|Year : 2015 | Volume
| Issue : 3 | Page : 19-26
Recent advances in imaging technologies in implant dentistry
Department of Oral Medicine, Diagnosis and Radiology, Dental and Maxillofacial Diagnostics, Ghaziabad and Gurgaon (Delhi-NCR), India
|Date of Web Publication||31-Dec-2015|
Department of Oral Medicine, Diagnosis and Radiology, Dental and Maxillofacial Diagnostics, Gurgaon, Haryana and Ghaziabad, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Dental implants have become a part of routine treatment plans in oral rehabilitation. Diagnostic imaging is critical to presurgical treatment planning and the success of implants. Various imaging modalities may aid the placing of implants in an appropriate location with relative ease and also obtain a predictable outcome. Cross-sectional imaging techniques such as computed tomography (CT) and, more recently, cone beam-computed tomography (CBCT) are invaluable during preoperative planning for endosseous dental implantation procedures. An understanding of geometric and software parameters, and image formatting options to maximize image display is necessary to optimize diagnostic yield while maintaining minimal patient radiation dose. Multiplanar CT or CBCT images contain far more detailed information of the maxillofacial region than do panoramic or other bidimensional (2D) images and necessitate a thorough knowledge of the tridimensional (3D) anatomy of the region and considerations of variability in the range of the anatomically normal. This article provides: (1) an overview of the fundamental principles of operation of maxillofacial CT and CBCT; (2) an understanding of image processing and display protocols specific to pre-implant bone assessment; (3) the basics of qualitative and quantitative bone evaluation; and (4) an introduction to image-guided implant surgery using custom or computer-generated surgical guides.
Keywords: Computed tomography (CT), cone beam-computed tomography (CBCT), dental implants, imaging
|How to cite this article:|
Sahai S. Recent advances in imaging technologies in implant dentistry. J Int Clin Dent Res Organ 2015;7, Suppl S1:19-26
| Introduction|| |
The growing older population and the consequent development of edentulism have increased the number of imaging studies performed for preoperative evaluation of dental implantation. Radiographic evaluation is essential for assessing bony support for endosseous dental implants. Several intraoral and extraoral radiographic methods, such as periapical, occlusal, panoramic, and motion tomography, are commonly available for evaluation of the implant recipient site, but the information is based on bidimensional (2D) geometric projections.
The ideal imaging technique for pre-implant bone evaluation should have several essential characteristics, including the following: The ability to visualize the implant site in the mesiodistal, buccolingual, and superioinferior dimensions; the ability to allow reliable, accurate measurements; a capacity for evaluation of trabecular bone density (bone mineral density, BMD) and cortical thickness; reasonable access and cost to the patient; and minimal radiation risk.
The most accurate technique for preoperative evaluation of dental implantation is dental computed tomography (CT), which has more recently been replaced by cone beam-computed tomography (CBCT). Multiplanar CT and CBCT can demonstrate the quantity of bone in three dimensions, the location of important adjacent anatomic structures (e.g., mandibular canal, dental inferior nerve, incisive foramen, mental foramen, maxillary sinus), and the quality of available bone with minimal geometric distortion. Drawbacks of conventional X-ray techniques, such as anatomic superimposition, and distortion are eliminated; therefore, possible complications such as injury of the neurovascular bundle and perforation of the maxillary sinuses, can be avoided.
Noninvasive and fast CBCT/CT imaging allows exact measurement of the length and width of the alveolar ridge due to its high spatial resolution. Locating sites suitable for implant placement and favorable for osseointegration, ascertaining the dimensions of the implant(s) required, and preoperative identification of areas where presurgical reduction or augmentation is needed to facilitate implant placement are also enabled. Such imaging is recommended by both the American Association of Oral and Maxillofacial Radiology (AAOMR) and the European Association for Osseointegration.
| Anatomic Considerations|| |
Important structures in the maxilla:
- Alveolar process of maxilla.
- Maxillary sinuses.
- Nasal fossa.
- Nasopalatine canal.
- Nasopalatine foramen.
