Finite element biphasic indentation of cartilage: a comparison of experimental indenter and physiological contact geometries

In experimental cartilage indentation studies, the indenter is typically a plane-ended or hemispherically ended cylinder and can be either porous or n on-porous. Joints such as the hip and knee, however, have much higher radii of curvature than those used in experimental indentation testing. In order to interpret the results from such testing in a physiological context it i s therefore useful to explore the effect of contact geometry on the pore pr essure and strain distribution generated in the cartilage layer. Articular cartilage can be described as a saturated porous medium. and can be conside red a biphasic material consisting of a porous, permeable solid matrix, and an interstitial fluid phase. This behaviour has been predicted in this Stu dy using the ABAQUS soils consolidation procedure. Four contact geometries were modelled: two typical experimental configurations (5 mm radii cylindri cal indenter and hemispherical indenters) and two effective radii represent ative of the hip and knee (20 and 100 mm). A 10 per cent deformation. or a load of 0.9 kN was applied over a ramp time of 2 s, which was then maintain ed for a further 100 s. The porous indenter generated less pore pressure co mpared with the equivalent non-porous indenter and produced higher values o f compressive strain in the solid matrix. The predictions made using the cy lindrical indenters. porous and nonporous. were dominated by the concentrat ions at the edge of the indenter and overestimated the peak compressive str ain in the tissue by a factor of 21 and a factor of 14 respectively when co mpared with the hip model. The hemispherical indenter predicted peak strain s in similar positions to those predicted using physiological radii. howeve r, the magnitude was overestimated by a factor of 2.3 when compared with th e knee and by 5.7 when compared with the hip. The pore pressure throughout the cartilage layer reduced significantly as the radius of the indenter was increased.