Orofacial Implants
Content
Few advances in dentistry have been as dramatic as the use of prosthetic implants to restore orofacial form and function. Implant technology has enabled the practitioner to help affected patients regain the ability to chew normally and function without embarrassment. With longterm success rates approaching 95% and higher, implant systems are rapidly entering the mainstream of dental practice. These implants typically function as part of a system combining metal fixtures integrated with bone, abutments fastened to fixtures, and a variety of dental appliances attached to the abutments. Because successful implantation depends on close integration of the fixture and the supporting bone, radiographic imaging is an important element of implant therapy. The burgeoning acceptance of these devices has been attributed in part to the increasingly sophisticated imaging techniques used in all phases of implant treatment, including preoperative treatment planning, intraoperative assessment (integration), and postoperative
assessment (function).
Dentists must be knowledgeable about contemporary implant imaging techniques and familiar with the radiographic appearance of various fixtures (Figs. 30-2 and 30-3). Implants encountered on routine dental radiographs
may range from fracture fixation devices to alloplastic materials used for augmentation. However, most of these devices are dental implants used to restore lost masticatory function by replacing missing teeth. Although subperiosteal and transosteal implant systems are still used occasionally, nearly all dental implants used today are root-form devices placed within bone (endosteal implants). Therefore this chapter focuses on the radiographic aspects of endosteal dental implants.
Radiographic Assessment of Dental Implants
Although useful and cost-effective, the conventional methods of implant imaging generally are considered inadequate for comprehensive implant evaluation. Newer techniques that permit cross-sectional
visualization and interactive image analysis may be considered the standard of care, especially for complex reconstructions. The choice of radiographic techniques often is a function of the various phases of the surgical and restorative procedures (Table 30-1). In every instance the imaging strategy most appropriate for a particular phase of the implant therapy should always
be a collective decision of the implant team-the restorative dentist, surgeon, and radiologist.
Preoperative Planning
Radiographic visualization of potential implant sites is an important extension of clinical examination and assessment. Radiographs help the clinician visualize the alveolar ridges and adjacent structures in all three dimensions and guide the choice of site, number, size, and axial orientation of the implants. Site selection includes consideration of adjacent anatomic structures such as the incisive and mental foramina, inferior alveolar canal, existing teeth, nasal fossae, and maxillary sinuses. Pathologic conditions, such as retained root fragments, impacted teeth, and osteomyelitis that could compromise the outcome must be identified and located relative to the site of the proposed implant. The variety of radiographic techniques available to assist the clinician includes intraoral radiography (film and digital), cephalometric radiography, panoramic radiography, conventional
tomography, computed tomography (CT), and stereoscopic (paired) x-ray imaging.
In evaluating a potential implant site, particular attention should be given both to the quality and quantity of bone required for placement of the fixture. The bone must have the necessary dimensions and quality to provide support for the implant fixture. Cortical bone typically is best suited to withstand the functional loading forces of dental implants. The thicker the cortical bone, the greater the likelihood of osseous integration and subsequent success. Bone quantity is assessed by documenting the height and width of available alveolar bone, as well as the morphology of the ridge. The chances of successful implantation increase as more bone is available for anchorage. A cross-sectional image to document the facial-lingual width and height of the ridge, along with, the inclination of the bone contours, is especially useful in the preoperative planning phase. Ridge width measurements aid in maximal engagement of cortical bone, and ridge height measurements aid in the selection of the longest appropriate fixture to maximize anchorage and distribution of masticatory forces. Frequently, morphologic features such as osseous undercuts and ridge concavities that is not immediately apparent on clinical examination become evident with cross-sectional imaging. This information may dictate the choice of implant and its axis of orientation.
Accurate bone measurements are essential for determining the optimal size and length of the proposed implants. The clinician should be aware that the magnification factor of radiographic images may vary with the imaging technique used. Except for reformatted CT, all radiographic images are magnified because the object is never in the same plane as the film. The clinician must consider this magnification factor when calculating the dimensions of the bone at the implant site. To obtain the actual dimensions of the available bone, the measurements obtained from the radiographs (usually in millimeters) are divided by the magnification factor (usually 1.0 to 1.8) for the particular imaging technique being used. The magnification factor of some techniques may be variable (periapical, panoramic) or fixed (conventional tomography). Reformatted CT images can be corrected to life size. If the magnification factor is constant, clear plastic overlays with 1 mm grids or diagrams of available implant sizes can be produced with the same magnification factor as the image.
