磷酸钙骨水泥强化椎弓根螺钉固定的生物力学特性

背景:聚甲基丙烯酸甲酯能改善椎弓根螺钉和周围骨质间界面的情况、显著提高螺钉固定强度,但它在术中和术后伴有聚合热损伤效应、毒性和不可吸收等缺点。磷酸钙骨水泥具有生物相容性和生物安全性好、可降解、不产生聚合热等优点,是一种较理想的聚甲基丙烯酸甲酯替代材料。目的:从生物力学方面评价磷酸钙骨水泥对椎弓根螺钉固定的强化作用。设计:随机对照,重复观察测量。单位:南京医科大学第二附属医院骨科。材料:实验于2002-08/2003-02在华中科技大学同济医学院完成。①由同济医科大学解剖教研室提供两具新鲜男性尸体椎骨,一具52岁,一具50岁。各取10个椎骨(T8～12,L1～5)分别构成52岁组和50岁组。摄X线片排除先天性畸形、骨折和肿瘤等病变。两组椎骨均为Ⅰ级骨质疏松,符合实验要求。②磷酸钙骨水泥固相主要成分是磷酸四钙和磷酸三钙超细粉末,液相主要成分是枸橼酸盐溶液,使用时按1g固相:1mL液相的比例进行配制,初步凝固时间15min,最终凝固时间12h,最大压缩强度介于45～57MPa。③椎弓根螺钉自制,螺钉直径5mm,螺纹段长34mm,螺距2mm,螺纹深0.8mm。方法:①磷酸钙骨水泥最终凝固时强化椎弓根螺钉固定的生物力学测试:取50岁组椎骨作为测试对象。对照侧:钉道直接置入椎弓根螺钉;强化侧:填入磷酸钙骨水泥再置入椎弓根螺钉。置钉后的椎骨在37℃恒温箱里放置12h,然后测定椎弓根螺钉的最大轴向拔出力。②磷酸钙骨水泥初步凝固时强化椎弓根螺钉固定的生物力学测试:取52岁组椎骨作为测试对象。用同样方法在椎弓根对照侧直接置入椎弓根螺钉,强化侧填入骨水泥后再置入椎弓根螺钉,37℃恒温箱里放置15min,测定椎弓根螺钉初步凝固时的最大轴向拔出力。③磷酸钙骨水泥强化松动椎弓根螺钉固定的生物力学测试:取测试后的50岁组椎骨,用磷酸钙骨水泥重新固定12h后拔松的椎弓根螺钉,测定其两侧的最大轴向拔出力。主要观察指标:①磷酸钙骨水泥最终凝固时强化椎弓根螺钉固定的生物力学测试结果。②磷酸钙骨水泥初步凝固时强化椎弓根螺钉固定的生物力学测试结果。③磷酸钙骨水泥强化松动椎弓根螺钉固定的生物力学测试结果。结果:①50岁组对照侧和强化侧的椎弓根螺钉最大轴向拔出力中位数分别为620N和1136N,强化侧较对照侧增加83%(P<0.01)。强化骨-螺钉界面的抗剪切应力中位数从1.16N/mm2增加到2.13N/mm2。②52岁组对照侧和强化侧的椎弓根螺钉最大轴向拔出力中位数分别为554.5N和859.5N,强化侧较对照侧增加55%(P<0.01)。强化骨-螺钉界面的抗剪切应力中位数从1.039N/mm2增加到1.61N/mm2。③50岁组椎骨对照侧和强化侧重新固定12h后最大轴向拔出力中位数分别为517N和876N,和同侧松动后轴向拔出力中位数比较,分别增加了63.6%和54.2%(P均<0.01)。结论:磷酸钙骨水泥初步凝固和最终凝固时能强化椎弓根螺钉的固定,并且椎弓根螺钉松动后使用磷酸骨水泥能使螺钉重新获得固定。椎体强化侧的椎弓根螺钉均从骨-螺界面剥离开来,不伴周边骨质和椎弓根的严重损害,有利于螺钉松动、拔出后的二次置入。

Biomechanical characteristics of calcium phosphate cement in the reinforcement of vertebral pedicle screw fixation

