Cross-sectional radiography for implant site assessment

Cross-sectional radiography for implant site c assessment Denise K. Kassebaum, DDS, MS,” Pirkka V. Nummikoski, DDS, MS,b R. Gilbert Triplet& DDS, PhD,” and Robert P. Langlais, DDS, MS,d Denver, Colo., and San Antonio, Texas THE UNIVERSITY OF COLORADO HEALTH SCIENCES CENTER AND THE UNIVERSITY OF TEXAS HEALTH SCIENCE CENTER AT SAN ANTONIO An accurate tomographic technique is described for acquisition of optimal cross-sectional images of implant sites before implant surgery. The described technique is applicable to tomographic systems equipped with a cephalometric head positioner. This cross-sectional tomographic technique was performed on a series of patients and the images of the first 20 patients subsequently evaluated. The cross-sectional images allowed for the characterization of the alveolar crest and visualization of anatomic structures in a buccolingual dimension while providing an accurate estimation of available vertical space from the crest. (ORAL SURC ORAL MED ORAL PATHOL 1990;705i74-8)

T

he placement of dental implants is an accepted treatment method for patients with edentulous spans of the jaws.lm3An essential step in the selection of appropriate implant sites is the presurgical radiographic examination. Radiographs reveal adjacent anatomic structures, such as the maxillary sinus and the mandibular canal, and indicate the optimal direction for the implant fixture insertion. A variety of radiographic techniques have been described to aid in presurgical implant site assessment.These include plain radiography,2 computed tomography (CT),4-‘0 and tomography. 1r-l3 The plain radiographs typically used are intraoral radiographs, a panoramic radiograph, and a skull profile or cephalometric radiograph.2 Thesefilms may not display the true distances betweenanatomic strucaAssistant Professor,Department of Diagnostic and Developmental Sciences, School of Dentistry, The University of Colorado Health Sciences Center. bAssistant Professor, Department of Dental Diagnostic Science, School of Dentistry, The University of Texas Health Science Center at San Antonio. cProfessor,Department of Oral and Maxillofacial Surgery, School of Dentistry, The University of Texas Health ScienceCenter at San Antonio. dProfessor,Department of Dental Diagnostic Science, School of Dentistry, The University of Texas Health Science Center at San Antonio. 7/16/19794 674

tures, such as the distance between the alveolar ridge and the mandibular canal, if the structures are not in the same plane perpendicular to the radiation direction. This will result in either overestimation or underestimation of the measured distances. Also, the plain films do not reveal the buccolingual dimensions of the bone in the area or the thickness of the cortical bone available. CT is a fast and convenient imaging method for obtaining data useful in implant surgery. Advances in CT software have led to the availability of programs that will yield axial, oblique, and panoramic reformations of the jaws.G* Although the inherent ability of CT to separate different soft tissue densities facilitates visualization of the mandibular canal, the true thickness of the cortical bone may not be reliably depicted becauseof the volume averaging and computer algorith limitations of CT procedures. Also, it may be difficult to accurately correlate the radiographic location to the clinical location at the time of surgery if there are no adjacent clinical landmarks, such as teeth. A number of tomographic techniques have been described. Petrikowski et a1.13used a hypocycloidal tomographic technique to radiograph the maxilla and mandible while patients wore specially designed splints. The authors relied on numerous stainless steel spheres mounted in the splints to depict the approximate location of each cut in a series of tomographic

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Fig.

Fig. 1. A, The Quint Sectograph (Denar Corp., Anaheim, Calif.). B, Optional fiber-optic positioning light for Quint Sectograph.

cuts. A similar radiographic technique was described by Eckerdal and Kvint.’ 1 The purpose of this study was to evaluate a tomographic technique for imaging implant sites that was developed to be performed on a linear tomography unit equipped with a continuously adjustable cephalostat. The technique employs many principles evaluated in previous tomographic studies, together with modifications that simplify the acquisition of consistently accurate images. MATERIAL

AND METHODS

The protocol for imaging implant sites to be described herein was first developed for use on the mandible of a human cadaver specimen. After an imaging protocol was developed, 20 patients were selectedfor participation in this project on the basis of the following criteria: (1) the patients were seeking the placement of implants to restore function in edentulous areas of the maxilla or mandible, and (2) the patients’ health made them good surgical candidates. This technique was developed for use on a linear tomography unit equipped with an adjustable cephalostat, and an optional fiber-optic light system that

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2. Stent with parapulpal pins in acrylic base.

