Mechanical anisotropy of the human knee articular cartilage in compression

Articular cartilage exhibits anisotropic mechanical properties when subjected to tension. However, mechanical anisotropy of mature cartilage in compression is poorly known. In this study, both confined and unconfined compression tests of cylindrical cartilage discs, taken from the adult human patello-femoral groove and cut either perpendicular (normal disc) or parallel (tangential disc) to the articular surface, were utilized to determine possible anisotropy in Young's modulus, E, aggregate modulus, Ha, Poisson's ratio, v and hydraulic permeability, k, of articular cartilage. The results indicated that Ha was significantly higher in the direction parallel to the articular surface as compared with the direction perpendicular to the surface (Ha = 1.237 +/- 0.486 MPa versus Ha = 0.845 +/- 0.383 MPa, p = 0.017, n = 10). The values of Poisson's ratio were similar, 0.158 +/- 0.148 for normal discs compared with 0.180 +/- 0.046 for tangential discs. Analysis using the linear biphasic model revealed that the decrease of permeability during the offset compression of 0-20 per cent was higher (p = 0.015, n = 10) in normal (from 25.5 x 10(-15) to 1.8 x 10(-15) m4/N s) than in tangential (from 12.3 x 10(-15) to 1.3 x 10(-15) m4/N s) discs. Based on the results, it is concluded that the mechanical characteristics of adult femoral groove articular cartilage are anisotropic also during compression. Anisotropy during compression may be essential for normal cartilage function. This property has to be considered when developing advanced theoretical models for cartilage biomechanics.