Patent Application
20090035739
The market for dental restorations is already commercially significant and is only likely to grow over the coining years as the public becomes more aware of the advances is aesthetic materials and dental implants as these become more affordable and readily available. For example, there are constant advances in the use of oxide and silicate ceramics that are currently used in restorative dentistry for inlays, onlays, crowns or bridges. However, despite the frequent use of such ceramics and other composites in dentistry their physical parameters, such as their modulus of elasticity, flexural strength, material hardness and their interactions with respect to wear and fatigue are often poorly understood.
Human mastication and associated repetitive dental loading have been proved to dramatically reduce the strength of dental components and restorative materials, especially ceramic restorations which tend to be brittle and have low tensile strength. This coupled with micro-cracking and frictional sliding during cyclical loading increases the risk of failure of the dental restorations. Therefore it is important to understand the mechanical properties of the materials used, such as their wear performance, to predict the likely life time of such dental restorations. However, medical and clinical trials are generally time consuming, with typical time spans of two to three years, and consequently expensive. It is therefore attractive to the dental restorative manufacturers to develop or make use of apparatus and methods for accelerated screening of the clinical wear at relatively low cost under controlled reproducible laboratory conditions.
Currently the most accurate testing methods take the form of a biaxial testing rig based around a design first developed in 1983 specifically for testing dental restorations. These test rigs can subject a single sample (such as a individual ceramic crown) to the three main types of jaw movement experienced, i.e. preparatory, crushing and gliding movements, and simulate the forces applied while doing so. However, a limitation of these methods is that the human masticatory cycle is an inherently triaxial process, whilst the testing rigs operate biaxially and thus deliver only an approximation to the type of wear experienced. Other mastication simulators have been proposed that more fully simulate the possible motion of the human jaw and achieve this by locating a plurality of mechanical actuators within the space defined by an artificial jaw. The space limitatiaons imposed by this arrangement limits the size of the actuators that can be used and thus limits the maximum force that may be applied to the artificial jaw and dental restorative accordingly.
The invention comprises a dental simulation apparatus comprising a six degrees of freedom motion actuator having an upper mobile portion, an artificial mandible secured to the mobile portion of the motion actuator, the artificial mandible being arranged to receive a plurality of artificial teeth or other dental restoratives, and an artificial maxilla rigidly located about the artificial mandible, the artificial maxilla also being arranged to receive a plurality of artificial teeth or other dental restoratives.
A compliance module may be located between the artificial mandible and the mobile portion of the motion actuator, the compliance module including at least one compressible portion arranged such that compression of the compressible portion results in a decrease in the separation of the artificial mandible and the mobile portion of the motion actuator. The compressible portion of the compliance module may be arranged to be compressed only when a force in excess of a first threshold value is applied. The compliance module may include an upper plate connected to the artificial mandible and a lower plate connected to the mobile portion of the motion actuator, the upper and lower plates being spaced apart by at least one compressive spring mounted there-between.
A force sensor may be located between the artificial mandible and the mobile portion of the motion actuator, the force sensor being arranged to measure the compressive force applied to the artificial mandible. The force sensor may also measure lateral forces applied to the artificial mandible.
The motion actuator may include a static portion and a plurality of linear actuators connected between the static and upper mobile portions. The apparatus may further include a control module arranged to control the position, speed and applied force of the linear actuators. Each linear actuator may include a position encoder and may be arranged to provide one or more encoder signals to the control module indicative of the position of the linear actuator, wherein the control module can process the encoder signals so as to provide closed loop control of the linear actuators. The control module may be further arranged to receive one or more signals indicative of the force applied by the actuators.
