Little s irregularity index (LII) was developed and

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1 ORIGINAL ARTICLE Little s irregularity index: Photographic assessment vs study model assessment Naif Almasoud a and David Bearn b Manchester and Dundee, United Kingdom Introduction: The aim of this study was to investigate whether photographic images are accurate and reliable to assess Little s irregularity index (LII) compared with study model measurements. Methods: Ten plastic models with different levels of incisor irregularity were photographed occlusally at 9 angulations ranging from 70 to 110 on 3 occasions. Clinical occlusal photographs and study models of 72 dental arches were selected from 16 patients participating in a study of incisor alignment. LII was measured from the models by using digital calipers and from the digital images with ImageJ software (Rasband WS, National Institutes of Health, Bethesda, Md; ). Results: Photographs of the plastic models and the direct measurements of the models showed high levels of reliability and repeatability (intraclass correlation coefficients [], and 0.983). Validity was shown by the limits of agreement between the plastic models and the photographs at 90,70, and 110, which were to , to , and to , respectively, and the were 0.993, 0.983, and 0.969, respectively. The intraexaminer and interexaminer for the clinical photographs were and 0.987, and intraexaminer agreement for the study models was The limits of agreement and the between the photographic method and study model method were to and Conclusions: LII can be measured from photographs with good reliability and repeatability. The limits of agreement indicate that, for the mean value of a group of subjects, the photographic method is valid, but care is needed in interpreting an individual measurement. (Am J Orthod Dentofacial Orthop 2010;138:787-94) Little s irregularity index (LII) was developed and published by Robert Little 1 in 1975, and it is widely used for assessing irregularity, crowding, relapse, and alignment of teeth. The LII measures the horizontal linear distance between anatomic contacts of the incisors in the labiolingual direction parallel to the occlusal plane, ignoring vertical displacement, starting from and ending at the mesial anatomic contacts of the canines. The sum of the 5 measurements is the LII score. The higher the index value, the more severe the labiolingual displacement of the teeth. Little developed his index on the mandibular incisors to look at relapse; however, researchers have used it to evaluate incisor irregularity in both arches in a range of settings. Most studies with the LII have recorded the score from study models by using calipers a Postgraduate student, Department of Orthodontics, School of Dentistry, University of Manchester, Manchester, United Kingdom. b Professor, Department of Orthodontics, Dental School, University of Dundee, Dundee, United Kingdom. The authors report no commercial, proprietary, or financial interest in the products of companies described in this article. Reprint request to: David Bearn, Dental School, University of Dundee, Park Place, Dundee, DD1 4HN, United Kingdom; , d.bearn@dundee.ac.uk. Submitted, August 2008; revised and accepted, January /$36.00 Copyright Ó 2010 by the American Association of Orthodontists. doi: /j.ajodo Other methods that have been used to measure the LII can be divided into 2-dimensional and 3-dimensional (3D) methods. Two-dimensional methods include occlusal scanning of study models with a flatbed scanner, 17,18 photocopies of the occlusal surfaces of the maxillary or mandibular study models followed by either direct measurement or digitizing the photocopied images. 19,20 These methods have the advantage that, as described by Little, vertical displacement is not considered. Three-dimensional methods include the reflex metrograph, 21,22 reflex microscope, 23 and use of a 3D digitizer. 24 These 3D methods are prone to errors because the distance between the contact points is recorded in both the horizontal and vertical planes and so might not be a valid representation of the LII. All of these methods require the availability of study models, produced from dental impressions a time-consuming and sometimes unpleasant procedure for the patient. Intraoral photographs have previously been used to measure tooth movement and rotations during active treatment, but the study did not report the repeatability or accuracy of this method. 25 Photographs provide a 2-dimensional image of a 3D object, the same as photocopying or scanning with a flatbed scanner. Photocopying and scanning with a flatbed scanner have good 787

