Key points

Removable partial dentures (RPDs) are used to restore patients’ oral function and maintain health. There is an increase in demand for comfortable, high-quality RPDs with the increase in an aging population. In a 2000 study, 76% of the patients surveyed stated lack of fit as a common source of dissatisfaction with a prosthesis. Only a limited number of studies have attempted to evaluate the fit of RPD frameworks. Perhaps this is because of the difficulty in evaluating the complexity of RPD framework designs.

Standards of care have been published related to principles, concepts, and practices in prosthodontics. These standards describe 8 categories for assessment: stress distribution, force control, base extension, occlusal contact, base support, rest seat form, framework fit, and retention.

In a study by Frank and colleagues, fit of the framework was assessed by visual and tactile examination using a mirror and explorer. Fit was rated good if all rest seats appeared to be seated, all rigid elements touched the teeth, and the major connector did not impinge on the underlying soft tissue or had visible relief space greater than 1 mm. Silicone material was placed beneath the framework to confirm displacement of the soft tissue when impingement was observed. Although 52% of the mandibular distal extension RPDs made in community practices failed to meet more than half of the 8 standards, researchers found that 32% of the frameworks evaluated had a poor fit.

Stern and colleagues evaluated rest seat adaptation. They attempted to quantify fit between castings and tooth structure by using polyether impression material samples, which represented the space between rest seat and rests collected from clinically acceptable RPDs. The polyether material was imbedded in a special medium to create samples that were measured in 4 zones: marginal ridge, the center, and the lingual and buccal contact zones.

A total of 79% of the 47 occlusal rests evaluated demonstrated at least 1 point where the distance between the RPD rest and the rest seat was 50 μm or less. Twenty-one percent of the rests did not contact at any point. Closer adaptation of the metal framework was found to exist on the marginal ridge zone of occlusal rests than the other zones for all types of RPDs. Mandibular Kennedy class I and II RPDs were significantly closer in average overall fit than mandibular Kennedy class III and IV RPDs. The difference was not identified between maxillary class I and II and maxillary class III and IV RPDs.

Twenty years later, the fit of conventionally fabricated RPD frameworks is still a problem. In a 2006 study, Dunham and colleagues evaluated the fit of RPDs using a methodology similar to the 1986 study by Stern and colleagues. When evaluating the fit between the framework rests and the prepared rest seats of the teeth, their findings showed that 76% of the rest seats had no contact at the deepest portion of the rest.

In the last 100 years, dental materials and dental technologies for the fabrication of prostheses have advanced remarkably. Manufacturing processing techniques and systems have been introduced into dentistry. In the 1970s and 1980s, computer-aided design (CAD)/computer-aided machining (CAM) systems were introduced for the fabrication of crowns and fixed partial dentures.

Several CAD/CAM software programs are now available commercially for virtual three-dimensional (3D) designing of dental restorations on a computer. When the design of the restoration is complete, the CAM software uses an additive rapid prototyping technology to produce a physical 3D object. This stereolithographic process is often called printing. The printed object (master pattern) can then be cast using conventional techniques, or can be produced directly in metal.

Dental laboratory work remains a labor-intensive and experience-dependent task. Many dental laboratories in the United States and Canada have adopted this technology into their RPD fabrication process. New innovative techniques such as CAD/CAM RPD systems could improve the quality of fit of RPD frameworks. The purpose of this report was to perform a systematic review of the literature to determine what level of evidence exists to support the current use of this technology.

Materials and methods

An electronic search was performed in PubMed using Boolean operators for articles published between 1950 and October 2012. The purpose of the search was to obtain all in vivo and in vitro articles published on the subject of CAD/CAM RPD frameworks. The following describes the key word combinations used and the number of articles produced through each search:

• 1.

  “CAD/CAM and removable partial dentures” 24 articles

• 2.

  “CAD/CAM and removable dentures” 130 articles

• 3.

  “CAD/CAM and removable prostheses” 40 articles

• 4.

  “CAD/CAM and removable frameworks” 10 articles

• 5.

  “Rapid prototyping and removable partial dentures” 8 articles

• 6.

  “Rapid prototyping and removable prostheses” 9 articles

• 7.

  “Rapid prototyping and removable frameworks” 5 articles

• 8.

  “Computerized and removable dentures” 30 articles

The titles from each search were reviewed for duplication yielding 181 publications. The electronic search was supplemented by searches for selected articles obtained from the references of the articles produced by the electronic search, and manual searches of articles published in 2012 in the Journal of Prosthetic Dentistry , Journal of Prosthodontics , and the International Journal of Prosthodontics . The titles and abstracts of all articles were reviewed for possible inclusion. Box 1 presents the inclusion criteria. On identification for inclusion, the full text of the article was reviewed. Each article was characterized by the study design and the level of evidence it provided.

Glover and colleagues further developed the hierarchy of evidence pyramid developed by Sackett and colleagues. In this refinement, the highest level of evidence is characterized as filtered information. The studies fall into 3 categories: systematic reviews, critically appraised topics (CATs) in which the evidence is synthesized, and critically appraised individual articles in which the publication is merely a synopsis of the literature. The types of articles in this category are listed in the hierarchy of evidence from highest to lowest.

The next level of evidence is considered unfiltered information. Randomized clinical trials (RCT), cohort studies, case-control studies/case series/reports, and background information/expert opinion are all considered unfiltered information. Within this category, RCTs are regarded as the highest level of evidence, whereas expert opinion is deemed to be the lowest level. Cohort studies, case-control studies, and case reports are listed in their respective hierarchy of evidence. In vitro studies are considered to be a lower level of evidence than in vivo studies.

Rosner reexamined the principles of evidence-based medicine citing the exclusion of numerous sources of research information (basic research, epidemiology, and health services research) as problematic. He therefore further refined the hierarchy pyramid to include animal research and in vitro studies. Combining the works of Glover and colleagues and Rosner, Table 1 defines the hierarchy of study design scores, whereas Table 2 defines the level of evidence score based on the Oxford Center for Evidence classification system.

Using these classifications, the articles were assigned a study design score ranging from 1 to 12 and a level of evidence score ranging from 1 to 5 (Oxford Center for Evidence ) independently by 2 reviewers. Where the scores assigned were different, a consensus score was then reach by the 2 examiners.