Imaging Techniques
The ideal imaging technique for dental implant radiography should have several essential characteristics, including the ability to visualize the implant site in the mesial-distal, facial-lingual, and superior-inferior dimensions; the ability to allow reliable, accurate measurements; a capacity to evaluate the density of trabecular bone and cortical thickness; a capacity to correlate the imaged site with the clinical site; reasonable access and cost to the patient; and minimal radiation risk. Usually a combination of radiographs is used. The following is a review of the imaging techniques applicable to dental implant case management.
INTRAORAL RADIOGRAPHY
Intraoral images may be acquired on film or as direct digital images. Periapical and occlusal radiographic films provide images with superior resolution and sharpness. Maxillary and mandibular periapical radiographs commonly are used to evaluate the status of adjoining teeth and remaining alveolar bone in the mesial-distal dimension. They also have been used for determining vertical height, architecture, and bone quality (bone density, amount of cortical bone, and amount of trabecular bone). Although readily
available and relatively inexpensive, periapical radiography has geometric and anatomic limitations. Periapical radiographs, made on a dentate arch, typically are made with the paralleling technique, creating an image with minimal foreshortening and elongation. Because an edentulous alveolar ridge may not have the same "long axis" as a tooth, positioning the film in a consistent and repeatable fashion is difficult, and the image may be foreshortened or elongated. Also, it frequently is difficult to place the film either superior or inferior enough to evaluate the entire maxillary or mandibular ridge all the way to the inferior cortical margin. One study reported that 25% of mandibular periapical radiographs did not demonstrate the mandibular canal. In cases when the canal was identifiable, only 53% of measurements from the alveolar crest to the superior wall of the mandibular canal were accurate within 1 mm.
Because periapical radiographs are unable to provide any crosssectional information, occlusal radiographs sometimes are used to determine the facial-lingual dimensions of the mandibular alveolar ridge. Although somewhat useful, the occlusal image records only the widest portion of the mandible, which typically is located inferior to the alveolar ridge. This may
give the clinician the impression that more bone is available in the crosssectional (facial lingual) dimension than actually exists. The occlusal technique is not useful in imaging the maxillary arch because of anatomic limitations.
LATERAL AND LATERAL-OBLIQUE CEPHALOMETRIC RADIOGRAPHY
Lateral cephalometric radiography provides an image of known magnification (usually 7% to 12%) that documents axial tooth inclinations and the dentoalveolar ridge relationships in the midline of the jaws. The soft tissue profile also is apparent on this film and can be used to evaluate profile alterations after prosthodontic rehabilitation. Although this projection provides a cross-sectional evaluation of the ridges, this dimension is seen only at the midline. The images of structures not in the midline are superimposed on the contralateral side, complicating the evaluation of other implant sites. Occasionally, lateral-oblique cephalometric radiography is used with one side of the body of the mandible positioned parallel to the film cassette. Image magnification on these views is not predictable, because the
body of the mandible is not the same distance from the cassette as is the rotation center of the cephalostat. Thus measurements made from these films are not reliable. In general, cephalometric radiographs are of limited use in the selection of implant sites.
PANORAMIC RADIOGRAPHY
Although the resolution and sharpness of panoramic radiographs are less than those of intraoral films, panoramic projections provide a broader visualization of the jaws and adjoining anatomic structures. Panoramic radiography units are widely available, making this imaging technique very popular as a screening and assessment instrument. Panoramic radiographs are useful in making preliminary estimations of crestal alveolar bone and cortical boundaries of the mandibular canal, maxillary sinus, and nasal fossa (Fig. 308). Information acquired from panoramic radiographs must be applied judiciously because this technique has significant limitations as a definitive presurgical planning tool. Angular measurements on panoramic radiographs tend to be accurate, but linear measurements are not. Image size distortion (magnification) varies significantly between films from different panoramic
units and even within different areas of the same film. Vertical measurements are unreliable because of foreshortening and elongation of the anatomic structures, since the x-ray beam is perpendicular neither to the long axis of the anatomic structures nor to the film plane. The negative vertical angulation of the x-ray beam also may cause lingually positioned objects such as mandibular tori to be projected superiorly on the film, which may result in an overestimation of vertical bone height. Furthermore, the anatomic vertical axis varies within the film image, particularly in nonmidline areas. Panoramic radiographs provide a two-dimensional image with no cross-sectional information.
