The aim of this study was to compare two methods of measuring mandibular asymmetry. The first method uses mirroring of the mandible in the midsagittal plane; the second uses mirroring of the mandible and registration on the cranial base

AlHadidi, A

Cevidanes, LHS

Ludlow, J

Mol, A

Styner, M

PubMed

Carolina Digital Repository

RESEARCHComparison of two methods for quantitative assessment ofmandibular asymmetry using cone beam computed tomographyimage volumesA AlHadidi*, LHS Cevidanes, A Mol, J Ludlow and M StynerUniversity of North Carolina, Chapel Hill, NC, USAObjectives: The aim of this study was to compare two methods of measuring mandibularasymmetry. The first method uses mirroring of the mandible in the midsagittal plane; thesecond uses mirroring of the mandible and registration on the cranial base.Methods: Surface models were constructed from cone beam CT (CBCT) scans of 50patients with asymmetry. For the first approach, a midsagittal plane was defined for eachpatient as the plane passing through nasion, anterior nasal spine and basion. Mirrors forboth halves of the mandible were created. The second approach consisted of mirroring theimage volume by flipping the left and right sides and then registering the mirrored image ontothe cranial base using a mutual information maximization method. Surface distances betweenhemimandibles and mirrors were calculated for nine regions.Results: There was no statistically significant difference between the mean surface distancemeasurements obtained with the two approaches and when comparing both halves in most areas.Conclusion: Both mirroring techniques provided similar quantification of mandibularasymmetry in this cohort.Dentomaxillofacial Radiology (2011) 40, 351–357. doi: 10.1259/dmfr/13993523Keywords: cone beam computed tomography; mandible; asymmetry; surface models; mirroringIntroductionFacial asymmetry is common and sometimes poses achallenge in craniofacial diagnosis and treatment plan-ning.1,2 It is aetiologically and pathologically hetero-geneous and may be localized or generalized. This widevariability in the aetiology and in the presentation ofthe disease necessitates that the management of thosepatients be a multifactorial stepwise decision makingprocess. Proper assessment and quantification of thedifferences between the right and the left sides are cru-cial for diagnosis, treatment planning and follow up.Conventional posterio-anterior cephalometric radio-graphs used in orthodontic practice to assess asymmetryhave inherent limitations as a result of superimposition,magnification and distortion.Three-dimensional (3D) imaging and associated imageanalysis methods developed recently carry a potential fordevelopment in this field. The use of relatively low-dosecone beam CT (CBCT) scanners in dental practicesfacilitates the examination of anatomical structures in amultiplanar view. It also provides a rich soil for the appli-cation of different image analysis techniques to furtherextract diagnostic information from available volumes.Creation of 3D virtual surface models from CBCT vo-lumes, registration of those models and measurements ofthe surface distances between different models is a welldocumented approach used to study growth, treatmentchanges and the stability of those changes.3–7Various approaches have been described in the lite-rature in an attempt to use 3D imaging for the detectionand quantification of craniofacial asymmetry.8–10 How-ever, most of these techniques depend on landmarkidentification and manual identification of a straight‘‘midsagittal plane’’, both of which proved to beextremely challenging and operator-dependent.In 2000, a method to automatically define the mid-sagittal plane of the brain in two-dimensional cross-sectional slices was presented.11 However, definition ofthe midsagittal plane in the maxillofacial area remains achallenge because the plane is a curved surface in somefaces with chin deviation. In 2004, an alternative method*Correspondence to: Dr Abeer AlHadidi, University of North Carolina, ChapelHill, NC, USA; E-mail: alhadidia@dentistry.unc.eduReceived 10 April 2010; revised 17 August 2010; accepted 24 August 2010Dentomaxillofacial Radiology (2011) 40, 351–357’ 2011 The British Institute of Radiologyhttp://dmfr.birjournals.orgfor determining a symmetry plane was presented, usingmirroring of the mandible in any arbitrary plane withrigid registration of the mirrored mandible to the originalimage.12 However, this procedure did not take intoconsideration the mandibular morphological relation-ships with the maxilla and the cranial base. A modifica-tion of this approach that first mirrors the image and thenregisters on the cranial base can provide information onthe entire facial structure rather than just the mandible.7The aim of this study was to compare the degree ofmandibular asymmetry, as obtained by surface distancemeasurement, using mirroring of the mandible in the‘‘midsagittal plane’’ vs mirroring of the mandiblefollowed by registration on the cranial base.Materials and methodsFollowing a protocol approved by the institutionalreview board for research involving human subjects, 50patients with clinically detectable asymmetry served asthe cohort of this study. Clinical asymmetry was definedas more than 2 mm of chin deviation or the presence ofcant of the occlusal plan before the start of theirorthodontic treatment. CBCT scans of all patients wereobtained using NewTom 3G cone beam scanner (AperioServices, Sarasota, FL). The 12 inch field of viewproducing 0.5 mm isotropic voxel sizes was used foracquiring the image volume. 