ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

AN ENHANCED DISCRETE ELEMENT ANALYSIS METHOD FOR
PREDICTING HIP CONTACT STRESSES
C.L. Abraham 1 , S.A. Maas 1,2 , B.J. Ellis 3 , J.A. Weiss 4 , C.L. Peters 5 , A.E. Anderson 6
1. ABSTRACT
Discrete element analysis (DEA) may provide an efficient alternative to finite element
analysis (FEA) for the study of contact stresses in the hip. Previous applications of
DEA have made simplifying assumptions that resulted in unrealistic predictions for the
magnitude and distribution of cartilage contact stress, precluding their clinical use. The
objectives of this study were to develop and verify an enhanced DEA model that
incorporates subject-specific bone and cartilage geometry and to compare predictions of
hip cartilage contact stresses predicted by DEA with FEA. CT image data of a
cadaveric hip were segmented to determine cartilage thickness and the geometry of the
bone-cartilage interface. Bones were represented as rigid bodies and cartilage was
represented as an array of springs. The same material properties were used by DEA and
FEA. DEA and FEA contact patterns both predicted irregular, complex contact
patterns, consistent with patterns observed in-vitro. Peak contact stresses for DEA and
FEA ranged from 9.8-13.6 and 6.2-9.8 MPa, respectively. Average contact stresses for
DEA and FEA ranged from 3.0-3.7 and 2.0-2.5 MPa, respectively. The use of subjectspecific
bone and cartilage geometry with the DEA method predicted realistic patterns
for contact stress. Because of its low computational expense, the DEA algorithm may
provide an alternative to FE modeling for the prediction of contact stresses in the hip for
surgical planning or to assess the mechanical effects of surgery.
2. INTRODUCTION
Chronic exposure of the articular cartilage in the hip to elevated pressure predicts the
progression and onset of osteoarthritis (OA) [1, 2]. Developing methods for accurately
quantifying cartilage stress is clinically relevant. For example, pressure maps may be
used to quantify differences between normal and pathologic hip joints, generate
preoperative surgical plans using optimization routines, and predict long-term prognosis
following surgical treatment. Computational modeling offers an approach to estimate
cartilage contact pressures, which cannot currently be measured in-vivo. Both finite
element analysis (FEA) [3-5] and discrete element analysis (DEA) [6-8] have been
developed to estimate hip cartilage pressures.
1 Ph.D. Student, 2 Lead Software Developer, 3 Staff Scientist, 4 Professor, 6 Corresponding Author, Adjunct
Assistant Professor, Department of Bioengineering, University of Utah, 50 S. Central Campus Dr., RM
2460 Salt Lake City, Utah, USA 84112. 5 Professor, 4 Adjunct Professor, 6 Assistant Professor, Department
of Orthopaedics, University of Utah. 590 Wakara Way, Salt Lake City, Utah, USA 84108.