Similarly, dimensional accuracy in the horizontal plane of panoramic radiographs is highly dependent on the position of the structures of interest relative to the central plane of the image layer. The horizontal dimension of images of structures located facial or lingual to the central plane but still within the image layer tends to be minimized or magnified. The degree of horizontal size distortion is difficult to ascertain on panoramic radiographs because the shape of the image layer is configured to a population average. However, the anatomic morphology of few individuals conforms totally to
that image layer. In summary, horizontal image magnification with panoramic radiographs varies from 0.70 to 2.2 times actual size, although some manufacturers still claim a 1.25 average magnification (at the central plane of the image layer). Errors in patient positioning can compound further the measurement limitation in the horizontal dimension. Compared with contact radiographs of dissected anatomic specimens, only 17% of panoramic measurements between the alveolar crest and superior wall of the mandibular canal were found to be accurate within 1 mm.
CONVENTIONAL TOMOGRAPHY
Conventional tomography provides reliable dimensional measurements at proposed implant sites, including the cross-sectional (facial-lingual) dimension. It also is reasonably widely available. Used as an adjunct to screening films, cross-sectional tomograms enhance visualization of the available bone. This technique produces a cross-sectional, flat-plane image layer that is perpendicular to the x-ray beam. Images of anatomic structures of interest are relatively sharp, and images of structures outside the image layer are blurred beyond recognition by the motion of the x-ray tube and film.
The thickness, orientation, and anatomic location of the image layer can be predetermined and manipulated. It is imperative that the image layer be a true cross-section of the dental arch, rather than oblique. Scout films (usually a submentovertex or panoramic projection) or wax bite registrations commonly are used to determine the appropriate cross-sectional angulation. The complex (multidirectional) tube motion of current conventional tomography units minimizes image superimposition and provides fixed, uniform image magnification, allowing for accurate measurements. Complex tube motion also permits use of a thicker image layer while retaining diagnostic quality. A thicker image layer is desirable to maximize image contrast, making the identification of structures such as the mandibular canal more predictable.
The dimensional accuracy of cross-sectional tomograms is particularly useful in measuring the distance between the alveolar crest and adjacent structures, such as the floor of the nasal fossa, maxillary sinus floor, mandibular canal, mental canal, and inferior mandibular cortex. The appropriate axis of insertion of the implant may also be predicted. Measurements are directly acquired from the films and subsequently
corrected by the magnification factor used. As an alternative, acetate overlays with appropriately magnified 1 mm grids may be used.
The clinical utility of conventional tomograms can be enhanced by the use of an imaging stent. The stent facilitates correlation of the tomograms to the scout film and provides a practical method of relating the radiographic information to the surgical site. The intended implant sites are identified by radiopaque spheres or rods (metal, composite resin, or gutta-percha) retained within an acrylic stent. The imaging stent subsequently may be used as a surgical guide. For optimal visualization, the width of the markers should be less than the thickness of the tomographic image layer.
Diagnostic dentures coated with barium paste also may be used during imaging. The site markers are visualized in a mesial-distal direction on the scout films and in the facial-lingual dimension on the cross-sectional conventional tomograms. Typically, two to three cross-sectional tomographic slices are required to image each intended implant site adequately. Conventional tomography is especially convenient in the planning of single site implants or those within a quadrant.
COMPUTED TOMOGRAPHY
Patients who are edentulous or who are being considered for multiple implants and augmentation procedures may be best imaged with CT. CT studies are planned on a lateral scout image of the selected jaw with alignment corrections made as needed. Direct axial images are then acquired as thin, overlapping axial scans with approximately 30 axial sections per jaw. These images usually are acquired perpendicular to the occlusal plane. The sequential axial images subsequently are manipulated to produce multiple two-dimensional images in various planes, using a computer-based process called multiplanar reformatting (MPR). In general, three basic images are reformatted: axial images with a superimposed curve, cross-sectional images, and panoramic-like curved linear images. An axial scan including the full contour of the mandible (or maxilla) at a level corresponding to the dental roots is selected as a reference for the reformatting process. The computer places a series of sequential dots on the selected scan and connects them to develop a customized arch or curve unique for each jaw. The computer program then generates a series of lines perpendicular to the curve. These
lines are made at constant intervals (usually 1 to 2 mm) and numbered sequentially on the axial image to indicate the position at which each crosssectional slice will be reconstructed. Cross-sectional reconstructions are made perpendicular to the curve, and panoramic (curved linear) reconstructions are made parallel with the curve. Three-dimensional representations may also be constructed in various orientations.
