Mechanical strength of trabecular bone at the knee.

Interest in the biomechanical properties of trabecular bone has expanded in response to the problems related to total and partial joint replacement with the knee joint constituting a main focus of attention. This relatively recent development has left a number of fundamental problems unanswered, especially related to the machining, storage and testing of trabecular bone specimens. Nevertheless, these studies have contributed to the understanding of the mechanical function of trabecular bone. Regarding the role of trabecular bone at the knee joint, the following conclusions may be emphasized (conclusions drawn from the author's previous studies (I-X) are shown in italics): (1) Trabecular bone is almost exclusively responsible for the transmission of load at the proximal tibial epiphysis from the knee joint to the metaphysis. The peripheral shell surrounding the epiphysis is not composed of cortical bone and plays a negligible role in load transmission. (2) The compressive strength and stiffness of trabecular bone is primarily dependent upon the apparent density, trabecular architecture and the strength of the bone material. Direct and indirect sources suggest that the true material strength of trabecular bone is less than that of cortical bone. The epiphyseal trabecular architecture, featuring a marked polarity with alignment of primary trabeculae at right angles to the joint surface, is responsible for functional anisotropy which points to the axial compressive properties as the more important mechanical parameters. (3) Tensile and shear properties are of special relevance to mechanical loosening of implants. These properties may be derived from the apparent density, and a close empirical relation to the axial compressive strength and stiffness is suggested. (4) The foam-like structure of trabecular bone is the basis for the large energy absorptive capacity. (5) The pattern of axial compressive stiffness and strength at the normal proximal tibia differs little among individuals. Supporting the medial tibial plateau is a large high strength area with maximal strength centrally and slightly anteriorly, while laterally there is a restricted area of relatively high strength posteriorly with a lower maximal value than medially. Bone strength is significantly reduced within ten millimeters of the subchondral bone plate, and this reduction continues distally at the lateral condyle. At both condyles strength is reduced towards the periphery with very low values being obtained at the margins of the condyles and at the intercondylar region. Absolute bone strength values are influenced by the level of physical activity.(ABSTRACT TRUNCATED AT 400 WORDS)