Dentistry – Prosthodontics – Impression taking or bite determination
Reexamination Certificate
2001-11-06
2003-07-15
Lewis, Ralph A. (Department: 3732)
Dentistry
Prosthodontics
Impression taking or bite determination
C433S029000
Reexamination Certificate
active
06592371
ABSTRACT:
1. Field of Invention
The present invention relates to intra-oral methods and apparatus for optically imaging a structure and creating representative 3D models from the images.
2. Background
Determination of the surface contour of objects by non-contact optical methods has become increasingly important in many applications. A basic measurement principle behind collecting range data for these optical methods is triangulation. Triangulation techniques are based on elementary geometry. Given a triangle with the baseline of the triangle composed of two optical centers and the vertex of the triangle the target, the range from the target to the optical centers can be determined based on the optical center separation and the angle from the optical centers to the target.
Triangulation methods can be divided into passive and active. Passive triangulation (also known as stereo analysis) utilizes ambient light and both optical centers are cameras. Active triangulation uses only a single camera and in place of the other camera uses a source of controlled illumination (also known as structured light). Stereo analysis while conceptually simple is not widely used because of the difficulty in obtaining correspondence between camera images. Objects with well-defined edges and corners, such as blocks, may be rather easy to obtain correspondence, but objects with smoothly varying surfaces, such as skin or tooth surfaces, with no easily identifiable points to key on, present a significant challenge for the stereo analysis approach.
To overcome the correspondence issue, active triangulation, or structured light, methods project known patterns of light onto an object to infer its shape. The simplest structured light pattern is just a spot, typically produced by a laser. The geometry of the setup enables the calculation of the position of the surface on which the light spot falls by simple trigonometry. Other patterns such as a stripe, or 2-dimensional patterns such as a grid of dots can be used to decrease the required time to capture the image surface.
The surface position resolution of structured lighting methods is a direct function of the fineness of the light pattern used. The accuracy of active triangulation methods depends on the ability to locate the “center” of the imaged pattern at each image capture step. A variety of real-world situations can cause systematic errors to be introduced that affect the ability to accurately determine the true imaged pattern “center”. Curved surfaces, discontinuous surfaces, and surfaces of varying reflectance cause systematic distortions of the structured light pattern on the surface which can increase the uncertainty in measuring the position of the surface being scanned.
Additional measurement uncertainty is introduced if a laser is used as the light source to create the light pattern. Due to the coherence of laser light, reflections from the surface create a random interference pattern, known as laser speckle, throughout space and at the image sensor. The result is an imaged pattern with a noise component that affects the “center” determination, causing measurement errors even from a flat surface. The difficulty of determining the “center” of the pattern is further compounded if the surface that the pattern is projected upon is not opaque but translucent. This type of surface can result in the projected pattern “blooming” at the illuminated surface because of the diffusion of light throughout the object. A tooth is an example of a translucent object that represents a challenging task from which to obtain a surface contour with active triangulation.
The dental and orthodontic field is one exemplary application for digitally generating 3D models of structures. In many dental applications, a working model of a patient's teeth is needed that faithfully reproduces the patient's teeth and other dental structures, including the jaw structure. Conventionally, a three-dimensional negative model of the teeth and other dental structures is created during an impression-taking session where one or more U-shaped trays are filled with a dental impression material. Impression materials include, among others, compositions based on alginates, polysulphides, silicones and vulcanizable polyether materials. The impression material is typically prepared by mixing a base component and a hardener or initiator or catalyst component. The impression tray containing the impression material, in its plastic state, is introduced into the mouth of the patient. To ensure a complete impression, an excessive amount of impression material is typically used. While the tray and impression material is held in place, the material cures, and after curing, the tray and material are removed from the mouth as a unit. The impression material is allowed to solidify and form an elastic composition, which is the negative mold after removal. The working model is obtained by filling this impression with a modeling material.
Dental patients typically experience discomfort when the dentist takes an impression of the patient's teeth. The procedure can be even more uncomfortable for the patient if the impression materials run, slump or are otherwise expelled into the patient's throat. Such situations can potentially cause a gag reflex reaction from the patient. In addition to patient discomfort, the impression process is time consuming. Additionally, the impression process can be error-prone. For example, when the impression material is not properly applied, the resulting working model may not accurately reflect features on the teeth. Moreover, the model can show air bubbles trapped during the impression taking session. Depending on the accuracy required, such working model may not be usable and additional dental impressions may need to be taken. Further, the mold and working model are fragile and can be easily damaged. The need to store the fragile models for future reference tends to become a logistical problem for a dental practice as the number of archived models accumulates.
Automated scanning techniques have been developed as alternatives to the mold casting procedure. Because these techniques can create a digital representation of the teeth, they provide the advantage of creating an “impression” that is immediately transmittable from the patient to a dental laboratory. The digital transmission potentially diminishes inconvenience for the patient and eliminates the risk of damage to the mold. For example, U.S. Pat. No. 6,050,821 discloses a method and apparatus for intraorally mapping the structure and topography of dental formations such as peridontium and teeth, both intact and prepared, for diagnosis and dental prosthetics and bridgework by using an ultrasonic scanning technique. As claimed therein, the method can provide details of orally situated dental formations thus enabling diagnosis and the preparation of precision moldings and fabrications that will provide greater comfort and longer wear to the dental patient. Also, as discussed therein, infra-red CAD/CAM techniques have been used to map impressions of oral structures and make single-tooth prosthetics.
Also, in certain applications such as restorative dentistry that is preformed on visible teeth, such as incisors, aesthetic considerations require that the prosthetic interface with the original tooth surface be underneath the gum (sub gingival) to eliminate the sight of the “joining line”. In preparation for the prosthetic, the patient's tooth must be shaped to create a ledge or margin beneath the gum line where the prosthetic will be sealed to the existing tooth. To prepare this surface, the dentist typically places a retraction cord between the tooth and gum. The retraction cord creates a working space that allows the dentist to machine the margin around the tooth of interest.
In order for the finished prosthetic to be correctly sized and properly seated on the prepared tooth, it is essential that the impression of the prepared tooth contain an accurate representation of the sub gingival margin. Improper resoluti
Durbin Dennis
Durbin Duane
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