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Dental implants and dental CT software programs
Dental Implants and Dental CT Software Programs James J. Abrahams and Arjun Kalyanpur Dental implants are titanium cylinders that are surgically implanted into the jaw to allow fixation of a permanent dental prosthesis. These have provided an attractive alternative to standard removable dentures and have become quite popular. To assess these patients preoperatively, CT software programs were developed that display multiple axial, cross-sectional, and panoramic images of the jaw. As a result, new dialogues and interactions were created between radiologists and dentists, and this in turn brought new territories and unfamiliar diseases to the radiologists' view. The purpose of this article is to familiarize the radiologist with dental implants, the surgical procedure, dental CT software programs, and related dental pathology, Copyright 9 1995 by W.B. Saunders Company
DENTULISM affects nearly half the population between the ages of 45 and 741 and is a common cause for oral dysfunction. Traditionally the treatment has been with removable dentures; these, whereas no doubt providing cosmetic benefits, are often associated with problems of impaired masticatory function and difficulty with speech. As a result, dentists developed nonremovable bridges that are attached to oral implants, metal posts surgically embedded in the jaw. Dental implant surgery has shown increasing popularity worldwide, with recent technologic advances resulting in progressively more viable implants. The success of this procedure engendered the need for an imaging technique that would provide the preoperative information necessary in planning these procedures, a need that has been met by the dental CT reformatting program described in this article. This technique provides multiple axial, panoramic, and cross-sectional projections affording precise anatomic and quantitative information as to the thickness, height, and contour of the alveolar ridge, and the location of the inferior alveolar canal and the maxillary sinuses relative to the alveolar margin. The objective of this article is to provide the reader with a comprehensive discussion of dental implants, CT reformatting programs, and their use in planning implant surgery.
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From Diagnostic Radiology, Yale University School of Medicine, New Haven, CT. Address reprint requests to James J. Abrahams, MD, Associate Professor and Director of Medical Studies, Diagnostic Radiology, Yale University School of Medicine, 20 York St, Room 2-123, South Pavilion, New Haven, CT 06520-8042. Copyright 9 1995 by W.B. Saunders Company 0887-2171/95/1606-000255. 00/0
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IMPLANT DESIGN
Dental implants are metal posts that are surgically implanted in the jaw to support a fixed dental prosthesis (Fig 1). Early long-term clinical studies with these osseointegrated implants showed high success rates of 91% in the mandible and 81% in the maxilla. 2 The results of these early studies were corroborated by others, 3 providing the basis for the osseointegrated implants used today. Microscopic sections through bone-containing titanium implants show that osteoblasts have the ability to grow and integrate with the posts resulting in osseointegration.4 Three types of implants are used in dental practice: blade (Fig 2), subperiosteal (Fig 3), and root form (Fig 1). Blade implants are generally rectangular and are similar in shape to a razor blade. From the long side of the rectangle, which is implanted into the bone via a linear osteotomy, one or more posts extend above the gingiva into the oral cavity to permit fixation of the prosthesis. Subperiosteal implants are metallic meshes that are custom designed to fit over the alveolar process and under the periosteum. Several metallic posts extend from the mesh into the oral cavity (above the gingival margin) to support the prosthesis. To customize these subperiosteal implants, the alveolar process must be surgically exposed so a plaster impression can be obtained to manufacture the implant. After the implant has been manufactured, the patient returns for a second surgical procedure to place the implant. Current imaging technology has made it possible to eliminate the first surgical procedure. Thin slice axial CT can be used to produce a three-dimensional (3-D) model of the jaw from which the prosthesis can be manufactured.
Seminars in Ultrasound, CT, andMRl, Vo116,No 6 (December),1995: pp 468-486
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Abutment
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Fig 1. View of the mandible illustrating several dental implants and the plane and orientation of the cross-sectional DentaScan (General Electric, Milwaukee, Wl) images. Three root-form implants are seen (arrowheads) supporting a fourtooth prosthesis. The abutment (thin arrow), which is attached to the fixture (broad arrow), raises it above the surface of the bone and gingiva and into the oral cavity. The top of the plane (curved arrow) would correspond to one of the numbered perpendicular lines on the axial image in Fig 6A. Note how the height and width of the alveolar process and the location of the mandibular canal can be readily determined on the crosssectional image. I, indicates inferior alveolar canal; m, mental foramen. (Reprinted with permission from Langer B, Sullivan D. Osseointegration: Its impact on the interrelationship of periodontics and restorative dentistry: Part 1. Int J Periodont Res 9:86, 1989.)
Root-form implants are osseointegrated cylinder-shaped implants that simulate the shape of the root of a tooth. They are the ones most frequently used today and are made up of several components (Fig 4). Fb:ture
The fixture is the portion of the implant that is surgically embedded in the osseous tissue of the jaw. It is made of titanium, a material that promotes osseointegration. Fixtures come in various sizes, typically ranging from 3.25 to 3.75 mm in diameter and from 7 to 10 mm in length. 5 The size of the implant chosen is dependent on the amount of available bone. Dentists prefer the largest possible implant because it increases the surface area and thus provides stronger anchorage and more successful osseointegration. At least 1 to 1.5 mm of bone should be present on either side of the implant and 1 to 2 mm of bone between the bottom of the implant and the adjacent structures, ie, maxillary sinus and mandibular canal. 5 Fixtures can be threaded, unthreaded, or even coated with hydroxyapatite. 6
The next component of the implant is the abutment (Figs 1 and 4), which is attached to the fixture to increase its height to a level above the gingival surface. This attachment is performed 3 to 6 months after the initial procedure, thus giving the fixture time to heal within the bone. The fixture is surgically exposed and the abutment is attached with an abutment screw (Fig 4). The top of the abutment screw has a small screw hole that will allow the dental prosthesis to be attached by a screw that runs through the prosthesis and into the abutment screw and abutment. This screw is designed as the weakest portion of the implant so if there is unforeseen stress, it will break rather than the fixture itself. Angled abutments are also available to correct for implants that are inserted at an angle rather than parallel to the residual teeth. Prosthesis
The prosthesis is composed of a strong metal framework (Fig 4) that supports the prosthetic teeth. Because the implants actually support this framework, one can have a full 14-tooth dental prosthesis supported by only six implants or a 3-tooth prosthesis supported by two implants, as seen in Fig 4, for example. IMPLANT SURGICAL PLACEMENT
Dental implant surgery is a two-stage procedure. 7 In the first stage, the fixture is installed within the bone, and in the second stage the abutment is attached to the fixture. A 4- to 6-month healing period is required between stages to allow time for osseointegration to occur. The procedures are typically performed in the dentist's office using local anesthesia. Fixture Placement
The first stage of surgery, fixture installation, is more extensive than the second. Its duration depends on the number of fixtures placed. Local infiltration with lidocaine and/or a nerve block are used for anesthesia. A linear incision is next made along the buccal or lingual surface of the alveolar ridge, and a soft tissue flap, incorporating the periosteum, is reflected back (Fig 5E). If the exposed bony ridge has a sharp or pointed
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Fig 2. Blade implant in mandible. (A) Plain film illustrating a single post-blade implant, Note how the blade is inserted in the mandible after a linear osteotomy while the post (P) extends above the level of the bone and gingiva. (B) Axial views demonstrating blade implant in mandible (arrow), (C) Cross-sectional views illustrating the blade within the mandible (black arrow) and the post (white arrow) extending into the oral cavity above the level of the bone and gingiva.
surface secondary to buccolingual atrophy, an alveolarplasty may be performed to remove the sharp edge and provide a broad surface to install the fixtures. The anticipated implant site that has been predetermined radiographically (Figs 5A through D) is then located on the patient either by measuring from an existing tooth or landmark that can be seen on the films and in patient or by using a stent with markers as shown in Fig 5B. (See "Radiographic and Surgical Stents" section.) Once the site has
been identified on the patient, a series of graduated drill bits are used to produce a hole in the bone and to progressively widen it to the appropriate size. The hole is then threaded with a titanium tap burr to accommodate the threaded fixture. As osseointegration can only occur with viable cells, drilling and threading are performed at extremely low revolutions per minute with the use of copious irrigation to prevent heating and destruction of osteoblasts. The top of the fixture has a threaded hole (Fig
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Fig 3. Subperiosteal implant. (A) Photograph of subperiosteal implant. The white portion fits under the periosteum and on top of the bone of the alveolar process while the metallic-appearing portion extends above the gingiva to support the prosthesis. The subperiosteal portion (white) is often coated with hydroxyapatite to facilitate bone growth. (B) Panoramic view demonstrating the subperiosteal implant on a severely atrophic mandible. The prosthesis has not been attached. (Courtesy of Wayne C. Jarvis, DDS, Dentofacial Surgery of Western New York, Williamsville, NY.)
5G) to accommodate the abutment screw, which is inserted in the second stage of the procedure. A healing or cover screw is used to prevent soft tissue and bone from growing into the hole during healing (Fig 5E). At this stage, the implant and cover screw are flush with the surface of the bone. After the fixtures are placed, the tissue flap is sutured closed and the implants are allowed to heal for approximately 4 months in the mandible and 6 months in the maxilla (Fig 5F) in order to promote the process of osseointegration, thus forming a strong bond between the bone and implant. After about 1 week, the skin sutures are removed and the
patient is fitted with an interim prosthesis, usually the one used before surgery. Abutment Connection The second stage of the procedure, which occurs 4 to 6 weeks later, tends to be less traumatic for the patient and is typically performed with local infiltration using lidocaine. A pointed probe is used to locate the cover screws. An incision is then made to expose and remove them or, alternatively, a circular soft tissue punch can be used to excise the tissue above the cover screw. The abutment is next attached to the fixture using the abutment screw. This
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A
Fig 4. illustration demonstrating the components of an implant (left) and two root-form implants supporting a threetooth prosthesis (right). For illustrative purposes, the black portion running through the prosthesis (arrow) represents the metal framework within the prosthesis. Prosthesis screw (Ps), abutment screw (As), abutment (A), and fixture (F).
extends the height of the implant to above the gingival margin. The top of the abutment screw has a screw hole that permits the prosthesis to be screwed into the fixture (Figs 4 and 5H). Finally, a surgical pack is applied and retained for a short period of time by a healing cap.
ure and creates the potential for antral infection. It is also important to know the dimensions of the alveolar process before surgery because atrophy of the alveolar ridge, which occurs in edentulous patients, can preclude the use of implants. Radiologists and dentists therefore began to evaluate the efficacy of using cranial CT to assess these patients. 8,9Axial and coronal images were only marginally helpful because .of the streak artifact created by dental restorations. However, the technique of reformatting images using thin slice axial CT was found to be extremely useful because it avoided the streak artifact, displayed the anatomy in multiple planes, permitted assessment of the width of the alveolar process, and allowed accurate millimeter measurements to be established. Reformatting software programs that display multiple panoramic and cross-sectional images soon became available 1~ (Figs 6 and 7) and have greatly facilitated the assessment of implant patients. These programs more recently have also become state of the art for evaluating lesions of the jaw 14-17 (Fig 8). The particular program used in this report is the DentaScan (General Electric, Milwaukee, WI).
