Constraints and fixation for implanted joint replacements

Observations made on both cemented and uncemented joint prostheses after about two years of use show a layer of fibrous tissue next to the bone. The fibrous layer smooths over surface features up to 1mm in size, and must be assumed to be weak in tension and shear. Prosthesis-bone interfaces should therefore be designed to transmit all forces acting on the prosthesis as compressive stresses. The forces acting across prostheses are controlled by the constraints exerted by the articulating surfaces; if the useful ligaments present are allowed to do their job of transmitting tension, the articulating surfaces can be designed to transmit only such forces and moments as can safely be transmitted across the prosthesis-bone interfaces.