3D surface models wereconstructed from the CBCT volumes using segmentationtools of the Insight SNAP software (http://www.itksnap.org/pmwiki/pmwiki.php).13 Following segmentation, a3D graphical rendering of the volumetric object allowednavigation between voxels in the volumetric image andthe 3D graphics with zooming, rotating and panning.Midsagittal plane approachNasion (Na), anterior nasal spine (ANS) and basion(Ba) were manually defined for each patient by oneFigure 1 Assessment of asymmetry following two different mirroring methodsTable 1 Descriptive statistics for the absolute difference in surfacedistance measurements between the two methods at the differentanatomical locationsn MeanStandarddeviationLateral pole of the left condyle 47 1.73 1.36Left ramus 50 2.00 2.50Left body 50 2.14 2.57Left symphysis 50 2.35 2.27Left inferior border 50 1.70 1.94Left posterior border 50 1.71 1.65Posterior surface of the left condyle 47 1.53 1.50Medial surface of the left condyle 47 1.41 1.79Anterior surface of the left condyle 47 1.35 1.77Lateral pole of the right condyle 48 1.84 2.18Right ramus 50 2.54 3.08Right body 50 2.82 3.21Right symphysis 50 2.58 2.34Right inferior border 50 1.42 1.57Right posterior border 50 1.48 1.37Posterior surface of the right condyle 48 1.65 1.90Medial pole of the right condyle 48 1.62 1.37Anterior surface of the right condyle 48 1.59 1.87Assessment of mandibular asymmetry352 A AlHadidi et alDentomaxillofacial Radiologyobserver. The midsagittal plane was defined as theplane passing through those three landmarks. Theresultant midsagittal plane was used to create mirrorsfor both halves of the mandible.Na, ANS, Ba, the midsagittal plane and the degree ofasymmetry were determined 5 times on 22 randomlyselected patients from the cohort. Differences betweenrepeated assessments of asymmetry served as a measureof reproducibility of midsagittal plane identification.Registration-based approachEach model was mirrored on an arbitrary plane. Themirroring is done by arbitrarily converting the imageorientation from right–left, anterior–posterior and inferior–superior to left–right, anterior–posterior and inferior–superior with a proprietary software called Imconvertertool (University of North Carolina, Chapel Hill, NC).The original and the arbitrarily mirrored images werethen registered on the cranial base. The registration wasaccomplished using IMAGINE software (National Insti-tutes of Health, Bethesda, MD) following a voxel-basedregistration method. This method utilizes maximizationof mutual information to avoid observer-dependenttechniques. After the software masks the maxillary andmandibular structures, it compares the grey level inten-sity of each voxel in the cranial base to register the twoCBCT images. The rotation and translation parametersthat are used to register the two greyscale images are alsoapplied to register the 3D surface models.CMFApp software (University of North Carolina,Chapel Hill, NC)14–17 was used to display the super-imposed images with the two approaches: (1) originalimage superimposed on the image mirrored on the mid-sagittal plane; (2) original image superimposed on theimage that was arbitrary mirrored and then registeredto the original image.Surface distances were compared between the resul-tant mirrors from both approaches and the original man-dible. The average surface distances of the right and leftside were calculated for nine anatomical regions ofinterest (ROI): the lateral pole; medial pole; anterior andposterior surfaces of both condyles; lateral surface of therami and corpora of the mandible; inferior and posteriorsurfaces of the mandible; and anterior surface of thesymphysis. Computation of surface distances was basedon iterative closest point algorithm. The 95th percentilesurface distance measurement of each ROI was con-sidered as a measurement of asymmetry at that location.The measures of surface distances were complementedby visualization of the 3D colour-coded maps (Figure 1).Figure 2 A box plot demonstrating the mean of the absolute difference in surface distance measurements (mm) using both mirroring methods oneach region of interest on the left side (* indicates statistical significance)Assessment of mandibular asymmetryA AlHadidi et al 353Dentomaxillofacial RadiologyFigure 3 A box plot demonstrating the mean of the absolute difference in surface distance measurements (mm) using both mirroring methods oneach region of interest on the right side (* indicates statistical significance)Figure 4 A box plot showing the mean of the absolute difference in surface distance measurements (mm) of the left and right sides of themandible using mirroring on the midsagittal plan (* indicates statistical significance)Assessment of mandibular asymmetry354 A AlHadidi et alDentomaxillofacial RadiologyPaired t-tests were used to assess statistical differ-ences between the surface distance measurementsobtained by the two approaches for each anatomicallocation. They were also used to test for differencesbetween the measurements obtained for the right andthe left sides for each approach. The Cochran–Mantel–Haenszel test was used to test for the effect of thedegree of asymmetry on the difference detected betweenthe two approaches (a 5 0.05).ResultsThe variability in surface distance measurements ob-tained from five repeated assessments of the midsagittalplane for each subject ranged from 0 mm to 1.63 mmwith a mean of 0.42 mm and a standard deviation of0.18 mm. Only five patients had a discrepancy largerthan 1 mm. The paired t-tests showed no statisticallysignificant difference in quantification of asymmetryamong the five measurement sets (P . 