Prosthodontic Procedure An impression of the jaw made of plaster or hydrocolloid is next made with the abutments in place. From this, a cast of the jaw is obtained and used by the prosthodontist to ensure that the screw holes in the implants line up exactly with those in the prosthesis and that the prosthetic teeth line up in the proper occlusal plane. After the prosthesis is manufactured, it is fixed to the abutments with screws, which typically insert into the central fossa of the prosthetic teeth. A white compound will finally be used to cover the screw holes (Fig 51). RADIOLOGIC ASSESSMENT
Radiologically, the precise height, width, and contour of the alveolar process and the location of the maxillary sinuses and mandibular canal must be determined in order for the oral surgeons and dentists to perform the surgery safely. Injury to the neurovascular bundle within the canal results in paresthesia or hyperaesthesia of the face, whereas perforation of the maxillary sinuses increases the likelihood of implant fail-
Dental CT Program Technical parameters. Having been instructed tq remain still and reassured that the procedure is painless, the patient is placed supine in the gantry using a head holder, chin strap, and sponges on either side .of the head to prevent motion. A lateral digital scout view is first obtained to define the upper and lower limits of the study and to determine whether the scan plane is parallel to the alveolar ridge. If it is not and the program does not permit angulation of the gantry, then the pat!ent should be repositioned and a repeat digital scan performed. Axial 1-mm cuts at an interval of 1 mm are obtained using a bone algorithm, in dynamic mode, with a 15-cm field of view, using a 512 x 512 matrix, at 140 KV and 70 MA. When both the mandible and maxilla are being evaluated, a separate run should be performed for each because the scan angle of the mandible is slightly different than that of the maxilla. Executing the dental program. Each program varies slightly depending on the manufac-
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turer. However, the following guidelines generally apply. First, the CT technologist selects one of the axial images as a reference image from which the program will be run. The image selected should be at the level of the roots of the teeth and should demonstrate the full contour of the mandible or maxilla. Next, the technologist superimposes a curved line on the image of the mandible or maxilla (Figs 6A and 7A). This is accomplished by depositing the cursor on one of the axial images at approximately six points along the curve of the jaw. Automatically, the program connects these points to produce a smooth curve that is superimposed on the jaw. This curved line defines the plane and location of the reformatted panoramic images (Figs 6C and 7D). Several images are reformatted buccal to this curve and several lingual to it. Reformatted cross-sectional images (Figs 6D through F and 7E and F) are defined by multiple numbered lines that the program automatically deposits perpendicular to the curved line (Figs 6A and 7A). The distance between the numbered perpendicular lines and thus between cross-sectional images can be varied by the technologist; however, 2-mm spacing is generally used. If a stent with radiographic markers is used, 1-mm slices may be necessary in order to visualize the markers. Streak artifact, which degrades visualization of bone on direct coronal images, does not degrade the reformatted cross-sectional images because the artifact is projected at the level of the crowns of the teeth and not over the bone (curved arrow in Fig 6E). When complete, three types of images are displayed: axial, cross-sectional, and panoramic. In a typical study, there are 30 to 50 axial images, 40 to 100 cross-sectional images, and 5 panoramic images. Images should be filmed in a consistent fashion to avoid confusion on the part of the referring dentist. Life-size crosssectional and panoramic images arepreferred. This can be accomplished with most programs by placing four screen saves on a 14 x 17 sheet of x-ray film. A millimeter scale displayed on the films (Fig 6D) can be used to verify that the images are near life-size. The scale is also used to obtain accurate measurements. One can place calipers on the bone to be measured and
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transfer this caliper setting to the millimeter scale. Accidental or intentional minification or magnification of the images will also minify or magnify the scale and not affect the measurement. Filming is optimally performed on a laser printer, allowing two original sets to be easily printed: one for the radiologist and one for the dentist. The author films the axial images with 12 screen saves per sheet of film, whereas the cross-sectional and panoramic images are filmed with 4 screen saves per sheet of film. Each cross-sectional and panoramic screen save contains multiple images. Windows of between 3,000 to 4,000 and levels of between 300 to 500 are typically used.
Interpretation of dental CT program images and measurements. In order to interpret the images appropriately, it should first be noted that each view can be related to the other by a series of (-) marks that appear on the films. These should not be confused with a scale for measurements, to which they appear similar. The marks that run along the side of the cross-sectional and panoramic images (Figs 6D and C) correspond to the direct axial slices that were used to reformat the images. For example, in Fig 6, 42 axial images were obtained to reformat the images, thus 42 marks appear along the side of the reformatted cross-sectional (Fig 6D) and panoramic images (Fig 6C) corresponding to the numbered lines drawn perpendicular to the curve superimposed on the axial image in Fig 6A. They therefore also correspond to the numbered cross-sectional reformatted images in Figs 6D, E, and F. To illustrate how one view can be related to another, note how the right mental foramen (M) in Fig 6E, which is seen on cross-sectional image 20 (lower right) at the level of the 14th mark on the side of the image, is also seen in Fig 6B on axial image 14 at the 20th perpendiculhr line. The same correlation can be made on the panoramic view. When the scai~ is performed as part of the preoperative workup of a dental implant patient, the status of the dentition and the desired implant sites are first established. In the partially edentulous patient, it is presumed that the implants will be placed in the edentulous seg-
Fig 5.
Fig 5. (G) Before obtaining this photograph, a small incision was made to remove the healing caps. Healing abutments, which were attached to the implants, have been removed. The threaded opening of the implant is visualized. (H) The permanent abutments, which raise the fixture above the gingival surface, have now been attached. The screw hole (arrow) in the center of the abutment accommodates the screw that fixes the prosthesis. (I) The prosthesis is now attached to the three implants. The screw heads are covered with a white compound, (Reprinted with permission. TM)
Fig 5. (cont'd,) Surgical implant procedure. (A) This patient, being evaluated for dental implants, is edentulous distal to the right maxillary canine (arrow). (B) A stent with six vertical markers has been placed over the alveolar ridge and residual teeth. The sixth marker (long arrow) is adjacent to the right canine (short arrow) and is demonstrated on the CT images, This marker appears as a dot on the axial image next to perpendicular line 32 (Fig 5C) and as a line on cross-sectional image number 32 (Fig 5D), (C) Axial view demonstrating the sixth marker (long thick arrow) at perpendicular line 32 (thin arrow) and adjacent to the right canine (short white arrow). Note the radiolucent pulp in the center of the teeth (black arrow). (D) Cross-sectional views demonstrating markers four (open arrow), five (straight arrow), and six (curved arrow) of the stent. Note that marker number six is adjacent to the right canine (open curved arrow). By placing the stent on the patient during surgery, the surgeon knows that the bone under marker 6 is as depicted by cross-sectional image 3Z. {E) An incision is made and the gingival and periosteal flap {arrowheads) is held back with sutures. This exposes the bone of the alveolar process (short thick 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). (F| The incision is sutured closed and permitted to heal for 4 months in the mandible and 6 months in the maxilla,
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Fig 6. DentaScan of mandible illustrating the use of the dental CT program and the anatomy. (A) Axial image of mandible with superimposed curve. The curve defines the plane and location in which the panoramic images in C are reformatted. Numbered lines drawn perpendicular to this curve (arrow) define the plane and location in which the cross-sectional images viewed in D through F are reformatted. Mental foramen (M). (B) Axial images illustrate the inferior alveolar canal (I), the mental foramen (M), and the genial tubercle (Gt). The numbers along the left side of the figure refer to the particular number of the axial image. Note that the mental foraman, which is seen on axial image number 14 at the 20th perpendicular line, is also seen in E on cross-sectional image 20 (lower right) at the level of the 14th tick mark on the side of the image. The tick marks and numbers allow one image to be correlated with another. (C) Panoramic views. The numbered tick marks along the bottom of the images correspond to the numbered perpendicular lines displayed on the axial image in A. The tick marks along the side of the image correspond to the axial images, which were used to reformat these images. Note that there were 42 axial images acquired and, thus, 42 tick marks along the side of this image. Inferior alveolar canal (I), mental foramen (M), second bicuspid (2B), first bicuspid (1B), and cuspid (C). (D through F) Cross-sectional views. (D) Images 1 through 10 are through the posterior right mandible (see perpendicular lines 1 through 10 in A).
ments. Measurements are therefore obtained in these regions. If the cross-sectional images are 2 mm apart, the author typically provides a measurement on
every 5th cross-sectional image. If the crosssectional images are 1 mm apart, measurements are provided for every 10th image in the edentulous areas. The height and width of the alveolar
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Fig 6. (cont'd.) (E) Images 11 through 20 are more anterior. (F) Images 21 through 30 extend to the midline. The images of the left half of the mandible are not shown. Arrowheads in image 17 (E) indicate that the height of the mandible distal to the mental foramen is measured from the top of the alveolar process to the top of the mandibular canal, whereas mesial to the foramen it is measured from the top of the alveolar process to the bottom of the mandible (arrowheads in F, image 30), Measurement of the width is also demonstrated in image 30 (F). M M at the bottom of D indicates the millimeter scale. Note how streak artifact from dental restoration (curved arrow in E) does not degrade visualization of the bone. A, alveolar process; C, cuspid; Cp, coronoid process; D, digastric fossa; GT, genial tubercle; I, inferior alveolar canal; L, lingula; M, mental foramen; Mf, mandibular foramen; Mg, mylohyoid groove; MI mylohyoid line; O, oblique line; Rf, retromolar fossa; Rt, retromolar triangle; S, submandibular fossa; SI, sublingual fossa; T, temporal crest; 1B, first bicuspid; and 2B, second bicuspid. (Reprinted with permission from Abrahams JJ: Anatomy of the jaw revisited with a dental CT software program: Pictorial essay. AJNR 14:979-990, 1993.)
