CHAPTER 4 Selection of the Proper Implant
Once the patient has been found to be physically and medically acceptable for implant therapy and the decision has been made to proceed, a thorough diagnostic evaluation is performed and a treatment plan is chosen to provide the proper approach. A visual examination should be the first step. The implantologist should view the edentulous areas and conceptualize the height, width, and length of the proposed operative sites. The amount of gingiva that is attached or keratinized (or both) is noted. In addition, the level of the lip line and exposed gingivae are noted, along with any muscle attachments. If natural teeth remain, they should be free of decay, and the periodontal tissues should be healthy. Neither infections nor localized areas of pathologic change can be permitted.
The next step in the diagnostic sequence is manual palpation. Using the thumb and index finger, the examiner palpates the edentulous ridges (Fig. 4-1), assessing the firmness and thickness of the soft tissues.
A determination of the uniformity of thickness over the entire height and length of the underlying bone is important. Concavities and convexities may be present that might not be evident on visual or digital examination. To clarify and define the presence and extent of such irregularities, the practitioner should prepare to sound the bone (or delineate the shape by closed examination). A 30-gauge needle is used to deposit a small amount of local anesthetic along the labial and lingual aspects of the edentulous areas under consideration for implant sites. A sharpened periodontal probe then is used to measure the thickness of the soft tissue at several points. These and all other calibrations should be recorded on a diagnostic chart. Next, a sterilized Boley gauge with sharpened beaks is used to puncture the soft tissues by squeezing the calipers directly through tissue to bone (Figs. 4-2 and 4-3). The beaks should oppose one another so that an accurate reading results. This produces a measurement of bone width at varying ridge sites. By obtaining measurements from superior to inferior and from medial to distal at 5-mm intervals, the clinician develops a topographic map of the soft and hard tissue dimensions of the areas into which implant placement is intended. When these measurements are used, an accurate, three-dimensional representation of the operative site can be sketched (Fig. 4-4).
At this point, full arch alginate impressions of both arches are made so that the dimensions can be transferred to the casts made from them. The impressions are poured immediately with dental stone, and a second (surgical planning) cast of the arch is made that will be restored with implants. A centric recording in the material of choice is needed to allow the casts to be mounted on a semiadjustable articulator (Whip Mix, Hanau). Proper articulation of casts is an essential part of every restorative procedure. Correct reproduction of the patient’s occlusal relationships on an articulator allows proper planning and saves a considerable amount of time that usually is wasted in adjusting prostheses. If the casts are not related to the patient’s condylar axis before they are mounted on the articulator, the bite record may be inaccurate.
The facebow, a relatively simple device to use, relates the maxilla to the same location on the articulator that is found in the patient’s skull. The facebow consists of three components: the bite fork, the bow, and the locator rods. To place the device properly, the practitioner must locate the patient’s condyles; this can be done with accuracy by palpating for these important structures while the patient repeatedly opens and closes the mouth. These points are marked on the skin with an indelible pencil so that the axis locator rods of the facebow can be placed against them. In addition to the palpation method, the external auditory meatuses can be used to locate the condyles. These meatuses are related consistently to the condylar head axes. Several systems provide earpieces on the facebow for this purpose.
Next, a U-shaped, softened sheet of wax is fixed to the bite fork. The patient’s mouth should be closed in centric relationship lightly into the wax so that indentations are made by the cusp tips of the teeth. For edentulous patients, stable base plates are used. The maxillary wax rim is attached to the fork, and the patient is guided into a centric closure of the proper vertical dimension. With the bite fork held in place by the patient’s teeth, the facebow is assembled. The axis locators should be allowed to contact the marks on the skin, and the assistant should tighten the set screws, locking the bite fork, locator, and facebow in place. The axis locators are then loosened; if the system is stable, the locator’s tips will not move. The patient may then open the mouth, and the entire assembly can be removed.
