骨质疏松绵羊腰椎膨胀式椎弓根螺钉与骨水泥强化椎弓根螺钉固定稳定性的动态比较研究

Biomechanical comparison of expansive pedicle screw and PMMA-augmented pedicle screw in osteoporotic sheep lumbar vertebra

中文关键词:  骨质疏松  膨胀式椎弓根螺钉  聚甲基丙烯酸甲酯  生物力学  稳定性

英文关键词:Osteoporosis  Expansive pedicle screw  Polymethylmethacrylate  Biomechanics  Stability

中文摘要:

  【摘要】 目的：比较骨质疏松绵羊腰椎膨胀式椎弓根螺钉（expansive pedicle screw，EPS）与骨水泥强化椎弓根螺钉（polymethylmethacrylate-augmented pedicle screw，PMMA-PS）固定的动态稳定性。方法：8只健康成年雌性绵羊，体重55.7±5.6kg，年龄5.5±0.7岁。行双侧卵巢切除术（去势手术）后1个月开始连续肌肉注射甲基强的松龙（0.45mg/kg/d）10个月，在建模前、激素注射结束后1个月（建模后）测量绵羊腰椎的骨密度（bone mineral density，BMD），BMD显著下降（>25%）时为骨质疏松动物模型成功建立。建模后将每只骨质疏松绵羊腰椎（L1~L6）随机分为3组，每组2个腰椎。普通椎弓根螺钉（conventional pedicle screw，CPS）组，直接拧入CPS；PMMA-PS组，向钉道内注入聚甲基丙烯酸甲酯（PMMA，1.0ml）后拧入CPS；EPS组，直接拧入EPS。螺钉置入术后6周和12周各处死4只绵羊，取出腰椎，剔除标本周围软组织，自各椎间盘处离断，游离成单个椎体。每个腰椎随机选择一侧的螺钉行轴向拔出实验，将椎体固定于MTS 858生物材料实验机上，沿椎弓根螺钉长轴方向以5mm/min的加载速度进行轴向拔出实验，测量螺钉的最大轴向拔出力（the maximum pullout strength，Fmax）和能量吸收值（energy absorbed value，EAV）。结果：建模前、后绵羊腰椎的BMD分别为1.14±0.10g/cm2和0.83±0.07g/cm2，建模后BMD显著下降（P<0.05），平均为27.2%（25.4%~28.9%），骨质疏松绵羊模型成功建立。置钉术后6周EPS组和PMMA-PS组的Fmax分别为1252.13±203.51N和1426.38±235.75N，EAV分别为2.48±0.45J和2.84±0.55J，均显著高于CPS组（827.88±139.22N和1.66±0.30J）（P<0.05）；置钉术后12周EPS组和PMMA-PS组的Fmax分别为1518.88±256.81N和1472.75±248.65N，EAV分别为3.09±0.59J和2.95±0.60J，均显著高于CPS组（906.63±152.50N和1.80±0.35J）（P<0.05）；置钉术后6周、12周EPS组的Fmax和EAV与PMMA-PS组比较差异均无统计学意义(P>0.05)。置钉术后12周CPS组和PMMA-PS组的Fmax和EAV与同组置钉术后6周比较无显著性变化（P>0.05），置钉术后12周EPS组的Fmax和EAV较同组置钉术后6周均有显著性提高（P<0.05）。结论：与CPS相比，EPS可显著提高螺钉在骨质疏松绵羊腰椎中的稳定性，并达到了与临床常用的PMMA-PS近似的固定效果。

英文摘要:

  【Abstract】 Objectives: To compare the biomechanical performance of expansive pedicle screw(EPS) and polymethylmethacrylate-augmented pedicle screw(PMMA-PS) in osteoporotic sheep lumbar vertebra. Methods: osteoporotic sheep model was established through bilateral ovariectomy combined with the intramuscular injection of methylprednisolone（0.45mg/kg/d）. The bone mineral density(BMD) of sheep lumbar was examined before the establishment of osteoporotic sheep model and one month after the injection of methylprednisolone, more than 25% decrease in BMD of lumbar spine was considered as the successful establishment of osteoporotic sheep. Six lumbar vertebrae(L1-L6) in each sheep were divided into three groups randomly(two vertebrae with four pedicles in each group) after the establishment of osteoporosis sheep. In CPS group, conventional pedicle screw(CPS) was inserted through the pilot hole into vertebral body without any augmentation. In PMMA-PS group, PMMA(1.0ml) was injected into the pilot hole prior to the insertion of CPS. In EPS group, EPS was inserted through pedicle into vertebral body. Four sheep were sacrificed and lumbar vertebrae(L1-L6) were harvested at the 6-week and 12-week post-operation, respectively. All the lumbar spines were disarticulated and dissected free of soft tissue, yielding individual vertebra. For two pedicle screws in each vertebra, one side was selected randomly for axial pullout tests. Once the specimen was tightly secured, each screw was pulled at a constant speed of 5mm/min by MTS 858 until purchase failure. The maximum pullout strength(Fmax) and the energy absorbed value(EAV) were recorded. Results: BMD value before and after the establishment of osteoporosis sheep was 1.14±0.10g/cm2 and 0.83±0.07g/cm2, respectively. There was a significant decrease on BMD(mean 27.2%, from 25.4% to 28.9%) between before and after the establishment of osteoporosis sheep(P<0.05), which demonstrated the successful establishment of osteoporosis sheep model. At 6-week post-operation, Fmax and EAV in both EPS group(1252.13±203.51N and 2.48±0.45J) and PMMA-PS group(1426.38±235.75N and 2.84±0.55J) were significantly higher than those(827.88±139.22N and 1.66±0.30J) in CPS group(P<0.05). At 12-week post-operation, Fmax and EAV in both EPS group(1518.88±256.81N and 3.09±0.59J) and PMMA-PS group(1426.38±235.75N and 2.95±0.60J) were significantly higher than those(906.63±152.50N and 1.80±0.35J) in CPS group(P<0.05). There were no significant differences on Fmax and EAV between PMMA-PS and EPS groups at both 6-week and 12-week post-operation(P>0.05). There were also no significant increments on Fmax and EAV in CPS and PMMA-PS groups at 12-week post-operation compared with those at the 6-week(P>0.05). In EPS group, however, there was a significant improvement on Fmax and EAV at the 12-week post-operation compared with those at the 6-week(P<0.05). Conclusions: Compared with CPS, EPS can markedly enhance screw stability in osteoporotic sheep lumbar vertebra with a similar effect to the traditional method of screw augmentation with PMMA in initial surgery in osteoporosis.