process are measured as described in the following paragraph. In the mandible, the height in the region distal (posterior) to the mental foramen is measured from the top of the alveolar process to the top of the mandibular canal (Fig 6E, arrowheads in image 17). The mandibular canal is marked with a wax pencil on the crosssectional images, and this set of films is sent to the referring dentist. Methods of locating the mandibular canal if it is not initially visualized on the cross-sectional images are discussed later. In the region mesial (anterior) to the mental foramen, the full height of the mandible is available to the surgeon because the mandibular canal ends at the mental foramen and is not present mesial to it. Measurements in this area are provided from the top of the alveolar ridge to the inferior surface of the mandible (Fig 6F, arrowheads in image 30). The width is measured near the top of the alveolar process (Fig 6F, arrowheads in image 30). Occasionally, if the alveolar process comes to a point secondary to atrophy, it may be difficult to measure. In this situation the surgeon may choose to remove the top-pointed portion of the ridge (alveolarplasty), providing a broad base for an implant. This obviously will affect the height of the alveolar ridge, and it is best to let the surgeon
estimate how much of the ridge will be removed and what the remaining height will be. In this situation, one may simply state that the ridge is pointed and not provide a measurement for the width. In the maxilla, the height in the distal (posterior) aspect is measured from the top of the alveolar process to the floor of the maxillary sinus (Fig 7E, arrows in image 7), whereas more mesially (anteriorly), the height is not limited by the maxillary sinuses and is measured from the alveolar ridge to the nasal fossa (Fig 7F, arrows in image 23). The width is measured in a similar manner as in the mandible (Fig 7F, arrows in image 22). In the completely edentulous patient, it is more desirable to place the implants centrally because the height of the alveolar process is not limited by the more distal mandibular canal or the maxillary sinuses. A sample dictated report. The referring dentist or surgeon should be provided with a complete and comprehensive report. The density and general health of the jaw are described, as are such conditions as maxillary sinus disease, periodontal disease, root canal procedures, extraction sockets, retained roots, atrophy, cysts, osteitis condensans, contour irregularities, surgical changes, and anomalies such as torus palatinus or mandibularus. The report defines
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Fig 7. DentaScan of maxilla illustrating the use of the program and the anatomy. (A) Axial image with superimposed curve. The curve defines the plane and location in which the panoramic images seen in D are reformatted. Numbered lines drawn perpendicular to this curve define the plane and location in which the cross-sectional images viewed in E and F are reformatted. (B) Axial views through the alveolar ridge and hard palate. A, alveolar process; As, inferior nasal spine; Gg, groove for greater palatine nerve; If, incisive foramen; Mb, maxillary bone; palatine process; Mp, median palatine suture; Ms, maxillary sinus; Nf, nasal fossa; Pb, palatine bone: horizontal plate; Ts, transverse suture; and T, tongue. (C) Axial views through the maxillary sinuses in the pterygopalatine fossa. G, greater palatine foramen; L, lesser palatine foramen; Ms, maxillary sinus; Nc, nasal conchi; Ns, nasal septum; Pt, pterygoid process; Tp, pterygopalatine fossa. (D) Panoramic views. A, alveolar process; IF, incisive foramen; MS, maxillary sinus; NC, nasal concha; NF, nasal fossa; NP, nasalpalatine canal; and NS, nasal septum.
the status of the dentition and provides measurements of the alveolar process. One should state whether the patient is completely or partially edentulous and, if partially, the location of the
edentulous segments should be noted. Measurements are provided in a tabular fashion, and the mental foramina and incisor foramen are identified for easy interpretation. For example, the
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Fig 7. (cont'd.) (E through F) Cross-sectional views. Images 1 through 15 (E) are through the posterior right maxilla (see perpendicular lines 1 through 15 in A); images 16 through 30 (F) are more anterior on the right and extend to the midline (see perpendicular lines 16 to 30 in A). The arrows in image 7 (E) indicate how the height of the distal alveolar process is measured from the floor of the maxillary sinus to the top of the alveolar process, whereas the mesial alveolar process (F in image 23) is measured from the floor of the nasal fossa to the top of the alveolar process (arrows in image 23). The arrows in image 22 (F) indicate how the width of the alveolar process is measured. A, alveolar process; As, anterior maxillary spine; G, greater palatine foramen; Gg, groove for greater palatine nerve; If, incisive foramen; Ms, maxillary sinus; Nc, nasal concha; Nf, nasal fossa; and Np, nasalpalatine canal. (Reprinted with permission from Abrahams J J: Anatomy of the jaw revisited with a dental CT software program: Pictorial essay. AJNR 14:979-990, 19933
following measurements were obtained from Figs 6E and F: Image 17: height 14 ram, width 4 ram. Image 20: left mental foramen. Image 25: height 26 ram, width 4 mm.
When a stent with radiopaque markers is used, then the markers on the images are numbered with a wax pencil, measurements are obtained on the cross-sectional images where the markers appear, and the image number of
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Fig 8. Mandibular DentaScan demonstrating a radicular cyst surrounding the root apex of the left canine. (A) Axial view again demonstrates the radicular cyst (black arrow). The periodontal space (Ps} typically not seen on CT can be visualized in this patient between the lamina durs (Ld) and the tooth. (B) Panoramic view illustrates an area of sclerotic osteitis condensans (arrowheads) surrounding the radicular cyst (black arrow). The cervical constriction (Cc), pulp chamber (Pc), root canal (Rc), and dense enamel (E) are nicely demonstrated on the right first molar. The mesial (M), occlusal (O), and distal (D) surfaces of the left canine are demonstrated as well as the mesiobuccai (Mb) and distobuccal (Db) roots of the left first molar. The lingual root of the left first molar is not seen in this plane and would be visualized on a more posterior slice. (C) Crosssectional views demonstrating the radicular cyst surrounding the root apex.
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this image is provided in the report. In Fig 5D for example, the measurements at the markers are as follows: Marker 4 (image 21): height 13 mm, width 6 mm. Marker 5 (image 27): height 12 mm, width 5 mm. At the end of the report, a brief impression is noted describing any pertinent pathology, the degree of atrophy, and a disclaimer stating that all measurements should be verified before surgery. This is included because it is difficult for the radiologist to know whether the numbers dictated were transcribed correctly onto the report and because the surgeon may choose to place the implant at a somewhat different angle than that measured by the radiologist.
Identifying the Mandibular Canal Identifying the mandibular canal on crosssectional images and providing measurements from the top of the alveolar ridge to the top of the canal is extremely important. If the canal is not properly identified, its injury during implant surgery can be quite debilitating, resulting in permanent paresthesia and hyperaesthesia of the face. Typically, the canal can be readily observed on cross-sectional images; however, there are some circumstances when portions of the canal or even the entire canal may be hard to visualize on the cross-sectional images. In this situation, the author has found the following methods extremely helpful in locating the canal. The first, the cortical niche sign, refers to an indentation along the inner cortical margin on the lingual surface created by the mandibular nerve as it traverses the mandible (Fig 9). This Fig 9. Cortical niche sign. This cross-sectional image of the mandible demonstrates an indentation on the lingual cortex of the mandible called the cortical niche sign (Cn), which is created by the mandibular nerve as it traverses the mandible. This sign, which is often more subtle than this, can be helpful in identifying the location of the mandibular canal. (Reprinted with permission from Abrahams J J: CT assessment of dental implant planning. Oral Maxillofac Surg Clin North Am 4:1-18, 1992.)
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can be quite subtle and is not identified in all patients. However, when present, it is very helpful for identifying the canal. Caution should be taken not to confuse other cortical irregularities with the cortical niche sign. The cortical niche is a continuous defect seen on multiple cross-sectional images, whereas other cortical irregularities are randomly situated and not seen consecutively on multiple images. If the canal is identified with the cortical niche sign, its location should be confirmed with the other methods described below. The next method, referred to as triangulation, uses the reference marks on the films to relate an anatomic structure on one view with that on another. In this fashion, the panoramic and axial views can be utilized to identify the canal on the cross-sectional view. For example, in Fig 10A, it is difficult to see the canal on the cross-sectional images, but it is readily observed in the panoramic view (Fig 10B). The information from the panoramic view can be used to triangulate the location of the canal on the cross-sectional image, as shown in Fig 10. If one wishes to locate the canal on cross-sectional image 14 (Fig 10A), one first refers to the 14th reference mark along the bottom of the panoramic view (Fig 10B). A vertical line from this point is then drawn up to the bottom on the canal and extended across in a perpendicular direction toward the reference marks on the left side of the panoramic image. Note that this line intersects the 21st mark along the left side of the image. The level of the canal in crosssectional image 14 can then be identified by counting up 21 marks along the side of the image (Fig 10A). This same process can also be used using the axial image.
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Fig 10. Triangulation. When the inferior alveolar canal is not initially seen on (A) the crosssectional images, its location can be determined by triangulating from the (B) panoramic or axial image. The inferior alveolar canal is indicated by arrows. (Reprinted with permission from Abrahams J J: CT assessment of dental iraplant planning. Oral Maxillofac Surg Clin North Am 4:1-18,1992.)
