International Journal of Computerized Dentistry, 03/2008

So far, CAD/CAM technology has not yet made any noteworthy inroads into removable dentures. One of the possible reasons for this may be the lack of a tried-and-tested and economically acceptable scanner technology. Our motivation for developing our own 3D digitization method was therefore to develop a really low-cost scanner under exclusive use of standard industrial components. The method presented here for spatial acquisition of edentulous jaws is based on the evaluation of the optical structure inherent in jaw impressions themselves. For this purpose, an optimized method of Dense Stereo Matching4 on three images, acquired from three different perspectives, is used. Keywords: full dental prosthetics, tooth arrangement, digitization, matching, stereo, scanner

Introduction: Individual bone quality depends on genetic, biological, and mechanical influencing factors, where the latter is accessible via Finite Element Simulation. This work is part of an interdisciplinary research project with the purpose of stepwise refinement towards anatomical reality. This approach opened the door for many interrelated applications such as atrophy of the jaw bone, periodontology, implantology, or TMJ disorders. This lecture is dedicated to the influence of dental anatomy on mandibular biomechanics. Materials and Methods: In general, biomechanical simulation requires reconstruction of the individual anatomy, implementation of the inhomogeneous and anisotropic material law of bone, and realization of the load case due to tooth, muscle and joint forces. The simulation chain ranges from image processing of CT data up to specifically adapted post-processing of the simulation results. In spite of ongoing research, there is still a fundamental difference of dental implants compared to natural teeth: the periodontal ligament (PDL) present at the interface between teeth and mandibular corpus. Due to its thickness of about 0.2 mm, the PDL was introduced to the simulation model by a special semiautomatic procedure. Results: Simulations "with and without PDL" proved remarkable force absorption due to the PDL, as well as qualitative changes of the stress/strain profiles of the alveolar ridge. Concerning the simulation without PDL, the observed high compressive strains at the adjacent bone were in agreement with regions of frequent implant failure. Conclusion: The PDL is essential for the structural behavior of the human mandible. Based on the mechanical adaptation of bone, the comparison of the simulation with and without PDL provided special insight to the changes due to dental implants, in particular implant loss and bone resorption. Finally, the simulation will serve as a virtual platform for further evaluation (a) of implant design (b) of implant placement. Keywords: mandible, biomechanics, finite element simulation, periodontal ligament, bone adaptation, dental implant

Purpose: Despite remarkable progress within the last decade, the treatment of mandibular fractures is still a highly discussed topic in oral and cranio-maxillofacial surgery. The possible traumatologic scenarios are characterized by high variability. A current project is focused on "resimulation" of traumatologic cases given by clinical radiographs by means of finite element method. Methods: The applied finite element model of the mandible is very refined, providing detailed dental anatomy especially of the periodontal ligament. The mandible was modelled as inhomogeneous and anisotropic. The temporomandibular joints were realized as simplified joint capsules, wherein the mandibular condyles are freely mobile with certain limitations. The user has the choice of 5 regions on the mandibular surface where the virtual injury can be inflicted. Power and direction of the impact force vector can be set at will. The masticatoy system including the digastrics and the mylohyoid muscles can be activated. Results: The situations given by radiographs could be "reproduced" by a simulation scenario characterized by high compressive strain at the location of fractures. If masticatory muscles were activated and teeth clenched, the stress/strain profiles were qualitatively changed. Discussion: The approach may be of benefit for optimized behavior with regard to certain sports or vocations. For forensic analysis, the method will contribute by elimination of scenarios not matching the given fracture locations. Nevertheless, the immediate purpose of our approach is a better understanding of the injured organ's condition. Fractures of bone as an adaptive biological tissue differ fundamentally from mechanical failure in engineering. Many of our trauma simulations showed elevated stress/strain around the fracture, leading to the suggestion of weakened bone there. This finding was confirmed by surgical observation.

John Naisbitt, in his 1982 book "Megatrends," postulated an important characteristic of our information and knowledge society: "We are drowning in information but starved for knowledge." Today, in the age of the Information and Knowledge society, we are faced with this problem every single day, because Web searches and information selection are highly time-consuming activities. Internet search engines attempt to employ intelligent search algorithms in order to optimize their search results. Nevertheless, the question remains how "qualitative knowledge" can be selected, ie, knowledge needed for supporting decisions in medicine and dentistry. Semantic search engines are one current approach to this problem. For this reason, a project entitled "Mr. Q, your personal Web Assistant" has been initiated and will be introduced in this paper. Keywords: knowledge society, information deluge, qualitative knowledge, Internet search engines, semantic search, Mr. Q, dynamic knowledge space, visualization of knowledge

The computer navigation system Robodent, which was developed for dental implants, was used for the placement of orthodontic anchorage screws in a phantom head. The deviation between the planned and finally achieved position of the screws did not exceed that reported for dental implants. In conclusion, the system seem to be suitable and precise enough for orthodontic screws. Keywords: orthodontic anchorage screws, computer navigation, computer assisted surgery