2 788 Almasoud and Bearn American Journal of Orthodontics and Dentofacial Orthopedics December 2010 Fig 2. The metal ruler in place for calibration of the image, showing the requirements for the best projection. Fig 1. Camera setup to control different photographic projections. reliability and validity compared with measurements from study models. 17 However, photographs can introduce errors of projection, since, unlike a scan or photocopy that is recorded perpendicular to the occlusal plane, the angulation at which the photograph was taken might vary so that a vertical component of the contact point displacement can be introduced to the measurement. In this study, therefore, we aimed to (1) evaluate the repeatability and reliability of measuring the LII from photographic images of study models compared with the repeatability and reliability of measuring the study models directly with digital calipers; (2) determine the validity of this photographic method by comparing the agreement of the measurements from these photographs with those from the study models; (3) assess the effect of the photographic projection on the measurement and its reliability and validity; and (4) assess the reliability and validity of clinical photographs to measure the LII. MATERIAL AND METHODS Ten plastic models with different levels of incisor irregularity were photographed occlusally on 3 occasions, 1 day apart. On each occasion, photographs were taken of each model at 9 angulations from 70 to 110 at 5 intervals, giving 270 images. The camera was mounted on a tripod and reset between each photograph by using the built-in protractor to control its angulation (Fig 1). The first photograph was taken parallel to the long axis of the incisor teeth (perpendicular position, 90 ),considered the ideal projection. This ideal projection should show the labial and lingual surfaces of the anterior teeth. Each of the 10 plastic models was photographed with a metal ruler crossing the dental arch, contacting the tips of the premolars to allow for calibration of the image (Fig 2). The models and photographs were randomly allocated codes and then measured twice by using digital calipers for the models and ImageJ software (Rasband WS, National Institutes of Health, Bethesda, Md; rsb.info.nih.gov/ij/, ) for the photographs, with a 2-week interval between measurements. For the photographs, the image magnification was calibrated by using 2 points 40 mm apart on the ruler; then the anatomic contact points were identified on the screen, and the contact point displacement was recorded by the program to allow calculation of the LII score. The observer (N.A.) was allowed to use the image zoom tools in the software when this aided in point identification. Clinical occlusal photographs and matched study models of 96 dental arches were available from 16 patients participating in a study of incisor alignment. This study was approved by the Salford and Trafford Research Ethics Committee of UK National Research Ethics Service, including the taking of photographs for this investigation.

3 American Journal of Orthodontics and Dentofacial Orthopedics Almasoud and Bearn 789 Volume 138, Number 6 Table I. values, reliability coefficients, and mean differences (mm) for photographs taken on 2 occasions at each angulation coefficient Mean difference (SD) Photo (0.857) Photo (0.387) Photo (0.816) Photo (0.696) Photo (0.341) Photo (0.521) Photo (0.583) Photo (0.647) Photo (0.939) The inclusion criteria for the sample were patients with maxillary and mandibular fixed appliances, minimum pretreatment LII scores equal to or more than 5 mm in either arch at the start of treatment, and all 6 maxillary and mandibular anterior teeth included in the appliance. We first developed a method of obtaining suitable photographic images for this study. Because of distortion and dimensional changes of objects in the photographs, a customized rigid reference scale was needed for calibration. A disposable millimeter ruler was developed by printing the scale on acetate and bonding this to a clear polypropylene sheet to give greater rigidity. The millimeter ruler was placed with wax to contact the cusp tips of the premolars to be in the occlusal plane, crossing the dental arch horizontally. To standardize the photographic projection of the images, the operators had training and were given a clinical guide. The occlusal photographs were taken, as far as possible, parallel to the long axis of the incisor teeth, and had to show the labial and lingual surfaces of the anterior teeth and the plastic ruler. All photographs were taken with a digital camera (S3, Fuji Film, Tokyo, Japan), a 105-mm macro lens (Nikon Corporation, Tokyo, Japan), and a ring flash (Speedlight, Nikon). Alginate impressions were recorded by using BluePrint Cremix (Dentsply, Detrey, Germany). Records were obtained every 4 to 6 weeks at the first 3 routine visits. The records were taken with the brackets in place but the archwire removed. The models were randomly allocated codes, measured with digital calipers, and the LII scores recorded. Two weeks later, the codes were reallocated and the models remeasured. A second observer then scored all the study models. To record the LII from the digital images, each file name was changed to a random code to ensure that the observer was blinded to the treatment Table II. values, reliability coefficients, and mean differences (mm) for the repeated measurements of plastic models and photographs of the models taken at the ideal projection (photo 90 ) and other projections coefficient Mean difference (SD) Study model (0.693) Photo (0.281) Photo (0.127) Photo (0.087) Photo (0.091) Photo (0.055) Photo (0.092) Photo (0.077) Photo (0.092) Photo (0.150) visit. The image was then imported into the ImageJ software and calibrated, and the data were recorded as discussed in the in-vitro study above. A second observer then recorded the LII scores for all digital images. Assuming a standard deviation of differences in the LII of 0.330, 17 a sample size calculation