Improved single- and multi-contact life-time testing of dental restorative materials using key characteristics of the human masticatory system and a force/position-controlled robotic dental wear simulator.

This paper presents a new in vitro wear simulator based on spatial parallel kinematics and a biologically inspired implicit force/position hybrid controller to replicate chewing movements and dental wear formations on dental components, such as crowns, bridges or a full set of teeth. The human mandible, guided by passive structures such as posterior teeth and the two temporomandibular joints, moves with up to 6 degrees of freedom (DOF) in Cartesian space. The currently available wear simulators lack the ability to perform these chewing movements. In many cases, their lack of sufficient DOF enables them only to replicate the sliding motion of a single occlusal contact point by neglecting rotational movements and the motion along one Cartesian axis. The motion and forces of more than one occlusal contact points cannot accurately be replicated by these instruments. Furthermore, the majority of wear simulators are unable to control simultaneously the main wear-affecting parameters, considering abrasive mechanical wear, which are the occlusal sliding motion and bite forces in the constraint contact phase of the human chewing cycle. It has been shown that such discrepancies between the true in vivo and the simulated in vitro condition influence the outcome and the quality of wear studies. This can be improved by implementing biological features of the human masticatory system such as tooth compliance realized through the passive action of the periodontal ligament and active bite force control realized though the central nervous system using feedback from periodontal preceptors. The simulator described in this paper can be used for single- and multi-occlusal contact testing due to its kinematics and ability to exactly replicate human translational and rotational mandibular movements with up to 6 DOF without neglecting movements along or around the three Cartesian axes. Recorded human mandibular motion and occlusal force data are the reference inputs of the simulator. Experimental studies of wear using this simulator demonstrate that integrating the biological feature of combined force/position hybrid control in dental material testing improves the linearity and reduces the variability of results. In addition, it has been shown that present biaxially operated dental wear simulators are likely to provide misleading results in comparative in vitro/in vivo one-contact studies due to neglecting the occlusal sliding motion in one plane which could introduce an error of up to 49% since occlusal sliding motion D and volumetric wear loss V(loss) are proportional.

The chewing robot: a new biologically-inspired way to evaluate dental restorative materials.

This paper presents a novel in vitro dental wear simulator based on 6-6 parallel kinematics to replicate mechanical wear formation on dental materials and components, such as individual teeth, crowns or bridges. The human mandible, guided by a range of passive structures moves with up to six degrees of freedom (DOF). Currently available wear simulators lack the ability to perform these complex chewing movements. In addition simulators are unable to replicate the normal range of chewing forces as they have no control system able to mimic the natural muscle function controlled by the human central nervous system. Such discrepancies between true in vivo and simulated in vitro movements will influence the outcome and reliability of wear studies using such approaches. This paper summarizes the development of a new dynamic jaw simulator based on the kinematics of the human jaw.

3D surface imaging in dentistry - what we are looking at.

3D imaging has been widely used within various fields of dentistry to aid diagnosis, in treatment planning and appliance construction. Whereas traditionally this has involved the use of impression materials together with plaster or stone models, modern techniques are continually evolving which use virtual 3D images. These electronic virtual images are created using either contact or non-contact optical scanning techniques, but there are limitations, the most important of which is that any new virtual surface image is created from a series of discrete data points. It is not created from a continuous stream of data relating to the original object. This means that computer software has to be used to recreate a possible best fit, virtual surface from the data obtained. This paper describes the principles behind 3D scanning technology, the limitations of 3D imaging as well as current and possible uses of such imaging in clinical dentistry.

Prototyping artificial jaws for the robotic dental testing simulator.

This paper presents a robot periphery prototyped for the six-degrees-of-freedom robotic dental testing simulator, simulating the wear of materials on dental components, such as individual teeth, crowns, bridges, or a full set of teeth. The robot periphery consists of the artificial jaws and compliance module. The jaws have been reverse engineered and represent a human-like mandible and maxilla with artificial teeth. Each clinically fabricated tooth consists of a crown and glass ceramic roots which are connected using resin cement. Normal clinical occlusion of the artificial jaws assembly was emulated by a dental articulator based on 'Andrew's six keys to occlusion'. The radii of the von Spee curve, the Monson curve, and the Wilson curve were also measured as important jaw characteristic indicators to aid normal occlusion. A compliance module had to be built between the lower jaw and the robot platform to sustain the fluctuating forces that occur during normal chewing in the occlusal contact areas, where these high bite forces are major causes of dental component failure. A strain gauge force transducer has been integrated into the machined lower jaw, underneath the second molars, to measure axial biting forces applied to the posterior teeth. The experiments conducted have shown that the sensor is able to sense small changes in the compression force satisfactorily, when applied perpendicular to the occlusal surfaces of the teeth.

Prototyping a robotic dental testing simulator.

A parallel robot based on the Stewart platform is being developed to simulate jaw motion and investigate its effect on jaw function to test the wearing away of dental components such as individual teeth, crowns, bridges, full set of dentures, and implant-supported overdentures by controlling chewing motion. The current paper only describes the comparison between an alternative configuration proposed by Xu and the Stewart platform configuration. The Stewart platform was chosen as an ideal structure for simulating human mastication as it is easily assembled, has high rigidity, high load-carrying capacity, and accurate positioning capability. The kinematics and singularities of the Stewart platform have been analysed and software developed to (a) test the control algorithms/strategy of muscle movement for the six degree of freedom of mastication cycle and (b) simulate and observe various design options to be able to make the best judgement in product development. The human replica skull has been analysed and reverse engineered with further simplification before integration with the Stewart platform computer-aided design (CAD) to develop the robotic dental testing simulator. Assembly modelling of the reproduced skull was critically analysed for good occlusion in CAD environment. A pulse-width modulation (PWM) circuit plus interface was built to control position and speed of the chosen actuators. A computer numerical control (CNC) machine and wire-electro-discharge machining (wire EDM) were used to manufacture the critical parts such as lower mandible, upper maxilla, and universal joints.

A dental vision system for accurate 3D tooth modeling.

This paper describes an active vision system based reverse engineering approach to extract the three-dimensional (3D) geometric information from dental teeth and transfer this information into Computer-Aided Design/Computer-Aided Manufacture (CAD/CAM) systems to improve the accuracy of 3D teeth models and at the same time improve the quality of the construction units to help patient care. The vision system involves the development of a dental vision rig, edge detection, boundary tracing and fast & accurate 3D modeling from a sequence of sliced silhouettes of physical models. The rig is designed using engineering design methods such as a concept selection matrix and weighted objectives evaluation chart. Reconstruction results and accuracy evaluation are presented on digitizing different teeth models.