monsour 2008 radiography implants

Australian Dental Journal 2008; 53:(1 Suppl): S11–S25

doi: 10.1111/j.1834-7819.2008.00037.x

Implant radiography and radiology

PA Monsour,* R Dudhia*

*School of Dentistry, The University of Queensland, Brisbane.

ABSTRACT

The practitioner placing dental implants has many options with respect to pre-implant radiographic assessment of the jaws.The advantages and disadvantages of the imaging modalities currently available for pre-implant imaging are discussed insome detail. Intra-oral and extra-oral radiographs are generally low dose but the information provided is limited as theimages are not three-dimensional. Tomography is three-dimensional, but the image quality is highly variable. Computedtomography (CT) has been the gold standard for many years as the information provided is three-dimensional and generallyvery accurate. However, CT examinations are expensive and deliver a relatively high radiation dose to the patient. The latestimaging modality introduced is cone beam volumetric tomography (CBVT) and this technology is very promising withregard to pre-implant imaging. CBVT generally delivers a lower dose to the patient than CT and provides reasonably sharpimages with three-dimensional information. A comparison between CT and CBVT is provided. Magnetic resonance imagingis showing some promise, but the examinations are not readily available, generally expensive and bone is not well imaged.Magnetic resonance imaging is excellent for demonstrating soft tissues and therefore may be of great use in identifying theinferior dental nerve and vessels. All of the above technology is of little value if the information required is not obtained andso information is also provided on imaging of some of the vital structures. Of particular interest is the inferior dental canal,incisive canals of the mandible, genial foramina and canals, maxillary sinus and the incisive canal and foramen of themaxilla.

Key words: Radiography, radiology, implants, computed tomography, cone beam volumetric tomography.

Abbreviations and acronyms: CBVT = cone beam volumetric tomography; CT = computed tomography; FOV = field of view; IC = incisivecanal; IDC = inferior dental canal; MRI = magnetic resonance imaging.

INTRODUCTION

It is essential to obtain appropriate information aboutthe jaws prior to implant placement and this includesassessment of bone grafts. It is also necessary to obtaininformation about consolidation of implants and posi-tioning following placement of implants in the jawsor adjacent bones. There are many imaging optionscurrently available, including intra-oral radiography,conventional extra-oral radiography, tomography,computed tomography (CT), cone beam volumetrictomography (CBVT) and magnetic resonance imaging(MRI). The appropriateness of each of the imagingoptions will be discussed and information will also beprovided on interpretation. In recent times we have seenthe emergence of CBVT units and as with any newtechnology many claims have been made to convinceprospective users of the benefits of the new technology.This paper will examine in some depth the benefits andfailings of CBVT as the technology currently stands.

Intra-oral radiography

Periapical radiographs and occlusal radiographs havebeen used to assess the jaws pre- and post-implantplacement. The use of the bisecting angle techniquefor taking periapical radiographs should be discour-aged because of the inherent distortion of theresultant image. The bisecting angle technique relieson a geometric trick to produce the image, but only aportion of the structures being imaged are dimension-ally accurate. The long cone paralleling technique fortaking periapical radiographs is the technique ofchoice for the following reasons: reduced skin dose;less magnification; a true relationship between thebone height and adjacent teeth is demonstrated; nosuperimposition of the zygoma over the upper molarregion. It should be remembered that to get the mostfrom the long cone paralleling technique it should beperformed with a film-focal distance of approximately30 cm.

ª 2008 Australian Dental Association S11

Occlusal radiographs have minimal application inimplant dentistry. Cross-sectional occlusal radiographsof the mandible give some information about the bucco-lingual dimension of the mandible, but this informationis only accurate with regard to the inferior aspect of thebody, not the width of the alveolar ridge where theimplant is to be placed. The use of cross-sectionalocclusal radiographs can be helpful when assessing theposition of the implant within the jaw followingplacement; this applies to both the mandible and maxilla.

Extra-oral radiography

Lateral cephalometric radiographs provide accurateinformation about the available bone in the mid-sagittalregion of the maxilla and mandible. Because of the longfilm-focal distances used in cephalometric radiographythe resultant image has minimal magnification. Figure 1shows a lateral cephalometric radiograph taken with atrial lower denture in place and radio-opaque materialdefining the proposed implant site in the anteriormandible. The cross-sectional dimensions and mor-phology of the ridge are shown accurately in the mid-sagittal plane of the anterior maxilla and mandible.