showed that a two-sided Fisher z-score of the null hypothesis (that the correlation coefficient would be 0.990, with no significant difference between the 2 methods, indicating near perfect agreement) would have 90% power to detect a correlation coefficient value of or less with a study sample size of 38. We therefore aimed to obtain a minimum sample of 38 matched photographic and study cast records. Statistical analysis For the statistical analyses in this study, we used interclass correlation coefficient (), reliability coefficients (Cronbach s alpha), and mean differences to assess the reliability of the repeated measurements. When comparing the photographic measurements with the study model measurements to determine the validity of the photographic measurements, we also used limits of agreement and plotted Bland-Altman graphs. 28 These graphs plot the mean difference against the mean value and are used when examining 2 methods of measuring the same variable, since they give more information than the simple correlation graphs in this situation. If the differences are normally distributed, 95% of the differences will lie between the limits of agreement. If the limits of agreement are judged to be narrow or can be regarded as unimportant clinically, we can be sure that the new method can be used interchangeably with the old method. 28 This judgment can be based on the

4 790 Almasoud and Bearn American Journal of Orthodontics and Dentofacial Orthopedics December 2010 Table III. values, reliability coefficients, mean differences, rms mean differences (mm), and limits of agreement for the mean (aggregated) plastic model measurements and the mean (aggregated) photographic measurements at different projections for the 10 cases coefficient Rms mean difference (SD) Mean difference (SD) Limit of agreement (mean difference SD) (mm) Study model photo (0.3792) (0.6994) Study model photo (0.5264) (0.9872) Study model photo (0.4048) (0.6876) Study model photo (0.2512) (0.4697) Study model photo (0.3042) (0.4752) Study model photo (0.1644) (0.2447) Study model photo (0.3566) (0.4299) Study model photo (0.5819) (0.6012) Study model photo (0.6951) (0.9396) amount of variation in the accepted or gold-standard method (in this case, study model measurement) and on current clinical opinion. For comparison between the plastic study model measurements and the photographs of the study models, we considered whether to aggregate the data from the separate recordings. Although this might give a truer value for the actual measurement and is a common practice in research studies, it might also reduce the error that we would detect and would be present if only 1 measurement was taken, as could occur in a clinical situation. To cover both possible scenarios research and clinical we therefore present the data from the aggregated measurements and also the first measurement. To give a more valid assessment of the mean difference between the methods, the values for the difference for each case were converted to positive values by using a root mean square (rms) method, and then mean differences were calculated. This ensures that errors on either side, one with a positive value and one with a negative value, do not cancel each other but, instead, summate. The unconverted data was used for the Bland-Altman plots, and this shows the direction and magnitude of the differences between the photographic and study model measurements. RESULTS Table I shows the repeatability for 1 measurement from each of 2 photographs taken at each angulation on the first and second occasions. These show high levels of agreement in all comparisons; the mean differences for all repeated measurements were less than 0.5 mm, with many less than 0.2 mm. This indicates that the test setup was reproducible. The results for the repeatability of the measurement of the same plastic model or same photograph of the model are shown in Table II. The correlation for the model and the photograph at the ideal projection (photo 90 ) are both high (0.983 and 0.999), with mean differences less than 0.2 mm at all projections except 80 (mean difference, mm). This indicates that both methods have excellent reliability. Validity is addressed in Table III, which shows the results for the comparison of measurements taken directly from the plastic models with those from each different photographic projection by using the aggregated data, showing the mean difference, the rms difference, and the limit of agreement for the 2 measurement methods. This shows that the limits of agreement for photographs at 85,90, and 95 are within 1 mm of the study model measurement, with a slight tendency for the score to be greater on the photographic measurement. The limits of agreement for all projections except 110 were within 2 mm. Figure 3 shows the rms mean difference at different projections and demonstrates an acceptable range of angulations between 85 and 105, with the mean difference below 0.5 mm. The rms mean difference for the plastic model repeated measurement was, for comparison, 0.51 mm. Figure 4 shows the Bland-Altman plot of the comparisons between model measurements and photographs at 85,95, and 105, showing no evidence of systematic bias, and few points outside the limits of agreement. Table IV shows the same data but for the first measurement of the records, which reflects the likely clinical usage, rather than the research usage of the method. This shows greater mean differences and wider limits of agreement compared with the aggregated values. The rms mean differences are all (except for the 75 projection) under 1 mm, but the limits of agreement were about 1.5 mm on each side of zero for the central projections and increased to 2.5 mm for the 110 angulation. Initially, 96 record sets were available for the 16 patients. Twenty-four record sets were excluded because

