A device with features for scanning

Abstract

A device and method for increasing the accuracy of an introral dental scan. The device comprises a plurality of marker posts (scan markers) attached to the dental abutments. The marker posts have a no

Description

Field of the invention The present invention relates to markers that aid 3D scanning, these markers may be used with software techniques to improve the accuracy of, and extrapolate from the 3D scan. Background Some patients without teeth, opt to have dental implants drilled into their jaw, these implants provide rigid mechanical fixing points for “bolt in dentures” to be attached (more formally called “full arch restorations”). This provides the patient with a robust and cosmetically immaculate set of artificial teeth. After the implants have been inserted their position is not precisely known. Their positions must be deduced, so that a perfectly fitting restoration can be designed and manufactured. Precise measurement is important. If a poorly fitting restoration was bolted into the patients jaw, it would be acting to strain the jaw causing patient discomfort. Traditionally the positions of the implants were captured using moulding material and then transferred into the restoration design. Though due to inaccuracies in the technique several test restorations may be needed and several test fittings [1], More recently, digital position measurement systems have been created called the “Pic Dental” and the “Imetric”. Special marker posts are attached to the implants and 3D cameras are used to locate the positions of the markers and therefore the positions and angle of the implants can be deduced [2]. However the scanners can be prohibitively expensive for many dental practices, require additional training for correct use, and still need an intra oral scanner for capturing the gum shape in a separate scan. Another approach is to scan implant marker posts with an intra oral scanner. Typically intra oral scanners can lose accuracy over long scanning distances (such as from one side of the jaw to the other). This error can cause the poor fitting of the restoration, but with special fiducial markers, the inaccuracies can be minimised [3], Fiducial markers connecting directly onto implants have been made by dental company TRI, and are called “Scanbridge” [4], However, these fiducial markers block the gum region, so a second gum scan is needed. These also require a long scan travel which may increase the scan error for an intra oral scanner. Other competitors [5, 6] also block the gums and require a second gum scan, but in addition, these are deficient in non repeating 3D texture that many intra oral scanners use to track their location with during scanning. Thus the inherent accuracy of the intra oral scan may be reduced. Hence there is a need for an implant system that maximises the accuracy of implant positions / angles, using standard intra oral scanners, whilst also capturing the gum data simultaneously. References [1] Dr. Cory Glenn, ‘Photogrammetry in Implant Dentistry- Introducing the "Grammee" Scanner', https: / / www.youtube.com / watch?v=ccRik4KUTLw&t=l57s&ab_channel=N.CoryGlenn, Feb 2024 [2] Pinto RJ, Casado SA, Chmielewski K, Carames JM, Marques DS. Accuracy of different digital acquisition methods in complete arch implant-supported prostheses: An in vitro study. J Prosthet Dent. 2023 Aug 22:S0022-3913(23)00466-3. doi: 10.1016,4.prosdem.2023.07.008. Epub ahead of print. PMID: 37620183. [3] Panos Papaspyridakos, Complete digital workflow in prosthesis prototype fabrication for complete-arch implant rehabilitation: A technique, https: / / mvw.researchgate.net / [4] "TRI Scanbridge", https: / / tri-implants.swiss / en / company / innovations / [5] "IOS connect" by Tru Abutment, https: / / www.cad-ray.com / product / truabutment-multi-unit-abutment-mua- scanb ody / [6] "Full Arch Implant Scanning: Reviewing Shining 3D Aoralscan Elite's Photogrammetry Workflow", https: / / www.youtube.com / watch? v=jvaaVHIRTTO&ab channel=InstituteofDigitalDentistry Summary of the invention Our invention is an implant attachment that acts as a marker for intra oral scanning, and associated software. The software can improve the accuracy of the intra oral scan based on the known true shape of the marker, and it can deduce the positions and orientations of the abutments that the markers are attached to. Thus dental prothesis can be designed from measurements acquired with an intra oral scanner with improved reliability, saving the time and costs associated with other techniques. One key element of our markers is that they comprises unique and non repeating 3D patterns substantially over the entire top surface of the marker. This gives the intra oral scanners features to improve the accuracy of the location tracking and for scan stitching. They also allow each marker to be uniquely identified so that the true known shape of the marker can be found in a database. A key element of some embodiments is that the bulk of the marker is held away from the gums, with the bare minimum of support structures to obstruct the gums. This allows the markers and the gums to be scanned at the same time. Scanning the markers and the gums in one scan, or even two rapid consecutive scans (with no hardware changes to the set up in the patients mouth), is a time saver for the dentist, and the patient. Minimising the time of what can be an unpleasant surgery is valuable to the patient. Other advantages should become apparent upon further study of this disclosure. To achieve the aims of the invention mentioned in this disclosure, and related aims, the device may contain but is not limited to the features described in this disclosure. The embodiments described in the following text indicate a few of the many ways in which the principles of the invention may be employed. The scope of the invention also includes both combinations and sub-combinations of the features described as well as variations and modifications which would occur to persons skilled in the art upon reading this disclosure. Examples of the invention will now be described by referring to the accompanying drawings: Figure 1 shows the preferred embodiment of the invention. Embodiment 1 The preferred embodiment of the invention is sketched in figure 1. The markers consist of a connector that attaches to the abutments in the patient’s mouth, a surface suitable for scanning from above that is substantially covered with non repeating 3D patterns (and / or non repeating visible patterns). Each pattern is unique for each marker in the set that will be used by the dentist for that surgery. The bulk of the marker pattern is held away from the abutment connector (which will be near the gum region), and it is supported by only minimal support structure so that the view of the gums remains a visible as possible to the scanner. The support structure may be thinner than 1.5mm in width, and raised from the gum by several millimetres so that the scanner can see the gum under the support structure when positioned at an angle away from the vertical. Figure 2a shows that the abutment connector can attach to a “factory scan base”. This base holds an abutment so that the marker can be attached and easily placed in to a desktop scanner of high