Rotational panoramic radiography (OPG) is anincredibly popular form of radiography in dentistrygenerally. No other imaging modality gives as muchinformation about the jaws with such a small radiationdose. With rare earth intensifying screens the dose froma single OPG is approximately 0.007 mSv using

analogue technology (Table 1). Panoramic radiographsprovide an excellent general overview of the dentitionand the jaws. However, OPGs have their limitationswhen being used for pre- and post-implant assessmentof the jaws. There are inherent problems with OPGswhich include distortion in the horizontal plane,magnification in the vertical plane, true relationshipsare not demonstrated well and the image is only two-dimensional. The accuracy of the image is largelyoperator dependent and varies greatly with patientpositioning. Figure 2 shows an OPG in which thepatient’s head is rotated to the left, resulting inhorizontal magnification of the structures on the left

Fig 1. Lateral cephalometric radiograph of an edentulous patientshowing the available bone in the mid sagittal plane of the maxillaand mandible (arrows) with the trial mandibular denture in place

(arrowhead). (Courtesy of Dr Gary Smith.)

Table 1. Radiation dose

MODALITY 1990� (mSv) 2005�� (mSv)

Background radiation39

(per annum)2.000 n ⁄ a

Intra-oral radiographyLong-cone parallelingperiapical radiograph40–42

0.001–0.010 0.005–0.015

Full-mouth periapicalsurvey8,10,42,43

0.013–0.150 n ⁄ a

Occlusal radiograph40,41 0.007–0.008(mx)

n ⁄ a

Extra-oral radiographyRotational panoramicradiograph8,10,11,15,40–46

0.0029–0.026 0.009–0.022

Lateral Cephalometricradiographs41,45

0.002–0.005 0.003

Tomography (4 slices)40,42–45,47 0.006–0.134 0.012Tomography (full survey)40,43,44 0.063–0.477 n ⁄ a

Cone beam volumetrictomography8,10,11,15,30,46,48

0.037–0.847 0.052–1.025

Multislice computedtomography8,30,40–44,46,48

0.104–2.100 n ⁄ a

Low-dose multislice computedtomography10,40,43,45,46,48

(most data pertains to singlearch scans)

0.100–0.760 0.924

Magnetic resonance imaging 0 0

�1990 guidelines for effective dose calculation do not apply an indi-vidual tissue weighting to salivary glands and brain tissue.��2005 draft guidelines apply an individual tissue weighting to sali-vary glands and brain tissue – increasing their relative weighting in theeffective dose calculation.n ⁄ a: not available.

Fig 2. Panoramic radiograph showing horizontal enlargement ofstructures on the left side due to rotation of the head to the left during

taking of the radiograph.

S12 ª 2008 Australian Dental Association

PA Monsour and R Dudhia

and a reduction in the horizontal dimension of struc-tures on the right. Figure 3 shows the effect onhorizontal dimension of having the patient positionedwith the chin up too high for an OPG. Figure 4 showsthe effect on horizontal dimension of the head being toofar forward and too far back in the OPG machine. Theinferior dental canal is not always well shown on

rotational panoramic radiographs and when it isshown, its relationship to the crest of the ridge maybe distorted. For example, if the inferior dental canallies close to the lingual cortex it will be projected higheron the film and therefore appear higher in the arch thanit really is (Fig 5). Due to the mode of operation of OPGmachines and the shape of the alveolar ridge, the ridgemay appear to have adequate vertical dimension for animplant, but the reality is very different (Fig 6). Also, asthe image on an OPG is only two-dimensional, it isdifficult to assess the available bone width (Fig 7).Other problems with OPGs include superimposition ofairway shadows, soft tissue shadows and ghost images,all of which can interfere with interpretation of theradiograph. As a general rule if the inferior dental canalis poorly visualized on a well-taken OPG it will bedifficult to localize, but not impossible, using otherimaging modalities.

Tomography

A number of multifunctional imaging machines arecurrently available that are capable of performingrotational panoramic, cephalometric and tomographic

(a)

(b)

Fig 3. (a) Panoramic radiograph with the occlusal plane inthe incorrect position (chin up). (b) Same patient with the chin

down further.