These reformatted images provide the clinician with two-dimensional diagnostic information in all three dimensions. Typical studies provide information on the continuity of the cortical bone plates, residual bone in the mandible and maxilla, the relative location of adjoining vital structures, and the contour of soft tissues covering the osseous structures. Studies have reported that 94% of CT measurements between the alveolar crest and wall of the mandibular canal were accurate within 1 mm. Three-dimensional reformations are particularly useful in the planning of augmentation procedures such as a sinus lift. Unlike conventional tomograms, reformatted CT images provide the radiographic density values of cortical plates and trabecular bone, which may be useful in managing the case. Reformatted CT
images also may be used with interactive software to simulate implant orientation and placement on a computer screen before surgery.
Reformatted CT studies provide diagnostic information on all available implant sites with a dental arch. The reformatted images typically are presented life-size on photographic prints or radiographic film. The panoramic (curved linear) images are helpful in identifying mesial-distal relationships and noncorticated mandibular canals. However, the quality of the reformatted CT study depends on the ability of the patient to remain still during image acquisition, because movement may result in subsequent geometric image distortion. Metallic restorations can cause streak image artifacts. However, the streaking is only within the axial plane and does not affect axial slices superior or inferior to it. As with conventional tomography, it is desirable to localize anticipated implant sites with imaging stents incorporating nonmetallic radiopaque markers (gutta-percha, composite resin). Barium-coated diagnostic dentures may also be used to establish the spatial relationships between the anticipated prosthesis and fixtures.
INTRAOPERATIVE AND POSTOPERATIVE ASSESSMENTS
Intraoral and panoramic radiographs usually are adequate for both intraoperative and postoperative assessments. If threaded root-form fixtures have been placed, the optimal radiographic image must separate the threads for best visualization. This may not always be a predictable procedure because the exact angulation of the implant is not known. The angulation of the x-ray beam must be within 9 degrees of the long axis of the fixture to open the threads on the image on most threaded fixtures. Angular deviations of 13 degrees or more result in complete overlap of the threads. In general, periapical radiographs are appropriate for longitudinal assessments. Mesial and distal marginal bone height is measured using known interthread measurements and comparing that with the bone level in previous periapical radiographs. The presence of relatively constant and distinct bone margins suggests successful osseous integration. Resorptive changes, if present, are evidenced by apical migration of the alveolar bone or indistinct osseous margins. These adverse changes are progressive and should be differentiated from the initial circumscribed resorptive osseous changes around the cervical area of the fixture induced by the surgical procedure itself. Studies suggest that the rate of marginal bone loss after successful implantation is
approximately 1.2 mm in the first year, subsequently tapering off to about 0.1 mm in succeeding years. Occasionally areas of marginal bone gain also may be noted.
A clinically stable fixture is invariably associated with the radiographic appearance of normal osseous tissue in intimate contact with the implant surface. The development of a thin radiolucent area that closely follows the outline of the implant usually correlates to clinically detectable implant mobility and is an important indicator of failed osseointegration. Changes in the periodontal ligament space of associated teeth (natural abutment) also are useful in monitoring the functional competence of the prosthesis implant system. Any widening of the periodontal ligament space compared with preoperative radiographs indicates poor stress distribution and forecasts implant failure. After successful implantation, radiographs may be made at regular intervals to assess the success or failure of the implant fixture. Advanced imaging studies may be necessary for adequate assessment in some cases.
Subtle areas of bone resorption adjacent to the fixture may be made evident with intraoral digital images by evaluating a density profile graph of radiographic density values, a feature available on most digital imaging units. If intraoral digital images are acquired at the time of surgery, they may be compared with subsequent digital images either by subjective visualization or digital subtraction. Digital subtraction is a computerized process that may reveal areas of bone resorption not apparent visually. Occasionally, stereoscopic plain films or scanograms, which provide the appearance of three dimensions, may be helpful in assessing multiple implant fixtures within a segment of the alveolar ridge. However, measurements may not be reliable on stereoscopic projections.
In summary, imaging is an integral part of dental implant therapy, and a variety of imaging techniques are used for implant assessment. Crosssectional imaging is increasingly considered integral to optimal implant placement, especially in the case of complex reconstructions. An initial assessment of the feasibility of implant placement may appropriately be made with panoramic radiography. If required, an intraoral radiograph can provide the higher resolution required to evaluate suspected areas of pathosis. Should
the initial assessment be favorable and a decision made to proceed with the placement of implants, a cross-sectional image is indicated. Conventional tomography is appropriate for single-implant sites, whereas reformatted CT is preferred for multiple sites or for an edentulous ridge in which all possible implant sites are to be considered. Assessment of implanted fixtures typically is performed with periapical and panoramic radiography. However, specific cases may require more advanced imaging studies, depending on the nature of the clinical concern.
assessment (function).
