RADIOGRAPHIC DETERMINANTSOF IMPLANT PERFORMANCE

MICHAEL S. REDDY*I-CHUNG WANG

Department of PeriodonticsSchool of DentistryUniversity of Alabama at BirminghamUAB Station 341919 7th Avenue South, Room 412Birmingham, Alabama 35294-0007, USA* Corresponding author

Adv Dent Res 13:136-145, June, 1999

AbstractThis paper reviews and compares the strengths andweaknesses of radiographic techniques including periapical,occlusal, panoramic, direct digital, motion tomography, andcomputed tomography. Practical considerations for eachmethod, including availability and accessibility, are discussed.To date, digital subtraction radiography is the most versatileand sensitive method for measuring boss loss. It can detect bothbone height and bone mass changes on root-form or blade-formdental implants. Criteria for implant success have changedsubstantially over the past two decades. In clinical trials ofdental implants, the outcomes require certain radiographicanalyses to address the hypothesis or clinical questionadequately. Radiographic methods best suited to the objectiveassessment of implant performance and hypothesis werereviewed.

Key words: Dental implants, radiographs, digital imaging,implant assessment.

Presented at the 15th International Conference on OralBiology (ICOB), "Oral Biology and Dental Implants ", held inBaveno, Italy, June 28-July 1, 1998, sponsored by theInternational Association for Dental Research and supportedby Unilever Dental Research

The assessment of bone support in endosseous dentalimplants is fundamental to the clinical utility ofimplants for restoration of function. Radiographs area critical tool for the assessment of bony architecture,and radiographs are used at each of three phases of implanttreatment, evaluation, and maintenance. The first phase is pre-surgical assessment of the bone at potential implant recipientsites during the treatment planning phase of therapy. Thesecond common use is intrasurgical assessment of theproximity of adjacent structures and parallelism of osteotomysites being prepared. The final use of radiographs is in long-term assessment of the success or failure of implant therapy.This paper focuses on the radiographic methods to evaluate thethird phase and briefly addresses the other methods. In implantresearch, the evaluation of longitudinal performance of theimplant is of primary importance in transferring newdevelopments to the practicing clinician.

DIAGNOSTIC METHODS READILYAVAILABLE

The radiographic methods available are considered from thesimplest (use of intra-oral films) to the more complexutilization (computed tomography). The radiographic methodscommonly available in longitudinal implant studies arepresented in Table 1. A vast amount of radiographic imagingassociated with dental implants is used for diagnosis of theimplant recipient site. The radiographic assessment of therecipient site ideally indicates the quantity of bone in threedimensions, the location of anatomical structures (such as themandibular canal and maxillary sinuses), and the quality ofbone available. Although an enormous body of literature (over1000 papers since 1990) addresses the pre-surgical planning ofdental implants, no technique truly satisfies this ideal goal(Fritz, 1996). In an attempt to achieve this ideal radiographicassessment, clinicians often combine multiple radiographicviews or utilize several imaging techniques.

Intra-oral films are utilized in pre-surgical planning ofimplant treatment, intra-operatively, and for longitudinalassessment. Occlusal films are used as a method of assessingthe buccal to lingual width of the edentulous ridge area duringthe pre-surgical planning phase. The use of occlusal films islimited to the anterior mandible, because the superimposition ofother anatomical structures tends to obscure the ridge in theposterior segment of the mandible. In addition, distortion ofmaxillary occlusal films is common. Periapical radiographs areused to assess limited areas or individual implant sites. Theperiapical films have minimal distortion if well-angulated andare suitable for the evaluation of bone height. A limitation ofperiapicals is that the area imaged is small, and adjacentanatomical structures may not be visible on the film (Fig. 1).