BACKGROUND: Polymethylmethacrylate (PMMA) can ameliorate the condition between vertebral pedicle screws and peripheral bone-matrix interfaces and notably enhance the strength of screw fixation. However, there are several disadvantages during and after operation such as polymerized thermal damaging effect, toxicity and unabsorbable etc. Calcium phosphate cement (CPC) is biocompatible and biodegradable with good biosafty and produce no heat of polymerization, which is a perfect substitute for PMMA.OBJECTIVE: To evaluate the reinforcing effect of CPC on vertebral pedicle screw fixation at biomechanical aspect.DESIGN: Randomized control and repetitive observed measurement.SETTING: Department of Orthopaedics, Second Affiliated Hospital of Nanjing Medical University.MATERIALS: The experiment was conducted in Tongji Medical College of Huazhong University of Science and Technology from August of 2002 to February 2003. ①Two fresh spines from male bodies respectively aged of 52 and 50 years were provided by the Anatomic Department of Tongji Medical College. Ten vertebrae in each spine were obtained (T8-12, L1-5) and taken as 52-year group and 50-year group. Radiographs of these vertebrae were taken to exclude congenital abnormality, fracture, tumor or other pathological changes. Vertebrae in both groups were osteoporoses of grade I and in accordance with experimental requirement.②Main components of solid phase of CPC were micropowder of tricalcium phosphate and tetracalcium phosphate (TTCP) and its main ingredients of liquid phase was citrate solution, which was prepared with solid phase in the ratio of 1g vs 1 mL.Primarily setting-time was 15 minutes and the final setting-time was 12hours with the maximum compressive strength between 45 Mpa and 57 Mpa. ③Diameter of self-made pedicle screws was 5 mm; Length of screw thread segment was 34 mm; Pitch was 2 mm; Depth of screw thread was 0.8 mm.METHODS: ①Biomechanical test of pedicle screw fixation at final solid time of CPC: Vertebrae of 50-year group were taken as testing subjects.Control lateral: vertebral pedicle screws were implanted directly in screw path; Strengthening lateral: vertebral pedicle screws were inserted after fillingwith CPC. After that, specimens were deposited in a thermostated container for twelve hours at 37 ℃. Maximum axial pull-out strength of vertebral pedicle screw was determinated. ②Biomachanical test of vertebra pedicle screw fixation when CPC primarily hardened: specimens in 52-year group were taken as testing subjects. In the same way, vertebral pedicle screw was implanted in the control lateral of vertebral pedicle,while that in the strengthen lateral was implanted after filling of cement,which were placed in the thermostated container for 15 minutes at 37 ℃,the maximum axial pull-out strength of vertebral pedicle in primary setting time were determinated. ③Biomechanical test of CPC in the reinforcement of loose vertebral pedicle screw fixation: vertebrae in 50-year group were selected. Loosened vertebral pedicle screws were re-fixed with CPC for 12 hours. Maximum axial pull-out strength of bilateral screws was tested.MAIN OUTCOME MEASURES: ①Biomechanical testing results of pedicle screw fixation at final solid time of CPC. ②Biomechanical testing results of vertebral pedicle screw fixation when CPC primarily hardened.③Biomechanical testing results of CPC in the reinforcement of loose vertebral pedicle screw fixation.RESULTS: ①Medians of maximum axial pull-out strength of vertebral pedicle screws in control and strengthening laterals in the 50-year group were 620 N and 1 136 N respectively. Compared with control lateral, that in the strengthening lateral increased by 83 % (P ＜ 0.01). Median of anti-cutting stress increased from 1.16 N/mm2 to 2.13 N/mm2 after being strengthened. ②The medians of those in the 52-year group were 554.5 N and 859.5 N respectively and that in the strengthening lateral increased by 55 % in comparison with that in the control lateral (P ＜ 0.01).The median of anti-cutting stress of reinforced bone-screw interface increased from 1.03 N/mm2 to 1.61 N/mm2. ③Maximum axial pull-out strength of vertebral pedicle screws in control and strengthening laterals in the 50-year group of 12 hours after re-fixation were 517 N and 876 N, which respectively increased by 63.6% or 54.2% (P ＜ 0.01) in comparison with median of that of loose screw in the same lateral.CONCLUSION: CPC can enhance vertebral pedicle screw fixation in primary and final setting time, with which loosened screws can be re-fixed.Vertebral pedicle screw in control lateral and strengthening lateral strips from bone-screw interface without peripheral bone and vertebral pedicle being destroyed seriously, which are beneficial to the second insertion of screw.