Dentists must be knowledgeable about contemporary implant imaging techniques and familiar with the radiographic appearance of various fixtures (Figs. 30-2 and 30-3). Implants encountered on routine dental radiographs
may range from fracture fixation devices to alloplastic materials used for augmentation. However, most of these devices are dental implants used to restore lost masticatory function by replacing missing teeth. Although subperiosteal and transosteal implant systems are still used occasionally, nearly all dental implants used today are root-form devices placed within bone (endosteal implants). Therefore this chapter focuses on the radiographic aspects of endosteal dental implants.
Radiographic Assessment of Dental Implants
Although useful and cost-effective, the conventional methods of implant imaging generally are considered inadequate for comprehensive implant evaluation. Newer techniques that permit cross-sectional
visualization and interactive image analysis may be considered the standard of care, especially for complex reconstructions. The choice of radiographic techniques often is a function of the various phases of the surgical and restorative procedures (Table 30-1). In every instance the imaging strategy most appropriate for a particular phase of the implant therapy should always
be a collective decision of the implant team-the restorative dentist, surgeon, and radiologist.
Preoperative Planning
Radiographic visualization of potential implant sites is an important extension of clinical examination and assessment. Radiographs help the clinician visualize the alveolar ridges and adjacent structures in all three dimensions and guide the choice of site, number, size, and axial orientation of the implants. Site selection includes consideration of adjacent anatomic structures such as the incisive and mental foramina, inferior alveolar canal, existing teeth, nasal fossae, and maxillary sinuses. Pathologic conditions, such as retained root fragments, impacted teeth, and osteomyelitis that could compromise the outcome must be identified and located relative to the site of the proposed implant. The variety of radiographic techniques available to assist the clinician includes intraoral radiography (film and digital), cephalometric radiography, panoramic radiography, conventional
tomography, computed tomography (CT), and stereoscopic (paired) x-ray imaging.
In evaluating a potential implant site, particular attention should be given both to the quality and quantity of bone required for placement of the fixture. The bone must have the necessary dimensions and quality to provide support for the implant fixture. Cortical bone typically is best suited to withstand the functional loading forces of dental implants. The thicker the cortical bone, the greater the likelihood of osseous integration and subsequent success. Bone quantity is assessed by documenting the height and width of available alveolar bone, as well as the morphology of the ridge. The chances of successful implantation increase as more bone is available for anchorage. A cross-sectional image to document the facial-lingual width and height of the ridge, along with, the inclination of the bone contours, is especially useful in the preoperative planning phase. Ridge width measurements aid in maximal engagement of cortical bone, and ridge height measurements aid in the selection of the longest appropriate fixture to maximize anchorage and distribution of masticatory forces. Frequently, morphologic features such as osseous undercuts and ridge concavities that is not immediately apparent on clinical examination become evident with cross-sectional imaging. This information may dictate the choice of implant and its axis of orientation.
Accurate bone measurements are essential for determining the optimal size and length of the proposed implants. The clinician should be aware that the magnification factor of radiographic images may vary with the imaging technique used. Except for reformatted CT, all radiographic images are magnified because the object is never in the same plane as the film. The clinician must consider this magnification factor when calculating the dimensions of the bone at the implant site. To obtain the actual dimensions of the available bone, the measurements obtained from the radiographs (usually in millimeters) are divided by the magnification factor (usually 1.0 to 1.8) for the particular imaging technique being used. The magnification factor of some techniques may be variable (periapical, panoramic) or fixed (conventional tomography). Reformatted CT images can be corrected to life size. If the magnification factor is constant, clear plastic overlays with 1 mm grids or diagrams of available implant sizes can be produced with the same magnification factor as the image.
Imaging Techniques
The ideal imaging technique for dental implant radiography should have several essential characteristics, including the ability to visualize the implant site in the mesial-distal, facial-lingual, and superior-inferior dimensions; the ability to allow reliable, accurate measurements; a capacity to evaluate the density of trabecular bone and cortical thickness; a capacity to correlate the imaged site with the clinical site; reasonable access and cost to the patient; and minimal radiation risk. Usually a combination of radiographs is used. The following is a review of the imaging techniques applicable to dental implant case management.
