Osteonal structure better predicts tensile strength of healing bone than volume fraction

Refractures and secondary fractures occur after removal of internal fixation devices even when the X-rays show well-mineralized bony unions. This indicates the unreliability of bone density as the only parameter to estimate the strength of healing bone. The aim of this study was to show that the strength of healing bone is more influenced by microstructural parameters than by bone density. A drill hole defect in the sheep tibia was used to investigate bone healing under stable conditions. After 4, 6, 9, 12, 24 and 104-week healing periods, bone specimens were taken from the healing zone and tested mechanically as well as histo-morphologically. The bone volume fraction in the defect increased at the beginning faster than the strength. In the later phase (> 24 weeks) the bone remodeling process dominated with little increase in bone volume fraction but an increase in strength. The increase in strength was linearly correlated to the orientation of the bone lamellae. Orientation of the bone lamellae which cannot be well visualized in a clinical X-ray, relates to strength more than density. Because only density and not microstructure can be well demonstrated in a clinical X-ray, a radiographically 'healed' defect may not reflect structural restoration for many years.