Lastly, if a canal is identified on some crosssectional images but not others, the images in which it is identified can be used to estimate the position of the canal on the other images. This can be performed because the distance from the bottom of the mandible to the bottom of the canal tends to be relatively constant. The only region where the distance is not constant is immediately adjacent to the mandibular foramen and the mental foramen. It is the distance from the top of the canal to the top of the alveolar process that varies secondary to atrophic changes. With this knowledge, one can extrapolate the location of the canal from the images in which it is visualized. For example, in Fig 6E, the distance from the bottom of the canal to the bottom of the mandible in image 11 is 5 mm, and in image 15 it is also 5 mm. Therefore, if the canal was not visualized in image 15, we would be able to estimate its location by using the position of the canal that was visible on image 11. The author has also found that the right and left hemimandibles are
bilaterally symmetrical and that the distance from the base of the mandible to the base of the canal on the right at a particular point is approximately the same as the distance measured at that same point on the left. Thus, if the canal is identified on the cross-sectional images of the right half of the mandible but not the left, this information can be used to estimate the position of the canal on the left. It is recommended that several of these methods be used to confirm the location of the canal rather than relying on one alone.
Radiographic and Surgical Stents Stents are clear acrylic devices that fit snugly over the residual teeth and alveolar ridge (Fig 5B). They are manufactured by the dentist after obtaining an impression of the teeth and ridge. Radiopaque markers are attached to the stent to provide reference points on the DentaScan images for the purpose of localization and measurement. At surgery, the same radio-
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I Fig 11. Illustration demonstrating a periodontal pocket and its repair. (A) Bacterial overgrowth has attacked the periodontal ligament (curved arrow) and resorbed bone (straight arrow) creating this periodontal pocket. (B) The periodontal pocket in A has now been packed with freeze-dried bone (curved arrow) and covered with a Gortex graft (straight arrow) to prevent ingrowth of soft tissue while the bone graft heals. The Gortex is removed in approximately 6 weeks.
graphic stent is used as a surgical template to assist in the placement of the implants. Radiographic markers are often made of gutta-percha, a radiopaque material commonly used by dentists as a temporary filling. Other radiopaque material such as pieces of wire also may be used for markers. Ideally the markers are 1 to 2 mm in diameter, vertically oriented, and without a mesial or distal tilt (Fig 5B). The vertical markers should be attached to the stent in such a way that they extend deep into the buccal vestibule. This helps their visibility on the cross-sectional images because they overlie the bone rather than the level of the crown of the tooth, which may have streak artifact from restorations. Other types of markers also have been used including coating a denture with barium. 18 Radiologists can be of assistance in
helping the dentist place the markers in a manner that yields the greatest information because most dentists are unfamiliar with CT scanning. Use of the stent is illustrated in Fig 5. After the patient is scanned, the markers appear as dots on the axial image (Fig 5C) and as lines on the cross-sectional images (Fig 5D). If surgeons want to put an implant just distal to the right canine (Fig 5A and C, short white arrow) they note that the sixth marker falls in this location (5B and C, long thick arrow). Next they look at the axial image to see on which cross-sectional image the marker appears. In this case, the sixth marker on the stent appears on the axial image at perpendicular line 32 (5C, thin arrow) and is therefore seen on cross-sectional image 32 in Fig 5D. The surgeon now knows that the bone
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Fig 12. Panoramic CT scan of mandible demonstrating periodontal disease. The radiolucency surrounding the root of the left first molar (arrowheads) is caused by bone resorption from periodontitis. The periapical lucency around the root apex of the right canine (long arrows) is a radicular cyst secondary to a periapical abscess. Note the metal post in the pulp chamber secondary to a prior root canal procedure (solid curved arrow) and compare this to the normal radiolucent pulp chamber of the left first premolar (open curved arrow). A zone of relatively dense sclerotic bone (osteitis condensans) is identified surrounding both areas of disease. (Reprinted with permission. TM)
under marker 6 corresponds to cross-sectional image 32. After the incision is made and the alveolar bone is exposed, the stent can be placed back on the patient and used as a template. This is accomplished by cutting a hole out over the alveolar aspect of the stent so the surgeon can mark the area for the implant (Fig 5B). The marker on the cross-sectional image can also be used to help the surgeon estimate the correct facial or palatal angulation of the implant. Stents and markers are only accurate if the stent fits tightly and if its position in the patient's mouth is reproducible and the same during scanning and surgery. To accomplish this, the dentist should give the patient careful instruction on how to insert the stent properly. The radiology staff should not be responsible for inserting stents. PERIODONTAL DISEASE AND ITS SEQUELAE
Most patients who are evaluated for dental implants have advanced periodontal disease, and it is this periodontal disease and its sequelae that are responsible for the edentulism. The process begins initially with gingivitis and the accumulation of plaque and bacteria around the teeth. Eventually the bacteria attack the periodontal ligament and track along the side of the root where erosion of bone occurs, thus forming a periodontal pocket 19(Fig 11A). Radiographically, this bone loss appears as a radiolucency adjacent to the side of the root (Fig 12A).
Routine dental hygeine is unable to access the periodontal pocket so it typically progresses unless treated appropriately by a periodontist. Bacteria can also gain access to the root through dental caries. The caries allow the bacteria to enter the pulp chamber where it then travels down the root canal and out the apical foramen to form a periapical abcess or granuloma that is often referred to as a radicular cyst2~(Figs 8 and 12). Radicular cysts are associated with nonvital teeth. This is because the tooth cannot expand so the edema and swelling from the pulpitis increases the pressure within the tooth and diminishes its blood supply. Radicular cysts, which appear as periapical radiolucencies, are treated by a root canal procedure in which the dentist drills down the root canal and drains the infection. The root canal then is sealed and filled with a metal post to add support to the tooth. Radiographically, the post appears as a radiopacity in the center of the tooth in contrast to the normal radiolucent pulp chamber (Fig 12). Periodontal pockets can be cleaned out surgically and filled with a freeze-dried bone graft that is covered with a Gortex (W. L. Gore Associates, Inc, Flagstaff, AZ) fabric barrier (Fig lIB). The Gortex is left in place for about 6 weeks and prevents the soft tissue from growing into the bone graft before it heals. Alternatively, periodontal pockets can be treated by resecting the gingiva and lowering the gum line to eliminate the pocket and allow access for home care dental hygeinc. The chronic inflammation from periodontal
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Fig 13. Cross-sectional views of the mandible in two different patients demonstrating bone resorption and atrophy. (A) Bone loss and atrophy of the height of the mandible in this patient causes the mandibular canal (arrow) to sit immediately under the gingival surface. The dotted line demonstrates how the bone contour might have appeared prior to atrophy. The patient has insufficient bone for implants. (Reprinted with permission.14) (B} The width of the mandible in this patient has been considerably diminished from atrophy. The dotted line again demonstrates how the bone might have appeared before atrophy. (Reprinted with permission from Abrahams J J: CT assessment of dental implant planning, Oral Maxillofac Surg Clin North Am 4:1-18, 1992.)
pockets and periapical infection often cause the surrounding bone to react by becoming sclerotic, a condition termed osteitis condensans, condensing osteitis, or focal sclerosing osteomyelitis 21 (Figs 8 and 12).
After the patient becomes edentulous, the normal vertical stress exerted on the bone is no longer present and atrophy occurs. This disuse affects both the height and buccolingual width of the alveolar process (Figs 13A and B).
REFERENCES 1. Laney WR, Tolman DE, Keller EE, et al: Dental implants: Tissue-integrated prosthesis utilizing the osseointegrated concept. Mayo Clin Proc 61:91-97, 1986
2. Adell R, Lukholm U, Rockier B, et al: A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Maxillofac Surg 10:387, 1981
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3. Albrektsson T, Bergman B, Folmer T, et al: A multicenter study of osseointegrated oral implants. J Prosthet Dent 60:75, 1988 4. Branemark PI: Introduction of osseointegration, in Branemark PI, Zarb G, Albrektsson T (eds): Tissue Integrated Prostheses. Chicago and Berlin, Quintessence, 1985 5. Delbalso AM, Greiner FG, Licata N: Role of diagnostic imaging in evaluation of the dental implant patient. Radiographics 14:699-719, 1994 6. Krauser JT: Hydroxyapatite-coated dental implants. Dent Clin North Am 33:879-903, 1989 7. Moy PK, Weinlaender M, Kenney EB, et al: Soft tissue modifications of surgical techniques for placement and uncovering of osseointegrated implants. Dental Clin North Am 4:665-699, 1989 8. McGivney GP, Haughton V, Strandt JA, et al: A comparison of computed-assisted tomography and datagathering modalities in prosthodontics. Int J Oral Maxillofac Surg 1:55-68, 1986 9. Wishan M, Bahat O, Krane M: Computed tomography as an adjunct in dental implant surgery. Int J Periodont Restor Dent 8:31-47, 1988 10. Rothman SLG, Chafetz N, Rhodes M, et al: CT in the preoperative assessment of the mandible and maxilla for endosseous implant surgery. Radiology 168:171-175, 1988 11. Schwarz MS, Rothman SLGM, Chatetz N, et al: Computed tomography in dental implantation surgery. Dent Clin North Am 33:555-597, 1989 12. Schwarz MS, Rothman SLGM, Rhodes ML: Computed tomography. Part I. Pre-operative assessment of the
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mandible for endosseous implant surgery. Int J Oral Maxillofac Surg 2:137-141, 1987 13. Schwarz MS, Rothman SLGM, Rhodes ML, et al: Computed tomography. Part II. Pre-operative assessment of the mandible for endosseous implant surgery. Int J Oral Maxillofac Surg 2:143-148, 1987 14. Abrahams JJ: The role of diagnostic imaging in dental implantology. Radiol Clin North Am 31:163-180, 1993 15. Abrahams JJ, Olivario P: Odontogenic cysts: Improved imaging with a dental CT software program. AJNR 14:367-374, 1993 16. Fogelman D, Huang AB: Prospective evaluation of lesions of the mandible and maxilla: Findings on multiplanar and three-dimensional CT. AJR 163:693-698, 1994 17. Yanagisawa K, Friedman C, Abrahams J J: Denta Scan imaging of the mandible and maxilla. Head & Neck J 15:1-7, 1993 18. Israelson H, Plemons JM, Watkins P, et al: Bariumcoated surgical stints and computer-assisted tomography in the preoperative assessment of dental implant patients. Int J Periodont Restor Dent 11:53-61, 1992 19. Burgett F: Periodontal disease, in Regezi JA, Sciubba JJ (eds): Oral Pathology: Clinical-Pathologic Correlations. Philadelphia, Saunders, 1989, pp 503-519 20. Regezi J, Sciubba J: Oral Pathology, ClinicalPathological Correlation (ed 1). Philadelphia, Saunders, 1989, pp 301-336 21. Regezi J, Sciubba J: Oral Pathology, ClinicalPathological Correlation (ed 1). Philadelphia, Saunders, 1989, pp 390-404
DENTULISM affects nearly half the population between the ages of 45 and 741 and is a common cause for oral dysfunction. Traditionally the treatment has been with removable dentures; these, whereas no doubt providing cosmetic benefits, are often associated with problems of impaired masticatory function and difficulty with speech. As a result, dentists developed nonremovable bridges that are attached to oral implants, metal posts surgically embedded in the jaw. Dental implant surgery has shown increasing popularity worldwide, with recent technologic advances resulting in progressively more viable implants. The success of this procedure engendered the need for an imaging technique that would provide the preoperative information necessary in planning these procedures, a need that has been met by the dental CT reformatting program described in this article. This technique provides multiple axial, panoramic, and cross-sectional projections affording precise anatomic and quantitative information as to the thickness, height, and contour of the alveolar ridge, and the location of the inferior alveolar canal and the maxillary sinuses relative to the alveolar margin. The objective of this article is to provide the reader with a comprehensive discussion of dental implants, CT reformatting programs, and their use in planning implant surgery.