(a)

(b)

Fig 4. (a) Cropped panoramic radiograph with the anterior teethinside the focal trough, resulting in horizontal enlargement and

blurring of the anterior teeth. (b) Panoramic radiograph with theanterior teeth in front of the focal trough, resulting in all structures

being compressed in the horizontal plane.

(a)

(b)

Fig 5. Cross-sectional diagram of the mandible showing thatstructures that are more lingual are projected higher on the film thanstructures that are more buccal. (a) shows the inferior dental canal

close to the buccal cortex and a relative indication of where the canalis projected onto the film, (b) demonstrates that when the inferior

dental canal is more lingual, the canal is projected higher on the film.The inferior line drawings depict the difference in appearance of the

canal when buccal or lingual, on the panoramic radiograph.



examinations. Cross-sectional images obtained usingspecially designed panoramic radiographic units havebeen shown to be acceptable for dental implantplanning.1 The tomography performed is usually linear,spiral or hypocycloidal. These devices are capable ofproducing thin (as small as 1 mm) cross-sectional slicesof the jaws that are suitable for pre- and post-implantassessment. Figure 8 shows a series of cross-sectionalimages of the mandible obtained using hypocycloidaltomography to demonstrate the available bone andthe location of the inferior dental canal. The imagesare produced at a constant known magnification andtherefore measurements may be taken directly fromthe images using a special ruler provided with theappropriate scale or in the case of digital images using ameasurement programme after calibration. The multi-functional units are also capable of providing imagessimilar to intra-oral radiographs. Limitations of thesetypes of units are that the examination times can bevery long (up to 20 minutes) and the patient is requiredto remain still for up to 20 seconds during tomographicacquisition of each site. Additionally, image detail maynot be sharp due to slight patient movement orsuperimposition of adjacent structures, tomographic

examinations are not widely available and the exam-ination can be uncomfortable for the patient due to therestraining devices and the length of the examination.

Trans-tomographic examinations when performedwith a radiographic reference guide during implantsurgery have been shown to provide accurate informa-tion for implant placement.2 This form of navigationsurgery allows the surgeon to rectify the drill’s orien-tation when needed. Trans-tomographic navigationprotocols may allow flapless surgical procedures to beutilized in a greater range of cases.

Computed tomography (CT)

For a long period of time CT has been the gold standardfor pre-implant assessment of the jaws. Modern CTunits have extremely fast gantry speeds and generatemultiple fan-shaped x-ray beams. As a result multisliceCT units have very short examination times andisotropic images can be reformatted in any plane. Thescan time using a 16-slice Toshiba CT unit is approx-imately five seconds for one arch. With appropriatesoftware packages, reformatted images are generated inthe panoramic plane and cross-sectional images aregenerated at right angles to the panoramic plane withintervals of between 1 and 2 mm. The CT pre-implantimaging software is designed to produce life-size images

(a)

(b)

Fig 6. (a) Cropped panoramic radiograph demonstrating considerablebone height in the left mandibular body. Three lines identified as 1, 2and 3 have been drawn vertically across the left mandibular body.

(b) Reformatted cross-sectional CT images corresponding to each linefrom (a), indicating there is far less usable bone height in the left

mandibular body than the panoramic radiograph suggests.

(a)

(b)

Fig 7. (a) Cropped panoramic radiograph demonstrating excellentbone height in the lower right molar region. (b) Reformatted

cross-sectional CT images showing reasonable bone height, but theridge is narrow bucco-lingually.



that can be used to assess the available bone, thelocation of vital structures and to present the images inan easy-to-read format. Figure 9 shows pre-implantimages generated using Toshiba’s pre-implant imagingpackage. The pre-implant imaging packages can beused to assess the bone in both jaws for implants, theavailable bone in the various bone donor sites priorto harvesting for ridge augmentation procedures and toassess the available bone in the malar bones priorto implant placement. The limitations of CT includea relatively high radiation dose compared to otherimaging modalities (Table 1), the appropriate softwareis not always available, the cost of the examination isrelatively high and not always rebateable from Medi-care, the inferior dental canal is not always shown welland beam hardening artefact or scatter from metalrestorations can obscure the regions of interest. Low-density structures such as osteoid are generally beyondthe resolution of CT units.