Although some articulators allow the transfer of intercondylar distance measurements, most have a preset distance. The locator rods are attached to the axes of the articulator, their set screws are fastened, and the maxillary cast is placed in the wax indentations on the bite fork. At this stage, the practitioner evaluates the position of the cast. The occlusal plane should be slightly higher in the posterior area. The cast should be supported with a block and attached to the upper member of the articulator with a mounting ring using model plaster. After the cast has set, the interocclusal bite record is used to relate the mandibular to the maxillary cast. It is affixed with plaster to the lower or fixed member of the articulator.
The implantologist should study the mounted casts with respect to interocclusal distances, existing occlusal relationships, and arch forms (Fig. 4-5). If a distance of 7 mm or less is found between the potential host site and the opposing natural or prosthetic occlusal surfaces (Fig. 4-6), an implant cannot be used unless additional space can be created by the following methods:
FIGURE 4-6. One cause of insufficient intermaxillary distance (7 mm or less) is supereruption of the opposing dentition. This problem may be solved by extraction, endodontic therapy, occlusal equilibration, an increase in the vertical dimension, or subapical en bloc resection (see Fig. 4-7).
With edentulous posterior mandibular quadrants, extrusion of the maxillary molars or premolars directly opposing the area often is a factor. If significant periodontal disease is not present, the alveolar ridge actually is thrust downward, carrying the teeth with it. In such cases, an en bloc segmental osteotomy can be performed to intrude the problematic area, increasing the intermaxillary space (Fig. 4-7).
FIGURE 4-7. A, The extended posterior maxillary segment prevents placement of implants or prosthetics in the opposing mandible. B, A segmental osteotomy permits intrusion of the posterior segment, which includes alveolus and teeth. After healing, a mandibular prosthesis can be placed. C, A postoperative radiograph shows implants placed and the maxillary quadrant successfully intruded.
If the articulation indicates crossbite or ridge procumbency, the implantologist must determine that the angulation of the implants will permit the final prosthesis to be in functional position. Implants at greater than 35-degree angulations from the long axis of the ridge present significant aesthetic and functional problems. If an angle greater than 35 degrees is created, forces are exerted that may be detrimental to the longevity of implant host sites. For this reason, some excellent ridges may be considered questionable if the angulation places implants in compromised positions. Custom casting using wax patterns for cementable abutments can be done. In addition, some implant designs (i.e., Straumann, Nobel Biocare) have abutments with angles of 15 degrees or even 30 degrees. However, seating these to the proper alignment requires special skills (see chapters 21 and 27).
The next step in the diagnostic sequence is a radiologic survey. The following radiographs and their purposes are listed so that the practitioner can select the fewest possible views required to attain the optimum data.
FIGURE 4-9. Periapical radiographs show detail of bony architecture and offer greater accuracy of measurement. Distortion, particularly in the maxillae, still exists but can be minimized by long cone techniques.
FIGURE 4-10. A lateral view of the mandibular symphysis, one of the most common sites of implantation, may be taken effectively with an occlusal film. It offers some information about cortical plate angulation, but the information is limited because only the greatest dimensions are outlined.
The Omnivac guide is retained, because at the time of surgery, the spheres can be removed; the template then can be sterilized, and after the soft tissues have been reflected, it can be used as an implant site locator (this technique is explained in more detail in Chapter 9).
An alternative method that can be used to measure available bone is simpler but slightly less accurate. First, the surgical planning cast is marked at the potential implant host sites. An Omnivac template then is made on the cast. Next, a No. 557 bur is used to drill through the plastic template at each of the implant sites. Cold-cure acrylic then is used to process a 0.045-inch orthodontic wire into each hole. The length of each wire is measured carefully at 5 mm (errors in measurement will be directly proportional to bone length distortion). The template is placed in the patient’s mouth, and the periapical radiographic survey is performed as in the previously described use of spheres. Because three factors are known, the unknown (the actual bone height) may be calculated using the formula given for the spheres.