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TABLE 1

RADIOGRAPHIC METHODS COMMONLY AVAILABLE IN LONGITUDINAL STUDIES OF DENTAL IMPLANTS

Radiographic Method Application Imaging Plane Limitations

Periapical(intra-oral film or direct digital)

Occlusal (intra-oral film)

Direct digital (intra-oral film)

Panoramic (extra-oral film)

Motion tomography(extra-oral film)

Computed tomography(extra-oral digital)

(1) Pre-surgical single site(2) Intrasurgical(3) Longitudinal assessment

(1) Pre-surgical

(1) Pre-surgical single site(2) Intrasurgical(3) Longitudinal assessment

(1) Pre-surgical multiple sites(2) Longitudinal assessment

(1) Pre-surgical limited sites(2) Longitudinal assessment

(1) Pre-surgical multiple sites

Buccal-lingual

Occlusal-apical

Buccal-lingual

Buccal-lingual

Mesial-distal

Buccal-lingual,mesial distal, axial,three-dimensional

Limited view of adjacent anatomyTwo-dimensional viewGeometry commonly non-standardized

Commonly distorted

Limited detector sizeInherent distortion

Geometric distortion and magnificationerrors. Decreased resolution

Limited availabilityRepositioning is difficult

Relative costAccess to CT servicesMetal artifact

The periapical image in Fig. 1 has minimal geometricdistortion, as illustrated by the spherical shape of the 5-mm ballbearings; however, the mandibular canal and the mentalforamen cannot be visualized. The use of a film such as thisdoes not provide adequate information for the planning of thesurgical procedure. Periapical films are particularly well-suitedfor the longitudinal assessment of implants. The lack ofdistortion and ability to standardize projection geometryenables them to be used in conjunction with a variety of linear,digital, and subtraction radiography techniques (Jeffcoat, 1992).

Direct digital periapicals utilize an intra-oral detector tocapture a radiographic image of the diagnostic area of interest(Jeffcoat, 1992; Reddy et ai, 1992a; Welander et al, 1993).The direct digital periapical image is used in a fashionanalogous to that of the film-based periapical for both pre-surgical planning and longitudinal assessment. The limitation ofthe diagnostic image area may even be more pronounced withthe digital image. This is largely due to the limited size of theintra-oral detectors which are currently available. Theresolution of the digital image is less than that available withconventional intra-oral film but is adequate for diagnosis andlongitudinal assessment of bone loss (Wenzel, 1994). There areat least two major categories of direct digital radiographymachines. The first uses a solid-state detector with or without alight pipe to amplify the signal. These detectors result in anearly instantaneous display of the radiograph on a monitorwith no chemical processing of conventional film. Signalamplification and detector sensitivity may result in majorreductions in ray dose compared with conventional non-screenintra-oral film. The second type of detector is a re-usable solid-state detector that is placed in a reading device, similar in

concept to an optical scanner. Following a brief (approximatelyone-minute) delay, the image appears on a monitor. Thetechnology of both the hardware and software associated withdirect digital imaging is rapidly evolving. Both the gray-levelresolution and the image accuracy are rapidly improving. Inaddition, the thickness of the detector, which was a previousproblem with patient acceptance, has decreased, making the

Fig. IA periapical film demonstrating good geometricprojection as illustrated by the spherical appearance of the 5-mm reference ball bearings. Some of the shortcomings of intra-oral periapical films are also apparent. The mental foramenand mandibular canal are not visible on the film, making pre-surgical planning difficult.

138 REDDY&WANG ADV DENT RES JUNE 1999

positioning of the detector much more comfortable for thepatient. Direct digital radiography offers several advantages forintra-operative use. The solid-state detector replaces film, sothere is no delay while the film is chemically processed. Thecontrast and brightness of the image may be adjustedretrospectively on the monitor so that different structures can bevisualized. Since the image is digital, it can be stored on a diskto facilitate measurements of bone loss along the root surface.This eliminates the step of indirectly digitizing a film through acamera or scanner.

One current limitation of digital systems concerns thedocumentation and archiving of images for medico-legalpurposes. Where film has been used as a permanent record ofpre-treatment conditions and treatment delivered, digital imagesmust now be archived. The more widespread use of digitalsystems and the expansion of disk storage space and back-upsystems will most likely alleviate this potential shortcoming inthe near future.

