Dental Imaging in Implant Treatment Planning

Abstract

Objectives: 

Proper implant treatment planning remains the first priority for implant success. Dental imaging is an important tool to accomplish this task. Traditional radiographs provide adequate information about proposed implant sites; however, limited film size, image distortion, magnification, and a 2-D view restrict their use in some cases. The purpose of this study is to provide an update about recent advancements in implant imaging to facilitate ideal implant treatment planning.

Search Strategy: 

A literature search was conducted using MEDLINE to identify studies related to this topic using the keywords of implant imaging, computed tomography (CT), cone beam CT (CBCT), and digital implant planning.

Results: 

Medical CT scan produce 3-dimensional replicas of anatomical areas with high resolution and accuracy. Although this type of imaging was introduced almost 20 years ago for implant planning, until recently it is widely used in most of the advanced procedures. CBCT is an advanced version of this technique. The advantages of CBCT are its specific design for the maxillofacial region, a reduced radiation exposure, cheaper, and excellent quality of images. Today, many companies are developing these cutting-edge machines and making it possible for dentists to use in their practices.

Conclusions: 

Coupled with converting software programs, CT/CBCT images may assist in selecting implant dimensions and predicting treatment outcomes. Understanding the up-to-date development of imaging aids could potentiate our ability in planning implant therapy.

One of the most important factors in determining implant success is proper treatment planning. Traditionally, conventional radiographic images (e.g., periapical and panoramic films) have been used to assist practitioners in planning implant treatment. Periapical radiographs present faithful images in terms of the size of the object examined as long as a parallel technique is used. Panoramic radiograph is an excellent tool for the overview of the maxillofacial area, including many of the vital structures, such as alveolar bones, maxillary sinus, inferior alveolar nerve (IAN), and temporomandibular joints. These conventional radiographics can also be used to identify pathologic abnormities. Conventional tomograms with cross-sectional views have expanded our ability to see 2-dimensional images to 3 dimensional levels. Although the earlier radiographic methods are commonly used, they have some inherent shortcomings, for example, a periapical film can only include about 3 teeth. Another limitation is the bending of the radiographic film to minimize patient discomfort, which may introduce some errors.¹ With regard to panoramic films, the lack of image sharpness and resolution, coupled with nonuniform distortion often leads to inaccurate interpretation and measurements.² The magnification of panoramic radiographs can be >30%, especially when patients are not in the optimal position.³ Although offering an additional view, tomography can be difficult to interpret due to the wide depth of field in which objects anterior and posterior to the target blend into the image, creating an out-of-focus image. Furthermore, their magnification rate can be 40%.⁴

With the advent of technology, computed tomography (CT) has lead to a new era of implant imaging. CT offers all ranges of images such as cross-sectional, panoramic, or 3-dimensional views and each has high resolution as well as accuracy. Nowadays, CT scans are commonly used for implant treatment planning. Nevertheless, CT scans are not without their limitations/concerns and radiation exposure and cost are the major two. Furthermore, because of the size of the machine, CT scans are usually reserved for hospital settings.

Cone beam CT (CBCT) scanners, newer generation machines specifically designed for the maxillofacial region, have allowed for reduction in the radiation absorbed by the patient. It uses a single 360 degrees rotation around the maxillofacial region and a cone beam, in comparison, a spiral CT, which makes several rotations and uses a fan beam. When matched up next to the conventional CT, the lower cost, radiation exposure, and in-office feasibility of CBCT render it the ideal model for oral and maxillofacial radiology.

Another distinct advantage of CT/CBCT scan is the ability to plan implant therapy virtually with special 3-dimensional programs.⁵ Some commonly used programs are Materialise's Simplant (Belgium), Nobel Biocare's Procera (Sweden), Implant Logic's VIP (USA), and iDent's Scan₂Guide (Israel). When those programs are applied, different diameters and length of implants can be “tried in” before the most optimal one is selected. Furthermore, the “placed” implant can be evaluated from several different viewpoints as well as 3-dimensional space. Moreover, once treatment planning is determined in the computer, it can be saved and applied to surgical sites by means of image-aided template production⁶ or image-aided navigation.⁷ The first aim of this review article is to summarize the current status of CT imaging, especially the advantages and the disadvantages of CT scan. The second aim is to discuss unique features of CBCT in comparison with CT scan. The third aim is the discussion on interactive 3-dimensional treatment planning for implant placement using software programs.