5 American Journal of Orthodontics and Dentofacial Orthopedics Almasoud and Bearn 791 Volume 138, Number 6 Fig 3. The mean difference between plastic model and photograph measurements at different photographic projections. alignment was complete when the models and photographs were taken. Of the remaining 72 sets, 4 study models were excluded because of poor quality, and 1 clinical photograph was excluded because the mesial surface of the canine was covered by the ruler. Therefore, we had 68 study models and 71 clinical photographs, forming 67 matched sets. The results for the interexaminer and intraexaminer reliabilities of the measurements by using and reliability coefficients are given in Table V. This shows excellent examiner reliability with both clinical photographs and study models, with the lowest agreement between the examiners using study models. The first measurement and the mean of the 2 measurements taken by 1 examiner were then used to compare the 2 methods, and the results are shown in Table VI. The mean differences between the 2 methods are comparable with the interexaminer and intraexaminer differences. Figure 5 is a Bland-Altman plot of mean differences plotted against mean values for clinical photographs and study models. The central line is the mean difference ( mm), and the upper and lower lines are the 95% limits of agreement. It can be seen that, for only 3 sets, the difference in value between the 2 methods is outside the lower limit of agreement, with differences around 3 mm. For one of these, the LII was high at over 20 mm; for the other two, the LII was approximately 10 mm. There were no large errors when the values were small. DISCUSSION We found that the measurement of LII using the photographic method described was repeatable and Fig 4. Bland-Altman plot of the comparison between plastic model measurement and photographs taken at A, 85, B, 95, and C, 105.

6 792 Almasoud and Bearn American Journal of Orthodontics and Dentofacial Orthopedics December 2010 Table IV. values, reliability coefficients, rms mean differences, mean differences (mm), and limits of agreement for the first plastic model measurements compared with the first photographic measurements at different projections coefficient Rms mean difference Mean difference (SD) Limit of agreement (mean difference SD) (mm) Study model photo (0.5339) (1.0156) Study model photo (0.7401) (1.3078) Study model photo (0.5099) (1.0255) Study model photo (0.6019) (0.8268) Study model photo (0.4098) (0.6483) Study model photo (0.3142) (0.5358) Study model photo (0.5869) (0.8894) Study model photo (0.6450) (0.9845) Study model photo (0.8331) 0.02 (1.2863) reliable with both photographs of study models and clinical photographs, and this was comparable with the results for the study models. The clinical photographs actually performed slightly better than the study model method with reference to interexaminer reliability. The ImageJ program was previously used by Tran et al 17 to measure images of study models scanned with a flatbed scanner. Their results showed good reliability and accuracy of measurements with ImageJ, and they suggested that the computer photographic measurement could be used interchangeably with conventional manual measurements on study models. We investigated this further and confirmed the accuracy and reliability ofthecomputerizedmethodwhenusedwithclinical photographs. We tested the repeatability of measurements taken from the photographs of the study model at angulations from 70 to 110, and we found that the photographic measurement was highly repeatable at a zone of 20, giving the best result at the middle of the scale ( ). The repeatability reduced gradually when the angle was outside this range. Inspecting the images for the in-vitro study showed that photographs taken outside the range could be easily identified, since they did not show both the labial and lingual surfaces of the incisors. This can therefore be used as a simple clinical guide to ensure satisfactory alignment of the photographs. The validity of the photographic method is whether it actually measures what it purports to measure, and this needs to be assessed separately from reliability and repeatability. Taking the measurement of the study model as the gold standard, we therefore investigated whether the actual measurements from the photographs were the same; if so, the method has validity. The assessment of validity on the plastic models showed excellent agreement between the 2 methods; this is best seen in the Bland-Altman plots and by the limits of agreement for the ideal projections that were Table V. Intraexaminer and interexaminer reliability values showing values, reliability coefficients, and mean differences (mm) for repeated measurements of clinical photographs and study models Intraexaminer photos (n 5 71) Interexaminer