A wealth of data have been collected over the last several decades describing the uniaxial mechanical properties of bone and cartilage. However, most biological responses and failure mechanisms occur in a multiaxial loading environment, and mechanical behavior/response to multiaxial loads have seldom been studied. Increasingly, the importance of torsional loading, and the shear properties of tissues are recognized as important properties which play a role in the mechanical behavior, physiologic response and failure of biologic materials. Uniaxial mechanical testing systems are available in many laboratories, but it is difficult to measure shear properties and apply torsional loads with these systems. Availability of a biaxial mechanical testing system would open significant new opportunities for collaborative research as well as supplement several ongoing NIH funded projects. A biaxial testing system would be applied to the following objectives: 1) Human femurs with simulated metastatic defects in the proximal femur would be tested to failure in torsion. The torsional failure strengths would be related to QCT derived structural parameters and integrated with data from failure tests using longitudinal compression to develop improved fracture risk predictors; 2) Human femurs with a wide range of age related changes would be tested to failure in torsion. Strains in the femur prior to failure would be used to validate finite element models, and the finite element model results would be used to refine QCT derived structural parameters which in turn could be used to predict fracture risk in the elderly. The results would also be applied to the design of fracture prevention strategies; 3) The static and fatigue properties of cortical and cancellous bone from human and animal donors would be measured to be used as input to finite element models of the proximal femur and lumbar spine. These models would then be compared to in vitro torsion tests to develop an improved understanding of the structural properties and failure processes important to fracture risk predictions and prevention; 4) Rat femurs from an osteoporotic animal model would be tested to failure in torsion. The relationship between torsional strength of the femurs and the extent of osteoporotic changes would be compared to similar relationships for the mandible. These data would be applied to understanding the importance of osteoporotic changes in the mandible; 5) The shear properties of tissues between fracture fragments in an animal model would be measured and used to assign material properties in finite element models. The torsional behavior of healing fracture would be used to verify finite element models; 6) The unconfined and confined uniaxial compressive properties, and the shear properties of articular cartilage would be measured. These data would be applied in mathematical models to predict hydrostatic pressure gradients, fluid flow and streaming potentials in articular cartilage, which would in turn be related to extracellular matrix synthesis, assembly and catabolism; 7) The shear strength between calcium phosphate coatings and a metallic substrate will be evaluated by torsional tests of cylindrical specimens bonded to a test fixture, and the shear strength of the bond between bone and the calcium phosphate coating will be evaluated by torsional tests of cylindrical specimens implanted into cancellous bone in an animal model. These data will be applied toward evaluation of two different calcium-phosphate coatings and a surface treatment procedure. The goal of this project is to improve orthopaedic and dental implant fixation in bone.