Panoramic radiography is one of the most commonlyutilized radiographic techniques in dental implantology.Panoramic images provide a global assessment for multipleimplant placement and are commonly used for initial treatmentplanning or screening. Clinical studies have utilized panoramicfilms either to score the presence or absence of bone loss or toquantify the amount of bone loss (Branemark et al., 1977; Adelletal., 1981; Naert, 1991; Quirynen etal., 1991; Mericske-Sternet al., 1994; Spiekermann et al., 1995). However, despite itswidespread use, panoramic imaging has a number of limitationsthat decrease its usefulness as a method for the longitudinalassessment of dental implants. Because film-based panoramicimages utilize screens, they have decreased resolution whencompared with intra-oral films, resulting in a decreased abilityto detect small changes in bone support along the implants(Backstrom et al., 1989). All panoramic films are magnifiedapproximately 30% when the patient is ideally positioned(Glass, 1991). When the patient is positioned as little as 5 mmfrom ideal, the magnification may range from 10 to 61%(Reddy et al., 1994). Additionally, if the patient is rotatedslightly, the magnification may differ from one side of the jaw

to the other (Fig. 2). Furthermore, the implant bone surface maybe out of the curved plane of the tomogram being created by thepanoramic machine, resulting in an inaccurate image of thebone-to-implant interface. Fig. 2 illustrates what appears to be apanoramic film of good diagnostic value and consistentmagnification. The panoramic film shown was made with 5-mm metal ball bearings attached to an acrylic vacuum-formedstent made from a diagnostic cast. The resultant image, whichappeared initially to have uniform magnification, is actuallyunevenly distorted from right to left. The second ball bearingfrom the right appears as a sphere, whereas the left sidedemonstrates geometric distortion, illustrated by the oblongappearance of the ball bearings. The errors of magnification andgeometric distortion become important in clinical researchwhich seeks to quantitate, from panoramic films, the amount ofbone loss over time. Panoramic films will most likely continueto be used to evaluate implants radiographically over timebecause of their availability, ease of use, and patientacceptance. The continued development of direct digitalpanoramic images may decrease some of the limitations foundwith the film-based machines. In longitudinal clinical trials,care should be taken to use a method that corrects or controlsfor geometric distortion errors.

Film-based motion tomography has been suggested as acost-effective method for pre-surgical evaluation of aprospective implant site (Miles and Van Dis, 1993). The imagesobtained are cross-sectional views that are useful for theevaluation of bone width and bone height from one image (Fig.3). In addition, the cross-sectional images are useful fordiagnosing bony undercuts not readily apparent on other

Fig. 2A second panoramic image with four 5-mm referenceball bearings in place. The image which initially appeareduniform in magnification is actually distorted, with non-uniformmagnification from right to left.

Fig. 3A cross-sectional view of the mandible obtained with afilm-based motion tomography machine. The image obtainedallows for the assessment of the available bone in twodimensions.

V0L.13 RADIOGRAPHICIMPIANT PERFORMANCE 139

radiographic views (Kassebaum et al., 1990). Film-basedtomography has not been utilized in the longitudinal assessmentof dental implants over time. The lack of use is largely due tothe limited availability of the machines themselves and thetechnical ability required to make images and use them tomeasure bone loss accurately. Linear or multidirectionaltomography machines are readily available at most university-based medical centers. Tomography machines are less likely tobe available in private dental practices due to the cost of theequipment. The lack of availability of the tomographicequipment may limit its usefulness in controlled clinical trialsas opposed to field trials. With the production of relatively low-cost tomography machines, their widespread use is likely toincrease.

Tomography machines which are intended for in-office orimaging laboratory (hospital) use are now available. Two basicprinciples should be considered in the selection of atomography machine that will produce the highest possibleimage quality. Both slice thickness and the amount of out-of-plane blurring contribute to image "readability". The longer thepath the tomography machine traverses, the thinner the slice inthe resultant image. Out-of-plane blurring is inherent to motion-based tomography. The more complex the path of the radiationsource, the less out-of-plane blurring. Thus, linear tomographywill produce more blurring than a machine using ahypocycloidal path. Furthermore, a machine designed to takeup little space in the office and which has a small tomographicpath will have a thick resultant image "slice thickness".

