Craniosynostosis is a disorder of skull development,

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1 Accuracy of Craniofacial Measurements: Computed Tomography and Three-Dimensional Computed Tomography Compared With Stereolithographic Models Julia Frühwald, MD,* Kurt A. Schicho, MD,1 Michael Figl, MD,2 Thomas Benesch, MMag, DI, DDr, Franz Watzinger, MD,1 Franz Kainberger, MD* Vienna, Austria In a retrospective study, distance measurements of nine children with craniofacial malformation were analyzed. The accuracy of measurements was compared when measured on a workstation using a 16-slice multidetector spiral computed tomography and on a stereolithographic model. Three different methods of defining distances were investigated: 1) on the stereolithographic plastic models, 14 distances connecting landmarks were identified with a digitizer (Polaris Tracker); 2) the same distances were defined at axial, coronal, and sagittal reformats of the computed tomography data set and measured using a Philips MX View workstation; and 3) the same 14 distances were defined at three-dimensional virtual reality models of the skulls at the same workstation. All measurements were performed with all three methods by three different readers. The following conclusions could be drawn: stereolithographic models provide a highly exact reproduction of the skull in children with craniofacial malformations. They are a reliable basis for all analytic and probatory endeavors preparing complicated surgical corrections. Threedimensional virtual reality display modes serve significantly better for exact distance measurements on the complex surface of the human skull than planar reformats of the same computed tomography data sets. Key Words: Multislice CT, craniofacial malformation, stereolithographic model Craniosynostosis is a disorder of skull development, the result of premature fusion of one or more cranial sutures. Surgical correction of craniosynostosis involves removal of the fused suture and correction of the asymmetry in early ages. The skull then can perform normal growth without danger of brain damage from increased intracranial pressure. Precise preoperative imaging and diagnosis is essential for the improvement of function and aesthetics as the operative outcome. Therefore, three-dimensional (3D) reconstructions of the skull are used for preoperative planning. 1Y10 For craniofacial modeling, data from computed tomography (CT) is used. The data from multidetector computed tomography are sent to a model manufacturer where a machine controls a laser beam to cure liquid resin layer by layer. Several studies have focused on the accuracy of these models and proved their feasibility for preoperative surgical planning, for example, in craniomaxillofacial surgery, ear, nose and throat surgery, neurosurgery, or traumatology. 10Y17 In this study, preoperative imaging was evaluated using CT reformats, a 3D virtual reality model and a stereolithographic skull of nine children with craniofacial asymmetry. From the *Department of Radiology, Division of Osteoradiology, the 1University Hospital of Cranio-Maxillofacial and Oral Surgery, the 2Center for Biomedical Engineering and Physics, and the Department of Medical Statistics, Medical University of Vienna, Austria. Address correspondence and reprint requests to Julia Frühwald, MD, Institut Frühwald, Kremsergasse 16A, 3100 St. Pölten, Vienna, Austria; Julia@fruehwald.at MATERIAL AND METHODS Nine children aged 5 to 51 months (mean age 14.6 months) with craniofacial malformation underwent CT scans between August 2001 and June Six patients suffered from craniosynostosis, one from plagiocephalous and lip and palate cleft, one from 22

2 ACCURACY OF CRANIOFACIAL MEASUREMENTS / Frühwald et al Pfeiffer syndrome, and one from microcephalus. We included all consecutive children examined in this period; as exclusion criteria, we defined a lack of quality. There was no need to exclude one single skull. Computed tomography was performed on a Phillips MX800 IDT CT scanner (Amsterdam, The Netherlands) with a 16-row detector. The children were examined under general anesthesia with 90 kv and 75 mas. Slice thickness was 0.8 mm, pitch 0.3 mm, collimation , rotation time 0.75 seconds, CTD 4.9, range 10 to 15 cm, and matrix The data were stored on CD-ROM and sent to a commercial manufacturer (Laserform Modellbau GmbH, Vienna, Austria), where the stereolithographic models were produced. The CT data were transferred to a computer that controls a laser beam to cure liquid resin layer by layer and finally forming a 3D plastic model of the infant s skull (Fig 1). All this was part of the routine preoperative examination of the Clinic of Cranio-Maxillo-facial Surgery and the Department of Radiology of the Medical University of Vienna. To investigate the accuracy of measurements on a CT console, a cooperative trial with three methods of distance measurements was undertaken. Three different methods of defining distances were investigated: 1) as a reference, 14 landmarks normally well visible on images of infant skulls Fig 2 Measurement of D2; three-dimensional virtual reality model. were chosen for analysis of measurement correctness. Those 14 landmarks were identified with a digitizer (Polaris Tracker; by NDI Northern Digital Inc., Ontario, Canada) on the stereolithographic plastic models. 2) The same distances were measured on planar CT reformats (axial, coronal, and sagittal) using a Philips MX View workstation (Fig 2). 3) The same 14 distances were defined at 3D virtual reality models of the skulls at the same workstation (Fig 3). All measurements were performed with all three methods by three different readers. This allowed for analyzing the accuracy of the three methods, the 14 different measurements, and for the interobserver variability. The data were statistically analyzed. Simple analysis of variance with post hoc Tukey test was used for assessing differences in methods and also in on a server. P G 0.05 was considered significant. Statistical calculations were carried out using SAS Version 8 (SAS Institute Inc., Cary, NC) and SPSS Version 12 (SPSS Inc., Chicago, IL) (Figs 4Y6). Fig 1 Measurement of D9; sagittal reformats. RESULTS Analysis of the 14 Distances Ninety-four percent of the distances could be measured on the plastic skulls; the mandible of one skull was broken so four distances could not be measured (during storage, after the patients treatment was completed). On the axial, coronal, and sagittal CT slices, five distances could not be measured because the 23

