Background. Children suffering from primary bone cancer necessitating resection of growth plates, may suffer progressive leg length discrepancy, which can be attenuated with extendable prostheses. A serious complication is catastrophic implant failure. Over time, bone will remodel, altering the stress pattern in the implant. By using finite element analysis we can model different bone remodeling conditions to ascertain the effect that this will have on stress distribution and magnitude. A finite element analysis was performed. Simplified computer generated models were designed of a cemented femoral Stanmore growing massive endoprosthesis. Three scenarios were designed, modelled on post-operative radiographs. Scenario 1 had a gap between the end of the femur and the implant collar, scenario 2 had no gap, but with no bone attachment into the collar, and scenario 3 had growth of the bone over the length of the collar with attachment. Physiological loading conditions were applied. The resultant stress in the implant for each scenario was measured, and compared to the strength of the material. Peak stresses were recorded at the stem-collar junction. The maximum stress recorded in the implant in scenario 1 was 3104.2Mpa, compared to 1054.4Mpa in scenario 2, and 321.2Mpa in scenario 3. Conclusions. Both accurate reduction and bone growth with attachment to the stem of a massive endoprosthesis will greatly reduce the resultant stress in the implant under loading conditions. The load is redistributed throughout the length of the bone. This may help to prevent catastrophic failure in the implant under loading conditions. Further investigations of patient findings are needed to ensure the model findings are verified. Level of Evidence. IIb (Theoretical)