(Radiology. 2001;219:334-345.)
© RSNA, 2001
Dental CT Imaging: A Look at the Jaw1
James J. Abrahams, MD
1 From the Department of Diagnostic Radiology, Yale University School of Medicine, 333 Cedar St (CB-30), PO Box 208042, New Haven, CT 06520-8042. Received May 13, 1999; revision requested June 29; revision received January 20, 2000; accepted February 11. Address correspondence to the author (e-mail: abrahams@biomed.med.yale.edu).
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ABSTRACT
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The jaw comprises two complex bony structures: the mandible and maxilla. Their curved or archlike configuration makes radiographic imaging difficult. Furthermore, the superimposition of dense teeth and roots may obscure underlying tissues, and streak artifacts from dental restorations often degrade computed tomographic (CT) images. Recently, dental CT reformatting programs that use thin transverse images of the jaw to reformat multiple panoramic and cross-sectional views were developed. Since images are reformatted, streak artifacts that degrade bone visualization at direct coronal CT are projected over the crowns of the teeth, permitting optimal viewing of bone. As a result, these programs have been successfully used to evaluate implants, cysts, tumors, and surgical procedures. They have created not only a new modality for viewing the jaw but also a new partnership between dentists and radiologists. Questions that cannot be answered in the dentist’s office with conventional radiographs are now answered in radiology departments with dental CT scans. The objective of this article is to trace, with a series of examples, the effect that this imaging modality has had on radiographic evaluation of the jaw and to provide the reader with knowledge that should enable him or her to become more involved in imaging of the jaw.
Index terms: Computed tomography (CT), computer programs • Computed tomography (CT), thin-section • Jaws, abnormalities, 241.38, 241.45, 243.38, 243.45 • Jaws, CT, 241.1211, 241.127, 243.1211, 243.127 • Jaws, neoplasms, 24.38 • Review
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INTRODUCTION
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Until recently, dentists have evaluated the jaw predominantly by using radiographs in their office. The development of dental computed tomographic (CT) reformatting programs, however, has completely revolutionized and changed the fashion in which we radiographically evaluate the jaw today.
What then, was the catalyst or motivating factor for the development of these programs? The catalyst was the dental implant. These implants are metallic cylinders that are surgically imbedded into the edentulous jaw to provide anchorage for a dental prosthesis (1). In this way, patients can have artificial teeth that are fixed in the jaw, which provide an attractive alternative to the standard removable denture.
Dentists and oral surgeons were experiencing difficulty with the use of conventional radiographs to determine whether there was sufficient bone in the jaw to accommodate these implants. They also found it difficult to determine the exact location of the mandibular nerve and other important structures. As a result, dentists soon began to work with their colleagues in radiology, and a dental CT reformatting program was developed to resolve these issues (2–4). Not only was a new method of evaluating the jaw developed, but also a new partnership was created between dentists and radiologists. Soon, questions that could not be answered in the dentist’s office were being answered in the radiology department by using dental CT scans.
Today, these programs are used to evaluate patients with dental implants; in addition, they are being used to assess tumors (5,6), cysts (7), inflammatory disease (8,9), oroantral fistulas (10), silicone implants (11), fractures (12), and surgical procedures. The programs are useful because they provide accurate information about the height and width of the jaw, as well as information about the location of vital structures, such as the mandibular canal, mental foramen, mandibular foramen, incisive foramen, and maxillary sinuses (13). In addition, detailed information about internal anatomy and the relationship between lesions and the cortical margins and roots of the teeth can be established. These images are also excellent because they eliminate the streak artifact from dental restorations that degrades direct coronal CT scans. The reason for this is that transverse images are used to reformat the coronal (cross-sectional) images, projecting the artifact along the crowns of the teeth rather than over the bone that is the region of interest. It should be noted that with the sophistication of CT workstations, evaluation of the jaw not related to implants can also be performed in this manner. These programs are optimally used as an adjunct to, rather than a substitute for, conventional dental radiography.
With a series of examples, this article will not only trace the effect that this imaging modality has had on the radiographic evaluation of the jaw but also provide the reader with a foundation of knowledge that should enable him or her to become more involved in imaging the jaw.
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DENTAL CT PROGRAMS
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Basically, dental CT programs, which are now commercially available through most scanner companies, use 1-mm transverse images of the jaw to reformat multiple cross-sectional and panoramic views (14) (Fig 1). The transverse images are scanned parallel to the alveolar ridge or occlusal plane of the teeth by using a bone algorithm, 15-cm field of view, and 512 x 512 matrix. The mandible and maxilla are each imaged with separate studies.
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Figure 1a. Dental CT scans in 24-year-old woman with periapical radiolucency in the left canine tooth from endodontal disease. (a) Transverse image shows where the cursor is deposited (curved arrows) for the program to produce a curved line (straight arrow) that defines the location for reformatting the panoramic image in c. Perpendicular numbered lines (arrowheads) define where cross-sectional images in d are reformatted. (b) Transverse image shows the appearance of the target due to the opaque root in the center of the radiolucency (arrowheads). (c) Panoramic image shows an area of sclerotic condensing osteitis (arrowheads) surrounding a periapical radiolucency (arrow). The dentine (D), dense enamel (E), pulp chamber (Pc), and root canal (Rc) are nicely depicted. (d) Cross-sectional views show the relation of the periapical radiolucency (arrowheads) to the buccal (solid arrow) and lingual (open arrow) cortex. (Reprinted, with permission, from reference 1.)