Cone beam volumetric tomography (CBVT) involves a higher radiation dose for the patient compared to that from a periapical or panoramic radiograph. However, it also provides greater clarity and accuracy in the radiologic survey of the surgical site (the slice thickness is 0.3 to 1 mm), and the images are free of magnification, superimposition of neighboring structures, and other problems inherent to a panoramic radiographic survey. As this technology becomes more widely accepted and more of these scanners are put to use in dental offices, both dental practitioners and patients will benefit from the convenience, ease of interpretation, and improved diagnostic utility of the resulting data.
CBVT and cone beam computed tomography (CBCT) scanners have been available for craniofacial imaging since 1999 in Europe and 2001 in the United States. These scanners use a cone-shaped x-ray beam rather than a conventional linear fan beam to provide images of the bony structures of the skull. Conventional medical CT scanners use a single row or a series of solid-state detectors (4, 8, 12, 32, 64 and now, 128) paired with a fan-shaped beam to capture the attenuated x-ray; CBCT scanners use a square, two-dimensional array of detectors to capture the cone-shaped beam. The medical CT scanner, therefore, provides a set of consecutive slices of the patient, whereas the CBCT scanner provides a volume of data. Reconstruction software is applied to the CBCT volumetric data to produce a stack of two-dimensional, gray scale–level images of the anatomy.
CBCT and CBVT maxillofacial imaging is indicated in a number of cases, such as for (1) assessment of the facial bones for infection, trauma, and congenital or developmental deformities; (2) quantitative and qualitative assessment of residual bone for primary implant stability, because there is no tissue superimposition and no image distortion (i.e., 1 to 1 scale images allow for accurate measurements); and (3) visualization of the mandibular condyle and the articulating components in the assessment of temporomandibular joint (TMJ) conditions.
All cone beam scanners have preinstalled software for image manipulation and added image functionality. For example with multiplanar reformatting (MPR), three-dimensional volume data usually are acquired in the axial plane (top to bottom slices). MPR creates sagittal, coronal, and transverse images from those axial images (basically, front view and side view slices). The images are displayed in formats that allow effective visualization of section to section change in the scanned structure. MPR images are available in all preinstalled cone beam scanner software. The software also allows the practitioner to take measurements on the slices and measure density in Hounsfield units.
Some scanners have user-friendly software targeted to dental problem solving and virtual implant planning through intuitive integration of the diagnosis, computer-aided therapy planning, and precise intraoperative implementation. Some implant planning software is able to virtually place the proposed implant, in the appropriate size, in the host site and evaluate it in all three dimensions. Important anatomic structures, such as the inferior alveolar canal and maxillary sinus, can be accurately localized and identified before surgery. Fabrication of surgical guides for implant surgery, based on the virtual implant planning, also is possible.
A radiographic bite plate contains fiduciary radiopaque reference markers and produces a patient-customized scan stent (Fig. 4-14). The dental technician manufactures a scan stent by fusing the radiographic bite plate with a vacuum-form pickup of the radiopaque die (25% barium sulfate)–impregnated cold-cure acrylic prosthetic mockup. This radiographic template is worn by the patient during a scan and is based on the bite plate and a diagnostic prosthetic workup on the gypsum model (Fig. 4-15). After the scan, the scan volume is reconstructed on the computer (Fig. 4-16). Based on the patient’s three-dimensional radiography, the implantologist can do the implant planning with the included (i.e., GALILEO Simplant) or third party (i.e., Materialize-Simplant, Nobel Biocare–Procera) right on the computer.
FIGURE 4-16. Panoramic view of the radiographic guide in place; notice the spherical radiopaque fiduciary markers of the radiographic bite plate. A virtual implant has been placed in the area of missing tooth #14 using the radiopaque acrylic mockup of the final prosthetic as a guide.
