When Do I Decide to Order A CT Scan?

I love the summer. It’s one of the seasons that I can enjoy the outdoors. It is also one of the seasons that many of my out of town friends tend to come back to Toronto for a visit and so many great get-togethers are happening this year. So I apologize for the lack of activity on my blog while I am enjoying the sun and my friends.

Recently, I got together with my friends from dental school. And one of them asked me a series of questions regarding ordering CT scans for my patients. At the end, she thanked me for clarifying her many questions she had regarding the application of CT scan for prosthetic dentistry.  Then I realized that what I know about CT scan is not common knowledge amongst the general dentists. I thought there is probably some value to share my conversation here, so if you are confused when to use a CT scan, then you may find the information here useful.

First of all, there are many reasons for ordering a CT scan. For a prosthodontist, I am mostly ordering to evaluate the volume of bone available for implant supported restorations. Even in this context, with the available technology, there is no limit to what you can do with the information. You can simply order a CT scan to evaluate the available bone. You can use the information available to print a surgical stent for guided surgery.

Ordering a CT scan is easy. Fill out a referral form and sending it off to the facility who provides this type of service. Or better yet, purchase one in your office and then you can start prescribing CT scan as you see fit.  There are regional restrictions as to who can own this type of equipment. Always check with your local dental authority before you explore this option in your practice.

Ordering a CT scan that has practical use requires some planning. Planning that if you don’t know what you are looking for, may render your CT scan not as diagnostic as this technology is capable of providing you.  The reality is when you order a CT scan of a patient without any radiographic guide or prosthetic reference, then all you get is a detailed picture of the bony architecture available for implant placement. This provides no information where the prosthetic teeth can be. And to me, that is useless information. To me it is important to have some type of prosthetic reference.   I will expand on that later.

For me, the planning for implant restorations begins with a series of questions? Before I start to evaluate the edentulous sites, I want to evaluate the overall stability of the dentition. I will look for any obvious signs of posterior collapse, unstable occlusal relationship, irregular occlusal plane, inadequate interocclusal space etc. These are issues, if present, I will have to address them first prior to planning for the replacement of the missing teeth. These are issues, if not dealt properly, may lead to more prosthetic complications in the future.

Then, I will look at where the missing teeth are? Are they in the posterior area?  Or are they in the anterior region where esthetics are more important? For me a basic clinical exam and the required radiographs would have allowed me to evaluate approximately their ridge width and ridge height of the edentulous sites. For me, the two reasons I order a CT scan would be to provide more accurate information regarding available bone and their proximity to vital structures of potential implant sites.

If the interested sites are still occupied by their corresponding teeth that are planned to be extracted, then I would also evaluate their current position and angulation. If I am satisfied with the current position and angulation of these non-restorable teeth, then these teeth will be my prosthetic reference. However, if I am not satisfied with the position and angulation of the current teeth, then they will not be my prosthetic reference. Instead, I may want to perform a teeth try in arrangement to establish my prosthetic reference more accurately.

If the teeth are missing already, then I will check to see if the patient has any temporary denture available. If it is available, I will evaluate if I am satisfied with their arrangement of the prosthetic teeth. If I am, I may use the temporary denture as my prosthetic reference. If I am not, I will most likely arrange for another teeth try in to improve the esthetics. This has to be tested out in the mouth rather than arbitrarily planned on the radiograph or the CT scan. This is especially true for anterior teeth. I don’t believe you can evaluate how the lip drapes over the teeth or evaluate the incisal plane or teeth display on the radiograph. To me, this is a very important diagnostic step that should not be overlooked. Once you have confirmed the esthetics and occlusal relationship are satisfactory, then I can either convert the teeth set up into a radiographic stent or another temporary denture. These radiographic stents or temporary dentures are now my prosthetic references.

As you can see, these prosthetic references can take on many forms. They can be the patient’s actual teeth prior to extraction. They can be patient’s existing denture teeth if deemed satisfactory occlusally and esthetically. They can be worked up from scratch using your denture knowledge by arranging some teeth in wax to determine where your missing teeth should be.Once you have the appropriate prosthetic reference. Then they can take on many form to be merged with your CT scan. This can be used to evaluate diagnostically the available bone and compared that to where your teeth can be.

Traditionally, the prosthetic reference is in the form of what we call a radiographic stent. They are usually made of clear acrylic and embedded with radiopaque markers. These markers can be anything radiopaque such as metallic balls, gutta percha or barium sulphate powder mixed into the acrylic in the area of relevant teeth and soft tissue. This radiographic stent will be worn by the patient while the CT scan is being done.

An example of a radiographic stent with radiopaque teeth

The resulting CT scan depicting where the prosthetic teeth should be in relation to the underlying osseous structure.

 

If I don’t want  to make a radiographic stent but I have a temporary denture of adequate fit and satisfactory occlusion and esthetics, than the temporary denture is my prosthetic reference.   I will advise the CT scan facility to apply scan markers to the denture and have the patient wear it while the CT scan is being done. Keep in mind that this would be possible if there is minimal metal in the denture. I have been advised, if there is too much metal, it would produce too much scatter that it may not work well.

Both approaches would allow you to see on the CT scan showing the underlying bony architecture in relation to where your planned prosthetic teeth are to be. In this approach, you can evaluate both the available bone and the estimated crown contour and possible angulation and position of implant sites.  Please note the outline of the denture is marked in green on the CT scan.

The other approach would be to take an impression or to scan the teeth with the patient wearing the denture. With the impression approach, it is recommended to take a PVS impression of the teeth with and without denture. With the scanning approach, you can either scan the impression or the corresponding model or the patients’ dentition in the form of a STL file. The files can then be merged with your CT scan file. Once the files are merged with your CT file, then you can evaluate the available bone and comparing it to where your prosthetic teeth will be.

The information you have obtained from the CT scan and the prosthetic reference can be so powerful that you can take a step further and fabricate a surgical template that way. There are many types of surgical stents available and depending on how precise you want it, anything is possible. In fact, recently, many companies have utilized this powerful information and develop patented surgical stent to facilitate implant surgeries from single tooth scenario all the way to full arch implant supported prosthesis.

Digital technology has really expanded on how much we can do at this diagnostic phase of implant treatment planning. Taking a CT scan without any prosthetic reference can only get you some baseline information on the osseous side of the picture. It only provides surgical implant related information. It is only when you are able to merge the information of your planned prosthetic teeth to the CT scan can you plan the placement of implants more predictably to give you a better prosthetic outcome.  I hope you will find this information useful. If you have any questions, please do not hesitate to reach out to me.

 

This is an example of a case where the existing missing teeth were arbitrarily replaced by denture teeth during orthodontic treatment.  I have to perform a new teeth try in and merge the information with the CT scan.  A Simplant surgical stent was fabricated to facilitate the placement of implants.  The final prosthetic outcome was much more predictable and favourable.

Please check out my other posts too.  Thanks for reading!