The use of CAD CAM For Fixed Partial Prostheses
Due to the increasing demand from patients and dentists for highly aesthetic and strong, metal-free restorations there has been a rapid increase in research into dental CAD CAM technique and zirconia based restorations over the last decade. Such new technology has the potential to take the place of conventional techniques and materials for fabricating indirect dental restorations in the future. In this PhD thesis, five laboratory studies were designed to investigate zirconia bridges constructed using dental CAD CAM. The studies concentrated on: 1. Ideal force applied by dentists for cementing zirconia bridges and the impact on seating. 2. The effect of firing cycles and zirconia thickness on the fit of zirconia bridges. 3. The effect of span length on the fit of three and four unit all zirconia bridges. 4. The effect of veneering on the strength of three unit zirconia bridges. 5. The fit of three unit all zirconia bridges produced by digital and conventional techniques. For these laboratory studies an ideal three unit (and four unit for study 3) fixed-fixed all ceramic bridge preparation was carried out on two plastic teeth and all SLA models and zirconia based bridges were made using the Lava COS and Lava™ CAD CAM system (3M, ESPE). In addition to the laboratory studies, a clinical audit was carried out to assess satisfaction (dentist, dental technician and patient) with zirconia based restorations (through a series of questionnaires) made and fitted at Dundee Dental Hospital and School. In addition, as part of this audit a simple cost analysis was carried out to explore the differences in cost between zirconia based restorations and high fusing gold alloy based metal ceramic restorations. Four of the studies (studies 1, 2, 3 and 5) investigated the internal and marginal fit of the zirconia based restorations under differing laboratory and clinical procedures and conditions. It was found that the seating force used to cement a zirconia based bridge had no impact upon fit (Study 1). Whilst the thickness of zirconia (all-zirconia bridge and un-veneered zirconia framework) did not affect the fit of the restoration, veneering the framework did lead to a statistically significant deterioration in fit (Study 2). Although leading to a poorer fit veneering did have a positive effect in strengthening the zirconia framework, but neither un-veneered nor veneered frameworks were as strong as monolithic/all zirconia bridges (Study 4). Despite the high shrinkage during post milling sintering and the potential for greater distortion on longer span bridges, the longer span bridges investigated in Study 3 did not impact upon fit. In study 1, 2, 3 and 4 the Lava COS intra-oral scanner was used to create a digital impression of the tooth preparations and study 5 confirmed that the fit of bridges made from these impressions were better than those made using conventional addition cured silicone putty and wash impressions (Study 5). The results of the questionnaires used in the audit revealed high satisfaction rates with all stake holders and the cost analysis showed that producing zirconia based restorations can be five to six times cheaper than conventional gold based restorations. Despite the variations in fit which were found in Studies 2 and 5, all bridges produced were within what would be regarded as clinically acceptable and comparable to those produced with more traditional techniques.
CAD CAM, Zirconia, Fit, Fixed partial prostheses, Bridges, Digital dentistry, 3D scanning, Ceramics, Impression, Milling, Internal, Marginal, Force, Strength, Cements, Preparation, Audit
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