Study on the relationship between stability of pedicle screw and injected volume of polymethylmethacrylate in severe osteoporotic lumbar vertebrae

English Keywords:Severe osteoporosis, Lumbar vertebrae, Pedicle screw, Polymethylmethacrylate, The maximum axial pullout strength

English Abstract:

  【Abstract】 Objectives: To compare the stabilities of pedicle screw augmented with different volumes of polymethylmethacrylate(PMMA) in severe osteoporotic lumbar vertebrae, and to investigate the relationship between screw stability and volume of PMMA in severe osteoporotic lumbar vertebrae. Methods: Eighteen lumbar vertebrae were obtained from four fresh-frozen human cadaveric spines(1 male and 3 female specimens with an average of 65±9 years old). Thirty-six pedicles from eighteen lumbar vertebrae were randomly divided into six groups(group A to group F) after measurement of bone mineral density(BMD) of all samples. A pilot hole was prepared in advance by using the same method in all samples. Then, the pilot hole was filled with different volume of PMMA(0ml, 1.0ml, 1.5ml, 2.0ml, 2.5ml, 3.0ml) followed by insertion of screw in group A, B, C, D, E and F, respectively. After that, X-ray and axial pullout test were performed in all groups, and the maximum axial pullout strength(Fmax) was measured. The one-way ANOVA and LSD test were used to detect the differences on BMD and Fmax among six groups. Relationship between Fmax and the volume of PMMA was assessed by using linear regression analysis. Results: All BMD values were less than 0.6g/cm2 and all T values were less than -3.5, therefore, all samples in six groups were diagnosed with severe osteoporotic lumbar vertebrae according to World Health Organization definition. There was no significant difference in BMD among six groups (P=0.799). No PMMA was found around screw in group A. PMMA was found evenly wrapping up screw in group B-F. No malposition of screw and leakage of PMMA were found in all lumbar vertebrae. The maximum axial pullout strength(Fmax) in group A-F was 358.50±86.00N, 442.67±96.02N, 532.00±103.18N, 740.67±120.90N, 841.50±133.42N and 1111.50±158.57N, respectively. Fmax in group B-F increased 23.48%, 48.40%, 106.60%, 134.73% and 210.04% respectively compared with that in group A. There were no significant differences for Fmax between group A and B, group B and C, and group E and F(P=0.230, P=0.203, P=0.152, respectively). While significant differences were found between every other two groups(P<0.05). There was significantly positive correlation between Fmax and the volume of PMMA(r=0.877, P<0.05). Conclusions: PMMA can enhance stability of pedicle screw in severe osteoporosis, and the screw holding strength increases with the increment of the PMMA volume in a limited range. Injection of 3ml PMMA is considered the preferred volume to improve the pedicle screw holding strength in severe osteoporotic lumbar vertebrae.