CT is also of value in assessing the quantity andsubjective quality of bone prior to harvesting fora bone graft or ridge augmentation procedure. Asimplants are not only placed in the jaws, CT is ofvalue in assessing other implant sites such as themalar bones prior to surgery. It will be necessary toliaise closely with medical radiology practices whenrequesting CT images for pre-implant assessment. Itshould be made clear to the CT radiographer in thepractice exactly what information is required and thisis especially true when the data is to be imported forfurther manipulation using pre-implant planning soft-

ware. CT radiographers should be encouraged to scanthe patient in a way that optimizes the informationobtained and that means orientating the patient tominimize artefact from metal restorations and avoid-ing gantry tilt wherever possible. The presence of apost in the tooth next to the region of interest orclose by may result in too much ‘‘beam hardening’’artefact or scatter to make the scan worthwhile.Computed tomography will not be of value inassessing integration of implants as a radiolucentband is usually present around the implant on CTimages (Fig 10), but the location of the implant canbe assessed in three dimensions using CT.

Cone beam volumetric tomography (CBVT)

Cone beam volumetric tomography was pioneered atthe Nihon University School of Dentistry during the1990s, and the first machines became commerciallyavailable during 2000.3,4 Since then, numerous ma-chines have been marketed and much research assessingthe usefulness of the technology in dentistry has beenperformed. As with any emergent technology, it cansometimes be difficult to separate fact from fiction.Cone-beam technology is progressing rapidly andscanners are constantly being refined and upgraded.Keeping abreast with the latest technologies andupgrades presents a significant challenge. A reasonablenumber of scanners have already been installed indental practices and radiology practices, and thisnumber is sure to grow in the future.

Fig 8. Cross-sectional and panoramic images of the 46 region produced using hypocycloidal tomography to demonstrate the inferior dental canal(arrows) and the available bone.



While CBVT permits three-dimensional visualizationof the dental hard tissues in a similar manner tomultislice CT,5 there are some fundamental differences.

With the majority of cone-beam machines, the patientis seated or standing rather than being supine. Conebeam volumetric tomography utilizes a cone-shaped

(a)

(b)

(c)

Fig 9. (a) Scout axial CT image of the mandible identifying the location of each reformatted panoramic and cross-sectional image usingpre-implant software. (b) Three of the usual five �life size� reformatted panoramic images demonstrating the ROI in the right mandibular body.(c) Consecutive reformatted �life size� cross-sectional images showing part of the ROI. The location of each cross-section is identified on the

panoramic provided with each series of cross-sectional images.



x-ray beam and either an image intensifier or flat paneldetector for volumetric image acquisition.6,7 During asingle rotation around the patient’s head, multiple basisimages are acquired at specific intervals. These aresubsequently reconstructed by a personal computerrunning proprietary software supplied by the machine’smanufacturer, and this enables the clinician to arbi-trarily reformat the data in any plane.8 Standard axial,coronal and sagittal views are available, as are pano-ramic reformats, cross-sectional cuts of varying thick-ness and 3D volume rendered images.6,8,9 Imagereformatting is identical to that available with multi-slice CT. This enables the clinician to easily assess animplant site in all three planes and perform accuratemeasurements using in-built measuring tools. Thevolumetric data can also be exported in DICOM 3format and viewed with numerous third-party pro-grammes including some that are freely available fromthe internet.

Image acquisition times vary and are specific toparticular models, but typically range between 10 to70 seconds.6–8,10–12 Acquisition time is also dependenton the selected field of view (FOV) and voxel size,which relates to the image resolution.11 Smaller voxelsizes theoretically equate to increased resolution. Fasterscan times typically result in reduced resolution (largervoxel sizes) and increased noise, but with a lowerradiation dose and decreased likelihood of motionartefact.8,13 This is achieved by decreasing the numberof basis images acquired prior to volume reconstruc-tion. Longer scan times utilizing an increased number ofbasis images permit increased resolution or a decreasein image noise but with a significantly higher radiationdose and an increased risk of patient movement.8 Thereare currently no clear guidelines for what scan param-eters produce acceptable image quality with the lowestradiation dose to the patient.