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Figure 1b. Dental CT scans in 24-year-old woman with periapical radiolucency in the left canine tooth from endodontal disease. (a) Transverse image shows where the cursor is deposited (curved arrows) for the program to produce a curved line (straight arrow) that defines the location for reformatting the panoramic image in c. Perpendicular numbered lines (arrowheads) define where cross-sectional images in d are reformatted. (b) Transverse image shows the appearance of the target due to the opaque root in the center of the radiolucency (arrowheads). (c) Panoramic image shows an area of sclerotic condensing osteitis (arrowheads) surrounding a periapical radiolucency (arrow). The dentine (D), dense enamel (E), pulp chamber (Pc), and root canal (Rc) are nicely depicted. (d) Cross-sectional views show the relation of the periapical radiolucency (arrowheads) to the buccal (solid arrow) and lingual (open arrow) cortex. (Reprinted, with permission, from reference 1.)
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Figure 1c. Dental CT scans in 24-year-old woman with periapical radiolucency in the left canine tooth from endodontal disease. (a) Transverse image shows where the cursor is deposited (curved arrows) for the program to produce a curved line (straight arrow) that defines the location for reformatting the panoramic image in c. Perpendicular numbered lines (arrowheads) define where cross-sectional images in d are reformatted. (b) Transverse image shows the appearance of the target due to the opaque root in the center of the radiolucency (arrowheads). (c) Panoramic image shows an area of sclerotic condensing osteitis (arrowheads) surrounding a periapical radiolucency (arrow). The dentine (D), dense enamel (E), pulp chamber (Pc), and root canal (Rc) are nicely depicted. (d) Cross-sectional views show the relation of the periapical radiolucency (arrowheads) to the buccal (solid arrow) and lingual (open arrow) cortex. (Reprinted, with permission, from reference 1.)
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Figure 1d. Dental CT scans in 24-year-old woman with periapical radiolucency in the left canine tooth from endodontal disease. (a) Transverse image shows where the cursor is deposited (curved arrows) for the program to produce a curved line (straight arrow) that defines the location for reformatting the panoramic image in c. Perpendicular numbered lines (arrowheads) define where cross-sectional images in d are reformatted. (b) Transverse image shows the appearance of the target due to the opaque root in the center of the radiolucency (arrowheads). (c) Panoramic image shows an area of sclerotic condensing osteitis (arrowheads) surrounding a periapical radiolucency (arrow). The dentine (D), dense enamel (E), pulp chamber (Pc), and root canal (Rc) are nicely depicted. (d) Cross-sectional views show the relation of the periapical radiolucency (arrowheads) to the buccal (solid arrow) and lingual (open arrow) cortex. (Reprinted, with permission, from reference 1.)
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After the transverse images are acquired, the technologist chooses an image that shows the curve of the jaw and deposits the cursor in the center of the jaw at approximately six locations along the curve of the jaw (Fig 1a). The program then connects these points to form a smooth curved line that is superimposed on the center of the jaw. This line defines where the center panoramic view will be reformatted (Fig 1c). Several other panoramic images (not shown) will be reformatted buccal and lingual to the center image. If the technologist does not deposit the cursor in enough positions or if they are not centered on the jaw, then the curved line may undulate when the left and right sides of the mandible are compared, as depicted in Figure 1a. Small degrees of undulation are not a problem, but if undulation is more severe, the technologist simply redeposits the cursor to produce a smooth curve before running the program.
Next, the program automatically draws a series of multiple numbered lines perpendicular to the curved line (Fig 1a). These numbered lines define where the numbered cross-sectional images (Fig 1d) will be reformatted. The distance between the perpendicular lines, and thus between the cross-sectional images, can be varied; however, it is typically 2 mm. The numbers along the bottom of the panoramic images (Fig 1c) correspond to the numbered perpendicular lines in Figure 1a, and thus to the numbered cross-sectional images, as seen in Figure 1d. The scale along the side of the panoramic images (Fig 1c) corresponds to the numbered transverse images that were used to reformat the panoramic view.
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DENTAL IMPLANTS
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Candidates for dental implants are preoperatively evaluated to determine if there is sufficient bone in the alveolar ridge to accept a titanium implant (1,15). The quantity of bone varies considerably because the edentulous regions undergo bone resorption due to disuse atrophy. This can considerably diminish the height and thickness of the alveolar ridge. Patients must also be evaluated to determine the precise location of the mandibular canal (neurovascular bundle), maxillary sinuses, and incisive foramen. Violation or damage to these structures can cause considerable complications. In Figure 2, one can appreciate how important it is to identify the mandibular canal and to determine the height and thickness of the jaw before the implant is placed. The components of the implant are seen on the left side of the figure. The fixture is the portion that is imbedded in the bone, and the abutment, which is held in place with the abutment screw, raises it above the level of the gingiva to permit the prosthesis to be screwed into the abutment with the prosthesis screw.
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Figure 2. Illustration of the components of an implant—prosthesis screw (Ps), abutment screw (As), abutment (A), and fixture (F)—and two dental implants supporting a three-tooth prosthesis. Black portion (long arrow) represents the metal framework within the prosthesis. Note how the location of neurovascular bundle in the mandibular canal (short arrow) must be established to prevent injury during implant placement. (Reprinted, with permission, from reference 1.)