Aim: Computer-assisted surgery (CAS) is used in implant dentistry for preoperative planning and intraoperative transfer of the correct implant position. Implant placement is performed via sleeve-guided templates in the case of static navigation procedures by taking into consideration prosthetics and the amount of local bone. The aim of the present in vitro study was to analyze the transfer precision of the computer-aided planning system Med3D as well as the possible improvement of transfer accuracy by using a second, template-guided drilling during osteotomy. Materials and Methods: A total of 48 implants were placed into 8 study models from calf ribs. Preoperative computed tomography (CT) scans were imported via the respective software and used for virtual planning of the implant position on the computer. CT control images were taken after the actual implant site preparation via sleeve-guided drilling templates and subsequent implant placement. The preoperative CT scans were compared to the post-operative CT images in order to analyze transfer precision. Vertical, radial and axis deviations were calculated for each implant. Results: The minimal deviations obtained between the planned and the achieved implant position have shown that a transfer accuracy within the range of 0.1 mm is feasible when utilizing the MED-3D system. However, maximal vertical deviations of up to 2 mm have to be considered. The maximal axis deviation may reach 16 degrees. Deviations from the planned implant position seem to be influenced especially by the local bone quality and quantity. Transfer precision may be significantly improved by adopting a second, sleeve-guided implant site preparation. Conclusion: Static procedures allow for a precise transfer of the virtual, computer-assisted implant planning to the surgical site. However, transfer deviations of up to 2 mm in all directions should be considered in order to prevent impairment of anatomical structures. Local bone quality and quantity seem to influence the deviations from the implant position planned. A significant improvement of the transfer precision can be achieved by applying a second sleeve-guided site preparation. Keywords: computer-assisted surgery (CAS), computerized tomography, transfer precision, static navigation

The advantages of minimally invasive dentistry are well established, especially for direct restorations. However, when it comes to indirect restorations traditional tooth preparation designs are still advised by most manufacturers. The purpose of this in vitro study was to investigate the ability of a CAD/CAM machine (Cerec) to produce minimal preparation designs and identify limiting parameters. Crown preparations based on the proposed minimal design were made using phantom teeth. Gauged burs (Intensiv SA; Meissinger, Germany) and a paralleling device were used to standardize preparations. Cerec Scan/Cerec 3D was used for scanning and designing. The materials tested were a resin composite (Paradigm MZ100, 3M ESPE) and two ceramic materials (ProCAD, Ivoclar Vivadent, and VITA Mark II, Vita). The morphology, marginal integrity, and materials' integrity were examined. The design was subjected to an interactive process as material property limitations and constraints imposed by the system became apparent. SEM, optical microscopy, and transillumination were used for the qualitative control of the crowns. The results of this study showed that only the composite material produced acceptable crowns with intact margins for the minimal design initially proposed. The ceramic materials required a wider preparation design in order to produce acceptable crowns. Within the limitations of this study, the null hypothesis was partially rejected as it was found that only the composite material could produce acceptable crowns based on the proposed minimal design. It was also found that the materials' properties, the milling mode, and cutting instruments are determining factors in establishing the extent of the minimal preparation.

The shape of a crown preparation is the prime determinant for the choice of material for an all-ceramic restoration. One essential factor is the available space for the restoration, which requires a certain occlusal thickness. The dentist's preparation design determines the available vertical clearance, and the dental technician has the responsibility of advising the dentist with regard to either choosing the right material to match the preparation or to preparing the tooth to match the material. Assuming a minimum static fracture strength of > 2000 N, the following materials can be used for all-ceramic crowns. Laboratory surveys have shown that in most situations, the available occlusal clearance in clinical reality is only 0.8 to 0.9 mm (after cementing). This shows that the available space will often be insufficient for providing monoblock crowns and still on the tight side for veneered oxide ceramics (In Ceram, zirconia, etc.). However, crowns made of veneered oxide ceramics are much more complex to fabricate and much more expensive. By simply providing a minimal occlusal thickness of 1.5 mm, the treatment provider could therefore easily facilitate the use of the much more economical monoblock crowns without compromising either esthetics or strength. Actually, crowns with veneered oxide ceramic copings do not offer any higher fracture resistance compared to Mark II crowns as long as the minimum thickness requirements are met. The flexural strength of CAD/CAM-fabricated lithium disilicate rods is about twice that of CAD/CAM-fabricated Mark II rods. When used for crowns with a wall thickness of 1.5 mm, however, both materials exhibit the same fracture strength of between 2000 and 2500 N. This is related to the different reinforcing action of the adhesive luting agent, which is essentially required for both these materials. When choosing a material, preparation shapes, technical complexity and cost should be thoroughly compared and scrutinized and should figure prominently in the discussions between dentists and dental technician. Unfavorable preparation shapes for single crowns will necessitate compromises in terms of the choice of materials that result in high cost but do not offer anything in the way of higher fracture resistance. What constitutes an appropriate all-ceramic restoration for a single tooth? Do all-ceramic single crowns require the same material bulk as multi-unit bridges? Everything would indicate that a suitable preparation geometry allows feldspathic ceramic monoblock crowns to be milled that do not require any extensive finishing efforts such as thermal annealing or in-laboratory veneering while at the same time demanding no compromises in terms of esthetics and load-bearing capacity.