Cad Cam and provisional temporary crowns

temporary-crownProvisional crowns and bridges have few important roles during the time between tooth preparation and  fitting the final restoration. They are  are very important part of prosthetic therapy not only for esthetic but also for pulp tissue protection against physical, chemical, and thermal impact. They also need to provide maintenance of positional stability and occlusal function.  Provisional restorations can even be a diagnostic tool not only a space maintainer. Properly contoured  they act as a diagnostic tool to determine occlusion. Because all off these requirements  need to be met, provisional restorations need to be as esthetic and as strong as the permanent restorations. They must satisfy biologic, mechanical, and esthetic demands including resistance to fracture, marginal fit,  wear resistance, tissue compatibility, ease of manipulation, and cost.  Good marginal fit of temporary crowns protects prepared tooth tissue and accurate margins that are neither over or under extended are need to prevent gingival hypertrophy, gingival recession, and hemorrhage during cementation. An optimal provisional fixed provisional restoration must protect the underlying preparation, pulp, and gums, and should provide health recovery to any traumatized soft tissues while the definitive restoration is being fabricated by the laboratory.

Among the requirements of a ideal provisional restoration material, marginal adaptation is the most important one. A good margin must provide preotect for the prepared tooth, as well as its gingival tissues which is necessary for further therapy. Marginal failure might lead to microleakage which is one of the the main causes of tooth sensitivity, postoperative sensitivity, and recurrent dental caries. This happens because there are many factors that can have infulence on marginal fit. The type of restorative material, dentinal fluids, material properties such as dissolution and coefficient of thermal expansion and polymerization shrinkage can affect marginal fit. Marginal failure may cause bacteria colonization  in dentinal tubules through the restorationtooth gap which can lead to pulpitis in vital teeth due to bacteria toxins. This directly affect  restoration longevity. Microleakage might be more present when the preparation margin is in dentin. This is quite possible in the cervical area and in elderly patients.

 

Pmma temporary crowns

designing-cadcam-temporary-crownsThere are many theories, materials and techniques for making provisional restorations in the dental literature. With the development of new temporary crown materials, it has become imperative to estimate their marginal fit and strength to select the ideal provisional crown material.

The most commonly used conventional chairside materials, used in direct and indirect restorative procedure are Polyethyl, Polymethyl methacrylates (PMMAs) and composite-based resins (CBR).  Bis-acrylic, based on multifunctional methacrylic acid esters, has evolved as the material of choice for provisional restoration because of its easy intraoral manipulation and good mechanical properties(picture1). Their chemical natures  is different.

Methacrylate resins are composed from liquid/powder  parts and need to be hand-mixed before use. Composite-based resins are usually auto mixed. The polymerization reaction of methacrylate resins initiates chemically (self-curing), while composite-based materials are available as both self-curing and dual-curing systems. Though the direct fabrication of provisional restorations is very common, it has its own flaws, for example, the mixing procedures may incorporate voids that could  affect the mechanical strength, surface texture, and precise marginal  fit of the restoration.

 

 

 

 

cad-cam-provisional-crownAnother method for fabrication of provisional restorations can is by computer-aided design and computer-assisted manufacturing (CAD/CAM) system(picture 2). Material for their fabrication is Polymethyl methacrylate (PMMA).   Polymethyl methacrylate blocks combined with digital processes reduce several obstacles that are present in process of making direct temporary restorations. With this method impression and mixing errors, polymerization shrinkage, and difficulties with cleaning are eliminated. These provisional restorations(picture 3) are semi-translucent, natural-looking, highly esthetic, extremely durable and they’re the strongest provisional restoration available .  Marginal adaptation of provisional restorations is a critical feature of these treatments. It is a function of the chemical composition, setting method, and aging procedures. Opinions about the maximum acceptable gap size includes a wide range of values. There are only few studies that have compared the marginal accuracy of different materials and methods used for temporary crowns and fixed partial dentures. According to American Dental Association specification marginal gaps >25-35 μm are unacceptable. The materials for provisional restorations are showing mean marginal discrepancy values of 71-106 μ immediately after fabrication, which is a wide range.

In CAD/CAM restorations these results are ideal. CAD/CAM provisional crowns have better marginal adaptation as compared to bis-acrylic composite-based auto-polymerizing resin material. The vertical gap mean value of CAD/CAM provisional is found to be only 15.026 ± 4.340 μm. These is probably because the Polymethyl methacrylate (PMMA) blocks are previously industrially polymerized under optimized conditions hence no further polymerization shrinkage is expected to happen.

On the other side some tudies have found tahte the vertical mean gap value of composite-based resins (CBR) is 145.418 ± 25.365 μm. This high value may be due to the polymerization shrinkage manifested by the material. The polimeryztion shrinkage  is a consequence of several factors, such as  the amount of monomer, the volume fraction of filler particles, filler size, the degree of conversion, the nature of the resin, stiffness and flow of resin, the rate of polymerization, curing characteristics, water sorption, and the intensity of the light used to polymerize the resin. Studies  suggested that the majority of gap formations occurred during the auto-polymerization phase of polymerization of dual polymerized material.

Polyethyl, Polymethyl methacrylates (PMMAs) and composite-based resins (CBR) material  and CAD/CAM provisional materials both have adequate flexural strength, but the marginal fit of temporary crowns fabricated by CAD/CAM is superior to the ones fabricated using bis-acrylic composite-based autopolymerizing resin materials. In cases where we need a short term restoration chairside provisionals can provide acceptable   hardness and  flexural strength.

The marginal adaptation is extremely important factor of provisional restoration for many reasons. The junction between a cemented restoration and the tooth is always a potential site for recurrent caries, tooth sensitivity and treatment failure. The more accurately the restoration is adapted to the tooth the bigger are the chances for long term clinical success of the final restoration. In conclusion to that we should choose CAD/CAM temporary restoration whenever possible, especially in cases when some kind of therapy or diagnostic is required before fiting the final restoration.