indicates the tomographic layer position in the patient (Quint Sectograph, Denar Corp., Anaheim, Calif.; Fig. 1). The cephalostat enables accurate and reproducible patient positioning, resulting in a well-controlled localization of the tomographic section. The accurate positioning allows for the use of small collimation, resulting in considerably reduced patient radiation dose compared with conventional tomography. Tomograms are taken in conjunction with a panoramic radiograph and a submentovertical view on each patient. To help correlate the tomographic imagesto the clinical field, a small stent is prepared for each patient (Fig. 2). The stent is made either of denture acrylic or vacuum-pressed plastic mouthguard material (Buffalo Dental Manufacturing Co. Inc., Syosset, N.Y .). At each proposedimplant fixture site a small parapulpal pin is inserted into the stent. The pins are inserted at different vertical angles to facilitate later identification of these in the resulting tomograms. This stent may be used at the time of surgery to correlate location of implant site radiographs to actual clinical location. The axial and panoramic films are taken with the stent in place becausethese localization markers are beneficial in confirming the locations of the implant sites. To obtain accurate cross-sectional images of the proposedimplant sites,it is necessaryto determine the head position in which the long axis of the implant site area is perpendicular with the plane of the film cassette or parallel with the central beam. This may be done visually, by positioning a tongue blade along the alveolar ridge for use as an indicator, or by calculations determined from the submentovertical

view.

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projection with labeling.

Fig. 5. Tomogram of mandible demonstrating location of mandibular canal.

Fig.

4. Collimator

and supplemental collimator.

The depths of cut that coincide with the pins on the stent must be determined. This may be done visually on units that have a fiber-optic positioning light, or the appropriate depths of cut may be calculated with the use of the axial projection (Fig. 3). The accuracy of the calculations made from the axial projection is improved by taking the radiograph with the stent in place, with the patient’s head positioned so that the Frankfort plane is parallel to the film, with the use of acrylic ear rod extensions containing lead ball markers on the cephalostat, to indicate the patient’s external auditory canals. The calculations to determine appropriate head position are completed on the axial projection by performing the following steps. A line is drawn through the lead ball markers, the intermeatal baseline (IBL). The midpoint or center of rotation (CR) is then determined. This midpoint is the point around which the patient is rotated when positioned in the machine. A line is drawn through the localization markers embedded in the stent along the alveolar process,to in-

dicate the long axis of the edentulous span or implant sites (line BCP). A line (P) is drawn perpendicular to line BCP and also intersects the center of rotation at midpoint (CR). The angle (8) between line ZBL and line P provides the calculated horizontal head rotation needed to achieve an optimum imaging direction. For determination of the appropriate depths of cut from the axial projection, the distance between the implant site and the intersection of the line P is measured. The obtained distance is divided by a magnification factor to obtain the true distance that will be set on the machine. The tomography is performed using KODAK LANEX Regular screens with KODAK T-Mat G film (Eastman Kodak, Rochester, N.Y.). The exposure factors usedare 78 to 80 kVp, 50 mA, and a time of 3.0 seconds.The amplitude of the movement of the Quint Sectograph unit is approximately 25 degrees, resulting in a 3 mm thick tomographic layer at the region of interest. The adjustable, centric collimator is supplemented with an off-center lead collimator that allows for accurate collimation to the side of interest (Fig. 4). The resulting tomographic images of proposed implant sites in the series of 20 patients were evaluated for acceptability with the following criteria: 1. The localization markers (parapulpal pins in the stent) should be visible on each of the tomographic cuts for identification of the location of the cut and subsequent correlation with the panoramic view.

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Fig. 7. Tomogram demonstrating knife-edge ridge on mandible.

found to be diagnostic, the implant site adequacy was determined by the criteria used with an endosseous threaded titanium implant system (Brinemark system.)2 RESULTS Fig. 6. Tomogram of maxilla demonstrating location of maxillary sinus.

2. The shape of the alveolar ridge crest must be demonstrated. 3. On mandibular tomographic cuts the inferior alveolar canal must be visible for subsequent assessmentof available distance to the alveolar crest. 4. On maxillary tomographic cuts the maxillary sinus or nasal cavity must be visible for subsequent assessmentof available distance to the maxillary alveolar crest. 5. The buccolingual orientation of the anatomic structures should be visible for subsequent implant site width assessment.This information must be present to determine the path of insertion for caseswith adequate widths of alveolar bone at the implant site. If the sets of tomographic images met the preceding criteria, they were considered to represent a “successful imaging outcome.” If the series of tomographic images on any one patient failed to demonstrate the specified criteria, the images were found to represent an “unsuccessful imaging outcome.” Once the tomographic images of a patient were

Twenty patients were radiographed by the aforementioned tomographic technique. Of the 20 sets of radiographs taken, 16 setsof images were found to be of sufficient diagnostic quality to represent successful imaging outcomes. (Figs. 5 and 6 depict diagnostic tomographic images of a mandibular case and a maxillary case, respectively.) Four of the sets of images were judged nondiagnostic because the images either failed to demonstrate a well-defined alveolar crest or failed to demonstrate needed information on the location of the mandibular canal. Six casesof the 16 sets of diagnostic tomographic images revealed information about proposed implant sites that precluded further treatment planning of dental implants for these patients. (Fig. 7 demonstrates a case with inadequate alveolar crest width.) Surgeries were successfully performed in the remaining cases. DISCUSSION

