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British Journal of Oral and Maxillofacial Surgery 52 (2014) 850–853

Biomechanical investigation of naso-orbitoethmoid trauma by finite element analysis Heike Huempfner-Hierl ∗ , Andreas Schaller, Alexander Hemprich, Thomas Hierl Department of Oral and Maxillofacial Plastic Surgery, Leipzig University, Liebigstrasse 12, 04103 Leipzig, Germany Accepted 28 July 2014 Available online 16 August 2014

Abstract Naso-orbitoethmoid fractures account for 5% of all facial fractures. We used data derived from a white 34-year-old man to make a transient dynamic finite element model, which consisted of about 740 000 elements, to simulate fist-like impacts to this anatomically complex area. Finite element analysis showed a pattern of von Mises stresses beyond the yield criterion of bone that corresponded with fractures commonly seen clinically. Finite element models can be used to simulate injuries to the human skull, and provide information about the pathogenesis of different types of fracture. © 2014 The British Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.

Keywords: naso-orbitoethmoid trauma; facial trauma; finite element analysis; biomechanics

Introduction The naso-orbitoethmoid complex comprises the confluence of the orbit, nose, maxilla, ethmoid sinuses, frontal bone, and floor of the frontal sinus. Markowitz et al classified fractures of the area into 3 types (I to III) according to the involvement of the medial canthal tendon.1 They can be part of a panfacial fracture, or localised. Kelley et al reported that they account for about 5% of all facial fractures and we have also found this in our experience.2 For many years, a large number of fractures of the naso-orbitoethmoid complex were sustained in road traffic accidents, but as cars have become safer they now occur less often.3 Nowadays, most facial fractures are caused by

∗ Corresponding author. Department of Oral and Maxillofacial Plastic Surgery, Leipzig University, Liebigstr. 12, 04103 Leipzig Germany. Tel.: +49 (0)341-9721163; fax: +49 (0)341-9721169. E-mail address: [email protected] (H. Huempfner-Hierl).

interpersonal violence and sports accidents,4,5 and these impacts differ considerably in velocity and power from those sustained in road crashes. Fractures caused by a single impact might seem of minor clinical relevance compared with those sustained in road accidents, which can be panfacial and associated with other life-threatening injuries, but the region consists of many anatomical structures and is close to parts of the anterior skull base where smaller fractures might have severe consequences – for example, intense bleeding from injury to the ethmoid vessels. We therefore examined the biomechanical performance of the bones in the area when hit by single impacts, and the distribution, direction, and extent of the progress of stress in a transient dynamic finite element analysis.

Methods We constructed a model of the midface to make a finite element analysis of fractures of the naso-orbitoethmoid

http://dx.doi.org/10.1016/j.bjoms.2014.07.255 0266-4356/© 2014 The British Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.

H. Huempfner-Hierl et al. / British Journal of Oral and Maxillofacial Surgery 52 (2014) 850–853

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Fig. 1. Impact by virtual impactors for study designs 1 and 2.

complex caused by a single impact. It consisted of 736 934 10-node tetrahedrons. Data were derived from the computed tomography (CT) dataset of a healthy 34-year-old white man (Siemens Somatom Plus 4 Volume Zoom, 1 mm contiguous slices). After manual segmentation the dataset was exported into VRML (virtual reality modeling language), triangulated (VWorks 4.0® Cybermed, Korea), and imported into ANSYS ICEM CFD 12.0.1® (ANSYS Inc, Canonsburg, USA).6 To assign individual variables of the bony material to each element, we translated grey-scale values of the CT Hounsfield scale into information on bone density, and used a BoneMat script to calculate Young’s modulus for each element.7,8 Two different study designs were chosen: the medial third of the infraorbital rim (Fig. 1), and the junction between the nasal bone, maxillary nasal process, and lacrimal bone (Fig. 2). To simulate the impact from one single fisticuff, a virtual brass impactor (weight 412 g, density 8.4 g/cm3 ), Young’s modulus of 100 000 MPa, and Poisson’s ratio of 0.37 were

Fig. 3. Von Mises stresses in study design 1 that correspond with paranasal fractures of the medial inferior orbital rim and fractures in the anterior orbital floor (scale is von Mises stresses in MPa).

modelled according to the experiments of Waterhouse et al.9 The velocity of impact was 6 m/second. We used a transient mode of simulation as the interaction between the skull and impactor depended on time. The model was fixed at the occipital condyles in all degrees of freedom. We assumed von Mises stresses of 150 MPa for the yield criterion of bone in the skull.10 According to the regulations of our Institutional Review Board, approval was not needed for this investigation.

Results In design 1, finite element analysis found a total impact of 7200 N for 1.3 msec. Von Mises stresses of more than 150 MPa, which corresponded with fractures, were seen in the medial inferior orbital rim paranasally, and in the anterior orbital floor (Fig. 3). In design 2, finite element analysis showed a total impact of 6980 N for 2.6 msec. Von Mises stresses of more than 150 MPa were seen in the medial orbital wall. The contralateral medial orbital wall was also affected, but here the yield criterion was not reached. High stresses even spread to the occipital bone (Fig. 4). In both designs minor stresses spread to the ipsilateral Le Fort I plane, but did not involve the anterior base of the skull or the bony optical canal. The pattern of the areas where von Mises stresses exceeded the yield criterion is consistent with typical fracture patterns seen in many patients (Figs. 5 and 6).