Each of the paired maxillae is composed of a body and four processes: The frontal, zygomatic, palatine, and alveolar processes. The alveolar processes are the tooth-bearing portions of the maxillae, located below the body of the maxilla and the nasal cavity. The maxillary sinus is located within the body of the maxilla. The inferior segment of the maxillary sinus, the alveolar recess usually extends into the alveolar ridge, often between the roots of the maxillary molars and occasionally between the premolar roots as well [Figure 1]. The maxillary sinuses expand inferiorly after the loss of one or more molars or premolars. The anterior portion of the alveolar ridge is located immediately below the nasal fossa [Figure 2]. The nasopalatine duct is located in the anterior midline of the maxilla, extending from the incisive (nasopalatine) foramen in the oral cavity to the nasal fossa. The nasopalatine duct contains the nasopalatine nerves and arteries [Figure 2].
|Figure 1: axial CBCT image at the level of cervical regions of maxillary teeth. note multiple normal anatomical landmarks: 1 = pterygomaxillary fissure, 2 = lateral pterygoid plate, 3 = medial pterygoid plate, 4 = ramus of mandible, 5 = nasopalatine canal|
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|Figure 2: multiplanar CBCT images through the maxilla. note multiple normal anatomical landmarks: 1 = maxillary sinus, 2 = inferior nasal turbinate, 3 = zygomatic body, 4 = zygomatic arch, arrows = vascular markings|
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Important structures in the mandible:
- Mandibular or inferior alveolar canal.
- Mental foramen.
- Incisive canal.
- Mylohyoid ridge.
- Genial tubercules.
The mandibular canal is within the central third of the mandibular body and extends from mandibular foramen on the medial aspect of the ramus to the mental foramen [Figure 3]. The mandibular canal contains the inferior alveolar nerve, a branch of the mandibular division of the trigeminal nerve (V3). The inferior alveolar nerve divides into two branches at the level of the mental foramen. These are the mental nerve, which exits the mental foramen, and the incisive nerve, which continues anteriorly in the incisive canal to innervate the canine and ipsilateral lateral incisors. The mental nerve provides sensory innervation to the skin of the lower lip and the skin overlying the mandible. The mental foramen is normally located in the lateral or buccal aspect of the mandible between the roots of the first and second premolar teeth, and is identifiable on tridimensional (3D) volume-rendered (VR) images of the mandible, on panoramic and cross-sectional reformatted images [Figure 3]. The mental foramen may be located on or close to the alveolar crest in severely atrophic mandibles. Violation of the mandibular canal by an endosseous implant that is either too long or improperly positioned usually results in permanent injury to the inferior alveolar nerve and permanent paresthesia of the lower lip. The mylohyoid ridge is a prominent bony ridge along the lingual aspect of the mandibular body, representing the point of origin of the mylohyoid muscle, and is often seen on cross-sectional reformatted images through the posterior mandible.
|Figure 3: right and left mental foramina. Clockwise from top left - coronal, sagittal, and axial sections showing the position of mental foramina (arrows). left mental foramen seen on VR image (bottom left)|
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The main arterial supply to the floor of the anterior mandible and gingival mucosa is the sublingual artery. This artery and its branches enter the mandibular foramen and accompany the inferior dental nerve, but do not exit through the mental foramen. A lingual vascular canal larger than 1 mm is more prone to hematoma formation. The genial tubercle is located on the lingual (medial) aspect of the mandibular midline and represents the point of insertion of the geniohyoid and genioglossus muscles. The genial tubercle is identifiable on 3D volumetric images of the mandible, cross-sectional reformatted images through the mandibular midline, and true axial images of the mandible [Figure 4].
|Figure 4: mandibular lingual incisive canal seen in mandibular midline region. clockwise from top left - coronal, sagittal, and axial sections showing the position of incisive canal (arrows)|
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| Image Acquisition and Reconstruction|| |
Nearly all current helical scanners are based on modifications of rotate-rotate designs. Typical scan times are on the order of a few seconds or less, and recent versions are capable of subsecond scan times [Figure 5]. Imaging is performed with the patient supine and with the head centered and immobilized by using a jaw strap or sponge pads on each side of the temporal region. The patients are instructed not to move, swallow, or chew during the acquisition process. Data acquisition is performed parallel to the hard palate or interocclusal plane/alveolar crests to allow better orientation of the cross-sectional images, as different transaxial planes might lead to incorrect height measurements of the bone for implant placement. Axial images are obtained with the following parameters: Detector coverage of 40 mm per rotation, 512 × 512 matrix with a 16-cm field of view; section thickness and separation between sections of 0.6-1.0 mm (64-row scanner) or 1.25 mm (four-row scanner); low pitch of 0.516:1; and rotation time of 0.35 s (64-row scanner) or 0.6 s (four-row scanner) [Figure 6] and [Figure 7]. Reconstructions are performed with a standard filter and with a bone filter. Images are processed with dental software to create panoramic and sagittal oblique (cross-sectional) reformatted images of the maxilla and mandible.