Computed Tomography

CT is a digital medical technique, which can generate 3-dimensional images of a patient's anatomy by reconstructing many axial slices. The newer generation of CT scans produces axial images perpendicular to the long axis of the patient by rotating a radiation source which emits fan-shaped beams 360 degrees around. The detectors capture x-rays, which transmit the subject and the data is processed by a computer. Because of the unique way, a CT scan acquires images and the reconstruction technology, CT imagines own high resolution and accuracy with minimum distortion and magnification.⁸

One of the prerequisites for proper implant treatment is to identify pertinent anatomical structures. Encroachment or damage of vital structures are unwanted complications and can be avoided. For example, in the posterior mandibular region, the IAN is an area of concern. CT scans can identify most inferior alveolar canals when multiple cross-sectional views are performed.⁹ Although only few cases are reported in the literature, lingual plate perforations in the interforaminal area and their subsequent damage to branches of sublingual artery can be fatal.¹⁰ Cross-sectional views obtained by CT scans can help clinicians to avoid these structures/problems. Tepper et al¹¹ have shown CT scans recognized at least 1 lingual perforating bone canal in the mandible. Therefore, this implies that the CT is a useful tool for planning implant procedures in the mandible to avoid complications.

CT scans can assist clinicians to select the proper implant diameter, length, and ideal position for placement. Images should be taken at one-to-one ratio without any distortion or magnification. CT scans have been shown to be very accurate with the magnification effect, the same for both the anterior and posterior area, from a range of 0% to 6% in horizontal as well as 0% to 4% in vertical dimension.³ It is not surprising that implants placed based on CT results were more consistent with the planning treatment outcome, compared with panoramic views, which have a tendency to underestimate the implant length due to distortion and magnification effects. Pecker et al⁴ compared the accuracy of 3 imagining methods: panoramic, conventional tomography, and CT for localizing the IAN. The various distances related to IAN were measured from different images and compared with direct measurements using a digital caliper. The result showed that the measurements obtained from CT were more consistent with direct measurements. The deviation from direct measurements was within 1 mm 97% of the time, compared with ∼80% from panoramic views or conventional tomography.

Exposure Concerns From CT

The New England Journal of Medicine has recently raised the concern about the radiation doses acquired with the prevalent use of CT scans.¹² As a clinician, we must first understand the measurements of radiation so the comparison of a CT scan with different radiographic techniques can be made. This allows for better communication to relieve patient's concern. The ultimate goal is to obtain essential diagnostic information while adhere to the ALARA principle (as low as reasonably achievable). Radiation dose is represented in several ways for different purposes. The relevant units for CT scans are absorbed dose and effective dose.

The absorbed dose is defined as the energy absorbed in per unit of mass of any type of matter and is measured in grays (Gy). One gray equals to 1 joule of radiation energy absorbed in per kilogram. Specifically, the organ dose is derived from absorbed dose and means the distribution of dose in an organ of interest. It determines the quantity of dose received by that organ and thus the level of risk of that organ.

The effective dose, expressed in sieverts (Sv), is designed to estimate the overall harm of radiation in humans. In other words, it measures the equivalent whole-body dose. Because each organ is not sensitive to radiation equally, the tissue weighing factor (the radiosensitivity of different tissues for cancer formation or heritable effect) is considered before adding up all doses in different organs.

Radiation doses from any given CT study depend on a number of factors. The most important are the number of scans, the axial scan range, the scan pitch (the degree of overlap between adjacent CT slices), the tube voltage in the kilovolt peaks (kVp), the tube current and scanning time in milliampseconds (mAs), the size of the patient, and the specific design of the scanner being used.¹²

There are several ways to compare radiation exposure from different imaging sources and the easiest is to compare the absorbed doses. These were obtained from in vitro studies using anthropomorphic phantoms with thermoluminescent dosimeters fixed at appropriate locations, mimicking radiation exposures during examinations. Comparisons can also be made through annual natural background radiation or estimated annual risk of death.¹³

Most of the quantitative information regarding the risks of radiation-induced cancer comes from cohort studies of survivors of the atomic bombs dropped on Japan in 1945. The survivors have a significant increase in the risk of cancer. The cancer risks associated with CT exposure can be estimated by calculating the organ doses involved and applying organ-specific cancer incidence or mortality data derived in these studies.