INTRAORAL RADIOGRAPHY
Intraoral images may be acquired on film or as direct digital images. Periapical and occlusal radiographic films provide images with superior resolution and sharpness. Maxillary and mandibular periapical radiographs commonly are used to evaluate the status of adjoining teeth and remaining alveolar bone in the mesial-distal dimension. They also have been used for determining vertical height, architecture, and bone quality (bone density, amount of cortical bone, and amount of trabecular bone). Although readily
available and relatively inexpensive, periapical radiography has geometric and anatomic limitations. Periapical radiographs, made on a dentate arch, typically are made with the paralleling technique, creating an image with minimal foreshortening and elongation. Because an edentulous alveolar ridge may not have the same "long axis" as a tooth, positioning the film in a consistent and repeatable fashion is difficult, and the image may be foreshortened or elongated. Also, it frequently is difficult to place the film either superior or inferior enough to evaluate the entire maxillary or mandibular ridge all the way to the inferior cortical margin. One study reported that 25% of mandibular periapical radiographs did not demonstrate the mandibular canal. In cases when the canal was identifiable, only 53% of measurements from the alveolar crest to the superior wall of the mandibular canal were accurate within 1 mm.
Because periapical radiographs are unable to provide any crosssectional information, occlusal radiographs sometimes are used to determine the facial-lingual dimensions of the mandibular alveolar ridge. Although somewhat useful, the occlusal image records only the widest portion of the mandible, which typically is located inferior to the alveolar ridge. This may
give the clinician the impression that more bone is available in the crosssectional (facial lingual) dimension than actually exists. The occlusal technique is not useful in imaging the maxillary arch because of anatomic limitations.
LATERAL AND LATERAL-OBLIQUE CEPHALOMETRIC RADIOGRAPHY
Lateral cephalometric radiography provides an image of known magnification (usually 7% to 12%) that documents axial tooth inclinations and the dentoalveolar ridge relationships in the midline of the jaws. The soft tissue profile also is apparent on this film and can be used to evaluate profile alterations after prosthodontic rehabilitation. Although this projection provides a cross-sectional evaluation of the ridges, this dimension is seen only at the midline. The images of structures not in the midline are superimposed on the contralateral side, complicating the evaluation of other implant sites. Occasionally, lateral-oblique cephalometric radiography is used with one side of the body of the mandible positioned parallel to the film cassette. Image magnification on these views is not predictable, because the
body of the mandible is not the same distance from the cassette as is the rotation center of the cephalostat. Thus measurements made from these films are not reliable. In general, cephalometric radiographs are of limited use in the selection of implant sites.
PANORAMIC RADIOGRAPHY
Although the resolution and sharpness of panoramic radiographs are less than those of intraoral films, panoramic projections provide a broader visualization of the jaws and adjoining anatomic structures. Panoramic radiography units are widely available, making this imaging technique very popular as a screening and assessment instrument. Panoramic radiographs are useful in making preliminary estimations of crestal alveolar bone and cortical boundaries of the mandibular canal, maxillary sinus, and nasal fossa (Fig. 308). Information acquired from panoramic radiographs must be applied judiciously because this technique has significant limitations as a definitive presurgical planning tool. Angular measurements on panoramic radiographs tend to be accurate, but linear measurements are not. Image size distortion (magnification) varies significantly between films from different panoramic
units and even within different areas of the same film. Vertical measurements are unreliable because of foreshortening and elongation of the anatomic structures, since the x-ray beam is perpendicular neither to the long axis of the anatomic structures nor to the film plane. The negative vertical angulation of the x-ray beam also may cause lingually positioned objects such as mandibular tori to be projected superiorly on the film, which may result in an overestimation of vertical bone height. Furthermore, the anatomic vertical axis varies within the film image, particularly in nonmidline areas. Panoramic radiographs provide a two-dimensional image with no cross-sectional information.
Similarly, dimensional accuracy in the horizontal plane of panoramic radiographs is highly dependent on the position of the structures of interest relative to the central plane of the image layer. The horizontal dimension of images of structures located facial or lingual to the central plane but still within the image layer tends to be minimized or magnified. The degree of horizontal size distortion is difficult to ascertain on panoramic radiographs because the shape of the image layer is configured to a population average. However, the anatomic morphology of few individuals conforms totally to
that image layer. In summary, horizontal image magnification with panoramic radiographs varies from 0.70 to 2.2 times actual size, although some manufacturers still claim a 1.25 average magnification (at the central plane of the image layer). Errors in patient positioning can compound further the measurement limitation in the horizontal dimension. Compared with contact radiographs of dissected anatomic specimens, only 17% of panoramic measurements between the alveolar crest and superior wall of the mandibular canal were found to be accurate within 1 mm.