The repositioning of the patient over time to obtain areproducible image suitable for clinical research is technicallydifficult with most standard techniques (Poon et al., 1992).Tomography machines that use a cephalostat such as the QuintSectograph (Denar Corp., Anaheim, CA, USA) may beadvantageous in longitudinal studies (Miles and Van Dis,

1993). Without replicate cross-sectional images at the sameplane on the implant, quantitative assessment of the bonesupport over time is likely to be highly inaccurate. Anadditional inherent limitation of film-based tomography is thelimited experience that most clinical researchers have with theinterpretation of tomograms. This may be overcome if theresulting images are digitized and electronically enhanced toimprove the contrast (Fig. 4). The use of tomograms inlongitudinal studies allows for the assessment of facial andlingual bone support that is not evaluated by intra-oral andpanoramic radiographic techniques (Geurs, 1995). Anadditional limitation of tomograms occurs when multipleimplants are close to each other: The implants tend to becomesuperimposed, rendering interpretation nearly impossible.

For pre-surgical assessment, the most accurate technique forsurgical site diagnosis is computed tomography (CT)(Petrikowski et al, 1989; Miller et al., 1990; Kassebaum et al,1992; Reddy et al., 1994). The CT images obtained haveminimal geometric distortion and are available in two-dimensional panoramic and cross-sectional formats as well asthree-dimensional images (Figs. 5, 6). Slice thickness in CTimaging is a function of the scanning protocol. Whenoverlapping slice protocols are utilized, at the expense ofadditional radiation doses, thinner slices with greater spacialresolution result. For longitudinal analysis of implant bonesupport, the virtue of CT imaging is greatly limited, due to astreaking artifact that occurs when metal is encountered duringthe CT imaging (Fig. 7). This artifact is due to the high densityof gold and silver alloys compared with that of the adjacentbone (Curry et al., 1990). In one study of 138 CT images fordental implant treatment planning, 34% of the images haddistortion due to metal restoration of the teeth (Mayfield-Donahoo et al, 1994). When dental implants are scanned after

Fig. 4The application of image enhancement to aid in theinterpretation of cross-sectional images.

Fig. 5Panoramic and axial cross-sectional images obtainedfrom computed tomography.

140 REDDY & WANG ADV DEW RES JUNE 1999

Fig. 6A three-dimensional image of the mandible reconstructedfrom a computed tomography image.

restoration, the high density of titanium and superstructuremetal renders the resulting images of little value in theassessment of longitudinal performance. Therefore, CTimaging is not a practical technique for use in routine follow-upclinical research of dental implants.

HISTORICAL RADIOGRAPHIC CRITERIAFOR IMPLANT PERFORMANCE

Complications in dental implantology tend to occur due tofailures in the prosthetic superstructure or a loss of supportingbone integration on the implant body itself. The assessment of

Fig. 7A streaking artifact on a three-dimensional CT imagedue to the presence of high-density metal restoration of the teeth.

implant performance generally relies on the detection ofmobility, the clinical signs of gingival inflammation,periodontal attachment loss and pocket formation, andradiographic bone loss. Radiographs are important tools in theevaluation and early diagnosis of implant-associated pathology.The use of radiographs allows for a quantitative assessment ofbone loss along the implant surface.

Historically, different criteria have been utilized to assessthe success of implants based on radiographic appearance ormeasurements (Table 2). The recommendations of the 1978Harvard Consensus Conference on Dental Implants utilized thefollowing categorical radiographic criteria: (1) no radiolucencyand (2) bone loss not greater than 1/3 of the implant length(Schnitman and Shulman, 1979). This was an initial attempt atobjective assessment of implant success or failure based onradiographs. Later, a publication from the Branemark groupestablished a success criterion of 0.2 mm of bone loss annuallyafter the first year of service (Albrektsson et al., 1986; Smithand Zarb, 1989). The problem with the 0.2-mm criterion is thatit was not an annual measurement but a retrospectivecalculation such as 2-mm loss over a ten-year period. Theauthors did not utilize a method that could actually measure0.2 mm or less. Further, when the observed bone loss isdivided over 10 years, it is assumed that it is a continuouslinear process. The American Dental Association (ADA)established a radiographic criterion of 2 mm of vertical boneloss at 5 years, measured by a technique with a resolution andvalidity that exceed the threshold for clinical success (ADA,1996). The ADA guidelines do not specifically exclude boneloss that occurs in the first year of service, but ratherincorporate all bone loss over a five-year period.