E
From Diagnostic Radiology, Yale University School of Medicine, New Haven, CT. Address reprint requests to James J. Abrahams, MD, Associate Professor and Director of Medical Studies, Diagnostic Radiology, Yale University School of Medicine, 20 York St, Room 2-123, South Pavilion, New Haven, CT 06520-8042. Copyright 9 1995 by W.B. Saunders Company 0887-2171/95/1606-000255. 00/0
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IMPLANT DESIGN
Dental implants are metal posts that are surgically implanted in the jaw to support a fixed dental prosthesis (Fig 1). Early long-term clinical studies with these osseointegrated implants showed high success rates of 91% in the mandible and 81% in the maxilla. 2 The results of these early studies were corroborated by others, 3 providing the basis for the osseointegrated implants used today. Microscopic sections through bone-containing titanium implants show that osteoblasts have the ability to grow and integrate with the posts resulting in osseointegration.4 Three types of implants are used in dental practice: blade (Fig 2), subperiosteal (Fig 3), and root form (Fig 1). Blade implants are generally rectangular and are similar in shape to a razor blade. From the long side of the rectangle, which is implanted into the bone via a linear osteotomy, one or more posts extend above the gingiva into the oral cavity to permit fixation of the prosthesis. Subperiosteal implants are metallic meshes that are custom designed to fit over the alveolar process and under the periosteum. Several metallic posts extend from the mesh into the oral cavity (above the gingival margin) to support the prosthesis. To customize these subperiosteal implants, the alveolar process must be surgically exposed so a plaster impression can be obtained to manufacture the implant. After the implant has been manufactured, the patient returns for a second surgical procedure to place the implant. Current imaging technology has made it possible to eliminate the first surgical procedure. Thin slice axial CT can be used to produce a three-dimensional (3-D) model of the jaw from which the prosthesis can be manufactured.
Seminars in Ultrasound, CT, andMRl, Vo116,No 6 (December),1995: pp 468-486
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Abutment
m
Fig 1. View of the mandible illustrating several dental implants and the plane and orientation of the cross-sectional DentaScan (General Electric, Milwaukee, Wl) images. Three root-form implants are seen (arrowheads) supporting a fourtooth prosthesis. The abutment (thin arrow), which is attached to the fixture (broad arrow), raises it above the surface of the bone and gingiva and into the oral cavity. The top of the plane (curved arrow) would correspond to one of the numbered perpendicular lines on the axial image in Fig 6A. Note how the height and width of the alveolar process and the location of the mandibular canal can be readily determined on the crosssectional image. I, indicates inferior alveolar canal; m, mental foramen. (Reprinted with permission from Langer B, Sullivan D. Osseointegration: Its impact on the interrelationship of periodontics and restorative dentistry: Part 1. Int J Periodont Res 9:86, 1989.)
Root-form implants are osseointegrated cylinder-shaped implants that simulate the shape of the root of a tooth. They are the ones most frequently used today and are made up of several components (Fig 4). Fb:ture
The fixture is the portion of the implant that is surgically embedded in the osseous tissue of the jaw. It is made of titanium, a material that promotes osseointegration. Fixtures come in various sizes, typically ranging from 3.25 to 3.75 mm in diameter and from 7 to 10 mm in length. 5 The size of the implant chosen is dependent on the amount of available bone. Dentists prefer the largest possible implant because it increases the surface area and thus provides stronger anchorage and more successful osseointegration. At least 1 to 1.5 mm of bone should be present on either side of the implant and 1 to 2 mm of bone between the bottom of the implant and the adjacent structures, ie, maxillary sinus and mandibular canal. 5 Fixtures can be threaded, unthreaded, or even coated with hydroxyapatite. 6
The next component of the implant is the abutment (Figs 1 and 4), which is attached to the fixture to increase its height to a level above the gingival surface. This attachment is performed 3 to 6 months after the initial procedure, thus giving the fixture time to heal within the bone. The fixture is surgically exposed and the abutment is attached with an abutment screw (Fig 4). The top of the abutment screw has a small screw hole that will allow the dental prosthesis to be attached by a screw that runs through the prosthesis and into the abutment screw and abutment. This screw is designed as the weakest portion of the implant so if there is unforeseen stress, it will break rather than the fixture itself. Angled abutments are also available to correct for implants that are inserted at an angle rather than parallel to the residual teeth. Prosthesis
The prosthesis is composed of a strong metal framework (Fig 4) that supports the prosthetic teeth. Because the implants actually support this framework, one can have a full 14-tooth dental prosthesis supported by only six implants or a 3-tooth prosthesis supported by two implants, as seen in Fig 4, for example. IMPLANT SURGICAL PLACEMENT
Dental implant surgery is a two-stage procedure. 7 In the first stage, the fixture is installed within the bone, and in the second stage the abutment is attached to the fixture. A 4- to 6-month healing period is required between stages to allow time for osseointegration to occur. The procedures are typically performed in the dentist's office using local anesthesia. Fixture Placement
The first stage of surgery, fixture installation, is more extensive than the second. Its duration depends on the number of fixtures placed. Local infiltration with lidocaine and/or a nerve block are used for anesthesia. A linear incision is next made along the buccal or lingual surface of the alveolar ridge, and a soft tissue flap, incorporating the periosteum, is reflected back (Fig 5E). If the exposed bony ridge has a sharp or pointed
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Fig 2. Blade implant in mandible. (A) Plain film illustrating a single post-blade implant, Note how the blade is inserted in the mandible after a linear osteotomy while the post (P) extends above the level of the bone and gingiva. (B) Axial views demonstrating blade implant in mandible (arrow), (C) Cross-sectional views illustrating the blade within the mandible (black arrow) and the post (white arrow) extending into the oral cavity above the level of the bone and gingiva.
surface secondary to buccolingual atrophy, an alveolarplasty may be performed to remove the sharp edge and provide a broad surface to install the fixtures. The anticipated implant site that has been predetermined radiographically (Figs 5A through D) is then located on the patient either by measuring from an existing tooth or landmark that can be seen on the films and in patient or by using a stent with markers as shown in Fig 5B. (See "Radiographic and Surgical Stents" section.) Once the site has
been identified on the patient, a series of graduated drill bits are used to produce a hole in the bone and to progressively widen it to the appropriate size. The hole is then threaded with a titanium tap burr to accommodate the threaded fixture. As osseointegration can only occur with viable cells, drilling and threading are performed at extremely low revolutions per minute with the use of copious irrigation to prevent heating and destruction of osteoblasts. The top of the fixture has a threaded hole (Fig
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Fig 3. Subperiosteal implant. (A) Photograph of subperiosteal implant. The white portion fits under the periosteum and on top of the bone of the alveolar process while the metallic-appearing portion extends above the gingiva to support the prosthesis. The subperiosteal portion (white) is often coated with hydroxyapatite to facilitate bone growth. (B) Panoramic view demonstrating the subperiosteal implant on a severely atrophic mandible. The prosthesis has not been attached. (Courtesy of Wayne C. Jarvis, DDS, Dentofacial Surgery of Western New York, Williamsville, NY.)
5G) to accommodate the abutment screw, which is inserted in the second stage of the procedure. A healing or cover screw is used to prevent soft tissue and bone from growing into the hole during healing (Fig 5E). At this stage, the implant and cover screw are flush with the surface of the bone. After the fixtures are placed, the tissue flap is sutured closed and the implants are allowed to heal for approximately 4 months in the mandible and 6 months in the maxilla (Fig 5F) in order to promote the process of osseointegration, thus forming a strong bond between the bone and implant. After about 1 week, the skin sutures are removed and the
patient is fitted with an interim prosthesis, usually the one used before surgery. Abutment Connection The second stage of the procedure, which occurs 4 to 6 weeks later, tends to be less traumatic for the patient and is typically performed with local infiltration using lidocaine. A pointed probe is used to locate the cover screws. An incision is then made to expose and remove them or, alternatively, a circular soft tissue punch can be used to excise the tissue above the cover screw. The abutment is next attached to the fixture using the abutment screw. This
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__
A
Fig 4. illustration demonstrating the components of an implant (left) and two root-form implants supporting a threetooth prosthesis (right). For illustrative purposes, the black portion running through the prosthesis (arrow) represents the metal framework within the prosthesis. Prosthesis screw (Ps), abutment screw (As), abutment (A), and fixture (F).
extends the height of the implant to above the gingival margin. The top of the abutment screw has a screw hole that permits the prosthesis to be screwed into the fixture (Figs 4 and 5H). Finally, a surgical pack is applied and retained for a short period of time by a healing cap.
ure and creates the potential for antral infection. It is also important to know the dimensions of the alveolar process before surgery because atrophy of the alveolar ridge, which occurs in edentulous patients, can preclude the use of implants. Radiologists and dentists therefore began to evaluate the efficacy of using cranial CT to assess these patients. 8,9Axial and coronal images were only marginally helpful because .of the streak artifact created by dental restorations. However, the technique of reformatting images using thin slice axial CT was found to be extremely useful because it avoided the streak artifact, displayed the anatomy in multiple planes, permitted assessment of the width of the alveolar process, and allowed accurate millimeter measurements to be established. Reformatting software programs that display multiple panoramic and cross-sectional images soon became available 1~ (Figs 6 and 7) and have greatly facilitated the assessment of implant patients. These programs more recently have also become state of the art for evaluating lesions of the jaw 14-17 (Fig 8). The particular program used in this report is the DentaScan (General Electric, Milwaukee, WI).