|Figure 5: third-generation CT scanner, which acquires data by rotating both the x-ray source with a wide fan-beam geometry and the detectors around the patient|
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|Figure 6: scout image of multidetector CT (64-slice) taken to define scan area for oral implant planning from cranial base till the hyoid bone|
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|Figure 7: selection of scan plane and gantry tilt. scan plane is oriented parallel to the hard palate or occlusal table|
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Early CBCT scanners for dental use were characterized by Mozzo et al. and Arai et al. in the 1990s. Since then, more commercial models have become available, inciting research in many fields of dentistry and oral and maxillofacial surgery. Most scans have a dose between 30 µSv and 80 µSv, depending on exposure parameters and the selected field of view (FOV) size. In comparison, standard panoramic radiography delivers 13.3 µSv and multidetector CT with a similar FOV delivers 860 µSv.
The introduction of CBCT has heralded a shift from a 2D to a volumetric approach in maxillofacial imaging in terms of technical data acquisition, reconstruction, image display, and image interpretation.
CBCT requires a 2D x-ray detector and a conical or pyramidal x-ray beam. CBCT imaging is performed using a rotating platform or gantry to which an x-ray source and detector are fixed.
The cone beam technique involves a single scan in which the x-ray source and a reciprocating area detector synchronously move around the patient's head, which is stabilized with a head/temporal support. At certain degree intervals, single projection images, known as “basis” images, are acquired. These are similar to lateral cephalometric radiographic images, each slightly offset from one other. This series of basis projection images are referred to as the projection data. Software programs incorporating sophisticated algorithms including back-filtered projection are applied to these image data to generate a 3D volumetric dataset, which can be used to provide primary reconstruction images in three orthogonal planes, i.e., axial, coronal, and sagittal [Figure 8].
|Figure 8: a series of 2D projections are acquired at incremental rotations about the object under synchronized computer control. these images are corrected by means of gain and offset calibration and filtration of pixel defects. volumetric reconstruction is performed using the feldkamp-davis-kress (FDK) method, in which the 2D projection data are filtered and back-projected in a fully 3D reconstruction space|
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| Image Processing|| |
Axial images obtained using the bone window setting with sharp or very sharp kernel are used in the dental software program or interactive reformatting software to create:
|Figure 9: paraxial curve created on axial section at the level of cervical regions of mandibular teeth. note fine (1 mm thick) sections created at 1.5-mm intervals using the dental CT software|
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|Figure 10a, 10b: panoramic (above) and paraxial (below) sections generated using dental curved reformatting software in CT|
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Axial CBCT images obtained after primary reconstruction of volumetric dataset, with a thickness of 0.18-0.3 mm, are used to create the paraxial or superimposed curve [Figure 11].
|Figure 11: curved multiplanar or dental image reformat using CBCT proprietory or third-party interactive software showing cross-sectional/paraxial/oblique sagittal sections (top left), paraxial curve superimposed over mandibular axial image (top right), and panoramic reconstruction (bottom left), VR (3D) image reconstruction (bottom right). note increasing numbers of cross-sections from right to left and mapping of the bilateral inferior alveolar canals (green solid lines)|
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Superimposed curve images
A curve is superimposed on an axial image of the maxilla or mandible approximately at the level of the tooth roots, thus defining the plane and location for reformation of the panoramic and sagittal oblique or paraxial/cross-sectional images. In patients with edentulism, the curve should be obtained where the outline of the mandible or maxilla is shown completely. The curve is obtained by positioning several contiguous points throughout the center of the mandible or maxilla and avoiding sharp angulations between any two successive points.
The perpendicular lines marked along the superimposed curve indicate the positions of the sagittal oblique reformatted images on the axial section and the levels of the reformatted images, thus providing data on the height and depth of every image. The numbering of these lines starts in the right posterior zone and ends in the left posterior zone [Figure 9] and [Figure 11].