If the procedure will involve the lower jaw, the implantologist may want to mark the inferior alveolar nerve as the first step (Fig. 4-17). The implant planning data then are stored on a removable computer medium (e.g., CD-ROM). The planning data, the scan stent, and the gypsum model of the surgical area, along with clear, detailed written instructions on the surgical drill guide order form, are sent to the laboratory that will fabricate the surgical drill guide (e.g., SiCat, Nobel Biocare, Biomet-3I) (Fig. 4-18).
FIGURE 4-17. Implant planning report as generated by the Galileos Implant planning software after the relevant vital anatomy in the surgical field has been identified and highlighted and the virtual implants placed as per the restorative or the surgical doctor’s preferences. This report can be shared with the entire implant team as well as with the patient as part of their education process.
FIGURE 4-18. Items submitted for the fabrication of the surgical guide include the study cast of the jaw where the implants will be done, the radiographic template, the CD-ROM with the implant planning loaded onto it and the prescription form for the fabrication of the surgical guide.
The laboratory manufactures a surgical drill guide based on the implant planning data and the scan stent. The guide is equipped with a pilot drilling system, a sleeve-in-sleeve system, or the outer sleeves of established surgical systems (Fig. 4-19).
The CBCT scanner has a compact size, is easy to use and allows easy patient positioning, has a fast acquisition speed (6 to 14 seconds), and involves a relatively low dose of radiation; all these factors make it ideally suited for imaging of the craniofacial region, including dental structures. This modality is emerging as the imaging standard of care for a number of diagnostic assessments of bony components of the face.
Three-dimensional imaging allows visualization of any area within the parameters of the scan. CT scanners provide a variety of views by making 0.5-mm slices through the bone, which are stacked by the program’s software like a deck of playing cards. When the three-dimensional subject is complete, the computer reformats it into coronal, cross-sectional, or panoramic images, which also are sliced. This program ensures that the surgeon is not confronted with any surprises during surgery. The amount of available bone (or lack of it) may be plotted to the millimeter. The amount of bone beneath the maxillary sinus and nasal cavity may be charted for width and height, and the density of bone may be assessed. In the mandible, the exact location of the mandibular canal in even its most tortuous course may be plotted before surgery.
This information enables the clinician to plan the proper implant types, numbers, sizes, and locations. Presurgical scanning minimizes the chance that the surgeon will have to inform the patient, midway through surgery, that the individual is an unsuitable candidate for implants.
Preferably, the patient should be sent to a radiologist who has a GE 9800 or GE 8800 CT unit. The radiologist also must have software that can create a three-dimensional reconstruction of the maxilla and mandible, such as the Columbia Scientific (3D Dental) program.
The scanner produces a series of images made from horizontal slices (Fig. 4-20). Each exposure is 0.5 mm wide. When the series of cuts is completed within the prescribed perimeters, they are stacked by the computer. The program uses this reconstituted three-dimensional structure to supply images made in the cross-sectional (Figs. 4-21 and 4-22), panoramic (Figs. 4-23 and 4-24), and occlusal modes (Figs. 4-25 and 4-26). These images may be interpreted, area by area, so that the arch in which the implants have been planned can be plotted to the millimeter in width and depth. In addition, exact locations of all vital structures are readily identified by using the images, which the process presents in 1-mm sections.
FIGURE 4-20. With the patient lying on the gurney, the computed tomography (CT) scanner makes horizontal radiographic cuts 1.5 mm wide. Each image is overlapped by 0.25 mm to permit accurate continuity and the presentation of views that are 1 mm wide.
FIGURE 4-22. A typical cross-sectional view of the mandible produced by CT scan dental software. Vital structures (e.g., the mandibular canal, cortical plates, and inferior border) can be reviewed clearly. With some systems (life-sized imaging), measurements can be made directly from the film.
FIGURE 4-23. A panoramic view is another valuable image produced by CT scanning. Usually five slices are made available, from buccal to lingual. This mandibular model shows the middle level of the five slices marked by pencil (see Fig. 4-24).
