Adaptive Remodeling Compensates for Strength Reductions Associated with Benign Tumors of the Femur

One reason that radiographic methods based on the absolute or relative size of the tumor alone fail to accurately predict fracture is because these methods fail to take into account the adaptive remodeling of the host bone in response to the tumor. This adaptation can occur when the tumor is slow growing but does not occur in rapidly growing tumors. The purpose of this investigation was to document the adaptive response of children with benign bone tumors in the femur by evaluating the structural properties of pathologic bone in the vicinity of the defect compared to the properties of non-adapted bone and to demonstrate the capability of our CT rigidity analysis to monitor changes in bone structural properties. 44 children (12.7±3.8 yrs) with benign femoral tumors were examined using helical CT of both the affected and contra-lateral normal femur. CT based structural analysis was performed on the affected bone which includes the effect of the hole on the bone structure and adaptive bone remodeling. The bone’s adaptive response to the defect was quantified by superimposing the defect onto images of the homologous cross-sections through the contralateral side and performing CT based structural analysis. Differences in the BMC, EA, EI, and GJ between the affected femur and the femur with the simulated defect alone were measured.

The effect of the defect on structural properties was significant for both cystic and fibrous bone tumors: mean BMC decreased 15% and 21% respectively. The bending moment of inertia decreased 24% for cystic and 19% for fibrous defects.  The adaptive responses for cystic defects included sclerotic margination, cortical expansion, and formation of septae that increased BMC 20% and EI 31%.  The adaptive responses for fibrous defects were cortical thickening and sclerotic margination that increased BMC 43% and EI 9%.  Compared to the normal side, the overall effect of the defect coupled with the compensatory response increased the mean BMC 5%, and mean EI 7% for cystic defects; however for fibrous defects mean BMC increased 22% but the mean EI decreased 10%.  Using standard radiographic criteria, 66% of the defects were predicted to fracture, but in only 27% of cases did fracture occur.  Those bones that did not fracture (65%) underwent adapted remodeling by periosteal expansion and/or new bone formation so that the bending rigidity of the affected bone was decreased less than 32% compared to normal.  Adaptation depends on patient age, activity level, tumor type, and tumor growth rate.  This study demonstrates the ability of CT based structural analysis to monitor changes in bone properties as a result of adaptive remodeling.