0.1 for all pairedcomparisons and P 5 0.44 when pooled measurementswere tested together).Of the 18 anatomical areas measured, the mandibularramus (P 5 0.04), body (P 5 0.01), symphysis (P 50.005) at the right side and the lateral pole of thecondyle at the left side (P 5 0.02) showed statisticallysignificant differences in the mean surface distancebetween the two approaches. The differences in themean surface distances ranged from 0.2 mm to 1.5 mm(Table 1 and Figures 2 and 3).A positive correlation between asymmetry measure-ments and the discrepancy between the methods wasobserved at the right inferior border (P 5 0.048), leftposterior border (P 5 0.04) and medial pole of the leftcondyle (P 5 0.03).There was no statistically significant difference in themean of surface distance measurements between the leftand the right side for all locations using the registeredmirror. However, a significant difference was observed inFigure 5 A box plot showing the mean of the absolute difference in surface distance measurements (mm) of the left and right sides of themandible using using arbitrary mirroring followed by registration on cranial baseAssessment of mandibular asymmetryA AlHadidi et al 355Dentomaxillofacial Radiologythe ramus (P 5 0.02), the body of the mandible (P 50.03) and the symphysis area (P 5 0.04) when mirroringwith the midsagittal plane was used (Figures 4 and 5).DiscussionAlthough cephalometric radiography has been and willprobably continue to be the routine radiographic exami-nation for any patient seeking orthodontic/orthognathictreatment, the ‘‘inappropriateness of conventional ce-phalometrics’’ was recognized as early as 1979.18 Thoseinadequacies are further accentuated when challengedwith bilateral differences in the anatomy. 3D cephalo-metry has the potential advantage of being able to betterdetect and localize existing asymmetries. However,quantification of asymmetries using 3D cephalometryheavily depends on the operator’s understanding of 3Dlandmark definition and the ability to reproduciblydefine those landmarks.8,9 The use of surface distancemeasurements between the two halves of the mandiblethat served as our asymmetry quantification outcomeovercomes the need to depend on points as surrogatesfor real structures.In this cohort, the use of either mirroring approach(midsagittal plane and registration-based approach)provided similar results in most areas. Most of thediscrepancies in surface distance measurements werelocated on the lateral surface of the mandible, whichsuggests that the mediolateral direction is most sensitiveto mirroring.It can be hypothesized that there should be nodifference in the absolute surface distance measurementsbased on whether the right or the left side of thea bdcFigure 6 Examples of patients with challenging asymmetries for quantification. Asymmetric cranial base (a) and cleft palate (b) would interferewith both mirroring protocols. Severe cant (c) and rotation (d) is hard to deal with using iterative closest point algorithmAssessment of mandibular asymmetry356 A AlHadidi et alDentomaxillofacial Radiologymandible is being mirrored. That was true whenmirroring using an arbitrary plane with registration onthe cranial base, but the use of a midsagittal planemirroring produced inconsistencies at some locations. Itfollows that most of those locations were the same onesthat showed discrepancies between the two approaches.The differences between the two mirroring approaches,although statistically insignificant, tended to increasewith more severe asymmetries. Application of both ap-proaches to an exclusive cohort of patients with severeasymmetries may further characterize this trend in termsof location and the cut-off point of maximal difference.Neither approach could be applied indiscriminatelyto any patient. Mirroring using the midsagittal plane isdifficult for patients suffering from conditions thatinterfere with the midline position of the points used todefine the midsagittal plane, i.e.cleft palate patients. Onthe other hand, registration on the cranial base inpatients with asymmetries involving the cranial basewould also result in suboptimal results.Applying both mirroring approaches to patients withrudimentary condyles, severe cants or a rotated man-dible would result in important discrepancies betweenthe mirrored image and the original. This complicatesthe interpretation of the surface distances obtainedat a specified location. The measured distance wouldnot represent the difference between correspondinganatomical locations, but rather the minimal diffe-rence between the models as computed by the iterativeclosest point algorithm used by CMFApp software(Figure 6).In conclusion, both mirroring using a midsagittalplane and using a registration-based approach providedsimilar quantification of mandibular asymmetry formost areas. One of these approaches is considered analternative to the other when certain structural featuresof the patient impede the use of either approach.Shape correspondence, a technology that allowsmeasuring of the surface distance between an area onone model to the same corresponding area in the othermodel regardless of the alignment of those models, iscurrently being investigated as a promising alternativeto overcome the shortcomings of the iterative closestpoint algorithm in situations of important discrepancy.AcknowledgmentsSupported by NIDCR DE017727 and DE018962.References1. 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Comparison of two methods for quantitative assessment of mandibular asymmetry using cone beam computed tomography image volumes

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