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Assessment with panoramic images, which were used previously, was inadequate because they provided no information relating to the thickness of the jaw. They had a distortion factor of about 25%, which made accurate measurements difficult at best. Dental CT reformatting programs, however, create images of the jaw in three planes that provide clear and comprehensive assessment with accurate measurements.
Figure 3 demonstrates how the dental CT reformatting program is clinically used to determine the optimal site for implants and to determine how much bone is available. It also shows the subsequent surgical procedure. In this particular case, a plastic stent with radiopaque markers was first placed over the edentulous region (Fig 3a). These markers appear on the scan (Fig 3b, 3c) and help the surgeons choose a suitable area for the implants. The same stent can be placed on the patient at the time of surgery to act as a template for identifying the implant sites. Once the implant sites have been established, an incision is made, the gingiva and periosteum are reflected back, and holes are drilled in the bone to accommodate the fixture (Fig 3d). After the fixtures are implanted in the bone, healing screws are placed over the top to prevent tissue from growing into the screw holes (Fig 3d). The incision is sutured closed, and the fixtures are permitted to heal for 3–6 months. In the second stage of the procedure, a small incision is made to expose the fixtures, and the healing screws are removed. The abutments are then attached with the abutment screws (Figs 2, 3e), and the prosthesis is screwed into the abutment (Fig 3f). This typically takes place in the dentist’s office, with local anesthesia.
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Figure 3a. Images show the clinical use of the dental CT program and show the dental implant surgical procedure. (a) Photograph shows a patient edentulous distal to the right maxillary canine (short arrow) being evaluated for dental implants. A plastic stent with six vertical markers is placed over the alveolar ridge and residual teeth. The sixth marker (long arrow) is adjacent to the right canine and is demonstrated in b and c. (b) Transverse CT image demonstrates the sixth marker as a dot (long thick arrow) at perpendicular line 32 (long thin arrow) adjacent to the right canine (short arrow). (c) Cross-sectional CT views demonstrate markers four (straight open arrow), five (large straight solid arrow), and six (curved solid arrow) of the stent as lines. Note that marker six is adjacent to the right canine (curved open arrow). By placing a stent on the patient during surgery, the surgeon knows that the bone under marker six is as depicted on cross-sectional image 32. Arrowheads indicate where alveolar bone width is measured; small straight solid arrows indicate where height is measured. (d) An incision is made, and the gingival and periosteal flaps (arrowheads) are held back with sutures, exposing the bone of the alveolar process (short arrows). Holes are drilled, and three titanium implants are inserted into the bone. Note that the implants are flush with the bone, and their openings are covered with healing screw caps (long arrow). (e) The incision is sutured closed, and after 4-6 months of healing, permanent abutments, which raise the fixture above the gingival surface, are attached with abutment screws. Screw hole in the center of the abutment (arrow) accommodates a screw that fixes the prosthesis. (f) Prosthesis is now attached to three implants. Screw heads are covered with white compound. (Reprinted, with permission, from reference 14.)
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Figure 3b. Images show the clinical use of the dental CT program and show the dental implant surgical procedure. (a) Photograph shows a patient edentulous distal to the right maxillary canine (short arrow) being evaluated for dental implants. A plastic stent with six vertical markers is placed over the alveolar ridge and residual teeth. The sixth marker (long arrow) is adjacent to the right canine and is demonstrated in b and c. (b) Transverse CT image demonstrates the sixth marker as a dot (long thick arrow) at perpendicular line 32 (long thin arrow) adjacent to the right canine (short arrow). (c) Cross-sectional CT views demonstrate markers four (straight open arrow), five (large straight solid arrow), and six (curved solid arrow) of the stent as lines. Note that marker six is adjacent to the right canine (curved open arrow). By placing a stent on the patient during surgery, the surgeon knows that the bone under marker six is as depicted on cross-sectional image 32. Arrowheads indicate where alveolar bone width is measured; small straight solid arrows indicate where height is measured. (d) An incision is made, and the gingival and periosteal flaps (arrowheads) are held back with sutures, exposing the bone of the alveolar process (short arrows). Holes are drilled, and three titanium implants are inserted into the bone. Note that the implants are flush with the bone, and their openings are covered with healing screw caps (long arrow). (e) The incision is sutured closed, and after 4-6 months of healing, permanent abutments, which raise the fixture above the gingival surface, are attached with abutment screws. Screw hole in the center of the abutment (arrow) accommodates a screw that fixes the prosthesis. (f) Prosthesis is now attached to three implants. Screw heads are covered with white compound. (Reprinted, with permission, from reference 14.)