As cone-beam technology was built on the platformof complex-motion tomography, the radiation dose is

typically lower than a multislice CT scan of thejaws.3,4,6,14 Exposure parameters, selected FOV andacquisition times differ markedly from one model toanother, and as such radiation dose is highly specific toeach individual machine and varies widely. A conse-quence of the low exposure parameters is that soft-tissue contrast is markedly decreased compared withthe higher-dose multislice CT examination.15–18 Fur-thermore, image noise is more intrusive in CBVTimages compared with multislice CT.15,17,19 Numerousauthors have written that the lack of soft tissue contrastwith CBVT is acceptable as these units are designed forhard tissue imaging,4,6,20,21 but the inability to changethe exposure parameters has implications when imag-ing larger patients.

The theoretical resolution of CBVT scanners is veryhigh; numerous manufacturers report a minimumvoxel size of between 0.1 and 0.2 mm3.9,19,22 Allscans are isotropic, with typical voxel sizes rangingbetween 0.2 and 0.4 mm3.7,19 Smaller voxel sizesnecessitate longer scan times and increased radiationdose to the patient. Isotropic multislice CT dataacquired with a 16-slice unit typically produces voxelsizes of 0.5 mm3, although 0.35 mm3 is achievablewith modern machines.5,7 As multislice spiral-CTacquisition is significantly faster than CBVT, move-ment artefact is less of a problem, and this ensureshigher image sharpness. Generally, CBVT scansperformed with larger voxel sizes result in subjectivelybetter image quality due to decreased noise. Numerousauthors have reported that CBVT offers higherresolution and better image quality compared withCT, but these studies all utilized either radiographictest-phantoms with soft-tissue simulation or cadav-ers.14,17,21 There was no assessment of how patientmovement may affect resolution, sharpness and imagequality. At present no studies compare the quality ofpatient images obtained from CBVT with high-quality,low-dose multislice CT using either a 16- or 64-sliceCT unit.17

The geometric accuracy of multislice CT scans iswidely accepted,22 and recent research indicates thatCBVT images are also of sufficient accuracy to use forpre-implant assessments.11,12,23–27 It was found that theerror in measurements obtained from CBVT scans wasless than 0.5 mm.26 Volume rendered images obtainedfrom CT data were found to be superior to those fromCBVT, but the difference was minimal and the CBVTimages were still of acceptable quality.28 One study hassuggested that soft-tissues may decrease the accuracy ofCBVT scans, but the authors did not feel that this wassignificant.12 Cone beam volumetric tomography stillsuffers the same volume-averaging effect as CT, andthis most likely accounts for the slight errors inmeasurements.27 Theoretically, higher resolution scanspermit improved accuracy of measurements, but

Fig 10. Axial CT image of the maxilla showing implants (arrows) inthe 14 and 24 regions with the typical CT radiolucent halo artefact.



increased noise and patient movement12 may negateany potential gains.

Cone beam verses multislice CT

Given that CBVT and multislice CT have similarcapabilities it is prudent to examine the differencesbetween the two modalities. Acquisition time with a16-slice CT scanner is shorter than the fastest CBVTscan, and newer 64-slice CT units reduce the scan timeeven further. This effectively minimizes the risk ofpatient movement. The theoretical resolution of CBVTis higher than CT,20 but the difference may not be assignificant as once thought due to the impact of patientmovement resulting from the increased scan times.

Image quality has been the subject of much debate,and there is no clear answer at present. Cadaver studiesdemonstrate the capabilities of cone-beam technol-ogy,14,21,29 but patient images have been less impres-sive.30 The low exposure parameters of CBVT result inpoor soft-tissue contrast compared with CT,8,31 and theinability to alter the exposure parameters in mostmachines means that image quality suffers in largerpatients. Furthermore, CBVT suffers from the samebeam-hardening artefact that CT does; limiting theusefulness of the exam in patients with metallicrestorations, posts or surgical plates.17 It has recentlybeen reported that dental implants produce a similarartefact on CBVT images.32 Sample cross-sectionalimages from CT, CBVT and hypocycloidal tomography

are shown in Fig 11 highlighting comparative imagequality.