FIGURE 4-24. Panoramic images at four buccolingual levels (view 31) on the right top are demonstrated in the panoramic reproductions (1 to 4) on the left. The lucent area in view 1 does not represent a lytic lesion. Line 1 in view 31 (upper right) (the most labial cut) shows that the scan passed through an area just anterior to the symphysis.
FIGURE 4-26. A, Diagnostic waxup of a full denture with vertical radiopaque markers for radiographic identification of implant sites. B, Transaxial (occlusal) views are used for the maxilla as well as the mandible. These eight images, numbered 10 through 17, offer dramatic representations of this geometrically complex structure at evenly spaced intervals (1.5 mm apart). The image at the upper right, marked A, known as a scout film, presents the inferior to superior perimeters of the numbered images.
If no such software programs are available, the radiologist may take a conventional scan of the patient on almost any CT scanner, and the resulting DICOM-compatible digital data can be sent to the Columbia Scientific Corporation. The company can translate such data into images containing the requisite three-dimensional information.
FIGURE 4-27. The Dentascan program allows computerized superimposition of implants of specific dimensions directly on images. The computerized implants may be placed on both cross-sectional and panoramic views (see also Fig. 4-54).
Before the patient is referred to the radiologist, intraoral splints designed to immobilize the lower jaw must be made if this area is to be scanned. However, this is not an absolute requirement for the maxilla. The splints are fabricated with three objectives in mind: immobilization, disocclusion, and orientation. The fixation devices are fabricated with the jaws in the resting position to allow the patient to keep the mandible comfortably immobilized for up to 30 minutes. Immobilization is necessary, because any movement causes blurring or distortion of the images.
If teeth are present in the maxilla and mandible, the splints should disocclude them so that a space is created on the images. No metal should be allowed to remain in the teeth or incorporated into the splint of the jaw being scanned, because metal (except for titanium) creates scatter (noise) and other masking artifacts on the images.
The proper plane of occlusion is established in the splints, and nonmetallic radiopaque markers (gutta percha or barium sulfate 25%) are placed parallel to and at the plane of occlusion. These markers tell the radiologist how to orient the patient’s head for the study. They also ensure that the angle at which the cross sections are reformatted through the scanned arch is 90 degrees to the occlusal plane. The accuracy of measurements taken from the scan depends on the orientation of the preliminary film, or scout film. Consequently, accurate placement of the opaque occlusal plane markers is essential. The radiography technician must position the patient’s head so that the scout film demonstrates its orientation lines parallel to the marker (see Fig. 4-42).
Each tooth represented in the template should have a 10-mm vertical gutta percha or barium sulfate marker processed into it, or the entire tooth can be fabricated with cold-cure acrylic mixed with barium sulfate in a 25% concentration. These are demonstrated in the occlusal, cross-sectional, panoramic, and transaxial images. When the template is placed in the patient’s mouth, each gutta percha marker can be traced to the images, so that anatomic localization is coordinated. This is particularly valuable during surgery, when planned host site dimensions can be pinpointed by relating them directly to the appropriately labeled cross-sectional views.
FIGURE 4-28. For a completely edentulous patient, trial dentures are placed with the teeth set in wax to check for esthetics and function and to locate the appropriate positions of significant teeth. These relate to potential implant sites on the scan.
FIGURE 4-30. In the curing of acrylic denture replicates, the pressure pot is filled with hot water to the point of overfilling and then closed tightly; this allows the acrylic to cure fully and without bubbles.
FIGURE 4-31. Grooves are drilled in the clear acrylic template to prepare for placement of radiographic markers in each of the 14 teeth of the surgical template. Each groove is made at a right angle to the occlusal plane marker, and the grooves are then filled with a radiopaque medium.
FIGURE 4-32. A, Gutta percha markers are inserted into the grooves of the radiographic or surgical template. Use of the Obtura device ensures a predictable density. B, Amalgam powder and acrylic powder (1:3 ratio applied with monomer and a paintbrush) is simple to insert into the grooves and creates a highly diagnostic image.