photos (n 5 71) Intraexaminer study models (n 5 68) Interexaminer study models (n 5 68) coefficient Mean difference (SD) (0.396) (0.821) (0.491) (1.0198) less than 1 mm on either side of zero. This means that we can be 95% certain that the measurement obtained by the photographic method is within 1 mm of the value we would have obtained by measuring the study model. The validity of the method in the clinical environment was assessed again by comparing the value from the clinical photograph with that from the study model. Here, the limits of agreement were slightly wider, as might be expected because of the variations introduced in the clinical setting. However, the mean difference in values was less than 0.5 mm, and the limits of agreement showed that any measurement is 95% certain to be within the range of 2.2 to 11.2 mm. Figure 5 shows that most measurements would be significantly inside this range. With the direct clinical photographic image, it is possible to control the angulations by using simple clinical guidelines, ensuring that the photograph is as near parallel to the long axis of the anterior teeth as possible. With digital photography, it is also possible to check the photograph immediately and repeat it quickly if necessary. The photographs of the plastic models showed increased variability and less validity as the angulation

7 American Journal of Orthodontics and Dentofacial Orthopedics Almasoud and Bearn 793 Volume 138, Number 6 Table VI. values, reliability coefficients, mean differences (mm), and limits of agreement for the measurements from the clinical photographs and the study models for single measurements of each and aggregated data for each coefficient Mean difference (SD) Limit of agreement (mean difference SD) (mm) Study model vs photo (n 5 67) (0.8891) Study model mean vs photo mean (n 5 67) (0.8295) moved away from the ideal, and a simple clinical rule is to ensure that both the labial and lingual surfaces of the incisors are visible on the photographic image. The photographic method eliminates the need to take impressions for study models, which is time consuming and can be unpleasant for patients. The use of this method was not, however, entirely without problems. These can be categorized as problems with the image and the measurement. The problems that could occur with the photograph include incorrect placement of the ruler, and the operator requires experience with the method to obtain goodquality photographs. The problems with the computerized measurement were that each photograph required calibration, and there is no automatic calculation of the total LII score in the software. However, during the study, a number of problems with the study models were also identified. These included quality of impressions affecting the study models, voids or air blows in the impression or the model that affected the ability to recognize the correct location of the contact points, difficulty in safely storing and transporting the study models, complexity of marking the study models for random assessment, difficulty in measuring the overlapped contact points, and deterioration of the study model surface quality with repeated measurements. The study models have long been regarded as the gold standard for recording the LII measurements. However, future studies with LII as the outcome measure should consider the use of a standardized photographic technique, such as that described in this study, rather than conventional study model analysis. CONCLUSIONS 1. Measurements of LII from plastic study models and digital photographs of the models were reliable and repeatable. 2. Measurements of LII from digital photographs of the plastic models were valid. 3. Measurements of LII from clinical photographic images were reliable and repeatable. Fig 5. Bland-Altman plot of the comparison between study model measurements and clinical photographs. 4. The use of clinical photographs for assessment of LII for groups of patients, such as in a research or audit project, has validity, but, for 1 patient, the result should be interpreted with caution. REFERENCES 1. Little RM. The irregularity index: a quantitative score of mandibular anterior alignment. Am J Orthod 1975;68: Puneky PJ, Sadowsky C, BeGole EA. Tooth morphology and lower incisor alignment many years after orthodontic therapy. Am J Orthod 1984;86: Harris EF, Vaden JL, Williams RA. Lower incisor space analysis: a contrast of methods. Am J Orthod Dentofacial Orthop 1987;92: Edwards JG. A long-term prospective evaluation of the circumferential supracrestal fiberotomy in alleviating orthodontic relapse. Am J Orthod Dentofacial Orthop 1988;93: Rossouw PE, Preston CB, Lombard CJ, Truter JW. A longitudinal evaluation of the anterior border of the dentition. Am J Orthod Dentofacial Orthop 1993;104: Canut JA. Mandibular incisor extraction: indications and longterm evaluation. Eur J Orthod 1996;18: Årtun J, Garol JD, Little RM. Long-term stability of mandibular incisors following successful treatment of Class II, division 1, malocclusions. Angle Orthod 1996;66: Azizi M, Shrout MK, Haas AJ, Russell CM, Hamilton EH Jr. A retrospective study of Angle Class I malocclusions treated orthodontically without extractions using two palatal expansion methods. Am J Orthod Dentofacial Orthop 1999;116:101-7.

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