Discussion

Fig. 2. Contact zones between impactor and bone for study designs 1 and 2.

When investigating the biomechanics of facial trauma it is difficult to generate a practical and ethically acceptable study design that will deliver valid and reliable information. In the past cadavers were often used. Nowadays, Le Fort’s studies of 190111,12 would not be feasible for ethical reasons and

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H. Huempfner-Hierl et al. / British Journal of Oral and Maxillofacial Surgery 52 (2014) 850–853

Fig. 4. Von Mises stresses in study design 2 that correspond with fractures of the medial orbital rim (scale is von Mises stresses in MPa).

because of the lack of human cadavers. Post mortem alterations, and in most cases the age at death of the cadaver, which often is not that of a typical patient with a facial injury, limits the use of such studies. Elhammali et al found a mean (SD) age of 29.7 (12.8) years in 147 patients with sportsrelated maxillofacial fractures,5 and Roccia et al reported a mean age of 28.5 years (range 11 – 72) in 138 patients with sports-related maxillofacial injuries (male:female ratio 8:1).4 Yamada and Evans found that the bony strength of cadaveric specimens with an age at death of 70 to 80 years was about 20% to 30% lower than specimens from cadavers with an age at death of 20 to 30 years.13 Our own model complied with the typical age and sex of a patient with facial injuries.4,5 Animal experiments, which use macaques and concern orthognathic questions,14 are not appropriate because of ethical reasons, and because they cannot be transferred to the human anatomy.

Fig. 5. Computed tomogram of patient showing fracture pattern (arrows) similar to stresses in study design 1. The injury was caused by interpersonal violence.

Fig. 6. Computed tomogram of patient with fracture pattern (arrows) similar to stresses in study design 2.

We therefore chose finite element analysis to generate valid models for biomechanical tests. Finite element analysis was introduced into biomechanical medical research in the 1990s. Takizawa et al reported a numerical computer simulation for the analysis of blowout fractures in 1988,15 but their model was only 2-dimensional and rather simple because of limited computing capacity, and data were derived from dry human skulls. In 1996 Voo et al presented their finite element models of the human head.16 They reported that the modelling of a biological structure such as a human head differs significantly from that of other structures. Nagasao et al published studies about blowout fractures on 3-dimensional models based on finite element analysis in 2006.17 They presented a detailed 3-dimensional model of about 240 000 finite elements, which allowed thorough analyses, but their CT-data were also derived from dry human skulls. Previously, most published studies on finite element models were based on cadaveric data, which have the advantage that the experiments can be repeated, whereas those on genuine cadavers cannot. However, the statements concerning post mortem alterations and the age of patients remain valid. Our experiments differed from other investigations in several respects. Our model of the midface consisted of 736 934 10-node tetrahedrons, which gave a high resolution. Unlike many finite element analyses that use uniform biomechanical values, we calculated individual biomechanical variables of bone, which improved accuracy and realism. Peterson et al found that different areas of bone in the dentate maxilla vary in thickness and material properties, and suggested that these should be incorporated into finite element models,18 as previously done by Szwedowski et al.8

H. Huempfner-Hierl et al. / British Journal of Oral and Maxillofacial Surgery 52 (2014) 850–853

Regarding impact, we chose a transient simulation to analyse the development of stresses within the model under time dependency as in an actual injury. Our model was designed to give more valid and reliable information than has been reported in earlier studies because our data were derived from a living person of a typical age, and because we calculated individual bony variables for each element and used a large number of elements. Inevitably there are limitations, and compared with studies on real models, finite element analysis has advantages and disadvantages. The main advantages are that experiments can be repeated, there are no ethical limitations, and the study design can be changed and adjusted according to need. However, the model is not real and there may be inaccuracies in segmentation, meshing, or variables in biomechanical materials. Szwedowski et al compared finite element analysis with real models.8 His model, which was equal to those in our study, accorded well with strain gauge measurements. The predictive value of simulations has to be measured with their clinical correlations. We investigated common fractures, but as it was the first study on this topic to our knowledge, comparison with other work was not possible. The simulation showed that forces resulting from one fistlike impact cause stresses beyond the yield criterion and accord with typical fracture patterns commonly seen clinically for both designs. Stress did not spread to the anterior base of the skull or the bony optical canal, but this could change if the point of impact was in a different place and the force was greater. High resolution finite element models can simulate traumatic insults to the human skull and should be used in further studies. They can also provide information about the pathogenesis of different types of fracture.

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Conflict of interest 16.

We have no conflicts of interest. 17.

References 18. 1. Markowitz BL, Manson PN, Sargent L, et al. Management of the medial canthal tendon in nasoethmoid orbital fractures: the importance of the

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Biomechanical investigation of naso-orbitoethmoid trauma by finite element analysis.

Naso-orbitoethmoid fractures account for 5% of all facial fractures. We used data derived from a white 34-year-old man to make a transient dynamic fin...
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