Generally, CT produced considerably higher doses than conventional tomography and other image techniques. A study showed that doses absorbed by most organs were 3 to 10 times higher in CT scans when compared to conventional tomography and by single organs, up to 200 times higher.¹⁴ A review article assesses the radiographic exposure from various methods and the associated risk to implant patients.¹⁵ The effective dose of CT when 1 jaw is exposed ranges from 250 to 560 uSv, compared with 60 uSv when a full mouth periapical view is taken and 30 uSv when a panoramic film is acquired. When only 1 site is examined by a conventional tomograph, the effective dose is measured ∼5 times less than CT scan. However, it is noted that when a whole maxilla or mandible is to be examined, the amount of radiation dose is similar between the 2 methods. The probability of death from CT scan reflects on those numbers. It is estimated that 12 to 28 people die from radiation-associated malignancies per million CT examinations.¹⁵ The radiation of a jaw exposed to CT scan is equal to 26 to 33 days of background radiation, compared with 1 day of background radiation during panoramic examination.¹⁶Tables 1 and 2 summarize radiation doses from various modalities and associated risk.

Table 1: Comparison of Adsorbed Doses (mGy) Received in Various Radiosensitive Organs

Table 2: Effective Dose and Associated Risk

Cone Beam CT

Because of higher radiation exposure, higher cost, huge footprint, and difficulty in accessibility associated with CT, a new type of CT, CBCT was developed.^17,18 CBCT was previously used in radiotherapy and have been applied in space, defense, and nuclear industry fields besides medicine. The primary difference between CBCT and CT is the shape of radiation beams and the mode of motion. As the name implies, CBCT generates cone-shaped beams and the imagines are acquired in 1 rotation by an imagine intensifier or flat panel detector, resulting in reasonably low levels of radiation dosage. In terms of patient's position, with CBCT machines, patients are seated or standing rather than supine. The theoretical resolution of CBCT is higher than CT. The voxel size, an indicator of resolution, can be as small as 0.1 mm for CBCT when compared to 0.5 mm for modern CT. In terms of economical aspect, CBCT is comparably more affordable for patients than CT and the estimate cost is about US$400 for both arches.¹⁹Table 3 summarizes comparisons between CT scan and CBCT.

Table 3: Comparison Between CT and CBCT

Radiation Dose Reduction

One of the features of CBCT is the reduced radiation dose compared with CT. Recent studies on radiation doses suggested that generally patients received less radiation burden from CBCT than CT. The dose differences are detailed in Table 2. One aspect which should be addressed is the dose variation among different brands of CBCT. The NewTom 3G (QR, Verona, Italy) had fewer radiation exposure than iCAT (Imaging Sciences International, Hatfield, PA) by a factor of 2 and had 10 times less than CB Mercuray (Hitachi Medical Systems America, Twinsburg, OH) in similar conditions.²⁰ The primary reason for the difference is the higher electric current used (10 mA compared with 1.5 and 5.7) and longer exposure time (10 sec in contrast to 5–6 seconds) associated with CB Mercury. Generally, the size of the images dictates radiation doses. Large field of view is usually a synonym for more radiation exposure. Although different machine specifications and settings make direct comparison difficult, a study has showed that CBCT is a dose-sparing method by reducing effective doses up to 10 times compared with CT under similar condition.²¹

Validation of CBCT

CBCT is able to identify the IAN in clinical studies. One study²² evaluated the ability of CBCT imagines to visualize IAN as well as alveolar crest in thirty patients without second premolar and all molars in the mandible. Seven observers examined 1 cross-sectional imagine around 1 mm posterior to mental foramen in each patient to evaluate whether the structures were clearly visible, probably visible, or invisible. The result showed that the visibility of anatomical structures examined as well as inter-examiner agreement was high with CBCT images. Another study compared the quality of CBCT imagines with panoramic images. Panoramic view reformatted from CBCT scan has shown better results in recognizing IAN than digital panoramic films when a 4-point subjective scale was used for evaluation.²³

