Application of two different pedicle screw augmentation techniques used for in vitro sheep osteoporotic vertebra model

Objective: To determine the effects of the two kinds of pedicle screws with cement augmentation used for in vitro sheep osteoporotic vertebra model. Method: Forty osteoporosis specimens of lumbar vertebrae demineralized by decalcification solution were complete randomly devided into four groups including Group A (only screw), Group B (whole trajectory filled with PMMA), Group C₁ (partial trajectory filled with PMMA) and Group C₂ (partial trajectory filled with PMMA). Osteoporosis specimens from four groups were tested by bone mineral density (BMD) and observed by spiral CT and MicroCT. The maximum force and energy absorption of pedicle screw were tested. Results: BMD in each group were averagely declined about 25%~30% (P>0.05), so osteoporotic vertebrae model was successfully established. Spiral CT and MicroCT showed that screw of Group B was fully surrounded by PMMA, which constituted screw-PMMA-bone interface, while screw of Group C₂ was partially surrounded by PMMA, which constituted partial screw-bone and partial screw-PMMA-bone interface. The axial pull-out test showed that the maximum force (F_max) and its energy absorption in Group B, C_1,2 were significantly higher than those in Group A(P<0.05), those in Group B, C₂ were higher than those in Group C ₁ (P<0.05), and those in Group B and Group C₂ had no significant difference(P>0.05). Conclusions: Both the whole and partial pedicle screw augmentation can significantly improve the strength of pedicle screw in the context of osteoporosis. The latter not only formed a special whole interface consisting of partial "screw-bone" and partial screw-CSC-bone interface, but produced the strengthening effect equivalent to the former by increasing injection doses of PMMA.