CONVENTIONAL TOMOGRAPHY
Conventional tomography provides reliable dimensional measurements at proposed implant sites, including the cross-sectional (facial-lingual) dimension. It also is reasonably widely available. Used as an adjunct to screening films, cross-sectional tomograms enhance visualization of the available bone. This technique produces a cross-sectional, flat-plane image layer that is perpendicular to the x-ray beam. Images of anatomic structures of interest are relatively sharp, and images of structures outside the image layer are blurred beyond recognition by the motion of the x-ray tube and film.
The thickness, orientation, and anatomic location of the image layer can be predetermined and manipulated. It is imperative that the image layer be a true cross-section of the dental arch, rather than oblique. Scout films (usually a submentovertex or panoramic projection) or wax bite registrations commonly are used to determine the appropriate cross-sectional angulation. The complex (multidirectional) tube motion of current conventional tomography units minimizes image superimposition and provides fixed, uniform image magnification, allowing for accurate measurements. Complex tube motion also permits use of a thicker image layer while retaining diagnostic quality. A thicker image layer is desirable to maximize image contrast, making the identification of structures such as the mandibular canal more predictable.
The dimensional accuracy of cross-sectional tomograms is particularly useful in measuring the distance between the alveolar crest and adjacent structures, such as the floor of the nasal fossa, maxillary sinus floor, mandibular canal, mental canal, and inferior mandibular cortex. The appropriate axis of insertion of the implant may also be predicted. Measurements are directly acquired from the films and subsequently
corrected by the magnification factor used. As an alternative, acetate overlays with appropriately magnified 1 mm grids may be used.
The clinical utility of conventional tomograms can be enhanced by the use of an imaging stent. The stent facilitates correlation of the tomograms to the scout film and provides a practical method of relating the radiographic information to the surgical site. The intended implant sites are identified by radiopaque spheres or rods (metal, composite resin, or gutta-percha) retained within an acrylic stent. The imaging stent subsequently may be used as a surgical guide. For optimal visualization, the width of the markers should be less than the thickness of the tomographic image layer.
Diagnostic dentures coated with barium paste also may be used during imaging. The site markers are visualized in a mesial-distal direction on the scout films and in the facial-lingual dimension on the cross-sectional conventional tomograms. Typically, two to three cross-sectional tomographic slices are required to image each intended implant site adequately. Conventional tomography is especially convenient in the planning of single site implants or those within a quadrant.
COMPUTED TOMOGRAPHY
Patients who are edentulous or who are being considered for multiple implants and augmentation procedures may be best imaged with CT. CT studies are planned on a lateral scout image of the selected jaw with alignment corrections made as needed. Direct axial images are then acquired as thin, overlapping axial scans with approximately 30 axial sections per jaw. These images usually are acquired perpendicular to the occlusal plane. The sequential axial images subsequently are manipulated to produce multiple two-dimensional images in various planes, using a computer-based process called multiplanar reformatting (MPR). In general, three basic images are reformatted: axial images with a superimposed curve, cross-sectional images, and panoramic-like curved linear images. An axial scan including the full contour of the mandible (or maxilla) at a level corresponding to the dental roots is selected as a reference for the reformatting process. The computer places a series of sequential dots on the selected scan and connects them to develop a customized arch or curve unique for each jaw. The computer program then generates a series of lines perpendicular to the curve. These
lines are made at constant intervals (usually 1 to 2 mm) and numbered sequentially on the axial image to indicate the position at which each crosssectional slice will be reconstructed. Cross-sectional reconstructions are made perpendicular to the curve, and panoramic (curved linear) reconstructions are made parallel with the curve. Three-dimensional representations may also be constructed in various orientations.
These reformatted images provide the clinician with two-dimensional diagnostic information in all three dimensions. Typical studies provide information on the continuity of the cortical bone plates, residual bone in the mandible and maxilla, the relative location of adjoining vital structures, and the contour of soft tissues covering the osseous structures. Studies have reported that 94% of CT measurements between the alveolar crest and wall of the mandibular canal were accurate within 1 mm. Three-dimensional reformations are particularly useful in the planning of augmentation procedures such as a sinus lift. Unlike conventional tomograms, reformatted CT images provide the radiographic density values of cortical plates and trabecular bone, which may be useful in managing the case. Reformatted CT
images also may be used with interactive software to simulate implant orientation and placement on a computer screen before surgery.