3 THE JOURNAL OF CRANIOFACIAL SURGERY / VOLUME 19, NUMBER 1 January 2008 less than 1 mm. There was no significant difference; this implicates a low interobserver variability (Fig 5). Fig 3 Stereolithographic model. landmarks to be connected were not on the same slice. A total of 44.71% of the distances were measured. We could measure 74.87% of the distances on the 3D virtual reality model. Problems occurred with distance D14 (nasion to protuberantia occipitalis externa), because the reference points could not be simultaneously visualized from any angle. In some distances, concurrence was better than in others (Fig 4). Interobserver Variability The deviation from the mean was analyzed for each skull and each observer. The average difference was Analysis of the Three Methods With the two most reliable distances, D7 (spina nasalis anterior to nasion) and D9 (processus coronoideus to condylus mandible sinister), we compared the concurrence of the three methods. There was no significant difference in distance D7 (P = 0.637); there was a range in the average of 1 mm. In D9, the average range was 1.7 mm; there was a significant difference between the methods (P = 0.017). Based on the results of a Tukey HSD, test we further analyzed the distance D9, the difference between method A (stereolithographic model) and method B (CT slices) was significant (P = 0.017). The difference between method B (CT slices) and method C (3D CT model) was significant also (P = 0.052). However, the difference between method A (stereolithographic model) and method C (2D CT model) was not significant (P = 0.934). There was a lightly higher dispersion in method A. Measurements on the stereolithographic model and the 3D CT model had a good concurrence and are equal (Fig 6). DISCUSSION As our results show, there is an excellent correlation between those plastic models and the multislice CT data. We proved that on a representative set of distances and in all situations in which the reference points could be identified, we had a good Fig 4 Each distance was measured three times by three readers: Methods A (SLA model), B (planar reformats), and C (three-dimensional model). Fig 5 The deviation of the mean of distance D7 among observers 1, 2, and 3 presented in a box plot. 24

4 ACCURACY OF CRANIOFACIAL MEASUREMENTS / Frühwald et al Fig 6 Comparison of the three methods: A (stereolithographic model), B (computed tomography slices), and C (three-dimensional computed tomography model). and statistical correlation. The mean imprecision in distance D7 was 1 mm corresponding to 3%; other authors reported inaccuracy from 0.6 to 6%. 12 Those results were observer-independent; no significant interobserver variability was found. In concordance with other authors, 18,19 we agree that exact measurements are much easier to perform on 3D images than on planar reformats. Especially in children, radiation protection is paramount. 20 Whereas the use of multidetector CT units leads to less artifacts, the use of dose-reduced protocols is mandatory. Craniosynostosis is not a very common disease, and because of radiation protection, only indefinite cases undergo CT scan. We compensated the lack of large numbers by measuring more distances on each case to increase the statistic evidence. Understanding of sutural anatomy, sutural fusion, and the resulting deformity is important for the diagnosis and potential surgical correction of those malformations. CT images are an exact reproduction of the human skull and can be used for measurements in craniofacial applications. 21Y23 Cavalcanti et al proved excellent correlation of 3D reformats and cadaver skulls. This compares favorably with our own results. Although other authors 24 advocate against the necessity of any imaging in certain infant skull malformation, we do consider imaging in the majority of cases useful. The technology of extracting the bony structures of the CT data set to manufacture an exact plastic replica of the infant s skull is of great value for the complex preoperative planning before correction of craniofacial malformations. The 3D CT models bridge the gap between radiology and surgery and the 3D CT models are even more concrete than virtual 3D reformats on a monitor screen. With preoperative model planning simulation, surgery can be performed and devices for osteosynthesis can be prefabricated, saving operative time. 25,26 Of course, a good correlation of the dimension in reality and the plastic model is mandatory for surgery. The production of the plastic model takes 2 days. This investment of time and money is especially justified in complex cases when time and money can be saved in avoidance of secondary correction interventions. Other techniques reproducing the human skull were used during the last 15 to 20 years, but only the new soft- and hardware technology available today is superior for fabrication of exact skull replicas. We recommend for the preoperative analyses of complex craniofacial malformations the use of highlevel CT equipment. Conventional films are of little value and can be avoided. The CT data of the normal investigative protocols can be used to manufacture exact stereolithographic models with no additional radiation. CONCLUSIONS Stereolithographic models are a very good reproduction of the skull of children with craniofacial malformation. Distance measurements are more exact using stereolithographic model or 3D CT reformats than planar CT reformats. REFERENCES 1. Girod S, Teschner M, Schrell U, et al. Computer-aided 3-D simulation and prediction of craniofacial surgery: a new approach. J Maxillofac Surg 2001;29:156Y Klein HM, Schneider W, Alzen G, et al. Pediatric craniofacial surgery: comparison of milling and stereolithography for 3D model manufacturing. Pediatr Radiol 1992;22:458Y Haers PE, Warnke T, Carls FR, et al. Preoperative diagnosis of complex craniofacial syndromes. Mund Kiefer Gesichtschir 1998;2(suppl 1):S13YS15 4. Fuhrmann R, Feifel H, Schnappauf A, et al. Integration of three-dimensional cephalometry and 3D-skull models in combined orthodontic/surgical treatment planning. J Orofac Orthop 1996;57:32Y45 5. Eppley BL, Sadove AM. Computer-generated patient models for reconstruction of cranial and facial deformities. J Craniofac Surg 1998;9:548Y D Urso PS, Barker TM, Earwaker WJ, et al. Stereolithographic 25

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