Predicting pathologic fracture risk in the management of metastatic bone defects

Guidelines for the prediction of pathologic fracture would facilitate the management of patients with metastatic bone defects. Unfortunately, existing clinical guidelines have not been validated, often run counter to engineering practice, and do not accurately predict the risk of fracture for many patients. To serve as a basis for improved guidelines, a factor of risk for a pathologic fracture is defined as the load applied to a bone divided by the load at which the bone fails. Failure loads for bones with metastatic defects have been measured in vitro, and depend on defect geometry, bone properties, and the type of loading. For a diaphyseal defect that destroys 50% of the cortex, strength reductions of between 60% and 90% can occur. The load-bearing capacity of a long bone with a diaphyseal defect also can be predicted using computer models if the geometry of the defect and properties of surrounding bone are known. Similarly, new methods that apply basic engineering principles to computed tomography data allow prediction of the load-bearing capacity of vertebrae with simulated defects. By contrast, the data presented here suggest that by using plane radiographs or computed tomographic (CT) examinations, experienced orthopaedic surgeons cannot accurately estimate the strength reductions or load-bearing capacity for proximal femurs with intertrochanteric defects. By combining new methods to predict the load-bearing capacity with estimated loads for activities of daily living, it is possible to calculate a factor of risk for pathologic fractures.