OSTEONAL STRUCTURE BETTER PREDICTS TENSILE-STRENGTH OF HEALING BONE THAN VOLUME FRACTION

Refractures and secondary fractures occur after removal of internal fi xation devices even when the X-rays show well-mineralized bony unions. This indicates the unreliability of bone density as the only paramete r 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 micros tructural parameters than by bone density. A drill hole defect in the sheep tibia was used to investigate bone healing under stable conditio ns. 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 h isto-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 b one volume fraction but an increase in strength. The increase in stren gth was linearly correlated to the orientation of the bone lamellae. O rientation of the bone lamellae which cannot be well visualized in a c linical X-ray, relates to strength more than density. Because only den sity and not microstructure can be well demonstrated in a clinical X-r ay, a radiographically 'healed' defect may not reflect structural rest oration for many years.