An Illustrative Study of the Role of Tomograms for the Placement of Dental Implants

ORAL IMPLANTOLOGYTuesday Mar 11 2003 02:04 PMAllen Press x DTPro SystemCLINICALEDITOR'S NOTE: This is the first in a series of Primers for New Implantologists. Otherarticles will follow.AN ILLUSTRATIVE STUDY OF THE ROLE OFTOMOGRAMS FOR THE PLACEMENT OF DENTALIMPLANTSKavas H. Thunthy, BDS, MS, MEdWilliam R. Yeadon, DDS, FAGDHisham F. Nasr, DDS, MScDKEY WORDSTomography, X rayDental implantsDental implantation, endosseousDental radiographyKavas H. Thunthy, BDS, MS, MEd, is aprofessor of Oral and Maxillofacial Radiology;William R. Yeadon, DDS, FAGD, is aclinical associate professor; and Hisham F.Nasr, DDS, MScD, is an assistant clinicalprofessor at the Louisiana State UniversitySchool of Dentistry in New Orleans, LA70119. Address correspondence to Dr Thunthy,Department of Oral Diagnosis, Medicine, andRadiology, Louisiana State University School ofDentistry, 1100 Florida Avenue, New Orleans,LA 70119-2799.ntal implants are widelyused in the restoration ofedentulous sites of dentalalveolar bone. Regardlessof the type of intraosseousimplant system used,the preoperative assessment requires aFIGURE 1. (a) Cross-sectional tomographic slices and (b) sagittal tomographic slices. A sagittalslice is mainly used for location and orientation of the multiple cross-sectional slices.orim 29_203 Mp_91File # 03emA. Norman CraninBefore dental implants are placed, an evaluation of a presurgical bone site withtomograms will reveal information not available from panoramic or periapicalradiographs. This article illustrates the importance of making tomograms beforethe placement of dental implants to determine the actual height, width,inclination, and undercut of alveolar bone; the shape, cortication, andirregularities of crestal alveolar bone; the density of alveolar bone; the relativelocation of anatomical landmarks, such as mandibular canal, maxillary sinus,nasal fossa, incisive canal, submandibular gland fossa, etc.; the bucco-lingualview of dental pathosis; the bucco-lingual evaluation of sinus graft followingsinus-lift surgery; and the evaluation of surgically placed dental implants.radiographic examination to facilitatethe planning of the placement of implants.1-8 Some clinicians evaluate theedentulous site solely on periapicaland/or panoramic radiographs, butthese 2 types of radiographs have amajor disadvantage: they do not showJournal of Oral Implantology 91TABLEORAL IMPLANTOLOGYTuesday Mar 11 2003 02:04 PMAllen Press x DTPro SystemDENTAL IMPLANT TOMOGRAPHYthe dental alveolar ridges in the thirddimension. Two other disadvantagesthat the 2 possess, especially with thepanoramic radiograph, is of distortionand uneven and unpredictable magni-fication.Some clinicians use the computerizedaxial tomography (CT) scan intheir evaluation of presurgical sites forImportance of tomograms for the placement of dental implants1. Actual height of alveolar bone (Figure 2).2. Actual width of alveolar bone (Figure 3).3. Inclination of alveolar bone (Figure 4).4. Undercut of alveolar bone (Figure 5).5. Shape of crestal alveolar bone (Figure 6).6. Cortication of crestal alveolar bone (Figure 7).7. Irregularities of crestal alveolar bone (Figure 8).8. Density of alveolar bone (Figure 9).9. Relative location of anatomical landmarks, such as mandibular canal, maxillarysinus, nasal fossa, incisive canal, submandibular gland fossa, etc (Figure 10).10. Bucco-lingual view of dental pathosis (Figure 11).11. Bucco-lingual evaluation of sinus graft following sinus-lift surgery (Figure 12).12. Evaluation of surgically placed dental implants (Figure 13).92 Vol. XXIX/No. Two/2003FIGURES 2-5. FIGURE 2. To determine the actual height of alveolar bone, the height of bone on the tomogram is multiplied by the magnificationfactor of the specific tomographic machine. Cross-sectional (a) and sagittal (b) slices show adequate alveolar bone height forimplant placement when measured from the crest of the ridge to the mandibular canal. Additional cross-sectional (c) and sagittal (d)slices show inadequate alveolar bone height. As a result, the implant projected into the maxillary sinus. The dentist could have avoidedthe error in implant placement if tomograms had been taken before treatment. FIGURE 3. The cross-sectional slice (a) shows the narrowbucco-lingual width of the alveolar ridge. The sagittal slice (b) is unable to show the bucco-lingual width of the ridge, illustrating theimportance of imaging in the third dimension, namely the cross-sectional slice. FIGURE 4. The cross-sectional slice (a) shows the lingualinclination of the alveolar ridge. The sagittal slice (b) is unable to show the inclination of the ridge, illustrating the importance of imagingin the third dimension, namely the cross-sectional slice. FIGURE 5. The cross-sectional slice (a) shows the severe undercut of the alveolarbone. The sagittal slice (b) is unable to show the undercut of the alveolar bone, illustrating the importance of imaging in the thirddimension, namely the cross-sectional slice.orim 29_203 Mp_92File # 03emlikely; a large area of face is X radiated;and the patient receives a large X-raydose.9-11For dental offices, panoramic machineswith the added capability ofmaking conventional linear motion tomogramshave been developed. Theadvantage of these panoramic/tomographymachines is that they aremuch less expensive than CT machinesand can be readily installed ina small area of a dental office. The disadvantagesare that the tomographicslices are very thick, images are notcustom-mapped for individual patients,and the layer depth depends onthe bone orientation (vertical, horizontal,or oblique bone structures wouldproduce vastly different section thicknesses).To overcome these disadvantages,complex motion tomographicmachines with a computer that assistsin custom-mapping the patient's dentheplacement of implants. The CTscan is an accurate imaging modality,but it has a number of liabilities: it isexpensive to patients; it produces scatterartifacts of metal restorations; transferringinformation from a surgicalstent is difficult; the interpretation ofimages is difficult; the chance of patientmovement during exposure isORAL IMPLANTOLOGYTuesday Mar 11 2003 02:04 PMAllen Press x DTPro Systemtal arch have been developed. The clinicianis thus able to obtain thin tomographicslices that are very accuratewith a constant magnification inall directions. This machine can alsomake panoramic radiographs. Thedisadvantage is that it is more expen-FIGURES 6-9. FIGURE 6. The shape of the crestal alveolar bone may be flat, knife-edged, or rounded. The cross-sectional slice (a) shows thebucco-lingual crestal bone to be knife-edged. The sagittal slice (b) is unable to show the knife-edged shape of the crestal bone, illustratingthe importance of imaging in the third dimension, namely the cross-sectional slice. Another cross-sectional slice (c) shows the bucco-lingualcrestal bone to be flattened, but the sagittal slice (d) is unable to show the flattened shape of the crestal bone, illustrating the importance ofimaging in the third dimension, namely the cross-sectional slice. FIGURE 7. The cross-sectional slice (a) shows the bucco-lingual corticationof the alveolar bone. The sagittal slice (b) does not show the cortication, illustrating the importance of imaging in the third dimension, namelythe cross-sectional slice. The cylindrical radiopacity is a marker for the implant site. FIGURE 8. The cross-sectional slice (a) shows the buccolingualirregularities of the crestal alveolar bone. The sagittal slice (b) does not show the crestal-bone irregularities, illustrating the importanceof imaging in the third dimension, namely the cross-sectional slice. FIGURE 9. The cross-sectional (a) and sagittal (b) slices show normalcancellous alveolar bone. The bone trabeculae are distinctly visible, and the bone appearance is not osteoporotic.orim 29_203 Mp_93File # 03emKavas H. Thunthy et alraphy is body section radiographythat shows more clearly a single layerof a structure by blurring out theshadows of superimposed structures.It is not a method of improving thesharpness of the image. On the contrary,it is a process of controlled blur-Journal of Oral Implantology 93sive than the conventional panoramic/tomography machine; an advantage,however, is that it is much lessexpensive than the CT machine.12-15Before discussing the importanceof presurgical tomograms, it is necessaryto define tomography. Tomog-ORAL IMPLANTOLOGYTuesday Mar 11 2003 02:04 PMAllen Press x DTPro SystemDENTAL IMPLANT TOMOGRAPHY94 Vol. XXIX/No. Two/2003orim 29_203 Mp_94File # 03emFIGURES 10-13. FIGURE 10. Before placing an implant, it is necessary to know the anatomy adjacent to the implantation site. The crosssectionalslice (a) shows the location of the mandibular canal and its distance from the crest of the alveolar bone. The sagittal slice (b)also shows the mandibular canal. A second cross-sectional slice (c) shows the nasal fossa and bucco lingual location of the incisive canal,and the sagittal slice (d) shows the nasal fossa and mesio-distal location of the incisive canal. A third cross-sectional slice (e) shows thedepression that the submandibular salivary gland forms in the lingual alveolar bone; however, the sagittal slice (f) does not show thedepression of submandibular gland fossa, illustrating the importance of imaging in the third dimension, namely the cross-sectional slice.FIGURE 11. To see the bucco-lingual extension of pathosis, a cross-sectional slice is necessary. The cross-sectional slice (a) shows the buccolingualextension of the inflammatory apical lesion. The sagittal slice (b) shows the