Cone beam CT applications in oral surgery

Dr. Bart Silverman discusses the benefits of an added dimension in imaging

At present, one can go into a cardi-ologist’s office with chest pain and — within minutes — a non-invasive, three-dimensional scan can be taken so the doctor can actually look at the great vessels and structures supplying the heart. The cardiologist can see details so intricate, they were previously seen only through the intra-arterial insertion of a camera.

Think about the treatment afforded to a patient with this state of the art technology; it is limitless. Now imagine in the future if we can use this same technology in the field of oral and maxillofacial surgery. Imagine seeing a patient’s jaw structure, surrounding nerves, and actual anatomy without any magnification or distortion prior to placing dental implants. Imagine the three-dimensional treatment-planning capabilities. Imagine if you could see the bony structure many times prior to actually bringing the patient in for surgery. You would be providing a terrific service for your patient, as treatment plans could be very specifically designed for that particular patient without having to generalize as “in most cases we do this.” Your surgery time as well as the patient’s postoperative swelling and discomfort would be greatly reduced. Imagine also in the future if you could use this three-dimensional technology to see where a patient’s mandibular nerve canal lies in reference to a lower third molar that needs to be removed. The information obtained could be priceless.

Well, the future is here today. We now have the ability to obtain a cone beam computed tomography (CBCT) scan of a patient performed by a unit solely made for dentistry. In the past, if we wanted to send a patient for a CT scan, the only option we had was to have them go to a hospital or radiology facility for a medical grade CT. These scans exposed patients to high doses of radiation and often produced a low resolution scan that made treatment planning a challenge. The cost, price, and radiation have made this somewhat prohibitive.

Now, with the advent of an in-office cone beam scanner, we can deliver high-resolution, low-dose scans packaged with software that is specific for the field of dentistry. The footprint of these newer machines is no greater than the size of a panorex unit, which easily fits in most offices. In contrast to a medical-grade CT scan, where patients would have to lie down and remain perfectly still for the whole scan, patients comfortably stand or sit when the cone beam CT scan is performed.

There are many advantages of having an in-office machine. The ease of being able to walk the patient to a part of your office and take a scan for them is great. No longer do you have to give the patients a prescription for a study and hope that they get it done, nor do you have to spend the time tracking down the scan. Think of the benefit of being able to do the scan, developing a treatment plan specific for the patient, reviewing the plan, and then scheduling the surgery all in the same visit.

Ever since Wilhelm Roentgen’s accidental — but momentous — discovery of X-rays in 1896, innovative imaging technologies have continued to dramatically improve health care. Digital radiography started in the field of dentistry with Dr. Francis Mouyen, a French dental student who developed it in 1982. Cone beam CT (CBCT) started around 2000 with the early adoptors being some dental schools, surgeons, and radiologists. Now one can hardly pick a dental journal without seeing two to three articles about the exciting technology of three-dimensional scanning. The increase in demand for imaging in the oral surgical practice has increased because of the need for better correlation between surgical placement and prosthetic restorability of dental implants as well as improved surgical techniques, which require three-dimensional visualization of the dental anatomy. We no longer have to treat three-dimensional patients based upon two-dimensional diagnostic means; rather we can develop three-dimensional treatment plans to treat our three-dimensional patients.

Cone beam CT: What is it?

Cone beam CT is a three-dimensional radiographic tool that can be used to obtain anatomically accurate information. This information, in turn, can help identify possibilities and limitations of treatment as well as provide us with a powerful communication tool to be utilized with our patients and referring doctors. In the past, say, in the posterior maxilla, when two-dimensional radiographs were obtained, you had superimposition of adjacent roots, posts, gutta percha, the zygomatic buttress, and floor of the maxillary sinus. It is almost like a series of window panes superimposed on top of each other. As dentists, we had to use our mind’s eye to pull out the structures we did not want to see and try to visualize only what we needed to see. With cone beam CT, on the other hand, we can pull out the window pane we want to see and clearly visualize the desired structures. A cone beam CT gives you an exact one-to-one representation of the clinical situation. When using a panorex radiograph, it is almost like when we were in elementary school and took a globe and unfolded it and laid it out on a table and made a map. Inherently, there is a degree of elongation or magnification built into the process. This does not happen with cone beam CT.

