4: Selection of the Proper Implant

CHAPTER 4 Selection of the Proper Implant

DIAGNOSTIC METHODS

Examination

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).

Study Casts

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.

All of the occlusal relationship records should be derived from the articulator. Final prosthesis balancing also is accomplished by following these articulator-related techniques.

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:

3. Reducing the alveolar height of the operative site by flattening the thin ridge (see chapters 6 and 8) while making sure that adequate bone height remains to provide sufficient dimension for seating of the planned implants

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).

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).

Radiology

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.

Panoramic radiograph (Fig. 4-8): A panoramic radiograph presents an overall view of both the mandible and maxilla. Normal anatomy and existing pathologic conditions of the dentoalveolar complex and adjacent structures can be noted. The remaining natural teeth are visualized. Unpredictable distortions of distances (25% or greater) are a constant characteristic of these films.
Periapical radiograph (Fig. 4-9): A periapical radiograph gives a view of higher resolution and greater accuracy than a panoramic study and indicates medullary and cortical bone density. Often measurements can be taken directly from these films, but distortions of up to 20%, depending on the angulation at which the PA was taken, can result in foreshortening or elongation.
Lateral cephalometric radiograph (Fig. 4-10): Lateral cephalometric radiographs are helpful if the patient has completely edentulous ridges. The cross-sectional morphology of the residual anterior ridge can be visualized, along with its angles of inclination. In addition, skeletal jaw relationships can be studied. This allows an estimate of labiolingual dimension. These views may be taken with occlusal films but are best produced with a cephalometric device using high-speed 8 × 10 cassettes.
Radiographic ball-bearing template: Periapical ball-bearing evaluations can be valuable. A template is prepared using the second (surgical planning) cast. The 5-mm diameter, standardized metal marking spheres (Biomet-3I, Ace Surgical) should be countersunk into the cast at the crest of the ridge at each potential implant site to a depth of 1 mm using a No. 6 round bur. Each is secured in place with sticky wax. An Omnivac machine then is used to mold a piece of 0.02-inch gauge, clear plastic material to the cast; this material incorporates the spheres (Fig. 4-11). After proper trimming, the template produced can be seated intraorally before periapical radiography (Fig. 4-12). If the template is nonretentive, a small amount of denture adhesive is used to stabilize it. After obtaining long cone, periapical radiographs with the template in place, the implantologist records the diameter of the spheres on the films (Fig. 4-13). If the spheres are 5 mm in diameter, the height and length of available bone can be measured accurately directly on the radiographs. If the spheres are not 5 mm in diameter on the film, a simple equation can be used to determine the actual bone dimensions:

image

where rs is the radiographic sphere measurement, rm is the radiographic bone measurement, rx is the actual bone measurement sought, and 5 is the actual sphere measurement.

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.

Instead of these methods, a single, accurate technique of bone measurement and morphology may be achieved using computed tomography (CT) scanning.

Cone Beam Volumetric Tomography

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.

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).

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).

RADIOGRAPHIC EVALUATION

Computed Tomography

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.

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.

Currently, software systems are available (Dentascan, Galaxis) that allow the dentist to reformat axial images in the office and superimpose appropriately sized implants on them (Fig. 4-27).

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).

Methods of Radiographic Splint Fabrication

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.

Edentulous Arch

The following steps make up the process for a patient with an edentulous arch.

6. Complete the dentures and replicate them in clear, self-curing acrylic using a Lang denture duplicator flask (Fig. 4-29). After preliminary polymerization, the acrylic replicate is fully cured in a pressure pot for 30 to 45 minutes (Fig. 4-30).

Partly Edentulous Arch

The steps for a partially edentulous are as follows.

13. Have the patient close into a centric relationship but stop at the resting vertical dimension (Fig. 4-40). The patient opens and closes the mouth to this position while the acrylic hardens. Complete the curing in the pressure pot (Fig. 4-41).

Jan 5, 2015 | Posted by in Implantology | Comments Off on 4: Selection of the Proper Implant
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