Reformatted CT studies provide diagnostic information on all available implant sites with a dental arch. The reformatted images typically are presented life-size on photographic prints or radiographic film. The panoramic (curved linear) images are helpful in identifying mesial-distal relationships and noncorticated mandibular canals. However, the quality of the reformatted CT study depends on the ability of the patient to remain still during image acquisition, because movement may result in subsequent geometric image distortion. Metallic restorations can cause streak image artifacts. However, the streaking is only within the axial plane and does not affect axial slices superior or inferior to it. As with conventional tomography, it is desirable to localize anticipated implant sites with imaging stents incorporating nonmetallic radiopaque markers (gutta-percha, composite resin). Barium-coated diagnostic dentures may also be used to establish the spatial relationships between the anticipated prosthesis and fixtures.
INTRAOPERATIVE AND POSTOPERATIVE ASSESSMENTS
Intraoral and panoramic radiographs usually are adequate for both intraoperative and postoperative assessments. If threaded root-form fixtures have been placed, the optimal radiographic image must separate the threads for best visualization. This may not always be a predictable procedure because the exact angulation of the implant is not known. The angulation of the x-ray beam must be within 9 degrees of the long axis of the fixture to open the threads on the image on most threaded fixtures. Angular deviations of 13 degrees or more result in complete overlap of the threads. In general, periapical radiographs are appropriate for longitudinal assessments. Mesial and distal marginal bone height is measured using known interthread measurements and comparing that with the bone level in previous periapical radiographs. The presence of relatively constant and distinct bone margins suggests successful osseous integration. Resorptive changes, if present, are evidenced by apical migration of the alveolar bone or indistinct osseous margins. These adverse changes are progressive and should be differentiated from the initial circumscribed resorptive osseous changes around the cervical area of the fixture induced by the surgical procedure itself. Studies suggest that the rate of marginal bone loss after successful implantation is
approximately 1.2 mm in the first year, subsequently tapering off to about 0.1 mm in succeeding years. Occasionally areas of marginal bone gain also may be noted.
A clinically stable fixture is invariably associated with the radiographic appearance of normal osseous tissue in intimate contact with the implant surface. The development of a thin radiolucent area that closely follows the outline of the implant usually correlates to clinically detectable implant mobility and is an important indicator of failed osseointegration. Changes in the periodontal ligament space of associated teeth (natural abutment) also are useful in monitoring the functional competence of the prosthesis implant system. Any widening of the periodontal ligament space compared with preoperative radiographs indicates poor stress distribution and forecasts implant failure. After successful implantation, radiographs may be made at regular intervals to assess the success or failure of the implant fixture. Advanced imaging studies may be necessary for adequate assessment in some cases.
Subtle areas of bone resorption adjacent to the fixture may be made evident with intraoral digital images by evaluating a density profile graph of radiographic density values, a feature available on most digital imaging units. If intraoral digital images are acquired at the time of surgery, they may be compared with subsequent digital images either by subjective visualization or digital subtraction. Digital subtraction is a computerized process that may reveal areas of bone resorption not apparent visually. Occasionally, stereoscopic plain films or scanograms, which provide the appearance of three dimensions, may be helpful in assessing multiple implant fixtures within a segment of the alveolar ridge. However, measurements may not be reliable on stereoscopic projections.
In summary, imaging is an integral part of dental implant therapy, and a variety of imaging techniques are used for implant assessment. Crosssectional imaging is increasingly considered integral to optimal implant placement, especially in the case of complex reconstructions. An initial assessment of the feasibility of implant placement may appropriately be made with panoramic radiography. If required, an intraoral radiograph can provide the higher resolution required to evaluate suspected areas of pathosis. Should
the initial assessment be favorable and a decision made to proceed with the placement of implants, a cross-sectional image is indicated. Conventional tomography is appropriate for single-implant sites, whereas reformatted CT is preferred for multiple sites or for an edentulous ridge in which all possible implant sites are to be considered. Assessment of implanted fixtures typically is performed with periapical and panoramic radiography. However, specific cases may require more advanced imaging studies, depending on the nature of the clinical concern.