How does it work?

Just like any other scan, the patient stands or sits by the machine, and the operator lines up the machine using a series of laser lines. The machine then rotates around the patient 360 degrees, and a cone beam of radiation is emitted against some type of imaging plate. The scans typically take less than 30 seconds. A primary reconstruction is obtained, and a series of DICOM files are formed. These DICOM files can be reconstructed and viewed as in the standard axial, saggital, coronal, or transaxial cuts, as with conventional CAT scans. The images can also be reconstructed to form 3D images. Once performed, a computer monitor is used to view the images.

Available systems

There are quite a few systems now available for use in the field of dentistry and oral surgery. With the advent of the newer scanners particularly made for dentistry, clinicians can have higher resolution scanners with less radiation. The scanners also now have software that is made specifically for dentistry. The scans are a lot faster, and reconstruction times have been greatly reduced.

Uses of cone beam CT in oral and maxillofacial surgery

Implants

The uses of cone beam CT in the oral and maxillofacial practice are almost limitless. Probably the number one use for cone beam CT for an oral and maxillofacial surgeon would be for implants. A cone beam CT can be used to determine the quantity and quality of the patient’s bone in order to assess the possibility and feasibility of implant placement. One can determine the amount of bone above the nerve canal or below the maxillary sinus, which can significantly reduce surgical morbidity. The clinician can assess bone morphology before taking a patient to surgery.

The degree of accuracy from these scans is so high that very precise surgical guides can be fabricated. This can increase accurate implant placement and allow for ease of prosthetic restorability. Surgical time is greatly reduced, and this is translated into less swelling and postoperative discomfort for the patient. By having this data set, the surgeon can reformat the DICOM files into different views many times prior to the procedure. This allows the surgeon to visualize the patient’s jaw structure before the actual surgery in order to potentially reduce unwanted results and untoward implant placement. A surgeon now can look at the patient’s bone and develop a treatment plan that is very specific for that particular patient. No longer do we have to generalize about what we may do at the time of surgery and then actually determine the exact plan once we open up the surgical site. We can now confidently say to our patients exactly what their treatment plan will be.

We also know how much time to set aside and can determine exactly what the patient’s cost will be based upon the exact treatment plan we will perform. At the same time, we are building confidence with our patients. Imagine if you went to a cardiologist and he or she said to you, “We have the technology to see and plan exactly what we’ll do to treat your chest pain, but we don’t need to do that. We can open you up, and if this vessel is occluded, we will do this. If this vessel is open, but this other one is blocked, depending upon the blockage, we may take a leg vein graft, or not, etc.” You would run out of there.

Once implants are placed, positioning can be confirmed by a follow-up cone beam CT scan. One must evaluate the potential gain versus the increase in radiation from a subsequent scan.

TMJ uses

A great use for cone beam CT is in the field of temporomandibular joint (TMJ) problems. Using this technology, one can assess morphological changes as well as see arthritic changes in the joint and surrounding bony structures. This is an improvement over diagnosing fractures of the TMJ using two-dimensional plane films, as hairline or greenstick fractures are difficult to visualize on many occasions. Also difficult to determine has been the position of the fractured condylar segment whether it is medial or lateral. On the other hand, superior positioning of the condyle into the middle cranial fossa following a severe motor vehicle accident can be very easily seen with the use of CBCT.

Impacted teeth

Probably the second biggest use for cone beam CT in the oral surgery field is impacted teeth. For those of us who — after determining the position of an impacted maxillary canine — had to start drilling palatally after starting buccally, you can really appreciate the embarrassment of not performing a cone beam CT scan. Multiple periapical radiographs taken at different positions and various angles in order to try to three-dimensionally locate the teeth are no longer necessary, as exact position of impacted teeth can be determined preoperatively. Surgical treatment plans and access can also be determined quickly and efficiently. By having an exact plan before starting the procedure, surgical time will be reduced.