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Figure 3c. Images show the clinical use of the dental CT program and show the dental implant surgical procedure. (a) Photograph shows a patient edentulous distal to the right maxillary canine (short arrow) being evaluated for dental implants. A plastic stent with six vertical markers is placed over the alveolar ridge and residual teeth. The sixth marker (long arrow) is adjacent to the right canine and is demonstrated in b and c. (b) Transverse CT image demonstrates the sixth marker as a dot (long thick arrow) at perpendicular line 32 (long thin arrow) adjacent to the right canine (short arrow). (c) Cross-sectional CT views demonstrate markers four (straight open arrow), five (large straight solid arrow), and six (curved solid arrow) of the stent as lines. Note that marker six is adjacent to the right canine (curved open arrow). By placing a stent on the patient during surgery, the surgeon knows that the bone under marker six is as depicted on cross-sectional image 32. Arrowheads indicate where alveolar bone width is measured; small straight solid arrows indicate where height is measured. (d) An incision is made, and the gingival and periosteal flaps (arrowheads) are held back with sutures, exposing the bone of the alveolar process (short arrows). Holes are drilled, and three titanium implants are inserted into the bone. Note that the implants are flush with the bone, and their openings are covered with healing screw caps (long arrow). (e) The incision is sutured closed, and after 4-6 months of healing, permanent abutments, which raise the fixture above the gingival surface, are attached with abutment screws. Screw hole in the center of the abutment (arrow) accommodates a screw that fixes the prosthesis. (f) Prosthesis is now attached to three implants. Screw heads are covered with white compound. (Reprinted, with permission, from reference 14.)
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Figure 3d. Images show the clinical use of the dental CT program and show the dental implant surgical procedure. (a) Photograph shows a patient edentulous distal to the right maxillary canine (short arrow) being evaluated for dental implants. A plastic stent with six vertical markers is placed over the alveolar ridge and residual teeth. The sixth marker (long arrow) is adjacent to the right canine and is demonstrated in b and c. (b) Transverse CT image demonstrates the sixth marker as a dot (long thick arrow) at perpendicular line 32 (long thin arrow) adjacent to the right canine (short arrow). (c) Cross-sectional CT views demonstrate markers four (straight open arrow), five (large straight solid arrow), and six (curved solid arrow) of the stent as lines. Note that marker six is adjacent to the right canine (curved open arrow). By placing a stent on the patient during surgery, the surgeon knows that the bone under marker six is as depicted on cross-sectional image 32. Arrowheads indicate where alveolar bone width is measured; small straight solid arrows indicate where height is measured. (d) An incision is made, and the gingival and periosteal flaps (arrowheads) are held back with sutures, exposing the bone of the alveolar process (short arrows). Holes are drilled, and three titanium implants are inserted into the bone. Note that the implants are flush with the bone, and their openings are covered with healing screw caps (long arrow). (e) The incision is sutured closed, and after 4-6 months of healing, permanent abutments, which raise the fixture above the gingival surface, are attached with abutment screws. Screw hole in the center of the abutment (arrow) accommodates a screw that fixes the prosthesis. (f) Prosthesis is now attached to three implants. Screw heads are covered with white compound. (Reprinted, with permission, from reference 14.)
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Figure 3e. Images show the clinical use of the dental CT program and show the dental implant surgical procedure. (a) Photograph shows a patient edentulous distal to the right maxillary canine (short arrow) being evaluated for dental implants. A plastic stent with six vertical markers is placed over the alveolar ridge and residual teeth. The sixth marker (long arrow) is adjacent to the right canine and is demonstrated in b and c. (b) Transverse CT image demonstrates the sixth marker as a dot (long thick arrow) at perpendicular line 32 (long thin arrow) adjacent to the right canine (short arrow). (c) Cross-sectional CT views demonstrate markers four (straight open arrow), five (large straight solid arrow), and six (curved solid arrow) of the stent as lines. Note that marker six is adjacent to the right canine (curved open arrow). By placing a stent on the patient during surgery, the surgeon knows that the bone under marker six is as depicted on cross-sectional image 32. Arrowheads indicate where alveolar bone width is measured; small straight solid arrows indicate where height is measured. (d) An incision is made, and the gingival and periosteal flaps (arrowheads) are held back with sutures, exposing the bone of the alveolar process (short arrows). Holes are drilled, and three titanium implants are inserted into the bone. Note that the implants are flush with the bone, and their openings are covered with healing screw caps (long arrow). (e) The incision is sutured closed, and after 4-6 months of healing, permanent abutments, which raise the fixture above the gingival surface, are attached with abutment screws. Screw hole in the center of the abutment (arrow) accommodates a screw that fixes the prosthesis. (f) Prosthesis is now attached to three implants. Screw heads are covered with white compound. (Reprinted, with permission, from reference 14.)
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Figure 3f. Images show the clinical use of the dental CT program and show the dental implant surgical procedure. (a) Photograph shows a patient edentulous distal to the right maxillary canine (short arrow) being evaluated for dental implants. A plastic stent with six vertical markers is placed over the alveolar ridge and residual teeth. The sixth marker (long arrow) is adjacent to the right canine and is demonstrated in b and c. (b) Transverse CT image demonstrates the sixth marker as a dot (long thick arrow) at perpendicular line 32 (long thin arrow) adjacent to the right canine (short arrow). (c) Cross-sectional CT views demonstrate markers four (straight open arrow), five (large straight solid arrow), and six (curved solid arrow) of the stent as lines. Note that marker six is adjacent to the right canine (curved open arrow). By placing a stent on the patient during surgery, the surgeon knows that the bone under marker six is as depicted on cross-sectional image 32. Arrowheads indicate where alveolar bone width is measured; small straight solid arrows indicate where height is measured. (d) An incision is made, and the gingival and periosteal flaps (arrowheads) are held back with sutures, exposing the bone of the alveolar process (short arrows). Holes are drilled, and three titanium implants are inserted into the bone. Note that the implants are flush with the bone, and their openings are covered with healing screw caps (long arrow). (e) The incision is sutured closed, and after 4-6 months of healing, permanent abutments, which raise the fixture above the gingival surface, are attached with abutment screws. Screw hole in the center of the abutment (arrow) accommodates a screw that fixes the prosthesis. (f) Prosthesis is now attached to three implants. Screw heads are covered with white compound. (Reprinted, with permission, from reference 14.)