Prosthodontic Procedure An impression of the jaw made of plaster or hydrocolloid is next made with the abutments in place. From this, a cast of the jaw is obtained and used by the prosthodontist to ensure that the screw holes in the implants line up exactly with those in the prosthesis and that the prosthetic teeth line up in the proper occlusal plane. After the prosthesis is manufactured, it is fixed to the abutments with screws, which typically insert into the central fossa of the prosthetic teeth. A white compound will finally be used to cover the screw holes (Fig 51). RADIOLOGIC ASSESSMENT
Radiologically, the precise height, width, and contour of the alveolar process and the location of the maxillary sinuses and mandibular canal must be determined in order for the oral surgeons and dentists to perform the surgery safely. Injury to the neurovascular bundle within the canal results in paresthesia or hyperaesthesia of the face, whereas perforation of the maxillary sinuses increases the likelihood of implant fail-
Dental CT Program Technical parameters. Having been instructed tq remain still and reassured that the procedure is painless, the patient is placed supine in the gantry using a head holder, chin strap, and sponges on either side .of the head to prevent motion. A lateral digital scout view is first obtained to define the upper and lower limits of the study and to determine whether the scan plane is parallel to the alveolar ridge. If it is not and the program does not permit angulation of the gantry, then the pat!ent should be repositioned and a repeat digital scan performed. Axial 1-mm cuts at an interval of 1 mm are obtained using a bone algorithm, in dynamic mode, with a 15-cm field of view, using a 512 x 512 matrix, at 140 KV and 70 MA. When both the mandible and maxilla are being evaluated, a separate run should be performed for each because the scan angle of the mandible is slightly different than that of the maxilla. Executing the dental program. Each program varies slightly depending on the manufac-
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turer. However, the following guidelines generally apply. First, the CT technologist selects one of the axial images as a reference image from which the program will be run. The image selected should be at the level of the roots of the teeth and should demonstrate the full contour of the mandible or maxilla. Next, the technologist superimposes a curved line on the image of the mandible or maxilla (Figs 6A and 7A). This is accomplished by depositing the cursor on one of the axial images at approximately six points along the curve of the jaw. Automatically, the program connects these points to produce a smooth curve that is superimposed on the jaw. This curved line defines the plane and location of the reformatted panoramic images (Figs 6C and 7D). Several images are reformatted buccal to this curve and several lingual to it. Reformatted cross-sectional images (Figs 6D through F and 7E and F) are defined by multiple numbered lines that the program automatically deposits perpendicular to the curved line (Figs 6A and 7A). The distance between the numbered perpendicular lines and thus between cross-sectional images can be varied by the technologist; however, 2-mm spacing is generally used. If a stent with radiographic markers is used, 1-mm slices may be necessary in order to visualize the markers. Streak artifact, which degrades visualization of bone on direct coronal images, does not degrade the reformatted cross-sectional images because the artifact is projected at the level of the crowns of the teeth and not over the bone (curved arrow in Fig 6E). When complete, three types of images are displayed: axial, cross-sectional, and panoramic. In a typical study, there are 30 to 50 axial images, 40 to 100 cross-sectional images, and 5 panoramic images. Images should be filmed in a consistent fashion to avoid confusion on the part of the referring dentist. Life-size crosssectional and panoramic images arepreferred. This can be accomplished with most programs by placing four screen saves on a 14 x 17 sheet of x-ray film. A millimeter scale displayed on the films (Fig 6D) can be used to verify that the images are near life-size. The scale is also used to obtain accurate measurements. One can place calipers on the bone to be measured and
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transfer this caliper setting to the millimeter scale. Accidental or intentional minification or magnification of the images will also minify or magnify the scale and not affect the measurement. Filming is optimally performed on a laser printer, allowing two original sets to be easily printed: one for the radiologist and one for the dentist. The author films the axial images with 12 screen saves per sheet of film, whereas the cross-sectional and panoramic images are filmed with 4 screen saves per sheet of film. Each cross-sectional and panoramic screen save contains multiple images. Windows of between 3,000 to 4,000 and levels of between 300 to 500 are typically used.
Interpretation of dental CT program images and measurements. In order to interpret the images appropriately, it should first be noted that each view can be related to the other by a series of (-) marks that appear on the films. These should not be confused with a scale for measurements, to which they appear similar. The marks that run along the side of the cross-sectional and panoramic images (Figs 6D and C) correspond to the direct axial slices that were used to reformat the images. For example, in Fig 6, 42 axial images were obtained to reformat the images, thus 42 marks appear along the side of the reformatted cross-sectional (Fig 6D) and panoramic images (Fig 6C) corresponding to the numbered lines drawn perpendicular to the curve superimposed on the axial image in Fig 6A. They therefore also correspond to the numbered cross-sectional reformatted images in Figs 6D, E, and F. To illustrate how one view can be related to another, note how the right mental foramen (M) in Fig 6E, which is seen on cross-sectional image 20 (lower right) at the level of the 14th mark on the side of the image, is also seen in Fig 6B on axial image 14 at the 20th perpendiculhr line. The same correlation can be made on the panoramic view. When the scai~ is performed as part of the preoperative workup of a dental implant patient, the status of the dentition and the desired implant sites are first established. In the partially edentulous patient, it is presumed that the implants will be placed in the edentulous seg-
Fig 5.
Fig 5. (G) Before obtaining this photograph, a small incision was made to remove the healing caps. Healing abutments, which were attached to the implants, have been removed. The threaded opening of the implant is visualized. (H) The permanent abutments, which raise the fixture above the gingival surface, have now been attached. The screw hole (arrow) in the center of the abutment accommodates the screw that fixes the prosthesis. (I) The prosthesis is now attached to the three implants. The screw heads are covered with a white compound, (Reprinted with permission. TM)
Fig 5. (cont'd,) Surgical implant procedure. (A) This patient, being evaluated for dental implants, is edentulous distal to the right maxillary canine (arrow). (B) A stent with six vertical markers has been placed over the alveolar ridge and residual teeth. The sixth marker (long arrow) is adjacent to the right canine (short arrow) and is demonstrated on the CT images, This marker appears as a dot on the axial image next to perpendicular line 32 (Fig 5C) and as a line on cross-sectional image number 32 (Fig 5D), (C) Axial view demonstrating the sixth marker (long thick arrow) at perpendicular line 32 (thin arrow) and adjacent to the right canine (short white arrow). Note the radiolucent pulp in the center of the teeth (black arrow). (D) Cross-sectional views demonstrating markers four (open arrow), five (straight arrow), and six (curved arrow) of the stent. Note that marker number six is adjacent to the right canine (open curved arrow). By placing the stent on the patient during surgery, the surgeon knows that the bone under marker 6 is as depicted by cross-sectional image 3Z. {E) An incision is made and the gingival and periosteal flap {arrowheads) is held back with sutures. This exposes the bone of the alveolar process (short thick 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). (F| The incision is sutured closed and permitted to heal for 4 months in the mandible and 6 months in the maxilla,
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Fig 6. DentaScan of mandible illustrating the use of the dental CT program and the anatomy. (A) Axial image of mandible with superimposed curve. The curve defines the plane and location in which the panoramic images in C are reformatted. Numbered lines drawn perpendicular to this curve (arrow) define the plane and location in which the cross-sectional images viewed in D through F are reformatted. Mental foramen (M). (B) Axial images illustrate the inferior alveolar canal (I), the mental foramen (M), and the genial tubercle (Gt). The numbers along the left side of the figure refer to the particular number of the axial image. Note that the mental foraman, which is seen on axial image number 14 at the 20th perpendicular line, is also seen in E on cross-sectional image 20 (lower right) at the level of the 14th tick mark on the side of the image. The tick marks and numbers allow one image to be correlated with another. (C) Panoramic views. The numbered tick marks along the bottom of the images correspond to the numbered perpendicular lines displayed on the axial image in A. The tick marks along the side of the image correspond to the axial images, which were used to reformat these images. Note that there were 42 axial images acquired and, thus, 42 tick marks along the side of this image. Inferior alveolar canal (I), mental foramen (M), second bicuspid (2B), first bicuspid (1B), and cuspid (C). (D through F) Cross-sectional views. (D) Images 1 through 10 are through the posterior right mandible (see perpendicular lines 1 through 10 in A).
ments. Measurements are therefore obtained in these regions. If the cross-sectional images are 2 mm apart, the author typically provides a measurement on
every 5th cross-sectional image. If the crosssectional images are 1 mm apart, measurements are provided for every 10th image in the edentulous areas. The height and width of the alveolar
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Fig 6. (cont'd.) (E) Images 11 through 20 are more anterior. (F) Images 21 through 30 extend to the midline. The images of the left half of the mandible are not shown. Arrowheads in image 17 (E) indicate that the height of the mandible distal to the mental foramen is measured from the top of the alveolar process to the top of the mandibular canal, whereas mesial to the foramen it is measured from the top of the alveolar process to the bottom of the mandible (arrowheads in F, image 30), Measurement of the width is also demonstrated in image 30 (F). M M at the bottom of D indicates the millimeter scale. Note how streak artifact from dental restoration (curved arrow in E) does not degrade visualization of the bone. A, alveolar process; C, cuspid; Cp, coronoid process; D, digastric fossa; GT, genial tubercle; I, inferior alveolar canal; L, lingula; M, mental foramen; Mf, mandibular foramen; Mg, mylohyoid groove; MI mylohyoid line; O, oblique line; Rf, retromolar fossa; Rt, retromolar triangle; S, submandibular fossa; SI, sublingual fossa; T, temporal crest; 1B, first bicuspid; and 2B, second bicuspid. (Reprinted with permission from Abrahams JJ: Anatomy of the jaw revisited with a dental CT software program: Pictorial essay. AJNR 14:979-990, 1993.)