While it is recognized that multislice CT is a higher-dose examination than CBVT, reports indicate thatlow-dose CT protocols result in significantly lessexposure than previously thought, without compromis-ing image quality significantly.33 A consequence of thelesser dose of the CBVT scan is reduced contrast andtherefore image quality. Image noise is also significant,especially with larger patients or higher resolutionscans. It is important to note that while the radiationdose from a CBVT scan may be less than from low-doseCT, the dose is still significantly higher than other formsof dental radiographic examination.8

Magnetic resonance imaging (MRI)

MRI has become accepted as a powerful imaging toolin medicine. Using the magnetic properties of thehydrogen atom, MRI units are capable of producingimages of the human body. As the technology isdependent upon the presence of hydrogen atoms MRIis particularly suited to imaging of soft tissues. Usingvarious radiofrequency pulse sequences and relaxationtimes, images may be produced to better demonstrateanatomy or pathology in the body. As MRI relies on theuse of a strong magnetic field, MRI examinations arecontraindicated in patients with metal foreign bodiesin the eyes, ferromagnetic intracranial aneurysm clips,cardiac pacemakers, cochlear implants and patients in

(a)

(c)

(b)

Fig 11. Comparison between cross-sectional images of the mandible obtained using hypocycloidal tomography (a), multislice CT (b) and CBVT (c).



the first trimester of pregnancy. The presence of certainmetals such as amalgam and non-precious alloys willresult in considerable artefact on the images and oftenrender the examination useless.34,35 Pure titaniumimplants show no artefact with MRI, but if there areany impurities in the titanium there will be artefact.Other considerations include the significant cost to thepatient for MRI examinations and claustrophobia is areal concern as the examinations are generally per-formed with the patient in a very confining tunnel.

Most studies using MRI for pre-implant imaginghave focused on the ability of MRI units to locate theinferior dental canal.36–38 With MRI the inferior dentalcanal appears as a black void within the high-signalcancellous bone (Fig 12). If the inferior dental canal issurrounded by sclerotic bone, visualization of the canalis more difficult with MRI as the presence of scleroticbone results in a low bone marrow signal. The reverse istrue for CT, as the presence of sclerotic bone in themandibular body makes the inferior dental canal moreobvious. Magnetic resonance imaging has potentialfor pre-implant imaging due to the lack of ionizingradiation, but acquisition times can be as long as30 minutes and there is limited bone informationavailable.

Radiographic interpretation

The primary role of any pre-implant imaging system isto provide adequate information regarding bony mor-phology and the location of structures that should beavoided when placing implants in the jaws. To a lesserextent pre-implant imaging may also give some mean-ingful information on the quality of the bone. Super-imposed over the above considerations is the need tokeep exposure of the patient to ionizing radiation aslow as possible in adherence with the ALARA principle(as low as reasonably achievable).

Inferior dental canal

The inferior dental canal (IDC) carries the neurovas-cular components that supply and innervate the teethand bone of the mandible. In 1992, Gowgiel49 studiedthe position and arrangement of the IDC. He foundthat the neurovascular bundle remained intact fromthe mandibular foramen to the mental foramen.Approaching the mental foramen the IDC turnedsharply from the lingual plate buccally toward themental foramen. Anterior to the mental foramenthe neurovascular bundle was smaller and close tothe labial cortical plate. In 1997, Wadu et al.50 showedthat in all cases they examined the inferior alveolarnerve divided into its incisive and mental branches inthe molar region, well before the mental foramen wasreached. They also demonstrated that before dividinginto incisive and mental branches the inferior alveolarnerve gives off a branch to supply the molars and intwo cases they found separate branches to the secondpremolar.

As a general rule if the IDC is well demonstratedon the OPG, it will usually be well demonstrated onother imaging modalities. Conversely, if the IDC ispoorly demonstrated on the OPG and the OPG is ofreasonable quality, the canal will be difficult tolocalize using other modalities. The IDC is identifiableon radiographs as a narrow radiolucent ribbonbordered by radio-opaque lines. Wadu et al.50 foundthat in a reasonable number of cases the radio-opaqueborder was disrupted in certain areas and in somecases absent radiographically. The superior borderwas more prone to disruption than the inferiorborder. There are a number of software programmescurrently available that can be used to help locate theIDC. Figure 13 shows a cropped OPG where the IDCis difficult to localize and a reformatted CT image inthe sagittal plane of the same mandible, showing thelocation of the canal. If the mandible is osteoporoticor the cancellous bone has few or very thin trabec-ulae, sometimes the only clue to the location ofthe canal is scalloping of the cortical plate on theendosteal surface. When in close relation to thelingual cortical plate in particular, the IDC may liein a groove in the endosteal aspect of the corticalbone (Fig 14a). On some occasions the IDC will notappear as a circumscribed area of reduced density,but as a circumscribed area of increased density(Fig 14b).

