Mechanical behavior of screws in normal and osteoporotic bone.

Fracture fixation in severe osteoporotic bone by means of implants that rely on screw anchorage is still a clinical problem. So far, a sufficiently accurate prediction of the holding capacity of screws as a function of local bone morphology has not been obtained. In this study the ultimate pullout loads of screws in the epi-, meta-, and diaphyseal regions of human tibiae were correlated to the cortical thicknesses and cancellous bone mineral densities at the screw axes determined from QCT densitometric data. Stepwise multiple linear regression showed that in regions with cortical thicknesses below 1.5 mm, cancellous density determined the ultimate pullout load (R2 = 0.85, p < 0.001), while in regions with cortices above 1.5 mm, cortical thickness alone significantly influenced the holding capacity of a screw (R2 = 0.90, p < 0.001). The findings of this study provide a basis for a bone morphology-related pre-operative estimation of the holding capacity of screws, which could help to improve their proper application in osteoporotic bone.