process are measured as described in the following paragraph. In the mandible, the height in the region distal (posterior) to the mental foramen is measured from the top of the alveolar process to the top of the mandibular canal (Fig 6E, arrowheads in image 17). The mandibular canal is marked with a wax pencil on the crosssectional images, and this set of films is sent to the referring dentist. Methods of locating the mandibular canal if it is not initially visualized on the cross-sectional images are discussed later. In the region mesial (anterior) to the mental foramen, the full height of the mandible is available to the surgeon because the mandibular canal ends at the mental foramen and is not present mesial to it. Measurements in this area are provided from the top of the alveolar ridge to the inferior surface of the mandible (Fig 6F, arrowheads in image 30). The width is measured near the top of the alveolar process (Fig 6F, arrowheads in image 30). Occasionally, if the alveolar process comes to a point secondary to atrophy, it may be difficult to measure. In this situation the surgeon may choose to remove the top-pointed portion of the ridge (alveolarplasty), providing a broad base for an implant. This obviously will affect the height of the alveolar ridge, and it is best to let the surgeon
estimate how much of the ridge will be removed and what the remaining height will be. In this situation, one may simply state that the ridge is pointed and not provide a measurement for the width. In the maxilla, the height in the distal (posterior) aspect is measured from the top of the alveolar process to the floor of the maxillary sinus (Fig 7E, arrows in image 7), whereas more mesially (anteriorly), the height is not limited by the maxillary sinuses and is measured from the alveolar ridge to the nasal fossa (Fig 7F, arrows in image 23). The width is measured in a similar manner as in the mandible (Fig 7F, arrows in image 22). In the completely edentulous patient, it is more desirable to place the implants centrally because the height of the alveolar process is not limited by the more distal mandibular canal or the maxillary sinuses. A sample dictated report. The referring dentist or surgeon should be provided with a complete and comprehensive report. The density and general health of the jaw are described, as are such conditions as maxillary sinus disease, periodontal disease, root canal procedures, extraction sockets, retained roots, atrophy, cysts, osteitis condensans, contour irregularities, surgical changes, and anomalies such as torus palatinus or mandibularus. The report defines
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Fig 7. DentaScan of maxilla illustrating the use of the program and the anatomy. (A) Axial image with superimposed curve. The curve defines the plane and location in which the panoramic images seen in D are reformatted. Numbered lines drawn perpendicular to this curve define the plane and location in which the cross-sectional images viewed in E and F are reformatted. (B) Axial views through the alveolar ridge and hard palate. A, alveolar process; As, inferior nasal spine; Gg, groove for greater palatine nerve; If, incisive foramen; Mb, maxillary bone; palatine process; Mp, median palatine suture; Ms, maxillary sinus; Nf, nasal fossa; Pb, palatine bone: horizontal plate; Ts, transverse suture; and T, tongue. (C) Axial views through the maxillary sinuses in the pterygopalatine fossa. G, greater palatine foramen; L, lesser palatine foramen; Ms, maxillary sinus; Nc, nasal conchi; Ns, nasal septum; Pt, pterygoid process; Tp, pterygopalatine fossa. (D) Panoramic views. A, alveolar process; IF, incisive foramen; MS, maxillary sinus; NC, nasal concha; NF, nasal fossa; NP, nasalpalatine canal; and NS, nasal septum.
the status of the dentition and provides measurements of the alveolar process. One should state whether the patient is completely or partially edentulous and, if partially, the location of the
edentulous segments should be noted. Measurements are provided in a tabular fashion, and the mental foramina and incisor foramen are identified for easy interpretation. For example, the
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Fig 7. (cont'd.) (E through F) Cross-sectional views. Images 1 through 15 (E) are through the posterior right maxilla (see perpendicular lines 1 through 15 in A); images 16 through 30 (F) are more anterior on the right and extend to the midline (see perpendicular lines 16 to 30 in A). The arrows in image 7 (E) indicate how the height of the distal alveolar process is measured from the floor of the maxillary sinus to the top of the alveolar process, whereas the mesial alveolar process (F in image 23) is measured from the floor of the nasal fossa to the top of the alveolar process (arrows in image 23). The arrows in image 22 (F) indicate how the width of the alveolar process is measured. A, alveolar process; As, anterior maxillary spine; G, greater palatine foramen; Gg, groove for greater palatine nerve; If, incisive foramen; Ms, maxillary sinus; Nc, nasal concha; Nf, nasal fossa; and Np, nasalpalatine canal. (Reprinted with permission from Abrahams J J: Anatomy of the jaw revisited with a dental CT software program: Pictorial essay. AJNR 14:979-990, 19933
following measurements were obtained from Figs 6E and F: Image 17: height 14 ram, width 4 ram. Image 20: left mental foramen. Image 25: height 26 ram, width 4 mm.
When a stent with radiopaque markers is used, then the markers on the images are numbered with a wax pencil, measurements are obtained on the cross-sectional images where the markers appear, and the image number of
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Fig 8. Mandibular DentaScan demonstrating a radicular cyst surrounding the root apex of the left canine. (A) Axial view again demonstrates the radicular cyst (black arrow). The periodontal space (Ps} typically not seen on CT can be visualized in this patient between the lamina durs (Ld) and the tooth. (B) Panoramic view illustrates an area of sclerotic osteitis condensans (arrowheads) surrounding the radicular cyst (black arrow). The cervical constriction (Cc), pulp chamber (Pc), root canal (Rc), and dense enamel (E) are nicely demonstrated on the right first molar. The mesial (M), occlusal (O), and distal (D) surfaces of the left canine are demonstrated as well as the mesiobuccai (Mb) and distobuccal (Db) roots of the left first molar. The lingual root of the left first molar is not seen in this plane and would be visualized on a more posterior slice. (C) Crosssectional views demonstrating the radicular cyst surrounding the root apex.
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this image is provided in the report. In Fig 5D for example, the measurements at the markers are as follows: Marker 4 (image 21): height 13 mm, width 6 mm. Marker 5 (image 27): height 12 mm, width 5 mm. At the end of the report, a brief impression is noted describing any pertinent pathology, the degree of atrophy, and a disclaimer stating that all measurements should be verified before surgery. This is included because it is difficult for the radiologist to know whether the numbers dictated were transcribed correctly onto the report and because the surgeon may choose to place the implant at a somewhat different angle than that measured by the radiologist.
Identifying the Mandibular Canal Identifying the mandibular canal on crosssectional images and providing measurements from the top of the alveolar ridge to the top of the canal is extremely important. If the canal is not properly identified, its injury during implant surgery can be quite debilitating, resulting in permanent paresthesia and hyperaesthesia of the face. Typically, the canal can be readily observed on cross-sectional images; however, there are some circumstances when portions of the canal or even the entire canal may be hard to visualize on the cross-sectional images. In this situation, the author has found the following methods extremely helpful in locating the canal. The first, the cortical niche sign, refers to an indentation along the inner cortical margin on the lingual surface created by the mandibular nerve as it traverses the mandible (Fig 9). This Fig 9. Cortical niche sign. This cross-sectional image of the mandible demonstrates an indentation on the lingual cortex of the mandible called the cortical niche sign (Cn), which is created by the mandibular nerve as it traverses the mandible. This sign, which is often more subtle than this, can be helpful in identifying the location of the mandibular canal. (Reprinted with permission from Abrahams J J: CT assessment of dental implant planning. Oral Maxillofac Surg Clin North Am 4:1-18, 1992.)
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can be quite subtle and is not identified in all patients. However, when present, it is very helpful for identifying the canal. Caution should be taken not to confuse other cortical irregularities with the cortical niche sign. The cortical niche is a continuous defect seen on multiple cross-sectional images, whereas other cortical irregularities are randomly situated and not seen consecutively on multiple images. If the canal is identified with the cortical niche sign, its location should be confirmed with the other methods described below. The next method, referred to as triangulation, uses the reference marks on the films to relate an anatomic structure on one view with that on another. In this fashion, the panoramic and axial views can be utilized to identify the canal on the cross-sectional view. For example, in Fig 10A, it is difficult to see the canal on the cross-sectional images, but it is readily observed in the panoramic view (Fig 10B). The information from the panoramic view can be used to triangulate the location of the canal on the cross-sectional image, as shown in Fig 10. If one wishes to locate the canal on cross-sectional image 14 (Fig 10A), one first refers to the 14th reference mark along the bottom of the panoramic view (Fig 10B). A vertical line from this point is then drawn up to the bottom on the canal and extended across in a perpendicular direction toward the reference marks on the left side of the panoramic image. Note that this line intersects the 21st mark along the left side of the image. The level of the canal in crosssectional image 14 can then be identified by counting up 21 marks along the side of the image (Fig 10A). This same process can also be used using the axial image.
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Fig 10. Triangulation. When the inferior alveolar canal is not initially seen on (A) the crosssectional images, its location can be determined by triangulating from the (B) panoramic or axial image. The inferior alveolar canal is indicated by arrows. (Reprinted with permission from Abrahams J J: CT assessment of dental iraplant planning. Oral Maxillofac Surg Clin North Am 4:1-18,1992.)
Lastly, if a canal is identified on some crosssectional images but not others, the images in which it is identified can be used to estimate the position of the canal on the other images. This can be performed because the distance from the bottom of the mandible to the bottom of the canal tends to be relatively constant. The only region where the distance is not constant is immediately adjacent to the mandibular foramen and the mental foramen. It is the distance from the top of the canal to the top of the alveolar process that varies secondary to atrophic changes. With this knowledge, one can extrapolate the location of the canal from the images in which it is visualized. For example, in Fig 6E, the distance from the bottom of the canal to the bottom of the mandible in image 11 is 5 mm, and in image 15 it is also 5 mm. Therefore, if the canal was not visualized in image 15, we would be able to estimate its location by using the position of the canal that was visible on image 11. The author has also found that the right and left hemimandibles are
bilaterally symmetrical and that the distance from the base of the mandible to the base of the canal on the right at a particular point is approximately the same as the distance measured at that same point on the left. Thus, if the canal is identified on the cross-sectional images of the right half of the mandible but not the left, this information can be used to estimate the position of the canal on the left. It is recommended that several of these methods be used to confirm the location of the canal rather than relying on one alone.