The technique was performed on an initial seriesof 20 patients at the University of Texas Health Sciences Center at San Antonio School of Dentistry. Currently the technique is used at this institution and at the University of Colorado School of Dentistry. This technique has been found to have several important advantages:it provides information regarding the thickness and quality of the bone present; it allows

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for the determination of appropriate fixture length; it aids in the surgeon’schoice of path of fixture insertion and the assessmentof available width of the implant site; it accurately characterizes the alveolar crest; the supplemental off-center lead collimator allows for accurate collimation to the site of interest resulting in a lower radiation dose;and the technique is available to a variety of dental practitioners in dental schools,private radiographic laboratories, and private offices. The radiographic correlations to the real dimensions of the bone and the proximity to anatomic structures were evaluated at the time of surgery. In some casesthe radiographs revealed information regarding ridge characteristics that prevented further consideration of treatment with the use of dental implants (Fig. 7). The distances for fixture length and the approximation of the location of anatomic structures were deemedaccurate by surgical confirmation. The technique has been demonstrated to be ideal for single implant or single quadrant implant site evaluation. The project was initiated with the intent to provide optimal pretreatment radiographs for treatment planning of endosseouscylinder-threaded implants. There are a few disadvantages of the technique: it is time consuming, multiple sites in a jaw necessitate additional calculations and changing of the angle of the patient’s head if the long axesof the proposedsites are not parallel to each other, and anatomic variations may result in images that are difficult to interpret. Estimation of the magnification factor present on the tomographic images necessitates the use of a range (from 6% to 10%). On someof the images of mandibular implant sites it is difficult to determine the exact location of the mandibular canal. The visibility of the sinus floor may be compromised on someof the maxillary images when the sinus wall curves upward and does not demonstrate a strong cortical outline. Similarly, if the central beam is off angle to the long axis of the cortical bone, the accuracy of the resulting image is compromised. For those casesor implant techniques which require complete arch tomographic evaluation or simultaneous assessmentof maxillary and mandibular arches, a CT scan of the proposed implant sites may be preferable. CONCLUSIONS

A technique that uses tomographic systems with cephalometric head positioners has beendescribed for

acquisition of optimal cross-sectional images of implant sites before implant surgery. This cross-sectional tomographic technique was used to produce images on a series of 20 patients, the images were subsequently evaluated, and, when indicated, the surgeries were completed. The tomographic images provided valuable information regarding the characteristics of the alveolar crest while allowing visualization of anatomic structures in buccolingual and vertical dimensions. REFERENCES

1. Adell R, Lekholm U, Rockier B, Branemark P-I. A 15year study of osseointegrated implants in the treatment of the edentulous iaw. Int J Oral Surg 1981:10:387-416. 2. Branemark”P-I, Zarb GA, Albrektsson T. Tissue integrated prostheses-osseointegration in clinical dentistry. Chicago: Quintessence, 1985. 3. Branemark P-I, Adell R, Albrektsson T, Lekholm U, LindStrom J, Rockier B. An experimental and clinical study of osseointegrated implants penetrating the nasal cavity and maxillary sinus. J Oral Maxillofac Surg 1984;42:497-505. 4. Andersson L, Kurol M. CT scan prior to installation of osseointegratedimplants in the maxilla. Int J Oral Maxillofac Surg 1987;16:50-5. 5. Fjellstroom C-A, Strom C. CT of the edentulous maxilla intended for osseo-integratedimplants. J Craniomaxillofac Surg 1987;15:45-6. 6. Schwarz MS, Rothman SL, Rhodes ML, Chaftez N. Computed tomography: part I. Preoperative assessment of the mandible for endosseousimplant surgery. Int J Oral Maxillofat Implants 1987;2:137-42. 7. Schwarz MS, Rothman SL, Rhodes ML, Chaftez N. Computed tomography: part II. Preoperative assessmentof the maxilla for endosseousimplant surgery. Int J Oral Maxillofac Implants 1987;2:143-8. 8. Rothman SLG, Chaftez N, Rhodes ML, Schwartz MS. CT in the preoperative assessmentof the mandible and maxilla for endosseousimplant surgery. Radiology 1988;168:171-5. 9. Engstrom H, SvendsenP. Computed tomography of the maxilla in edentulous patients. ORAL SURC ORAL MED ORAL PATHOL 1981;52;557-60.

10. North A, Rice J. Computed tomography in oral and maxillofacial surgery. J Oral Surg 1981;39:199-207. 11. Eckerdal 0, Kvint S. Presurgical planning for osseointegrated implants in the maxilla. Int J Oral Maxillofac Surg 1986; 151722-6. 12. Engelman MJ, SorensonJA, Moy P. Optimum placement of

osseointegrated implants. J Prosthet Dent 1988;59:467-73. 13. Petrikowski CG, Pharoah MJ, Schmitt A. Presurgical radio-

graphic assessmentfor implants. J Prosthet Dent 1989;61:5964. Reprint requests to:

Dr. Denise K. Kassebaum University of Colorado School of Dentistry 4200 E. Ninth Ave., Box C-284 Denver, CO 80262