Influence of the Superficial Tangential Zone for Cartilage Modeled in Finite Deformation and With Tension/Compression Nonlinearity

Creation of replacement tissue to repair articular surface defects remains a challenge. Normal zonal characteristics of articular cartilage throughout its thickness, particularly the superficial tangential zone (STZ), and normal material properties have not been reproduced in vitro in scaffolds nor in vivo in repairing defects. Without sufficient quality, such transplanted scaffolds in vivo may be doomed mechanically from the outset. Removal of the STZ from normal cartilage negatively affects the remaining cartilage’s ability to support axial loads and retain fluids [1–3]. Previous studies have modeled excessive axial deformation of repair cartilage [4–5]. Studies have shown that modeling the STZ of normal cartilage as transversely isotropic provides better agreement with indentation experimental results than isotropic models [6–9]. Others have modeled experimental conditions by incorporating tension and compression nonlinearity [10]. Previous analyses have indicated that strain-dependent permeability within the STZ can positively affect the ability of free-draining normal and repair models to withstand imposed surface loads [11,12]. This finite element study further examined the role of an STZ with strain-dependent permeability on the behavior of normal and repaired articular surfaces under contact loading from rigid permeable and impermeable spheres. Nonlinear geometry permitted finite deformations to occur while the differential stiffness in tension and compression was also represented.