INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING Int. J. Numer. Meth. Biomed. Engng. (2013) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/cnm.2586 PAPER PRESENTED AT ECCOMAS-3MBM - MULTISCALE AND MULTIPHYSICS MODELLING IN BONE MECHANOBIOLOGY Bone remodelling in the natural acetabulum is influenced by muscle force-induced bone stress Justin Fernandez 1,2, * ,† , Massimo Sartori 3 , David Lloyd 4 , Jacob Munro 1,5 and Vickie Shim 1 1 Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand 2 Department of Engineering Science, The University of Auckland, Auckland, New Zealand 3 Department of Neurorehabilitation Engineering, University Medical Center Göttingen, Göttingen, Germany 4 Griffith Health Institute, Griffith University, Queensland, Australia 5 Department of Orthopaedic Surgery, Faculty of Medicine, The University of Auckland, Auckland, New Zealand SUMMARY A modelling framework using the international Physiome Project is presented for evaluating the role of mus- cles on acetabular stress patterns in the natural hip. The novel developments include the following: (i) an efficient method for model generation with validation; (ii) the inclusion of electromyography-estimated mus- cle forces from gait; and (iii) the role that muscles play in the hip stress pattern. The 3D finite element hip model includes anatomically based muscle area attachments, material properties derived from Hounsfield units and validation against an Instron compression test. The primary outcome from this study is that hip loading applied as anatomically accurate muscle forces redistributes the stress pattern and reduces peak stress throughout the pelvis and within the acetabulum compared with applying the same net hip force with- out muscles through the femur. Muscle forces also increased stress where large muscles have small insertion sites. This has implications for the hip where bone stress and strain are key excitation variables used to initiate bone remodelling based on the strain-based bone remodelling theory. Inclusion of muscle forces reduces the predicted sites and degree of remodelling. The secondary outcome is that the key muscles that influenced remodelling in the acetabulum were the rectus femoris, adductor magnus and iliacus. Copyright © 2013 John Wiley & Sons, Ltd. Received 8 April 2013; Revised 12 July 2013; Accepted 18 July 2013 KEY WORDS: bone remodelling; acetabulum; finite elements; hip muscles 1. INTRODUCTION The acetabulum is the articulating region at the pelvis where forces are transmitted from the lower limb. The hip has been reported to endure 2–4 times body weight during walking and stair ascent [1] and measured up to 6 times body weight using instrumented in vivo devices [2]. When healthy, it can receive a lifetime of loading but disease, injury and a change in loading pattern can initiate a cycle of bone degeneration. A wealth of studies in the literature focuses on artificial hip and implant intervention [3]. However, with increased focus on early intervention and non-invasive treatments, an evaluation of the bone stress in the natural hip can provide an understanding of the homeostatic state of stress provided by muscle forces. Diseases such as osteoarthritis are often initiated by deviation from this natural state. Multibody dynamics offers a robust tool for calculating whole body dynamics and muscle forces. *Correspondence to: Justin Fernandez, Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand. † E-mail: j.fernandez@auckland.ac.nz Copyright © 2013 John Wiley & Sons, Ltd.