Axial cyclic behavior of the bone-screw interface.

Screw fixation strength is investigated by using a pullout test. Despite many screw pullout studies, the effects of loading rate on the pullout behavior of pedicle screws are not known. The objective of this study was to assess the effects of loading rate on the pullout stiffness and strength of pedicle screws. Sixty pedicle screws were inserted in foam blocks and pulled out at four different rates: 0.1, 1, 5 and 50 mm/min. Twenty of these 60 screws were cycled non-destructively at four different rates sequentially, i.e., 0.1, 1, 5 and 50 mm/min prior to pullout. Ten additional pedicle screws were inserted in five calf lumbar vertebrae, cycled as in foam group, and pulled out at a rate of either 0.1 or 50 mm/min. The results showed that the stiffness was higher at all rates compared to 0.1 mm/min in foam model but in bone model only 1 and 5 mm/min groups were higher compared to 0.1 mm/min. The pullout strength in 50 mm/min group was higher than that in 0.1 mm/min group in both foam and bone model. The results suggested that loading rate influenced the mechanics of the bone-screw interface. Therefore, a fair comparison between the pullout studies can be achieved under same loading rate conditions. Moreover, the cycling of the pedicle screws in axial direction within a pre-yield region showed an unusual hysteresis curve. Further studies are needed for a better understanding of the mechanics of the screw-bone interface.