, which may have resulted in additional time and expense. For the same reason, a clinical remount procedure was also performed in this patient to optimize the lingualized occlusion scheme before attachment to the implant abutments. A slight discrepancy was also noted in the maxillary and mandibular midlines of the dentures compared to the digital preview, probably due to minor movement of the AMD while making or transferring the records. As the final occlusion and esthetic goals were satisfied, this was not an issue for patient or the clinician. Future clinical experiences and clinical research can help determine the best method of utilizing CAD-CAM technology for implant-retained overdentures.

This clinical report described the successful use of CAD-CAM technology for fabrication of a maxillary complete denture and a mandibular implant retained overdenture in 2 clinical appointments. Mandibular implant-retained overdentures have proved their clinical and functional superiority over conventional dentures. By utilizing CAD-CAM technology for fabrication of the mandibular implant-retained overdenture, it was possible to complete this treatment in only 2 clinical visits. This method could potentially become the routine method of care for complete denture and overdenture therapy in the future. The ability to manufacture complete dentures using computer-aided technology has innumerable clinical possibilities for the future.

Abbreviations

AMD

anatomic measuring device

CAD-CAM

computer aided design-computer aided machining

OVD

occlusal vertical dimension