Biomechanical properties of hip cartilage in experimental animal models.

The material properties of normal adult articular cartilage were determined in the femoral head and acetabulum of baboons, dogs, and bovines, and were compared with those of normal human hip cartilage. In situ creep and recovery indentation experiments were performed using an automated creep indentation apparatus. To curvefit the entire creep curve, a numerical algorithm based on biphasic finite element methods and nonlinear optimization was developed. This effort represents the first successful use of 100% of the creep indentation curve to obtain the mechanical properties of normal articular cartilage. The results show that material properties of articular cartilage exhibit significant topographical variations in the femoral head and acetabulum, and between these two bone structures. Furthermore, significant differences exist in the mechanical properties of hip cartilage among the 4 species. Specifically, in all species the smallest aggregate modulus is found in the inferior aspect of the femoral head. Among all species, human hip cartilage is the stiffest in all test sites; bovine tissue is the softest. Human tissue has the smallest Poisson's ratio and permeability in all test sites. The aggregate modulus of human hip cartilage is closely resembled by that of baboon hip cartilage. Anatomically, canine and baboon hips exhibit similar characteristics to the human hip joint; the bovine hip joint is distinctly different. Based on this study's data, the baboon represents the most appropriate animal model of normal human hip articular cartilage.

Comparative study of the intrinsic mechanical properties of the human acetabular and femoral head cartilage.

Biphasic creep indentation methodology and an automated indentation apparatus were used to measure the aggregate modulus, Poisson's ratio, permeability, thickness, creep and recovery equilibrium times, and percentage of recovery of normal articular cartilage in 10 human hip joints. These properties were mapped regionally to examine the mechanical factors involved in the development of site-specific degenerative lesions in the acetabulum and femoral head. The results indicate that there are significant differences between these properties regionally in the acetabulum and femoral head and between the two anatomical structures. Specifically, it was found that cartilage in the superomedial aspect of the femoral head has a 41% larger aggregate modulus than its anatomically corresponding articulating surface in the acetabulum. In addition, the superomedial aspect of the femoral head has the greatest aggregate modulus (1.816 MPa) within the hip joint. During sitting, the inferior portion of the femoral head is in contact with the anterior acetabulum, and the anterior acetabulum has a 53% greater aggregate modulus than the inferior femoral head. This area below the fovea on the femoral head has the least aggregate modulus (0.814 MPa) within the hip joint. These mismatches in the compressive modulus of opposing articulating surfaces may contribute to degeneration of cartilage in the superomedial acetabulum and the inferior femoral head. Our findings support the clinical observation that these areas are frequent sites of early degeneration.