Effect of superficial collagen patterns and fibrillation of femoral articular cartilage on knee joint mechanics—A 3D finite element analysis M.E. Mononen a,b,n , M.T. Mikkola a , P. Julkunen c , R. Ojala d , M.T. Nieminen d,e , J.S. Jurvelin a , R.K. Korhonen a a Department of Applied Physics, University of Eastern Finland, POB 1627, FI-70211 Kuopio, Finland b Department of Clinical Physiology and Nuclear Medicine, Kuopio University Hospital, Kuopio, Finland c Department of Clinical Neurophysiology, Kuopio University Hospital, Kuopio, Finland d Department of Diagnostic Radiology, Oulu University Hospital, Oulu, Finland e Department of Medical Technology, University of Oulu, Oulu, Finland article info Article history: Accepted 6 November 2011 Keywords: Articular cartilage Split-line pattern Osteoarthritis Magnetic resonance imaging Finite element analysis abstract Collagen fibrils of articular cartilage have specific depth-dependent orientations and the fibrils bend in the cartilage surface to exhibit split-lines. Fibrillation of superficial collagen takes place in osteoar- thritis. We aimed to investigate the effect of superficial collagen fibril patterns and collagen fibrillation of cartilage on stresses and strains within a knee joint. A 3D finite element model of a knee joint with cartilage and menisci was constructed based on magnetic resonance imaging. The fibril-reinforced poroviscoelastic material properties with depth- dependent collagen orientations and split-line patterns were included in the model. The effects of joint loading on stresses and strains in cartilage with various split-line patterns and medial collagen fibrillation were simulated under axial impact loading of 1000 N. In the model, the collagen fibrils resisted strains along the split-line directions. This increased also stresses along the split-lines. On the contrary, contact and pore pressures were not affected by split-line patterns. Simulated medial osteoarthritis increased tissue strains in both medial and lateral femoral condyles, and contact and pore pressures in the lateral femoral condyle. This study highlights the importance of the collagen fibril organization, especially that indicated by split-line patterns, for the weight-bearing properties of articular cartilage. Osteoarthritic changes of cartilage in the medial femoral condyle created a possible failure point in the lateral femoral condyle. This study provides further evidence on the importance of the collagen fibril organization for the optimal function of articular cartilage. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Collagen fibril meshwork is known to affect significantly the mechanical response of cartilage, especially under dynamic and impact loading (Li et al., 2000; Silver et al., 2001; Korhonen et al., 2003; Mononen et al., 2011). In adults, articular cartilage can be divided into three depth-dependent zones on the basis of its characteristic collagen network architecture; superficial, middle and deep. In the superficial zone the fibrils are oriented parallel to the cartilage surface, in the middle zone the fibrils have a less organized random-appearing orientation, and in the deep zone the fibrils are oriented vertically to the cartilage surface (Benninghoff, 1925). In addition, cartilage surfaces exhibit specific fibril patterns in the plane parallel to the surface, this being indicated by the split-lines, which are believed to orient according to joint movements and principal stress directions, resisting effectively the loads exposed to cartilage (Bullough and Goodfellow, 1968; Below et al., 2002; Goodwin et al., 2004; Leo et al., 2004; Bottcher et al., 2009). The importance of the depth-dependent collagen architecture and split-lines on carti- lage mechanics has been demonstrated by 2D finite element (FE) models at the tissue and joint level (Julkunen et al., 2008; Mononen et al., 2011) as well as in 3D in vitro simulations (Li et al., 2009). However, the effects of superficial fibril patterns on knee joint stresses have not been investigated in 3D joint models. In osteoarthritis (OA), the superficial fibrils lose their organi- zation and functional integrity (Buckwalter and Mankin, 1997; Temple-Wong et al., 2009; Saarakkala et al., 2010). During OA progression, collagen fibrillation progresses deeper into the Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics 0021-9290/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2011.11.003 n Corresponding author at: Department of Applied Physics, University of Eastern Finland, POB 1627, FI-70211 Kuopio, Finland. Tel.: þ358 40 3553260; fax: þ358 17 163266. E-mail address: mika.mononen@uef.fi (M.E. Mononen). Journal of Biomechanics 45 (2012) 579–587