In this blog, we will walk you through how to prepare for a successful intraoral scan for general prosthodontics.
To get accurate scan data, you need to follow four steps: tooth preparation, gingiva control, fluid control, and scan data check.
While these steps are also required for silicone rubber impressions, they are vital for intraoral scans.
Tooth and Inlay Preparation
Keep zirconia’s properties in mind when making a prothesis with an oral scanner. Zirconia is hard but thin, and therefore fragile. A zirconia-made crown should have a minimum thickness greater than 1.0mm.
In the case of a zirconia inlay, the shape of the cavity should be a simple “open box” without an undercut. When you work with an intraoral scanner, it is model-less, which avoids the undercut in the preparation teeth. As a stone model is not used, it is not possible to block out the undercut (cannot control the size of the block out when editing CAD scan data).
You need to be careful of two aspects. First, if the margin line is sharp like a ‘J’ shape (J margin or fin), it will need to be removed during the margin preparation. If not, the shape won’t be milled accurately, resulting in a faulty fit on the margin area. Second, sharp cusps should be avoided as they will be difficult for the milling bur to go in, which may cause the final product to be too thin.
You should take the bur thickness of the milling machine into account and make sure the anterior teeth’s incisors and posterior teeth’s cusps aren’t prepared too sharply. If so, the zirconia crown will be thinner as the inside of it will have been excessively milled.
In case of equigingival or subgingival margins, gingival retraction is necessary. If the margin is subgingival, the line is not easy to scan as it is under the gingiva. Hence, supragingival margins are easiest to work with during margin placement.
With silicone impressions, there is a gap between the tooth and the gingiva caused by the impression material. The impression can be taken with cord packing as the gingiva is separated with applied pressure. However, digital impressions require gingival retraction because you will scan the oral condition as it is at the time. Therefore, thorough gingiva conditioning is required to capture accurate scan data.
There is a way to expose the margin line using cord packing or a laser, but it is best for the margin placement to be supragingival. In case the margin is subgingival, insert two cords for proper gingival retraction. Right before scanning, remove one cord.
Finally, it is essential to control saliva and blood before scanning. You should remove stagnant saliva on the teeth. Although you don’t want the mouth to be completely dry, it needs to be dry enough to eliminate any fluid on the teeth’s surface and prevent bubble creation. Make sure to control for blood. Remember that blood will also be scanned, which would lower scan accuracy and even if scanned, the data won’t be usable. Gingiva must also be healthy. Unhealthy gum tissue makes it tougher to obtain accurate scan data due to liquid leaking from the tissue. If there is any tissue fluid or inflammation, it is difficult to obtain scan data and thus less likely to create proper prosthetics.
Scan Data Check
After scanning, check the scan data. There are four points to check: the preparation tooth, soft tissue, adjacent teeth, and occlusion.
- Working tooth
After scanning, check the margin line, make sure there is no undercut, and that it was sufficiently prepped.
- Soft tissue
With soft tissue, some parts are deleted automatically but recognition is slower without proper soft tissue retraction such as: did not align well during the scan, did not locate the bite well and incorrectly aligned it, the soft tissue data was attached to the teeth scan, or the data volume was too large (post processing speed slows down as the data volume increases).
Also, when unnecessary data accumulates, capacity increases. This results in scan data not aligning to the correct position.
When retracting soft tissue, use a mirror or finger to ensure enough retraction. You could also use a retraction appliance such as OptraGate. For plastic openings, avoid using the scanner because of its limited space in the mouth.
- Adjacent teeth
As is the case with silicone impressions, digital impressions need adjacent teeth information to make an accurate prosthesis. If a preparation tooth is a posterior tooth, you have to scan 1.5-2 adjacent teeth. If it is an anterior tooth, it would be better to get the full anterior scan data.
When scanning adjacent teeth, get the occlusal surface, contact points, and height contour. Scan the shape of the adjacent teeth for the design.
After the occlusion scan, check if it is aligned in the same position as the physical occlusion. Look at the contact points on the occlusal surface with a tool or by checking the visible areas as you rotate the scan data.
If you keep the listed steps in mind, you should be able to produce accurate scans in no time! You can also find a link to a video-version of this blog here or under the 'learning' section on the website.