The apparatus preferably includes a plurality of artificial teeth arranged to be located in either the artificial mandible or maxilla, each tooth having a root comprising a profiled upper root portion and a locating pin. The profile of each upper root portion is different from the remaining teeth and the artificial mandible and maxilla includes a plurality of root receiving cavities, each root receiving cavity having a profiled section having a profile that is complementary to only one of the upper root portions, such that each tooth may only be located in a single cavity. Each upper root portion may have an upper planar surface for a dental crown to be secured to. The length of the upper root portions preferably varies from tooth to tooth such that when each tooth is located in the corresponding cavity of the artificial mandible or maxilla, the upper surfaces of the upper root portions define a Curve of Spee. The upper root portion may include an outer layer of resilient material. Additionally, the root may include a layer of resilient material interposed between the upper root portion and the locating pin. The locating pin may have a rounded end.
Referring to
When chewing, a combination of both force and position control is required. The human masticatory system achieves this through a combination of neural control and inherent dynamics emergent from the characteristics of muscle and their geometric information. The human muscle structure of the jaw area can alter the inherent stiffness of the jaw and can do so very rapidly. It is this property, for example, that prevents damage to the teeth and jaw when a hard piece of food suddenly gives way. In embodiments of the present invention force and position control are achieved by appropriate control of the six telescopic actuators 6 of the Stewart platform, together, in the illustrated embodiment, with the use of the compliance module 14 to control contact forces. The compliance module allows the simulator to sustain the high bite forces that occur in the contact areas between the upper and lower teeth during normal chewing that are the main causes of dental component failure. Typical chewing forces experienced in the jaw arrange from approximately 9 to 180N. However, for failure testing this force can be as high as 2500N. As noted with reference to
As previously noted the position of and force applied to the artificial mandible via the mobile upper plate of the Stewart platform is primarily controlled by the operation of the telescopic actuators 6.
Each actuator motor 36 is therefore controlled using an inner-loop current controller and an outer-loop position controller. Additional force measurements from a load cell suitably located on the simulator may also be used by the control apparatus indicated in
In some embodiments of the present invention one or more strain gauge load cells 42 may be provided located between the lower mandible 24 and the sensor plate 22, as indicated in
The teeth of the dental simulator comprise a root portion and a crown portion.
The length of each upper root portion 44 varies from tooth to tooth. This variation in height is required to ensure that when the completed the teeth are assembled the crowns of the teeth with occlude. Occlusion in dentistry relates to the way upper maxilla and lower mandible meet during mastication. As the teeth are clenched the upper and lower sets should come together evenly. One factor in achieving good occlusion is to accurately reproduce the “curve of spee” for the maxilla and mandible set of teeth. The curve of spee relate to the natural curvature of the mandibular and maxillary arches, with the curve of spee for the maxilla being convex and for the mandible being concave. Whilst the exact shape of the curve of spee varies from individual to individual, it can generally be defined as “the atomic curve established by the aclusal alignment of the teeth, as projected on the minimum plane, beginning the cusp tip of the mandibular canine and following the buccal cusp tips of the premolar and molar teeth”. In practice, the curve can be made by drawing a straight line between the mesiolingual cusp of the second molar and the buccal cusp of the canine. From there, lines normal to the cusp tips of the premolars, the first molar and second molar, can be drawn. The longest of these distances represents the depth of the curve of spee. Based on these measurements, the radius r of the curve can be calculated by applying standard geometry where s is the distance between the mesiolingual cusp of the second molar tooth and buccal cusp of the canine tooth. The depth of the curve h is thus given by the following formula:
r[h
2+(s/2)2]/2h
In embodiments of the present invention this curve is reproduced by adjusting the lengths of the upper root portions 44 for each individual tooth. This is indicated in
Healthy human teeth are able to deflect within the jaw by approximately 0.1 to 0.4 mm. To replicate this movement in some embodiments of the present invention the upper root portion 48 is slightly under dimensioned and a layer of resilient material 50 is bonded around the periphery of the upper root portion, the thickness of the resilient layer 50 being such that the required degree of deflection is provided when the teeth are subject to a mean mastication load. This is illustrated in
The dental simulator of the present invention is capable of simulating the wear of dental restorative material by controlling the simulator chewing motion to a high degree of accuracy than is provided by the known prior art apparatus.