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RADIOLOGIST’S REPORT
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A complete and comprehensive report should be provided for the referring surgeon or dentist. If the patient is referred for dental implants, then one might start with a paragraph describing the general condition of the jaw. Is the patient partially or completely edentulous? If partially edentulous, are there teeth on the right or left side, and are they central or distal? Conditions such as maxillary sinus disease, periodontal and endodontal disease, root canal procedures, extraction sockets, retained roots, condensing osteitis, contour irregularities, surgical changes, and anomalies (such as palatine or mandibular torus) should also be reported.
Next, measurements of the alveolar bone are obtained in the edentulous regions where implants are likely to be placed. This is best done in a tabular fashion. The height and width are obtained from the cross-sectional images at approximately 1-cm intervals. The width is measured near the top of the alveolar ridge, as indicated in Figure 3c. If the ridge comes to a point, one might state this rather than give a measurement. The height is measured from the top of the ridge to the bottom, as indicated in Figure 3c. In the mandible, the mandibular canal containing the neurovascular bundle enters the lingual surface of the ramus and travels anteriorly to exit the mental foramen on the facial surface near the first premolar. Distal to the mental foramen, the height is therefore measured from the top of the ridge to the top of the mandibular canal. Proximal to the mental foramen and the canal, it is measured from the top of the ridge to the bottom of the mandible. Since numbers can occasionally be transcribed incorrectly on the report, it is wise to state that all measurements should be verified prior to surgery. In general, surgeons like the bone to be at least 5 mm wide and 7 mm high to place implants.
If the scan is obtained to evaluate a jaw lesion rather than implants, then measurements of the alveolar bone need not be obtained. One should describe the relationship of the lesion to the mandibular canal, mental foramen, incisive canal, and maxillary sinuses. Also, describe whether there is root resorption or displacement, if the cortex is expanded or destroyed, and if the lesion is low- or high-attenuating.
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ANATOMY
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The anatomy of the teeth and jaw, as seen on images obtained with dental CT programs, has been previously described (13), and only a brief review of this anatomy is presented here and illustrated in Figures 1c and 4a. The teeth are embedded in a horseshoe-shaped bony ridge called the alveolar process of the mandible or maxilla. The dense cortical lining of the bony socket for the tooth is called the lamina dura, and the dense outer surface of the root of the tooth is called the cementum (Fig 4a). Between the lamina dura and cementum, and attached to both, is the periodontal ligament, which is responsible for holding the tooth in its bony socket. The cementum continues over the crown of the tooth and thickens to become the dense enamel. Beneath the enamel and cementum is the softer dentine, and, finally, in the center of the tooth are the pulp chamber and root canal, which contain the neurovascular bundle. The neurovascular bundle enters the tooth at the root apex via the apical foramen and travels up through the root canal to enter the pulp chamber (Fig 4a). Radiographically, the enamel is opaque, the dentine is less opaque, and the root canal and pulp chamber are lucent, as depicted in Figure 1c.
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Figure 4a. (a) Illustration of normal tooth anatomy. A = apical foramen, C = cementum, D = dentine, E = enamel, G = gingiva, L = lamina dura, N = neurovascular bundle, P = periodontal ligament, Pc = pulp chamber, R = root canal. (b) Illustration of dental caries (curved solid arrow), which allow bacteria to enter the pulp chamber and travel down the root canal to the apical foramen (arrowhead), where a periapical radiolucency develops (open arrows). Note how increased pressure from pulpitis compromises the blood supply to the tooth (straight solid arrow), rendering it nonvital. (Reprinted, with permission, from reference 8.)
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Figure 4b. (a) Illustration of normal tooth anatomy. A = apical foramen, C = cementum, D = dentine, E = enamel, G = gingiva, L = lamina dura, N = neurovascular bundle, P = periodontal ligament, Pc = pulp chamber, R = root canal. (b) Illustration of dental caries (curved solid arrow), which allow bacteria to enter the pulp chamber and travel down the root canal to the apical foramen (arrowhead), where a periapical radiolucency develops (open arrows). Note how increased pressure from pulpitis compromises the blood supply to the tooth (straight solid arrow), rendering it nonvital. (Reprinted, with permission, from reference 8.)
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INFLAMMATORY DISEASE
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Many patients who require dental implants frequently have dental infections that are the underlying cause of their edentulism. This inflammatory disease, which causes resorption of bone, is readily seen on dental CT scans and can be divided into periodontal and endodontal disease (7). Frequently, both periodontal and endodontal disease coexist; the former affects predominantly the bone along the side of the root, and the latter, the bone at the root apex. To facilitate understanding of the disease process, it may be helpful to first familiarize oneself with the anatomy of the teeth and surrounding regions (13,16). This is briefly illustrated in Figures 1c and 4a.
Endodontal Disease
Endodontal disease refers to an acute (abscess) or chronic (granuloma) infection surrounding the root apex of the tooth. Radiographically, it appears as a fairly well-defined radiolucency at the root apex, which is typically smaller than 1 cm in diameter (Fig 1). On the transverse image, it may have a targetlike appearance, with the radiopaque root apex in the center of the radiolucency (Fig 1b). Smaller lesions may simply appear as crescent-shaped radiolucencies capping the root apex (Figs 5, 6).