RADIOGRAPHIC ANALYSIS OF IMPLANTBONE LOSS IN CLINICAL TRIALS

One of the major problems of using radiographic analysis toquantitate bone loss from radiographs is geometric distortiondue to misangulation of the film or misangulation of the x-raybeam. The first misangulation error may occur when the filmangulation is changed at different radiographic exams while

VOL. 13 RADIOGRAPHIC IMPLANT PERFORMANCE 141

the x-ray source is held constant. The distortion produced bythis error may be retrospectively corrected with the aid of acomputer matrix transformation algorithm (Jeffcoat et aL,1984; Webber et ai, 1984). The second misangulation error,an x-ray direction error, occurs when the radiographic sourceis moved and the implant and film are held in consistentgeometry. The first and second types of misangulation errorsoccur together in most conventional clinical radiographs madeto assess dental implants.

The errors of film position and x-ray direction as sources ofgeometric distortion may be minimized by the use ofstandardized radiographs in a clinical trial. Standardized filmsare made by controlling the projection geometry of the imagecreated. An occlusal index or stent is commonly used to registerthe implant or prosthesis position. The stent also incorporates afilm holder and is attached or electronically coupled to theradiographic cone (Hausmann etaL, 1985,1992). Alternatively,a cephalostat or video feedback system may be utilized toreposition the patient's head in combination with a long film-to-object distance (Jeffcoat et ai, 1987; Reddy et a/., 1991).

Minor errors that occur in angulation may be corrected withthe application of a matrix transformation algorithm. Errorsdue to the tilt of film or detector may be retrospectivelycorrected by "warping" the second image onto the first. Theradiographic assessment of implants is ideally suited to theapplication of warping algorithms, because the implants are ofpre-defined dimension and known anatomy. The identificationof clear landmarks on implant images is easily accomplishedbecause of the known dimension. Landmarks are identified onboth images before the transformation to be applied. Thismethod allows for corrections in geometry without the strictrequirement of standardization of the radiographs. Thelimitation of these techniques is that they allow for warpingprojection in only two dimensions. Therefore, as the film isbent in the patient's mouth in one or both radiographic

TABLE 2

RADIOGRAPHIC CRITERIA FOR IMPLANT SUCCESS

Radiographic Criteria Criteria Established

1/3 implant length

0.2 mm annuallyafter first year

1.4 mm over 3 years2.0 mm over 5 years

NIH Consensus Conference

Branemark retrospective studies

ADA Council on Dental Materials

examinations to be compared, the image will be distorted inthree dimensions. A full three-dimensional distortioncorrection has been achieved with tomosynthesis (Horton etal., 1996), and the use of software application on invariantstructures has also been reported (Ostuni et aL, 1993). To date,we are not aware of the application of either of thesetechniques to large-scale clinical implant trials. The utilizationof direct digital techniques may have an advantage in thisregard. Since the image detector is rigid and cannot bend,distortions of the implant should be limited to two dimensions.

Approaches to evaluating dental implants in clinical studiesare outlined in Table 3. One method for assessing implantperformance by high-quality radiographs is bone heightanalysis of the amount of bone loss relative to the implantlength (Fig. 8). There is a major difference in using thisapproach in studies of dental implants compared with studiesmeasuring bone height around teeth. The length of the naturaltooth root is unknown, whereas the length of the implant isknown. If the bone height on the implant is expressed as apercentage of the implant length, it may be easily convertedinto a millimeter measurement of the bone loss present. In this

TABLE 3

APPROACHES TO RADIOGRAPHIC EVALUATION OF DENTAL IMPLANTS IN CLINICAL STUDIES

MethodAbility to

Requirements for Input Radiograph Detect Change Comments

(1) Measurement of %,mm, bone loss

High-quality clinical radiographs Moderate Uses implant as reference scale,ideally should be digitized andmeasured under software control

(2) Measurement of boneloss by counting threads

(3) Grid overlay to detectmesial-distal and apical-coronal bone loss

(4) Digital subtractionradiography

High-quality clinical radiographs

High-quality clinical radiographs

Standardized image

Low

Requiresspecializedsoftware

High, can detect changestoo small to see

Low tech.Very easy

Time-consuming,requires specialized software

Requires specialized software

142 REDDY & WANG ADV DENT RES JUNE 1999

Fig. 8The application of bone height analysis on an implantas a proportion of overall implant length. The implant has alength of 10 mm, and the bone loss from the top of the implantis 4.4 mm.