By reducing surgical time, we are ultimately reducing the time patients are under anesthesia as well as swelling. Patients will heal faster, and these surgical outcomes can translate into additional referrals from our patients and referring doctors.

Juxtaposition of third molar roots with the inferior alveolar canal can also be determined. When discussing the risks of surgery, particularly paresthesia, a surgeon can be very specific with their patients and their patients’ family members. You can visualize the position of the impacted tooth within the alveolar bone and its location relative to the adjacent teeth and surrounding neuro-vascular structures. This may also help determine if it is wise to remove asymptomatic third molars and whether it is prudent to remove these teeth in the office or hospital settings.

There was a great study performed by Park and Choi1 where they looked at the cortical integrity of the inferior alveolar canal as a predictor of paresthesia after third molar extraction. The prevalence of paresthesia was higher where third molar to inferior alveolar canal continuity was disrupted. The frequency of nerve damage increased with the number of cone beam CT image slices showing loss of cortical continuity. By using a cone beam CT, surgeons can help plan their surgeries and educate their patients on a realistic risk-versus-benefit situation.
Surgical orthodontic uses

Of course, an obvious use in oral surgery in conjunction with orthodontics is in orthognathic surgery. The cephalometric view (if the scanner has a full-field view or a cephalometric arm) can be used to analyze each case, and the cone beam CT component can be used once a treatment plan is decided upon to evaluate the patient’s bone morphology and determine if there are any anatomical limitations to the proposed treatment. The surgeon can also look at the patient’s bone anatomy where the proposed osteotomies will be performed, in order to see if they are conducive to the planned surgery.

Temporary anchorage devices, or TADS, are used to aid the orthodontist in tooth movements where there may be lack of posterior anchorage or in patients that are — or would be — non-compliant with an external head gear. Three-dimensional volumes give a much better view of the special relations of adjacent roots in suggested TAD placement areas than a 2D periapical radiograph. Three-dimensionally, the colleagues can discuss movement of adjacent teeth prior to TAD placement to help insure optimum positioning for the patient and less risk of placement into the roots of the adjacent teeth.

Sleep apnea

Oral nasal airways can be measured, and three-dimensional volumes can be determined using cone beam CT technology. Appliances can be fabricated and, after a certain time frame, evaluated based on a subsequent scan to see if the airway measurements or volumes have changed. Now, there can be a concrete method used to determine the effectiveness of surgical or non-surgical therapies.

Pathology and trauma

With cone beam CT scans, a surgeon can take a look at a lesion, such as a fairly large jaw cyst, before the patient even comes into the surgical suite. This technology allows surgeons to see if the buccal and lingual cortices are intact as well as to evaluate teeth or nerve canal displacement if any impacted teeth are associated with the lesion. If a primary reconstruction with bone grafting is planned, an exact measurement of the bony volume that needs to be replaced can be determined and planned for ahead of time. All of this planning ahead of time allows you to be more confident in your surgical approach and allows for a quicker surgery, which translates into less surgical swelling and discomfort for our patients. In evaluating an odontoma, it is beneficial to see where it extends and what other structures may be involved when planning the case.

With tooth avulsions, luxations, fractures or other dentoalveolar trauma, a high-resolution, three-dimensional focused-field volume would aid your diagnosis better than a two-dimensional periapical radiograph. You can now visualize alveolar fractures and no longer have to feel along the alveolar ridge for movement when you palpate the involved dentition.

With maxillofacial trauma, one cone beam CT scan can be obtained, and the DICOM files can then be reviewed, obviating the need for four or potentially five other X-rays. Radiation to the patient is limited. Once the DICOM files are taken, they can be reconstructed many times without subjecting the patient to more and more radiographs. The cone beam CT can also be performed postoperatively, which can allow the surgeon the ability to immediately see if the fractured bones and fixation were performed adequately.