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Figure 5. Panoramic dental CT scan in a 52-year-old man with a small periapical lesion (straight white arrows) from endodontal disease that surrounds a zone of condensing osteitis (arrowheads). Note that the radiopaque post (black arrows) from a root canal procedure fills the root canal. Compare this with the normal radiolucent root canal in the adjacent tooth (curved white arrow). (Reprinted, with permission, from reference 8.)
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Figure 6. Panoramic dental CT scan in a 60-year-old man with advanced periodontal (arrowheads) and endodontal (straight arrows) lesions. Note how periodontal disease travels down the sides of the root, while endodontal disease affects the root apex. A radiopaque post from a root canal procedure is seen in a tooth with an endodontal lesion (curved arrow). (Reprinted, with permission, from reference 14.)
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Typically, the infection starts with dental caries, which destroy the enamel and dentine and permit bacteria to enter the pulp chamber containing the neurovascular bundle (Fig 4b). From here, the infection travels down the root canal and out the apical foramen (opening for neurovascular bundle), where it propagates and causes bone resorption. Such endodontic lesions are typically treated with a root canal procedure. In this procedure, the endodontist drills an access hole in the tooth to clean out the pulp and root canal and to allow the infection to drain. The apical foramen and root canal are then sealed with a radiopaque material called gutta-percha that causes the normally radiolucent pulp chamber to appear radiopaque on the images (Fig 5). The procedure is completed by placing a crown over the tooth. Bone surrounding the endodontic infection often responds by forming a zone of sclerosis called condensing osteitis (Figs 1c, 5). This is also seen with periodontal disease, as described later. To the unwary, condensing osteitis can potentially be confused with an osteoblastic lesion because of its sclerotic appearance.
It has been shown (9) that, by using dental CT reformatting programs, patients with dental infections (including both endodontal and periodontal infections) have a twofold increase in maxillary sinus disease. Specifically, mounds of inflammatory soft tissue can be seen in the maxillary sinus centered over the root apex. On the transverse image, these areas look like polyps or retention cysts (Fig 7a), but the dental CT reformatting program reveals the true nature of these lesions to be related to dental infection (Fig 7b).
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Figure 7a. CT images in a 64-year-old patient with endodontal and periodontal disease extending into the maxillary sinus. (a) On transverse view, inflammation has the appearance of polyps or retention cysts (arrow). (b) The panoramic view, however, reveals mounds of inflammation (straight arrows) centered over the root apex of the teeth, which are affected by advanced endodontal and periodontal disease. Note the resorption of bone due to the infection (arrowheads) and the empty socket (curved arrow), where the tooth was lost because of periodontal disease. m = maxillary sinus, n = nasal turbinate. (Reprinted, with permission, from reference 8.)
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Figure 7b. CT images in a 64-year-old patient with endodontal and periodontal disease extending into the maxillary sinus. (a) On transverse view, inflammation has the appearance of polyps or retention cysts (arrow). (b) The panoramic view, however, reveals mounds of inflammation (straight arrows) centered over the root apex of the teeth, which are affected by advanced endodontal and periodontal disease. Note the resorption of bone due to the infection (arrowheads) and the empty socket (curved arrow), where the tooth was lost because of periodontal disease. m = maxillary sinus, n = nasal turbinate. (Reprinted, with permission, from reference 8.)
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Periodontal Disease
The second method in which bacteria can affect the teeth is by attacking the periodontal ligament. This ligament surrounds the root of the tooth and holds it in its bony socket (Figs 4a, 8a). The infection, which starts as gingivitis, travels along the ligament, causing bone resorption and formation of a periodontal pocket or space in which the infection propagates (Fig 8a). A clinical example of a periodontal pocket is depicted in Figure 9a, in which the gingiva has been surgically reflected back to expose the pocket. Once the pocket forms, routine dental hygiene is insufficient to cleanse this area, and the infection will progress unless surgical intervention is undertaken. Radiographically, this bone loss appears as a radiolucency adjacent to the surface of the root that can travel down as far as the root apex (Fig 6).
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Figure 8a. Illustrations of a periodontal pocket and its treatment. (a) On the right, a normal periodontal ligament is depicted (thick arrows). On the left, the portion affected by periodontal disease results in a periodontal pocket (thin arrow), with resorption of adjacent bone (arrowheads). (b) The periodontal pocket is treated by filling it with freeze-dried bone (curved arrow), by placing a expanded polytetrafluoroethylene membrane (straight arrow) between soft tissues, and by performing a bone graft to prevent soft-tissue ingrowth. (Reprinted, with permission, from reference 8.)
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Figure 8b. Illustrations of a periodontal pocket and its treatment. (a) On the right, a normal periodontal ligament is depicted (thick arrows). On the left, the portion affected by periodontal disease results in a periodontal pocket (thin arrow), with resorption of adjacent bone (arrowheads). (b) The periodontal pocket is treated by filling it with freeze-dried bone (curved arrow), by placing a expanded polytetrafluoroethylene membrane (straight arrow) between soft tissues, and by performing a bone graft to prevent soft-tissue ingrowth. (Reprinted, with permission, from reference 8.)
