Finite element analysis was performed on an implanted femur, using loads of daily living activities, performed by total hip replacement patients. A probabilistic bone fatigue failure model was utilized to analysis the risk of post-operative femoral fracture in different patient groups, depending on bone fatigue strength for young and elderly patients, and activity levels for normal and active patients. Different bone-implant interface conditions were considered: after surgery, osseointegrated, fibrous tissue covering, and loose. For young patients, the probability of failure is less than 4% for all cases. While in elderly patients, high of 28.2% and 57.9% are reported for normal and active groups respectively. For both age groups and activity levels, loose stems had the highest probability of failure, while osseointegrated had the lowest.