Surgical endodontic uses

Think of the last time a patient presented to your office with an alveolar swelling opposite his/her maxillary first molar: A periapical radiograph is taken and demonstrates a periapical lesion associated with what looks like only the MB root of a previously endodontically treated tooth. A sulcular or vestibular incision is made, thereby exposing the periapical area. Not only is the MB root affected, but you must chase the periapical pathology back to the palatal root, only to find out this was the cause of the abscess. If a focused-field cone beam CT was performed preoperatively, one could then review the volume and see on the coronal and sagittal slices where the pathology is emanating from. You can use the axial views and see if the pathology extends into the sinus or not. A more definitive treatment plan can be determined to better treat the patient. Is it best to have the canals retreated, or to perform an apicoectomy or to remove the tooth, bone graft, and place an implant?

Diagnosis and treatment planning

When we decide to pursue a career in dentistry, one of the first things we have to do is take a dental admission test. As future dentists, we are tested on our three-dimensional perception. Those of us who excel are admitted into a dental school. We then spend 4 years looking at two-dimensional radiographs in order to treat three-dimensional patients. Aside from not making sense, if one can see problems three-dimensionally, it certainly can change our treatment plans.
This patient came to our office with a cystic area seen superiorly to an impacted mandibular left third molar on a periapical radiograph taken by his general dentist. I took a panorex radiograph to fully visualize the tooth (Figure 3).

We then decided to obtain a cone beam CT scan of the area to see if the cystic area affected the buccal or lingual plates of the bone before we performed a biopsy. If one looks at the cone beam CT transaxial view (Figure 4), not only does the tooth extend most of the vertical portion of the mandible, but also the whole buccal lingual width. If you decided to remove this tooth based upon the two-dimensional panorex, you would be in for a huge awakening when you found out there was no bone on the buccal or lingual when you were removing the tooth, thereby leaving the jaw in a severely weakened state.

In the following case, I received a call from an orthodontist about a patient we had treated several years earlier (Figure 5). He reported that the cyst that I removed had recurred. As it was an eruption cyst, I didn’t think it was possible. The orthodontist then sent over a copy of the panorex he had taken.

It appeared to demonstrate a large radiolucency of the right mandibular ramus. A cone beam CT scan (Figure 6) was taken to help delineate the borders of the lesion before removal or marsupialization was performed. The cone beam scan showed no right ramal expansion or bone thinning, as would be expected. The triplanar views actually demonstrated a bowing of the cortex (Figures 7 and 8).

A follow-up MRI yielded no soft tissue tumors as well. If a cone beam was not used to validate the suspected lesion, unnecessary surgery would have been performed.

In this last case, the patient had presented from his orthodontist with a series of three panoramic radiographs taken 2 years apart. The orthodontist was concerned about the non-eruption of the maxillary left second molar (Figure 9).

Reviewing the two dimensional radiographs, I didn’t know why the tooth did not erupt. It was decided to do a cone beam radiograph (Figures 10 and 11).

Upon review of the cone beam scan, one can see the reason for the unerupted maxillary left second molar was its fusion to the impacted maxillary left third molar.

Computer-generated surgical guides

The use of cone beam CT in the fabrication of computer-generated surgical guides is probably one of the most exciting uses for this technology. In the past, our planning of multiple-placed implants would involve taking a panoramic radiograph with an acrylic tray with some gutta percha placed. Now, surgical-guided techniques allow the fabrication of computer generated guides through the use of cone beam CT to allow prosthetically driven implant placements (Figures 12 and 13).

As you can see, there are many uses for a cone beam CT scanner in the oral surgical field. It seems as every day, more and more uses come about. It has definitely changed the way we practice. I question if we would be able to provide the same care for our patients without it.

Reference
1. Park W, Choi JW, Kim JY, Kim BC, Kim HJ, Lee SH. Cortical integrity of the inferior alveolar canal as a predictor of paresthesia after third-molar extraction. J Am Dent Assoc. 2010;141(3): 271-278.