antero-posterior extension of the lesion. The apicallesion is thus seen in all 3 dimensions. Another cross-sectional slice (c) shows the mesio-distal fracture line of the mandibular first molar;however, the sagittal slice (d) cannot show the mesio-distal fracture line, illustrating the importance of imaging in the third dimension,namely the cross-sectional slice. FIGURE 12. The cross-sectional slice (a) shows the bucco-lingual extension of the sinus graft. The sagittalslice (b) shows the antero-posterior extension of the graft. The graft material is thus seen in all 3 dimensions. FIGURE 13. If presurgicaltomographic evaluation of the implant site is not undertaken, the implant may inadvertently be placed in the wrong position. The firstcross-sectional slice (a) shows the implant penetrating the lingual cortical plate and extending to the floor of the oral cavity. The firstsagittal slice (b) gives the illusion of the implant being in bone, illustrating the importance of imaging in the third dimension, namely thecross-sectional slice. The circular radiopacity in both slices is a site marker in an acrylic stent. Both of the second slices, cross-sectional(c) and sagittal (d), show the tip of the implant penetrating the mandibular canal. Both of the third slices show the penetration of theORAL IMPLANTOLOGYTuesday Mar 11 2003 02:04 PMAllen Press x DTPro Systemring that merely leaves some parts ofthe image less blurred than others. Fordental implants, tomograms are madein cross-sectional and sagittal modes(Figure 1). A cross-sectional slice (orcut) is made in a bucco-lingual directionanalogous to a loaf of bread cutbreadthwise in the traditional fashion.A sagittal slice (or cut) is made in ananterior-posterior direction, analogousto a loaf of bread cut lengthwiseresulting in an image similar to thatof a panoramic radiograph. The sagittalslice assists in the correlation ofcross-sectional slices. In practice, severalcross-sectional slices are madealong with a corresponding sagittalslice. To determine the correct heightand width of the alveolar ridge, measurementsmade on tomograms aremultiplied by a magnification factor.Some manufacturers supply a magni-fied measuring scale that when placedon a tomogram gives the actual sizeof the applicable anatomy.Some clinicians place implantsbased solely on the information derivedfrom periapical and/or panoramicradiographs. It is not the intentof this article to suggest that cliniciansmake tomograms of all dental-implantpatients. Each clinician must makethat decision based on one's expertiseand judgment. Suffice it to say thattechnology exists to obtain informationof the dental alveolar arches inthe third dimension. Considerationshould also be given to medico-legalramifications that may result from inadequatepresurgical evaluation.The Table lists the importance ofpresurgical tomograms for dental implants.Figure 1 shows the directionsof the cross-sectional and sagittal slic-implant into the maxillary sinus. The cross-sectional slice (e) shows the bucco-lingual penetration of the sinus. The sagittal slice (f) showsthe antero-posterior penetration of the sinus. The penetration of the sinus by the implant is thus seen in all 3 dimensions. The fourthcross-sectional slice (g) shows 2 implants: 1 in the alveolar bone and the other inadvertently pushed into the maxillary sinus, showingthe bucco-lingual relationship of the implant located in the sinus. The fourth sagittal slice (h) shows the antero-posterior relationship ofthe implant located in the sinus. The fifth cross-sectional slice (i) shows the implants placed labial to the alveolar ridge. The fifth sagittalslice (j) gives the illusion that the 2 implants are correctly placed in the alveolar bone, illustrating the importance of imaging in the thirddimension, namely the cross-sectional slice.es. Figures 2 through 13 are cross-sectionaland sagittal tomographic slices,which illustrate the importance of tomogramsstated in the Table.All tomograms in this article weremade using a complex motion tomographicmachine, the CommCAT modelIS2000 (Imaging Sciences International,Hatfield, PA). It produced a constantmagnification of 26% in all directionson cross-sectional and sagittalslices.REFERENCES1. Tyndall AA, Brooks SL. Selectioncriteria for dental implant site imaging:a position paper of the AmericanAcademy of Oral and MaxillofacialRadiology. Oral Surg Oral Med OralPathol Oral Radiol Endod. 2000;89:630-637.2. Pietrokovski J. The bony residualridge in man. J Prosthet Dent. 1975;34:456-462.3. Grondahl K, Ekestubbe A,Grondahl HG, Johnsson T. Reliabilityof hypocycloidal tomography for theevaluation of the distance from the alveolarcrest to the mandibular canal.Dentomaxillofac Radiol. 1991;20:200-204.4. Kassebaum DK, NummikoskiPV, Triplett RG, Langlais RP. 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