Biomechanical response of the human mandible to impacts of the chin Matthew Craig a , Cynthia Bir a,Ã , David Viano a , Scott Tashman b a Bioengineering Center, Wayne State University, 818 W. Hancock, Detroit, MI 48202, USA b Department of Orthopaedic Surgery, University of Pittsburg, Pittsburg, PA, USA article info Article history: Accepted 24 July 2008 Keywords: Mandible Impact biomechanics Response corridors abstract The purpose of this study was to determine the force–time and force–displacement response of the human mandible under direct loading at the chin. Sub-fracture response of the mandible and temporomandibular joint (TMJ) were analyzed from 10 cadavers that were impacted at the chin with a 2.8-kg mass at drop heights of 300, 400 and 500 mm and a 5.2-kg mass at 500 mm. Motion of radio- opaque markers adhered to the surface of the bone was recorded at 1000 Hz by a bi-planar X-ray and converted to three-dimensional coordinates. Peak force ranged from 0.90 to 4.54 kN causing chin displacement of 1.2–4.4mm. A bi-linear response was observed with stiffness of 475.17199.8 kN/m for chin displacement resulting from loading up to 0.6 kN and 2381.67495.7 kN/m for loads from 0.6 to 3.25 kN. This defines the biomechanical response of the mandible for chin motion under impact loading. The response of different segments of the mandible and TMJ are also documented. Force–time and force–displacement response corridors for the mandible can be used for finite element model and/or the development and validation of a biomechanical surrogate. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction In efforts to document the conditions leading to mild traumatic brain injury (MTBI) in American football, Pellman et al. (2003a, b) re-enacted 31 National Football League (NFL) cases with anthro- pometric test devices (ATD) to describe the head impact response players were experiencing on the field. Four average impact conditions were described. Only one condition, Condition A, involved the potential for indirect loading of the mandible through the chin strap. Prior research has studied mandible fracture tolerance (Hodgson, 1967; Nahum et al., 1968; Schneider and Nahum, 1972; Nahum, 1975; Huelke and Compton, 1983; Hopper et al., 1994; Unnewehr et al., 2003) and the response of the condylar fibrocartilage and temporomandibular joint (TMJ) articulating disk to load (Lai et al., 1998; Hu et al., 2003; Beek et al., 2001; Tanaka et al., 2001). However, none of these studies described the gross deformation of the mandible versus time under chin loading. This response is critical for developing a human surrogate headform with an articulating jaw that can be used to evaluate football head and mouth protection equipment including mouthguards, facemasks and chin straps with regard to energy transferred via the jaw to the skull that may lead to concussion. Response corridors have been widely used as a requirement for finite element model and/or biomechanical surrogate validation (Kroell et al., 1971; Hardy et al., 2001; Stemper et al., 2001; Maltese et al., 2002; Bir et al., 2004; Wheeldon et al., 2006). The aim of this research was to develop force–time and force–displacement response corridors of the mandible and TMJ under chin loading. Others have studied biofidelity improvements to the Hybrid III ATD headform (Newman and Gallup, 1984; Melvin and Shee, 1989; Melvin et al., 1995) with emphasis on facial loading response. This study provides the response requirements for loading at the chin that can be used for further refinement of the Hybrid III or other surrogates. 2. Methodology 2.1. Chin impact test method development The goal of this study was to document the response of the mandible to drop mass impacts that produce comparable jaw loading to that seen in NFL Condition A (Pellman et al., 2003a, b). A Hybrid III 50th percentile male headform was retrofit by Robert A. Denton, Inc. (Rochester Hills, Michigan) for Biokinetics and Associates, Ltd. (Ottawa, Canada) with a fixed triaxial load cell (Model 6080, Robert A. Denton, Inc.) capable of measuring jaw loads (Fig. 1a). This headform was impacted in Condition A using the helmeted surrogate pendulum impact methods described by Pellman et al. (2006). The resulting force–time history data were then used to establish a drop mass impact condition to the chin of the same Hybrid III headform using a refined version of a test stand previously developed by Walilko (2004). This drop mass condition would then be used on postmortem human subjects (PMHS) ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics 0021-9290/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2008.07.020 Ã Corresponding author. Tel.: +1313 5773830; fax: +1313 577 8333. E-mail address: cbir@wayne.edu (C. Bir). Journal of Biomechanics 41 (2008) 2972–2980