Biomechanical comparison of expansive pedicle screw and polymethylmethacrylate-augmented pedicle screw in osteoporotic lumbar vertebrae, a cadaver study

English Keywords:Osteoporosis  Expansive pedicle screw  Polymethylmethacrylate  Biomechanics

English Abstract:

  【Abstract】 Objectives: To compare the biomechanical characteristics of expansive pedicle screw(EPS) and polymethylmethacrylate-augmented pedicle screw(PMMA-PS) in osteoporotic cadaveric lumbar vertebrae. Methods: A total of sixteen vertebrae(L1-L4) was obtained from four fresh-frozen human cadaveric spines with a mean of 63 years(range, 51 to 78 years). There were 3 females and 1 male specimens. Anterior-posterior and lateral radiographs of each vertebra were obtained to exclude vertebral fracture, deformity and osteolysis resulting from malignancy. One markedly deformed vertebra was excluded from this study. After the measurement of bone mineral density(BMD) of all vertebral bodies, fifteen vertebrae were randomly divided into three groups. After the preparation of pilot hole by using the same method, the conventional pedicle screw(CPS) was inserted in CPS group, the pilot hole was filled with PMMA followed by CPS insertion in PMMA-PS group, and EPS was inserted in EPS group. Twenty four hours later, X-ray and CT scan were performed to exam the positions of screws and distribution of PMMA. Then, all vertebrae were tightly fixed on MTS 858, each screw was pulled at a constant speed of 10mm/min until the failure of the pedicle screw, and the maximum pullout strength(Fmax) and energy absorbed value(EAV) were measured. Results: There was no significant difference in BMD among three groups(P>0.05). According to World Health Organization definition, vertebrae in three groups were osteoporotic with all BMD values less than 0.8g/cm2 and T values was between -3.5 and -2.5. No malposition of screw was detected in all groups and no cement leakage was detected in PMMA-PS group under X-ray and CT examination. In CPS group, screw was surrounded with bone tissue directly. In PMMA-PS group, screw was wrapped up by PMMA and PMMA existed in bone tissue around the CPS which shaped like a spindle-shaped structure in vertebral body. In EPS group, anterior part of EPS presented an obvious expansion in vertebral body and formed a clawlike structure. Fmax in CPS group, PMMA-PS group and EPS group was 751.50±251.37N, 1521.70±513.27N and 1175.20±396.51N, respectively. Fmax in PMMA-PS group and EPS group was significantly higher than that in CPS group(P<0.001, P=0.026). However, there was no significant difference in Fmax between PMMA-PS group and EPS group(P=0.064). EAV in CPS group, PMMA-PS group and EPS group was 1.47±0.51J, 3.09±0.93J and 2.46±0.69J, respectively. EAV in both PMMA-PS group and EPS group was significantly higher than that in CPS group(P<0.001, P=0.005). However, there was no significant difference in EAV between PMMA-PS group and EPS group(P=0.067). Conclusions: EPS can markedly enhance screw strength with a similar effect with traditional screw augmentation with PMMA for osteoporotic lumbar vertebrae.