Radiographic and Surgical Stents Stents are clear acrylic devices that fit snugly over the residual teeth and alveolar ridge (Fig 5B). They are manufactured by the dentist after obtaining an impression of the teeth and ridge. Radiopaque markers are attached to the stent to provide reference points on the DentaScan images for the purpose of localization and measurement. At surgery, the same radio-
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I Fig 11. Illustration demonstrating a periodontal pocket and its repair. (A) Bacterial overgrowth has attacked the periodontal ligament (curved arrow) and resorbed bone (straight arrow) creating this periodontal pocket. (B) The periodontal pocket in A has now been packed with freeze-dried bone (curved arrow) and covered with a Gortex graft (straight arrow) to prevent ingrowth of soft tissue while the bone graft heals. The Gortex is removed in approximately 6 weeks.
graphic stent is used as a surgical template to assist in the placement of the implants. Radiographic markers are often made of gutta-percha, a radiopaque material commonly used by dentists as a temporary filling. Other radiopaque material such as pieces of wire also may be used for markers. Ideally the markers are 1 to 2 mm in diameter, vertically oriented, and without a mesial or distal tilt (Fig 5B). The vertical markers should be attached to the stent in such a way that they extend deep into the buccal vestibule. This helps their visibility on the cross-sectional images because they overlie the bone rather than the level of the crown of the tooth, which may have streak artifact from restorations. Other types of markers also have been used including coating a denture with barium. 18 Radiologists can be of assistance in
helping the dentist place the markers in a manner that yields the greatest information because most dentists are unfamiliar with CT scanning. Use of the stent is illustrated in Fig 5. After the patient is scanned, the markers appear as dots on the axial image (Fig 5C) and as lines on the cross-sectional images (Fig 5D). If surgeons want to put an implant just distal to the right canine (Fig 5A and C, short white arrow) they note that the sixth marker falls in this location (5B and C, long thick arrow). Next they look at the axial image to see on which cross-sectional image the marker appears. In this case, the sixth marker on the stent appears on the axial image at perpendicular line 32 (5C, thin arrow) and is therefore seen on cross-sectional image 32 in Fig 5D. The surgeon now knows that the bone
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Fig 12. Panoramic CT scan of mandible demonstrating periodontal disease. The radiolucency surrounding the root of the left first molar (arrowheads) is caused by bone resorption from periodontitis. The periapical lucency around the root apex of the right canine (long arrows) is a radicular cyst secondary to a periapical abscess. Note the metal post in the pulp chamber secondary to a prior root canal procedure (solid curved arrow) and compare this to the normal radiolucent pulp chamber of the left first premolar (open curved arrow). A zone of relatively dense sclerotic bone (osteitis condensans) is identified surrounding both areas of disease. (Reprinted with permission. TM)
under marker 6 corresponds to cross-sectional image 32. After the incision is made and the alveolar bone is exposed, the stent can be placed back on the patient and used as a template. This is accomplished by cutting a hole out over the alveolar aspect of the stent so the surgeon can mark the area for the implant (Fig 5B). The marker on the cross-sectional image can also be used to help the surgeon estimate the correct facial or palatal angulation of the implant. Stents and markers are only accurate if the stent fits tightly and if its position in the patient's mouth is reproducible and the same during scanning and surgery. To accomplish this, the dentist should give the patient careful instruction on how to insert the stent properly. The radiology staff should not be responsible for inserting stents. PERIODONTAL DISEASE AND ITS SEQUELAE
Most patients who are evaluated for dental implants have advanced periodontal disease, and it is this periodontal disease and its sequelae that are responsible for the edentulism. The process begins initially with gingivitis and the accumulation of plaque and bacteria around the teeth. Eventually the bacteria attack the periodontal ligament and track along the side of the root where erosion of bone occurs, thus forming a periodontal pocket 19(Fig 11A). Radiographically, this bone loss appears as a radiolucency adjacent to the side of the root (Fig 12A).
Routine dental hygeine is unable to access the periodontal pocket so it typically progresses unless treated appropriately by a periodontist. Bacteria can also gain access to the root through dental caries. The caries allow the bacteria to enter the pulp chamber where it then travels down the root canal and out the apical foramen to form a periapical abcess or granuloma that is often referred to as a radicular cyst2~(Figs 8 and 12). Radicular cysts are associated with nonvital teeth. This is because the tooth cannot expand so the edema and swelling from the pulpitis increases the pressure within the tooth and diminishes its blood supply. Radicular cysts, which appear as periapical radiolucencies, are treated by a root canal procedure in which the dentist drills down the root canal and drains the infection. The root canal then is sealed and filled with a metal post to add support to the tooth. Radiographically, the post appears as a radiopacity in the center of the tooth in contrast to the normal radiolucent pulp chamber (Fig 12). Periodontal pockets can be cleaned out surgically and filled with a freeze-dried bone graft that is covered with a Gortex (W. L. Gore Associates, Inc, Flagstaff, AZ) fabric barrier (Fig lIB). The Gortex is left in place for about 6 weeks and prevents the soft tissue from growing into the bone graft before it heals. Alternatively, periodontal pockets can be treated by resecting the gingiva and lowering the gum line to eliminate the pocket and allow access for home care dental hygeinc. The chronic inflammation from periodontal
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Fig 13. Cross-sectional views of the mandible in two different patients demonstrating bone resorption and atrophy. (A) Bone loss and atrophy of the height of the mandible in this patient causes the mandibular canal (arrow) to sit immediately under the gingival surface. The dotted line demonstrates how the bone contour might have appeared prior to atrophy. The patient has insufficient bone for implants. (Reprinted with permission.14) (B} The width of the mandible in this patient has been considerably diminished from atrophy. The dotted line again demonstrates how the bone might have appeared before atrophy. (Reprinted with permission from Abrahams J J: CT assessment of dental implant planning, Oral Maxillofac Surg Clin North Am 4:1-18, 1992.)
pockets and periapical infection often cause the surrounding bone to react by becoming sclerotic, a condition termed osteitis condensans, condensing osteitis, or focal sclerosing osteomyelitis 21 (Figs 8 and 12).
After the patient becomes edentulous, the normal vertical stress exerted on the bone is no longer present and atrophy occurs. This disuse affects both the height and buccolingual width of the alveolar process (Figs 13A and B).
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2. Adell R, Lukholm U, Rockier B, et al: A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Maxillofac Surg 10:387, 1981
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3. Albrektsson T, Bergman B, Folmer T, et al: A multicenter study of osseointegrated oral implants. J Prosthet Dent 60:75, 1988 4. Branemark PI: Introduction of osseointegration, in Branemark PI, Zarb G, Albrektsson T (eds): Tissue Integrated Prostheses. Chicago and Berlin, Quintessence, 1985 5. Delbalso AM, Greiner FG, Licata N: Role of diagnostic imaging in evaluation of the dental implant patient. Radiographics 14:699-719, 1994 6. Krauser JT: Hydroxyapatite-coated dental implants. Dent Clin North Am 33:879-903, 1989 7. Moy PK, Weinlaender M, Kenney EB, et al: Soft tissue modifications of surgical techniques for placement and uncovering of osseointegrated implants. Dental Clin North Am 4:665-699, 1989 8. McGivney GP, Haughton V, Strandt JA, et al: A comparison of computed-assisted tomography and datagathering modalities in prosthodontics. Int J Oral Maxillofac Surg 1:55-68, 1986 9. Wishan M, Bahat O, Krane M: Computed tomography as an adjunct in dental implant surgery. Int J Periodont Restor Dent 8:31-47, 1988 10. Rothman SLG, Chafetz N, Rhodes M, et al: CT in the preoperative assessment of the mandible and maxilla for endosseous implant surgery. Radiology 168:171-175, 1988 11. Schwarz MS, Rothman SLGM, Chatetz N, et al: Computed tomography in dental implantation surgery. Dent Clin North Am 33:555-597, 1989 12. Schwarz MS, Rothman SLGM, Rhodes ML: Computed tomography. Part I. Pre-operative assessment of the
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mandible for endosseous implant surgery. Int J Oral Maxillofac Surg 2:137-141, 1987 13. Schwarz MS, Rothman SLGM, Rhodes ML, et al: Computed tomography. Part II. Pre-operative assessment of the mandible for endosseous implant surgery. Int J Oral Maxillofac Surg 2:143-148, 1987 14. Abrahams JJ: The role of diagnostic imaging in dental implantology. Radiol Clin North Am 31:163-180, 1993 15. Abrahams JJ, Olivario P: Odontogenic cysts: Improved imaging with a dental CT software program. AJNR 14:367-374, 1993 16. Fogelman D, Huang AB: Prospective evaluation of lesions of the mandible and maxilla: Findings on multiplanar and three-dimensional CT. AJR 163:693-698, 1994 17. Yanagisawa K, Friedman C, Abrahams J J: Denta Scan imaging of the mandible and maxilla. Head & Neck J 15:1-7, 1993 18. Israelson H, Plemons JM, Watkins P, et al: Bariumcoated surgical stints and computer-assisted tomography in the preoperative assessment of dental implant patients. Int J Periodont Restor Dent 11:53-61, 1992 19. Burgett F: Periodontal disease, in Regezi JA, Sciubba JJ (eds): Oral Pathology: Clinical-Pathologic Correlations. Philadelphia, Saunders, 1989, pp 503-519 20. Regezi J, Sciubba J: Oral Pathology, ClinicalPathological Correlation (ed 1). Philadelphia, Saunders, 1989, pp 301-336 21. Regezi J, Sciubba J: Oral Pathology, ClinicalPathological Correlation (ed 1). Philadelphia, Saunders, 1989, pp 390-404