accuracy. A high accuracy scan of the marker is made so that is size and shape are known, and also the centre point and orientation of the abutment are known relative to the marker. This information will be used later to deduce the position of the patient’s abutment centre point and orientation. Figure 2b shows a cross section of the screw and abutment connector. The screw can be fitted to the marker post by screwing the thread through the connector, but then because the screw thread is thinner at the head, when the screw is full installed it becomes loose and can spin freely in the abutment connector. This means that screw will not fall out of the abutment connector (which would be a choking hazard for the patient), yet it will be held in position ready for connection to the abutment. During connection to the abutment the marker can spin freely for positioning, until finally it is clamped down and held tight by the retaining screw. Figure 3 shows a set of 4 markers installed on 4 abutments in the patients mouth. These are then scanned with the intra oral scanner. The scan can capture the gums and the markers simultaneously in one scan saving time. Figure 4 shows the operation of the software algorithm that (optionally) performs accuracy improvement over the original scan, and deduces the positions of the abutments. First the corner tips of each marker are found since the pattern is known from the factory base scan and then can be found in the intra oral scan by algorithms such as iterative closest point alignment, and global alignment techniques to give a starting point. One such global technique would be to find peaks in the 3D marker pattern, match them to the known peaks from the factory scan in a manner similar to that of Fast Point Feature Histograms (FPFH) for 3D registration. Peaks can be found in a 3D scan by estimating surface normals for each point in the point cloud by using nearby surrounding points. The normals can also be estimated using points over more distance points, in which case the surface normal will represent the overall marker shape rather than the local 3D texture. Subtracting the nearby normals, from the long distance normals gives the local surface gradient. Finding local maxima in the local surface gradient will identify the location of the peaks, and this process will work regardless of the orientation of the markers in the scan. Next the distances between tips 1 &2, tips 3 &4, tips 5 &6, and tips 7 &8 can be corrected based on the known distances between these tips from the factory scan. This means that the majority of the distances measured in the scan come from the high accuracy desktop scanner, not the intra oral scanner which is prone to in accuracies due to stitching errors that occur over long distances. The only reliance on the intra oral scan is for orientations, and the relation between tips 2 to 3, tips 4 to 5, tips and 6 to 7, and so the intra oral scan error is minimised. With the location and orientation of the markers corrected and understood, the patient’s abutment positions can be deduced as we understand the relationship between the marker position and the abutment position from the factory base scan. Finally the location, orientation, and shapes of the patient’s abutments are exported, and can be used as a basis for designing the prothesis. Additionally, the patient’s abutment export can be merged with the patients gum scan data to further aid the prothesis design process. Note that instead of scanning the gums and markers in one scan, there may be an accuracy advantage in scanning first the markers only, then in a second scan the gums. This is because the motion of the soft tissue may cause more stitching errors than if the scan only focuses on the markers. However because of our single scan accuracy improvement techniques, this approach is unlikely to be recommended. Further, the markers may also be designed to connect directly onto a patients implants rather than onto the abutments that sit on the implants. The main surface of the markers may be positioned on the inside or the outside of the gum region. In addition to the 3D structure on the marker surface, (or instead of), a non repeating intensity pattern may be used. For example by printing randomly located triangles on the main marker surface. The shape and / or colour of the overall marker could also help to distuighish each marker from the others. The markers may be coloured. For example a green colour would help the markers standout from the red gums and white teeth, and their colour could be used to filter out the background making the markers easier to find. The same correction technique may be applied to any type of scanner such as a desktop scanner. Even without using specialist software accuracy correction, the features on the marker can act to increase the accuracy of the scan. Embodiment 2 In another embodiment the marker has a wide support structure that intentionally covers the gum region (an example is shown figure 5a). The accuracy improvement of the intra oral scanner depends on the marker providing 3D structure over the full area of the marker. This is useful to aid the stitching process during the scan, and simultaneously the marker covers up parts of the mouth that are hard to scan such as moving soft tissue around the tongue and featureless regions on the roof of the mouth. The positions of the abutments may be deduced using only by considering the tip of the marker in figure 5a, since the tips of the marker are close together when fitted in the mouth, the intra oral scanner is less likely to accumulate long range stitching errors, further increasing the accuracy of the method. Embodiment 3 The marker in figure 5b is designed under a similar principle to 5a, but there is a minimal support structure over the gum region so that the gums can be scanned at the same time as the tips of the marker. The positions of the abutments may be deduced using only by considering the tips of the marker, but the soft tissue gum region data can be collected in the same scan.

Claims

1. A pluerality of scan markers comprising;• a substantially flat upper surface, that is substantially covered with a non repeating 3D texture,• a connector that enables them to be attached to dental abutments or implants.

2. Scan markers as in claim 1, where the main surfaces of the markers are held substantially away from the gum region, having only a minimal support structure across the gum region.

3. Scan markers as in claim 1, where the marker shapes are known, and used to provide information about the relative positions and angles of each of the dental abutments or implants.

4. Scan markers as in claim 2 where the main surfaces of the markers for a track around the inside of the outside of the gum region.

5. Scan markers as in claim 1, where the non repeating 3D texture includes a non repeating intensity pattern.

6. A method using the scan markers in claim 1, where a 3D scan is made of the markers and the accuracy of the scan is determined by comparing the scan to the known dimensions of the marker.

7. A method as in claim 6, wherein the inaccuracies in the scan are retrospectively corrected based on knowledge of the actual marker dimensions.A

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