Strain patterns induced by press-fitting and by an external load in hip arthroplasty: A photoelastic coating study on bone models

The success of prostheses is highly dependent upon load transfer and bone stresses. Reflection photoelasticity is a viable technique for measuring stress patterns in vitro. However, when a cementless stem is tested, the resultant stresses under load are the sum of the implantation stresses and of those induced by load application. Separating these two components is a critical issue that so far has not been thoroughly solved using photoelasticity. Implantation strains and strain under load were measured separately by a suitable procedure: the photoelastic coating was applied to the femur after stem press-fitting. Thus, when an external load is applied (e.g. to simulate a physiological activity), the fringe patterns indicate the effect of the external load alone. Implantation strains are measured independently, after load removal; the stem is extracted and stresses are released, causing a new fringe pattern. The method was used to investigate the stress pattern caused by press-fitting of two cementless hip stems and that caused by a load applied to the implanted femora. Different isochromatic and isoclinic patterns were observed for the strain distribution caused by load application and by press-fitting for two stem designs. Differences in press-fit pattern and load transfer were successfully detected between the two designs.