Mental foramen

Typically the mental foramen is located in the buccalcortex of the mandible in the premolar region. Theinferior dental canal usually rises quite sharply to theforamen. In cases where the patient has been edentulous

(a) (b)

Fig 12. Magnetic resonance imaging demonstrating the left inferiordental canal as a low signal line (arrow heads) surrounded by highsignal cancellous bone in the ramus (a) and as a small black void

(arrow) in a cross-section of the left mandibular angle (b).



for a considerable period of time and the ridge hasatrophied, the inferior dental canal may run very closeto the crest of the ridge and the mental foramen mayopen onto the crest (Fig 15). The IDC may extendanteriorly past the mental foramen and then loop backto the foramen. The extent of this looping of the IDC isvery variable and not always visible on conventionalradiographs.51,52

Incisive branch of the inferior dental canal

The anterior region of the mandible is generallyconsidered to be a relatively safe area for implantsurgery due to little chance of significant damage toneurovascular structures. Previous studies however,have reported life-threatening complications caused byprofuse bleeding after implant placement between themental foramina.53–55 A number of other compli-cations have been reported following placement ofimplants in the inter-mental region and some of thesehave been attributed to damage of the incisive canal(IC).56,57 Kohavi and Bar-Ziv56 describe a case wherean implant was placed through a large lumen ICresulting in pain.

The incisive branch of the IDC extends anteriorlyfrom the mental foramen to supply the lower anteriorteeth. The incisive branch is usually poorly demon-strated on conventional radiographs.51,52 Generally, theIC extends anteriorly and inferiorly from the mentalforamen. The IC has been shown to be located onaverage 9.7 mm (SD 1.8 mm) from the lower border ofthe mandible and continues toward the incisor regionin a slightly downward direction with a mean distanceto the lower border of 7.2 mm (SD 2.1 mm).58 Thediameter of the IC has been found to range from0.48 mm to 2.90 mm.52 As the incisive canal is ananterior extension of the IDC, it should be consideredto contain the same neurovascular elements.58

(a)

(b)

Fig 14. (a) Reformatted cross-sectional CT images demonstratinggrooving of the endosteal surface of the lingual cortical plate (arrows)as a guide to the location of the inferior dental canal. (b) The inferiordental canal may appear as a small circular opacity on reformatted

cross-sectional images of the mandible (arrow heads).

(a)40 45

(b)

Fig 15. Reformatted panoramic (a) and cross-sectional (b) CT imagesof an edentulous and atrophic mandible showing the inferior dental

canal very close to the crest of the ridge (arrows) and the mentalforamen at the crest (arrow head).

(a)

(b)

Fig 13. (a) Cropped panoramic radiograph with poor visualization ofthe left inferior dental canal. (b) Reformatted, corrected sagittal CTimage demonstrating the inferior dental canal (arrow heads) not seen

on the panoramic radiograph shown at (a).



Genial foramina

The anatomical structures worthy of note between themental foramina of the mandible include the midlineforamina. These foramina may be denoted as thesuperior and inferior genial foramina with their asso-ciated canals. The typical appearance of the genialforamina on reformatted CT images is shown in Fig 16.Liang et al.59 found that 98 per cent of the 50mandibles assessed had at least one genial foramen.Only one mandible lacked a genial foramen and in onemandible there were three foramina. In those cases withonly one foramen, the superior genial foramen was

much more common (72 to 28 per cent). The canals hada mean length of 6.5 mm (SD 2.4 mm). In most casesthe canal has a downward course toward the labialplate, but in a reasonable number of cases the canal wasdirected upwards toward the labial side. The genialforamina and canals are not always shown on refor-matted cross-sectional CT images.60 The superior andinferior genial foramina have been shown to containneurovascular elements and this has obvious implica-tions for pre-operative planning of surgical proceduresin the anterior mandible.59,60

Submandibular depression

Below the mylohyoid ridge on the lingual aspect ofthe mandibular body is a concavity known as thesubmandibular fossa or depression. There is consider-able variation in length, height and depth of thesubmandibular fossae. The submandibular fossae arewell demonstrated using tomography, CT and CBVT.