Sagittal oblique or paraxial images
These are the most representative and important images that enable the determination of shape, angulation, and measurements of height and thickness of the alveolar bone; they also allow the determination of optimal fixture lengths required to engage the cortical bone and remaining distances from vital structures. Images are generally obtained from multiple 1-mm-thick axial CT sections with a 1.5-mm or 2-mm interval  or from multiple 0.18-0.3 mm thick axial CBCT sections with a 1-mm or 1.5 mm interval [Figure 10]b and [Figure 11]. The images are numbered from right to left.
| Advantages and Drawbacks|| |
Contraindications for CT include
Tremors and tics
Disabling conditions that might cause a patient to be uncooperative.
Drawbacks of CT include the following
- The radiation dose has been an issue of concern. Although most of the edentulous population are older than 40 years, there might be younger patients who require dental restoration, mostly due to trauma.
- Highly radiosensitive organs are near the area of exposure, such as the thyroid gland, parotid gland, bone marrow, and lens of the eye.
- Recent advances in multidetector CT technology have made the acquisition of isotropic data feasible, with a trade-off in the form of increased radiation dose to the patient and prolonged scanning time.
- CT dental software programs could resection volumetric datasets in relation to the dental arches, and these enabled the standard visualization protocol for preoperative assessment of the maxilla and mandible. This forms the basis of current image display formats [Figure 9] and [Figure 10]. However, while facilitating an integrated approach to visualization, CT images were most often static, provided on analog film, and thus not interactive.
Advantages of CBCT
- Isotropic voxels with a spatial resolution of 0.125-0.4 mm and restricted FOV that limits the radiation dose with detector array sizes of 4-22 cm [Figure 12].
- Total data acquisition is performed in 10-80 s with a mean effective radiation dose of 36-50 μSv, a value equivalent to that of 4-15 panoramic radiographs.
- Clinicians have a pivotal role in imaging, either in acquiring the image (by prescribing or directly performing CBCT scans) or in the manipulation of software to provide task-specific display of images for the assessment of bone characteristics and anatomic structures.
- Proprietary or third-party software via Digital Imaging and Communications in Medicine (DICOM) export that affords greater sophistication in analysis and facilitates planning by providing interactive methods enabling implant selection and virtual prosthetic positioning.
|Figure 12: Isotropic nature of CBCT volumetric elements (voxels) and anisotropic (cuboidal) voxels of conventional or spiral CT. recent advances in multidetector CT technology have made the acquisition of isotropic data feasible, with a trade-off in the form of increased radiation dose to the patient and prolonged scanning time|
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Drawbacks of CBCT
- Artifacts, noise, poor soft-tissue contrast, and partial volume averaging - all of which may reduce the diagnostic yield of reformatted images for implant site assessment.
- Lack of user experience and what is currently a relatively small body of related literature.
- Because of the low radiation dose, CBCT can only provide bony detail and is unable to provide images of the soft tissues.
- Controversy surrounding office-based imaging, which is usually performed and interpreted by nonradiologists often without the accreditation, training, or licensure associated with the radiology community.
| Bmd Evaluation|| |
The term “bone quality” is commonly used in implant treatment and in reports on implant success and failure. Lindh et al. (2004) emphasized that BMD and bone quality are not synonymous. Bone quality encompasses factors other than BMD, such as skeletal size, the architecture and 3D orientation of the trabeculae, and matrix properties. Bone quality is a matter not only of mineral content but also of structure.
Each individual element of a CT image is called a voxel, which has a value referred to in Hounsfield units (HU) that describes the density of the CT image at that point. HU, also known as CT numbers, range from -1000 (air) to +3000 (enamel), each corresponding to a different level of beam attenuation.,, The density of structures within the image is absolute and quantitative, and can be used to differentiate tissues in the region (i.e., muscle 35-70 HU; fibrous tissue 60-90 HU; cartilage 80-130 HU; bone 150-1800 HU) and characterize bone quality (D1 bone >1250 HU; D2 bone 750-1250 HU; D3 bone 375-750 HU; D4 bone <375 HU).
Voxel values obtained from CBCT images are not absolute values, unlike the HU values obtained using CT, and various methods have been proposed to evaluate the BMD.,, HU values provide a quantitative assessment of BMD as measured by its ability to attenuate an x-ray beam. To date, there has not been any standard system for scaling the gray levels representing the reconstructed values.