Fig. 9A digital subtraction radiography image illustratingbone loss around a failing image. The initial radiograph issubtracted from the subsequent image, and the resultantimage is neutral gray, where no change has taken place.Areas of bone loss appear as a darker gray image.

way, the implant acts as its own ruler to compensate partiallyfor geometric foreshortening or elongation distortion of theradiographic image during the measurement. The use of boneheight measurements has an advantage in that it is very simpleand does not require standardized radiographs. The image ofthe implant in Fig. 8 indicates the linear loss of bone along theimplant. The resulting data obtained may be expressed as apercentage of overall implant length or in mm of bone loss ifone knows the length of the implant. The use of bone heightmeasurements is a linear, one-dimensional measure that islimited to the analysis of bone loss that can be visualized on theradiograph. Subtle changes may be overlooked due to a lack of

contrast between gray levels. The contrast of slight areas ofbone loss may be enhanced by the application of digitalimaging techniques.

A low-technology approach utilized in longitudinal studieshas been simply to count the number of threads exposed bybone loss. The thread-counting approach is limited in resolutionto the spacing of the threads. For example, if the threads arespaced at 0.6 mm, and measurement error is estimated at twicethe resolution of the technique, only a 1.2-mm change in boneloss can be detected. In addition, the comparison of cylindricalimplants and threaded implants with non-threaded collarsrepresents additional complications and precludes the simple

TABLE 4

RADIOGRAPHIC METHODS FOR DIFFERENT IMPLANT STUDY DESIGNS

Type of Study/Clinical Question

RequirementsDesign to Detect Change Method Comments

Following an implantover time, natural history

Comparing two implanttypes, superiority

Single arm, Dependent on hypothesisno blinding or criteria to be satisfied

Parallel arms Requires high sensitivitybecause most implantsare successful

(1) Measurement of mm(4) Digital subtraction(2) Thread counting

(1) Measurement of mm(4) Digital subtraction

Equivalency of two systems Parallel arms Requires high sensitivity, and (4) Digital subtraction,or types of implants a large (n) number of implants (1) Measurement of mm

to have sufficient power

Comparison of implant types Block design Requires high sensitivitywithin the same patient within subjects

(4) Digital subtraction(1) Measurement of mm

Most frequently usedHigh resolutionInsufficient resolutionto satisfy

"No significantdifference" is not thesame as "equivalent"

VOL. 13 RADIOGRAPHIC IMPLANT PERFORMANCE 143

Fig. 10A threshold has been applied to the binary image toobtain area of change and added back to the original radiograph.

use of this methodology.A variation of bone height analysis is the use of a grid to

assess bone loss in two dimensions (Reddy et al., 1992b). Thegrid analysis is primarily useful for analyzing blade implants inwhich wide saucer-like bone loss tends to occur. A grid overlayis superimposed on the digital image of the blade and is basedon the known dimensions of the blade. Multiple measurementsare made along the grid lines from the radiolucency to theimplant surface. The shortcomings of the grid method are that itis very time-consuming and, as with bone heightmeasurements, measures only visual change.

One of the most versatile methods for measuringradiographic bone loss on both root-form and blade implants isthe use of digital subtraction radiography. Subtractionradiography was introduced to dentistry in the 1980s (Webberet al., 1982; Grondahl et al., 1983; Hausmann et al., 1985;Jeffcoat et al., 1987). Subtraction radiography is used tocompare two standardized radiographs taken at sequentialexamination visits. All structures that have not changedbetween examinations, such as the implant, are subtracted. Theresultant computer image shows areas of bone change against aneutral gray background (Fig. 9). Areas of bone loss areconventionally shown in dark shades of gray, whereas areas of

