Augmentation of pedicle screw fixation strength using an injectable calcium sulfate cement:an in vivo study

Objective:To investigate the long-term in vivo biomechanical performance of pedicle screws aug-mented with calcium sulfate cement.Method:10 pedicle screws were implanted into the lumbar vertebrae of 15 adult female landraces weighing 105~115kg.The pedicle srcrews in the left pedicles of the L1 vertebrae and the right pedicles of the L5 vertebrae were not augmented as control groups,the pedicle srcrews in the right pedicles of the L1 vertebrae and the left pedicles of the L5 vertebrae were augmented with polymethyl methacrylate(PMMA) as PMMA groups,the pedicle srcrews in the pedicles of the L2,L3 and L4 vertebrae were augmented with the calcium sulfate cement as calcium sulfate groups.The landraces were randomly divid-ed into 3 groups based on study periods including immediate(n=5),6 weeks(n=5) and 12 weeks(n=5).At the end of each assigned study period,the animals were sacrificed and axial pull out tests and histological analy-ses were conducted on the isolated specimen vertebrae.The bone mineral density of the vertebral bodies were measured using dual-energy X-ray absorptiometry before the biomechanical tests.The Oneway ANOVA was used to evaluate the difference in the bone mineral density,pull-out strength and the thickness of the bone walls around the screws.Result:Mean bone mineral densityof all groups was similar.The maximum pull out strength(POS) of the immediate control group,calcium sulfate group and PMMA group was 1.03±0.16kN,1.92±0.27kN and 3.63±0.55kN respectively.The maximum POS of the 6-week control group,calcium sulfate group and PMMA group was 1.26±0.09kN,1.85±0.28kN and 3.75±0.56kN.The maximum POS of the 12-week control group,calcium sulfate group and PMMA group was 1.32±0.10kN,2.04±0.34kN and 3.72±0.61kN.Among which significant differences were found at each time point(P0.001).Histologically,the thickness of the bone tissue in the immediate control group,calcium sulfate group and PMMA group was 231±33μm,244±28μm and 255±25μm respectively,among which no significant difference was found(P0.54).The thickness of the bone tissue in the 6-week control group and PMMA group was 256±24μm and 267±37μm respectively,no signifi-cant difference was found between them(P0.69).The thickness of the bone tissue in the 12-week control group,calcium sulfate group and PMMA group was 244±44μm,972±62μm and 253±29μm respectively,no sig-nificant difference found between control and PMMA groups,however the bone tissue in the 12-week calcium sulfate group was significantly thicker than that of the other two groups(P0.002,P0.001),and also signifi-cantly thicker than that of the immediate calcium sulfate group(P0.001).Conclusion:The injectable calcium sulfate cement can significantly improve the immediate pull out strength of pedicle screw fixation,and this effect can be maintained even if the calcium sulfate cement has been absorbed completely as it can be substituted by autogenous bone ingrowth.Calcium sulfate cement may be a good alternative to PMMA for the augmentation of pedicle screw fixation.