Elsevier

Journal of Biomechanics

Volume 45, Issue 11, 26 July 2012, Pages 1861-1868
Journal of Biomechanics

Stress profile of infant rib in the setting of child abuse: A finite element parametric study

https://doi.org/10.1016/j.jbiomech.2012.05.049Get rights and content

Abstract

The primary goal of this study is to advance our current understanding of infant rib injuries in the setting of child abuse. To this end, we employed finite element model simulations to determine the sensitivity of an infant rib's stress response to varying material properties and under varying degrees of anterior–posterior chest compression. Using high-resolution chest CT images obtained from a 6-day-old infant, we constructed a simplified geometric model consisting of bone and cartilage structures. To simulate the lateral gripping of an infant in child abuse, an anterior–posterior chest compression load was applied to cause increased stresses along the costovertebral articulation, a classic site for inflicted rib fractures. A sensitivity analysis was conducted to quantify the effects of varying Young's modulus and Poisson's ratio of the bones and cartilages. In addition, we varied the amount of anterior–posterior chest displacement to assess the sensitivity of this parameter to the rib's stress profile. We found that varying Young's modulus of the bone and cartilage not only changed the magnitude but also the shape profile of the rib's stress response. In contrast, varying the degree of chest compression only changed the magnitude of the stress response and not the shape profile. We also discovered that by varying Poisson's ratio of the bone and cartilage, no appreciable change was seen in the magnitude or the shape profile of the rib's stress response. Finite element modeling shows promise as a tool to elucidate the mechanisms of rib fractures in abused infants.

Introduction

Unlike adults, in whom rib fractures are frequent and often caused by direct impact, rib fractures in infancy (12 months old) are uncommon and often caused by child abuse (Worn and Jones, 2007). Although direct impact may occasionally play a role, manual thoracic compression is generally assumed to be the cause of most inflicted infant rib fractures (Betz and Liebhardt, 1994, Kleinman, 1990, Kleinman et al., 1996, Kleinman and Schlesinger, 1997, Reece, 1993, Reece, 2002, Worn and Jones, 2007). Fractures near the costovertebral articulations are highly associated with infant abuse, and although anterior–posterior compression of the rib cage during assaults is supported by perpetrator confessions and injury morphology, rigorous laboratory research that characterizes the type and magnitude of force required to cause inflicted rib fractures is lacking (Boal, 2008, Kleinman, 1990, Lonergan et al., 2003, Reece, 1993). Attempts to understand infant rib fracture with in vivo experimentation are not feasible. Finite element (FE) modeling offers an attractive alternative to develop a scientific foundation for investigating these theories. Thus, the goal of this paper is to begin the development of an FE model that can be used to advance our current understanding of infant rib fractures and better understand the biomechanical response of infant ribs during child abuse.

While there are numerous reports of the biomechanical response of adult ribs under stress using the FE modeling (Charpail et al., 2005, Ito et al., 2009, Kemper et al., 2005; Li et al., 2010a, Li et al., 2010b; Niu et al., 2007, Ruan et al., 2003, Yoganandan and Pintar, 1998), similar information on infant ribs is lacking, mostly because there is very little material property data on infant ribs. To our knowledge, there is only one study that reported the material properties of pediatric ribs (Pfefferle et al., 2007). In that study, Young's modulus, yield force, and rib stiffness of the pediatric rib bone were explored in 13 subjects ranging in age from 1-day-old to 72-months-old. As is typical in studies with limited human specimen availability, variability is large and identifying representative properties for the FE modeling is challenging. Therefore, the objective of this study was to identify the material properties that most influence the infant rib stress response during abusive chest compression, and ascertain whether changes in chest compression depth significantly alter patterns of stress. These findings will provide preliminary data that can direct future research for predicting rib fracture in children.

Section snippets

Geometry

For anatomic fidelity of an infant's chest, the geometry of the FE rib model was constructed based on a chest CT of a 6-day-old boy (approved by the Institutional Review Board at Children's Hospital Boston), with 0.28×0.28mm in-plane resolution, 2 mm slice thickness, 80 KVp, 90 mAs, and pitch of 1. This chest CT, acquired on a Siemens Sensation 64 multi-detector CT (Siemens Medical Solutions, Malvern, PA), was initially performed to investigate a small lucent lung lesion found on a chest

Mesh convergence study

The average element edge length was varied from 0.416 mm (employing 831,812 elements) to 1.438 mm (employing 49,222 elements). A total of 11 simulations, each using a different mesh size, were performed. The percent difference in the average stress within each rib segment (relative to the densest mesh model) was then plotted as a function of mesh size (Fig. 5). Specifically, Δim, the percent difference in stress in rib segment i using average element edge length m, was defined asΔim=σvmimσvmiMσvm

Discussion

To build a foundation for investigating infant rib fracture, we developed an anatomically detailed FE model, and performed a sensitivity analysis to evaluate the effects of bone and cartilage material properties on stress during an abusive type of chest compression. We found that Young's modulus for both bone and cartilage had a significant effect on magnitude and distribution of stress in the bone, but variations in Poisson's ratio for either material had no effect. This might be expected

Conflict of interest statement

The authors do not have any financial or personal relationships with other individual or organizations that could inappropriately influence or bias this work.

Acknowledgments

This work was supported in part by Department of Radiology, Children's Hospital Boston. The authors would like to thank Nancy Drinan for her help in the preparation of this manuscript.

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