bone gain are shown in light shades of gray. Alternatively,areas of bone loss or gain may be displayed in contrastingcolors (Reddy et al., 1991). From the subtraction image,changes in bone height may be measured or two-dimensionalareas of bone change may be calculated. In order to obtain anestimate of bone loss in three dimensions, investigators havecarried out a quantitative analysis of the gray scale changes(Ruttimann and Webber, 1987; Bragger, 1988). The use of anarea measurement from the subtraction image and the grayscale difference at the bone change has been applied to implantperformance analysis (Jeffcoat, 1992). In brief, a referencewedge is used to convert the gray scale levels and areacalculation to a bone loss or bone gain mass. The referencewedge is incorporated into the first radiograph and not thesecond, so that the resultant subtraction image has a negativeimage of the wedge along with an image of the bone loss. Theimage of the wedge is used to determine the thickness of thewedge that corresponds to the same gray level change as thebone loss. The mass of the lesion is then calculated bymultiplying area x thickness x aluminum density x aluminum-to-bone-density conversion factor.

As a final step, the software is used to improve thevisualization of the area of change (Fig. 10). A threshold hasbeen applied to the black-and-white (binary) image to obtain anarea of change and added back to the original radiograph. Thismethod has been validated by multicenter clinical trials throughthe calculation of the mass of cortical bone chips in skulls(Jeffcoat et al., 1992, 1996). The correlation between thecalculated mass of the bone chips and the actual mass wasfound to be excellent (r2 > 0.90).

Clinical studies in dental implants have been designed toanswer different questions related to the performance of theimplants over time (Table 4). The design of the study requiresthat different radiographic analyses be utilized to address thehypothesis or clinical question adequately. A common studydesign is to follow a specific type of implant over time. In astudy of the natural history of the implant, several of theradiographic methods described in Table 3 may be used. Themost frequently used method is to measure millimeters of boneloss along the implant surface over an interval of years. Thistechnique uses digital measurement of the bone height and thelength of the implant to help compensate for elongation andforeshortening (Verdonschot et al., 1991). Digital subtractionradiography could also be used to follow an implantlongitudinally. Since subtraction techniques require carefullystandardized radiographs, the decision to use digitalsubtraction radiography would have to be made prospectively(Bragger, 1988). Historically, investigators have simplycounted the threads on implants of known geometry to assessbone loss in this type of longitudinal study (Albrektsson et al.,1986). However, counting threads may provide insufficientresolution to address the question of implant performance in acontemporary clinical study.

If the study design is to compare two implant types in anattempt to test for superiority, a high-resolution method needsto be utilized (Listgarten, 1992). The high-resolutionradiographic techniques become necessary in this instance,because bone loss around dental implants tends to be small for

144 REDDY&WANG ADV DENT RES JUNE 1999

the most part (Imrey, 1992). The fact that most implants aresuccessful makes determining the differences between implantseven more difficult. The methods useful for superioritycomparisons would be digital measurement of mm of bone lossor digital subtraction radiography. A study to assess eqivalencyof two implants may be even more challenging than superioritytesting. The equivalency study needs the highest resolution thatcan be achieved, and, in addition, a sufficient number ofsubjects must be utilized (Imrey and Chilton, 1992). If aninsufficient number of implants is used or the resolution of theradiographic method is too low, no significant differencebetween two implants will be observed, even if a differenceactually exists.

Comparing implants of different types, surface coatings, ordesign within the same subject presents an additional indicationthat the highest-possible resolution should be used (McKinneyet aL, 1988). Again, since the bone loss at any modern implantis not likely to be great, a high-resolution technique and asubject population with sufficient power will be necessary. Theposition of the implants within the arch will also need to bedetermined with a random block design to ensure that the sameimplant does not always get the most favorable position in thearch.

CONCLUSION

Radiographic methods are essential for assessing bony supportin endosseous dental implants. However, each technique has itsown advantages and drawbacks. Different criteria have beenutilized to determine the success or failure of implantperformance based on radiographic appearance ormeasurements. In the design of clinical trials for dentalimplants, standardized radiographs should be utilized and thehighest-resolution technique available must be considered. Atpresent, digital subtraction radiography is an accurate andlegitimate technique for the detection of minor bony changearound dental implants.

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