CT is considered the gold standard for its accuracy.²⁴ Comparison of linear accuracy of CBCT with CT have found CBCT is more accurate than CT.²⁵ The mean error was 4.7% (in dry mandible) and 2.3% (in sucrose solution) for CBCT, compared with 8.8% and 6.6% for CT, respectively. Chen compared ridge mapping, CBCT images and direct surgical access measurement in 16 patients with 25 implant planned sites.²⁶ Ridge mapping showed 89% to 94% of measurement deviations within 1 mm, while CBCT had 55% to 70% for the same deviation range. The authors questioned reliability of using CBCT in determining bone width. Nevertheless, it is noted that the slice thickness was 2 mm in this study, which may be too thick for the examination. On the basis of the earlier findings, CBCT can also identify some critical anatomical structures relevant to implant placement and provide accurate images for implant planning.

With the popularity of CBCT, more and more companies are developing new models to improve the properties of images. Table 4 features some of currently available CBCT machines in the market.

Table 4: Comparison of Some of Currently Available CBCT Devices

Interactive Implant Treatment Planning

The philosophy of prosthodontic-driven implant placement has revolutionized how implant dentistry is practiced.²⁷ The idea of placing implants based upon available bone has long gone. In this new era, functional, esthetic, and prosthetic applicability are all incorporated into overall implant diagnosis and treatment planning. The prosthetic designs dictate the position of dental implants. Model-based treatment planning with the assistance of CT images has been developed to fulfill this purpose.⁴ After a diagnostic cast and a preplanned wax-up, diagnostic templates are fabricated or modified from existing dentures.²⁸ The implant position as well as direction is determined based on final restoration position using radiopaque material, such as gutta percha²⁹ or metal pins¹⁹ to mark the spots. Images are then evaluated for available bone height, width and related vital anatomical structures. According to this information, changes can be made to accommodate final implant position. A second guide can be fabricated manually based on the modification or this stent can be used as a surgical guide during the operation. This approach has shown to be a successful technique. In a cadaver study, the mean angular error was 1.3 degrees (max: 4 degrees) and the mean horizontal error was 0.4 mm (max: 1.5 mm).³⁰ A clinical study evaluated the transfer error of a surgical template by comparing the proposed and actual direction. The deviation is on average 5 degrees, with a range of 0.5 degrees to 14.5 degrees.³¹

With the aid of interactive software, another approach for the transfer of implant planning to the surgical site is to use computer-aided design/computer-aided manufacturing technique. Many software programs are currently available (Table 5). These programs enable a clinician to transfer CT/CBCT findings into surgical area.

Table 5: Comparison of Some of Commonly Used Software Programs for Implant Planning

Using one of these programs, the implant treatment team has the ability to “try in” different diameters and length of implants and selects the ideal one. Furthermore, the placed implant can be evaluated from several different viewpoints as well as from 3-dimensional view. It can also be rotated and tilted on any axis to adjust its position. For multiple implant placements, the parallel relationship to simplify prosthetic reconstruction can be ascertained. Moreover, once treatment planning is determined in the computer, it can be saved and applied to surgical sites by means of image-aided template production⁶ or image-aided navigation.⁷ The primary interest in the literature is the accuracy of transferring treatment plan into surgical field. CT based implant planning and its transfer to the surgical field via a surgical guide has resulted ∼1 mm of mean linear deviation and 3degrees mean angulation difference. Although computer-aided-implant placement is a promising technique, the unexpected high linear deviation which sometimes reaches 4 mm and angulation deviation 17 degrees³² can be a major concern. Hence, more research is needed before this approach can be widely used. Figure 1 illustrates how CT/CBCT can assist in implant planning.

Fig. 1.: Procedures of software-aid implant therapy.

Conclusions

CT assists clinicians in identifying bone volume, jaw tomography important anatomical, and landmarks. However, because of its higher radiation dose and cost, CBCT was introduced to overcome these shortfalls. With images acquired from CBCT and the assistance of software programs, clinicians may now place implants in the ideal position while avoiding vital structures so they can fulfill esthetic, functional, and biologic demands.

Disclosure

The author(s) claim to have no financial interest in any company or any of the products mentioned in this article.

Acknowledgments

This article was partially supported by the University of Michigan Periodontal Graduate Student Research Fund.