Bone quality is broken down into four groups according to the proportion and structure of compact and trabecular bone tissue. Bone quality is categorized into four groups: Groups 1-4 or types I-IV (bone quality index, BQI).
- Type I: Homogeneous cortical bone.
- Type II: Thick cortical bone with marrow cavity.
- Type III: Thin cortical bone with dense trabecular bone of good strength.
- Type IV: Very thin cortical bone with low-density trabecular bone of poor strength.
Subjective assessment of bony quality using the BQI can provide a reasonable idea of the bone type in prospective implant sites. More important than an objective density value or scale is the knowledge of rarefied, osteopenic or sclerotic, avascular bone type in the region of interest, as these may significantly alter the stability and prognosis of the implant fixture during healing and functional loading [Figure 13].
|Figure 13: paraxial sections through the maxilla and mandible showing moderate to high density due to small, condensed, closely spaced alveolar trabeculae and mild or moderate endosteal thickening of adjoining cortices|
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| Bone Quantity Assessment|| |
Bone quantity of the mandible is classified into five groups (from minimal to severe, A-E), based on residual jaw shape, differing rates of bone resorption following tooth extraction. During all stages of atrophy of the alveolar ridge, characteristic shapes result from the resorptive process [Figure 14].
|Figure 14: cross-sectional/paraxial images through the molar region of the maxilla. normal ridge morphology in dentulous site (top left); mild to moderate ridge resorption (top right); advance residual ridge resorption with focal effacement of the sinus floor and focal sinus mucosal thickening (bottom left); near-total residual ridge resorption with intact sinus floor and polypoidal mucosal thickening in overlying maxillary sinus (bottom right)|
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Successful implant placement and osseointegration requires
- 1-1.5 mm bone on either side of fixture.
- 1-2 mm bone between base of fixture and adjacent structure such as nasal fossa, maxillary sinus floor, and mandibular canal.
- Determination of ridge angulation with respect to the vertical axis of ridge.
- Occlusal force vector acting on the implant fixture parallel or almost parallel to the vertical axis through the ridge.
Interactive third-party or proprietary CBCT software can be used to measure the available bone height from the ridge crest till the nearest vital structure (i.e., nasal fossa/maxillary sinus floor or inferior alveolar/incisive canal) and width at the alveolar crest [Figure 15]. Simulation of virtual implants on the scan images can also be performed. Most of the commonly available software applications contain an implant library to select implant diameter-length, design, and manufacturer, based on available bone quantity and quality.
|Figure 15: bone quantity assessment in edentulous prospective implant site. note measurement of available bone width at the alveolar crest and available bone height from alveolar crest till inferior alveolar canal|
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| Radiological Scan Prosthesis|| |
Imaging-based preoperative implant planning can be further improved by the use of an imaging stent that helps relate the radiographic image and its information to a precise anatomic location or a potential implant site.
The intended implant sites are identified by radiopaque markers retained within an acrylic stent, which the patient wears during the imaging procedure so that images of the markers are created in the diagnostic images. The imaging stent subsequently may be used as a surgical guide to orient the insertion angle of the guide burr and hence the angle of the implant. Generally, nonmetallic radiopaque markers are used in CT and CBCT imaging [Figure 16].
|Figure 16: CBCT mandible with curved multiplanar or dental image reformat showing GP markers in the radiological scan prosthesis. images reconstructed using CBCT third-party interactive software|
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Scan appliances are also used along with a dual-scan protocol for fabrication of surgical guides/stents using stereolithography or rapid prototyping. The dual-scan protocol requires two separate scans:
- Scan 1 - Patient wearing radiological scan prosthesis.
- Scan 2 - Radiological scan prosthesis only.
The planning software merges the two scans by matching gutta percha (GP) markers and aligning the radiopaque markers so that the prosthesis will be visible over the available osseous anatomy.
| Conclusion|| |
Multiplanar imaging of the dental arches has transformed the depth and accuracy of pre-implant bone assessment, and enhanced the predictability of oral implantology. Pre-implant multiplanar imaging began with CT dental reformatting software and has evolved tremendously since the introduction of CBCT, with which selectable scan sizes, interactive software, and virtual implant simulations became possible.
CBCT is the current imaging modality of choice for pre-implant bone assessment; however, due to wider availability, CT continues to be in use for this purpose.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16]