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Figure 9a. Photographs in a patient with a large periodontal pocket treated with an expanded polytetrafluoroethylene membrane and freeze-dried bone. (a) Soft tissue is retracted, exposing bone (arrowheads) and the periodontal pocket (straight arrows). Note the exposed root (curved arrow) caused by bone resorption. (b) The defect is packed with freeze-dried bone (arrowheads). (c) The membrane is placed over the bone graft, and the soft tissue is sutured closed to cover it. The edge of the membrane (arrowheads) is depicted between the bone graft and soft tissue. (Reprinted, with permission, from reference 17.)
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Figure 9b. Photographs in a patient with a large periodontal pocket treated with an expanded polytetrafluoroethylene membrane and freeze-dried bone. (a) Soft tissue is retracted, exposing bone (arrowheads) and the periodontal pocket (straight arrows). Note the exposed root (curved arrow) caused by bone resorption. (b) The defect is packed with freeze-dried bone (arrowheads). (c) The membrane is placed over the bone graft, and the soft tissue is sutured closed to cover it. The edge of the membrane (arrowheads) is depicted between the bone graft and soft tissue. (Reprinted, with permission, from reference 17.)
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Figure 9c. Photographs in a patient with a large periodontal pocket treated with an expanded polytetrafluoroethylene membrane and freeze-dried bone. (a) Soft tissue is retracted, exposing bone (arrowheads) and the periodontal pocket (straight arrows). Note the exposed root (curved arrow) caused by bone resorption. (b) The defect is packed with freeze-dried bone (arrowheads). (c) The membrane is placed over the bone graft, and the soft tissue is sutured closed to cover it. The edge of the membrane (arrowheads) is depicted between the bone graft and soft tissue. (Reprinted, with permission, from reference 17.)
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Treatment frequently requires periodontal surgery to eradicate the periodontal pocket. More recently, guided tissue regeneration has been used, whereby the pocket is cleaned by means of curettage, and an expanded polytetrafluoroethylene membrane barrier (Gore-Tex; W.L. Gore, Flagstaff, Ariz) is placed between the healing bone and surrounding soft tissue (Figs 8b, 9b, 9c). The membrane acts as a physical barrier, preventing the ingrowth of rapidly growing soft tissue and allowing bone to heal. Packing the pocket first with a bone graft or freeze-dried bone appears to improve results. In about 5–6 weeks, the membrane is removed. Radiographically, these areas simply appear as areas of normal bone that previously had a defect. This should not be confused with condensing osteitis, which involves the cancellous bone and appears opaque against a lucent background.
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SURGICAL PROCEDURES AND JAW AUGMENTATION
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When periodontal and endodontal lesions are left untreated, the bone surrounding the teeth will continue to be resorbed, and the teeth will eventually be lost. Edentulism is not the end of the process, however, for when the teeth are lost, the normal vertical stress placed on the bone is also lost. This leads to disuse atrophy and continued resorption of alveolar bone that creates difficulty with dentures and precludes the use of dental implants. Fortunately, a number of augmentation and bone graft procedures have been developed to increase the amount of bone available for dental implants. An example is the sinus lift procedure (18,19) (Fig 10). This involves the creation of an osteotomy in the lateral wall of the maxillary sinus to produce a superiorly hinged bony flap, which is pushed inward with the sinus membrane (Fig 10a). The space created in the floor of the maxillary sinus is then packed with bone graft material to increase the height and width of the available bone (Fig 10b). The cross-sectional dental CT image demonstrates how the available bone for implants, including its height and width, is increased by the graft (Fig 11).
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Figure 10a. Coronal diagrams of the maxillary sinus show the sinus lift procedure. (a) The bone flap (arrow) created by means of osteotomy is pushed inward with the maxillary sinus membrane (arrowheads) to create a space (S) that is packed with the bone graft material depicted in b. (b) Image shows the bone graft filling space created with the inward displacement the osteotomy and sinus membrane. (Reprinted, with permission, from reference 8.)
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Figure 10b. Coronal diagrams of the maxillary sinus show the sinus lift procedure. (a) The bone flap (arrow) created by means of osteotomy is pushed inward with the maxillary sinus membrane (arrowheads) to create a space (S) that is packed with the bone graft material depicted in b. (b) Image shows the bone graft filling space created with the inward displacement the osteotomy and sinus membrane. (Reprinted, with permission, from reference 8.)
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Figure 11. Cross-sectional dental CT image shows the sinus lift procedure in an edentulous patient with severe alveolar process atrophy. Note how the graft (G) has created ample bone for implantation in a patient who initially had almost no available bone. (Reprinted, with permission, from reference 20.)
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In addition to the sinus lift procedure, the jaw can be augmented with bone grafts. This is commonly done in the mandible by using an iliac crest graft. Hydroxyapatite, a major mineral constituent of bone that can be produced synthetically, has also been used for augmentation. More recently, guided bone regeneration, which has been used to treat periodontal defects, has had considerable success.
Dental CT imaging has also been excellent for evaluating other surgical procedures. Plastic surgeons may augment the mentum, or chin, by performing a horizontal osteotomy and by anteriorly advancing the inferior aspect of the mandible (Fig 12). The dental CT reformatting program beautifully demonstrates the osteotomy site of the genioplasty and can, therefore, potentially provide important information regarding healing and/or infection in patients who have postoperative difficulties, such as pain or swelling. This information would be difficult to obtain with transverse CT, because the scan is in the plane of the osteotomy, or with direct coronal CT, because of streak artifact created by dental restorations.