Incisive foramen and canal

The size and morphology of the incisive canal and theincisive foramen is extremely variable (Fig 17).61 Theincisive foramen has been described as a funnel-shapedhole between the two halves of the maxilla palatal tothe upper central incisors.62 The incisive canal containsthe nasopalatine nerve and the descending palatineartery. It has been shown that the descending palatine

(a) (b)

Fig 16. Reformatted cross-sectional CT images demonstrating thesuperior (a) and inferior (b) genial foramina (arrows) and canals

(arrow heads).

Fig 17. Cropped axial CT images demonstrating some of the variations found in the morphology of the incisive canals of the maxilla (arrows) incross-section.



artery lies in an anterior canal and there are branchessprouting from the left and right sides of the canal thatcontain connective tissue and blood vessels.62

Maxillary sinuses

The maxillary sinuses are the first of the paranasalsinuses to form and they usually develop symmetricallywith only minor variations. Unilateral hypoplasia ofthe maxillary sinuses has been reported to occur in1.7 per cent of people and bilateral hypoplasia in 7.2per cent of people.63 The posterior superior alveolarnerve enters the maxillary sinus through the posteriorwall, then runs forward and downwards in a smallcanal to supply the molars. Usually the maxillarysinuses do not extend anteriorly beyond the apex ofthe upper canine, but the maxillary sinus may on occa-sion extend almost to the midline of the maxilla. Themaxillary sinus is visualized as an air-filled space, asthe healthy mucosal lining is not visible on radiographs.The most common pathology noted in the maxillarysinus is thickening of the mucosal lining of the sinus. Thismucosal thickening may sometimes take the form ofpolypoidal thickening or circumferential mucosal thick-ening. Often sinus changes on the floor of the maxillary

(a) (b)

(d)(c)

Fig 18. (a) Reformatted CT image showing mucosal thickening on the floor of the right maxillary sinus. (b) Reformatted CBVT image showingconsiderable mucous in the right maxillary sinus (coronal plane). (c) Reformatted CT image showing circumferential mucosal thickening in both

maxillary sinuses (axial plane). (d) Reformatted CT image showing fluid on the floor of the right maxillary sinus.

Fig 19. Reformatted cross-sectional images of the mandibular bodyshowing marked resorption of the alveolar ridge resulting in a thin

plate of bone (arrows) on the lingual aspect of the ridge.



sinus develop in response to adjacent pathology such asperiodontal disease or pulpal pathology. The radio-graphs of choice for plain film evaluation of the maxillarysinuses are the Waters view, Caldwell view and lateralsinus view. Sinus pathology is best demonstrated on CTimages and some examples are shown in Fig 18.

Ridge form

Information obtained from intra-oral radiographs androtational panoramic radiographs typically give verylittle information about the available bone width asthey are two-dimensional. The anterior alveolar ridgemay have excellent bone height but be extremely thin inthe labio-lingual dimension. Another common findingin the edentulous arch is marked flattening of the ridgein the lower premolar ⁄ molar region and the formationof a thin plate of bone on the lingual aspect (Fig 19).It is also not uncommon to find a concavity at the crestof the ridge in the lower molar regions that can bepredicted from an OPG, but is best demonstrated withthree-dimensional imaging (Fig 20).

CONCLUSIONS

The decision on what pre-implant imaging is appropri-ate for each case must be considered carefully due to theradiation involved and the cost of each examination.Although in the opinion of the authors, multislice CT is

the gold standard, this type of examination cannot bejustified for every implant case. Cone beam volumetrictomography has great potential with regard to pre-implant imaging. In deciding on what imaging isappropriate, the clinician should not be swayed entirelyby the dose of radiation the patient will receive. There isvery little to be gained by opting for pre-implantimaging where the dose is very low, if the end result iscompromised because of a lack of reliable information.The risk-to-benefit ratio should be determined on anindividual basis so as to maximize success.

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Address for correspondence:Dr P Monsour

X-Ray DepartmentSchool of Dentistry

The University of Queensland200 Turbot Street

Brisbane, Queensland 4000Email: [email protected]



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