Human cortical bone is a complex composite material that displays timedependent deformation. The response of bone to loads in the transverse orientation is not well understood and has implications to the stability of a press-fit hip implant. Total hip arthroplasty is a surgical procedure to replace the hip joint. It involves inserting an implant into the shaft of the femur. One method of implant fixation is a press-fit of the implant into the bone canal. Press-fit fixation relies on the elastic response of the proximal femur to hold the implant in place until new bone growth occurs. The radial load may cause the bone to experience creep, deformation due to a constant load, which can influence initial implant stability. The objectives of this study were to (a) study the time-dependent hoop response of femoral bone to an intramedullary radial load, (b) study the creep response of specimens under constant load until failure, and (c) assess damage morphology and determine the damage mechanisms in cortical bone due to a radial load. A test fixture was used to apply internal pressure to cylindrical bone specimens.

Hoop strain was measured on the surface of the cylindrical specimens. Creep strain, creep strain rate and permanent strain exhibited similar linear behavior at low stress, until a particular stress level, or threshold, where nonlinear behavior began. This deformation, which occurred at low hoop stress levels, may change the press-fit between the implant and bone, result in the loss of initial implant fixation, and may cause creep fracture. A relationship between hoop stress and time to failure was obtained and a hoop creep function was determined to model the creep strain behavior as a function of hoop stress and time. Fractured specimens were prepared for microscopic evaluation of damage morphology. Tensile hoop and compressive radial stresses resulted in extensive radial matrix microcracking combined with osteonal delaminations and may serve as a damage mechanism in cortical bone. The results from this study will provide fundamental knowledge regarding the creep behavior of bone in a similar loading environment to invivo press-fit loading