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Figure 12. Cross-sectional dental CT images of the mandible show healing after genioplasty in a patient suspected of having nonunion. Note the osteotomy (arrows), which allows anterior advancement of inferior mandible (arrowheads).
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The chin can also be augmented by surgically placing silicone implants under the soft tissue and adjacent to the anterior aspect of the mandible. Although these implants are relatively benign, they can cause erosion of the mandible, which at times can expose the root apex and jeopardize the tooth (11). Cross-sectional images exquisitely depict the relationship of the silicone implant and the root apex (Fig 13) and are thus recommended for use in the evaluation of symptomatic patients (11). Patients may present with pain related to the central incisors, even years after the procedure. Since the procedure is relatively simple and benign compared with the alternative therapy (osteotomy with genioplasty), it is still often used.
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Figure 13. Cross-sectional dental CT images of the mandible show a silicone implant (arrows) eroding the mandible and compromising the root apex (arrowhead). (Reprinted, with permission, from reference 11.)
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Another surgical procedure that can be evaluated with dental CT reformatting programs is mandibular reconstruction after mandibulectomy. The fibular and/or iliac crest grafts that are frequently used can be assessed, and information regarding healing and potential sites for dental implants can be obtained. Dental CT programs are particularly useful for the latter because they provide accurate measurements and information about the altered anatomy. The fixation plates and screws used do not create substantial streak artifact, and in fact, the individual screws can often be clearly delineated.
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OROANTRAL FISTULAS
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An oroantral fistula is an abnormal communication between the maxillary sinus and oral cavity. It is frequently caused by tooth extractions, but it can be secondary to infection (21–23) and trauma (24). These lesions have traditionally been hard to image because the sinus floor is oriented parallel to the plane of the transverse scan, and direct coronal images often have extensive streak artifact caused by dental restorations projected over the bone. The dental CT program, however, can clearly delineate these lesions because it uses transverse images that are then reformatted into coronal or cross-sectional images, causing the streak artifact to be projected horizontally over the crowns of the teeth rather than vertically over the bone (10) (Fig 14). In this fashion, the exact size and location of the fistula can be established, and surgical repair can be undertaken more readily. Radiographically, these fistulas appear as small defects in the alveolar bone between the maxillary sinus and mouth (Fig 14). The ipsilateral sinus will typically contain fluid.
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Figure 14. Cross-sectional dental CT image of the maxilla shows an oroantral fistula (arrow). Note that a streak artifact (arrowheads) is projected horizontally over the crowns of the teeth rather than over the bone. Also note fluid in the sinus (S). (Reprinted, with permission, from reference 10.)
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TUMORS AND CYSTS
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The differentiation of a benign cyst or tumor from a malignant one can be difficult with conventional radiographs. Dental CT programs, however, are able to display the jaw in three planes without streak artifact and, thus, provide information about cortical margins, extent of lesion, and involvement of surrounding structures. Slow-growing benign lesions often expand the bone and thin the cortex, while rapidly growing malignant lesions, such as squamous cell carcinoma of the oral cavity, typically create a sharp defect in the bone, destroying the cortex without expansion. These changes are easily seen on dental CT images (7). In Figure 15, which shows a benign calcifying odontogenic cyst, note how the jaw is expanded, the cortex is thinned, and the mandibular canal is displaced inferiorly. Knowledge of the cortical margin and teeth involved and the position of the mandibular canal (neurovascular bundle) is important for surgical planning.
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Figure 15a. Images in a 9-year-old girl with a calcifying odontogenic cyst. (a) Panoramic radiograph shows a double cortex (arrowheads) due to cortical expansion, but one cannot determine whether buccal or lingual cortex is involved. Also, the margins of the lesion are poorly defined, the neurovascular bundle is not intensified, and the superimposition of an ectopic tooth (arrow) makes it difficult to determine if resorption of left first bicuspid (b) root has occurred. (b) Panoramic dental CT scan permits better visualization of the lesion (solid arrow), ectopic tooth (open arrow), and mandibular canal (arrowheads). b = first left bicuspid. (c) Cross-sectional dental CT scans clearly demonstrate expansion of the buccal cortex (arrowheads), two unerupted teeth within the lesion (open and thin solid arrows), an erupted left first bicuspid (b), and displacement of mandibular canal (thick solid arrows). (Reprinted, with permission, from reference 7.)
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Figure 15b. Images in a 9-year-old girl with a calcifying odontogenic cyst. (a) Panoramic radiograph shows a double cortex (arrowheads) due to cortical expansion, but one cannot determine whether buccal or lingual cortex is involved. Also, the margins of the lesion are poorly defined, the neurovascular bundle is not intensified, and the superimposition of an ectopic tooth (arrow) makes it difficult to determine if resorption of left first bicuspid (b) root has occurred. (b) Panoramic dental CT scan permits better visualization of the lesion (solid arrow), ectopic tooth (open arrow), and mandibular canal (arrowheads). b = first left bicuspid. (c) Cross-sectional dental CT scans clearly demonstrate expansion of the buccal cortex (arrowheads), two unerupted teeth within the lesion (open and thin solid arrows), an erupted left first bicuspid (b), and displacement of mandibular canal (thick solid arrows). (Reprinted, with permission, from reference 7.)
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TOP
ABSTRACT
INTRODUCTION
