骨质疏松尸体腰椎中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比较研究

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

目的:比较骨质疏松尸体腰椎膨胀式椎弓根螺钉(expansive pedicle screw,EPS)固定与骨水泥强化椎弓根螺钉(polymethylmethacrylate-augmented pedicle screw,PMMA-PS)固定的稳定性。方法:16个腰椎标本取自4具新鲜尸体的脊柱(L1~L4)。年龄51~78岁,平均63岁,其中女性3具,男性1具。所有标本经X线检查排除畸形、骨折等病变,其中1个腰椎因严重畸形被剔除。测量各椎体的骨密度值(bone mineral density,BMD)后,将15个椎体随机分为3组。采用相同方法制备钉道,普通椎弓根螺钉(CPS)组直接置入CPS;PMMA-PS组先向钉道内注入PMMA,再置入CPS;EPS组直接置入EPS。置钉后24h,对标本进行X线检查和CT扫描,观察螺钉位置及骨水泥分布情况;然后将椎体固定于MTS 858上,沿椎弓根螺钉的长轴方向以10mm/min的加载速度进行拔出实验,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:所有腰椎的BMD均小于0.8g/cm2,T值为-3.5^-2.5,均为骨质疏松椎体,3组之间BMD的差异无统计学意义(P>0.05)。X线检查和CT重建显示各组螺钉位置均良好,PMMA-PS组中未见PMMA渗漏现象;CPS组螺钉被周围的骨质直接包绕;PMMA-PS组螺钉被PMMA所包裹,PMMA存在于螺钉周围的骨质中,在椎体内形成了"纺锤样"结构;EPS组螺钉的前端在椎体内明显膨胀,形成了"爪状"结构。CPS组、PMMA-PS组和EPS组的Fmax分别为751.50±251.37N、1521.70±513.27N和1175.20±396.51N,PMMA-PS组和EPS组均显著高于CPS组(P<0.001,P=0.026),而PMMA-PS组和EPS组之间的差异无统计学意义(P=0.064)。CPS组、PMMA-PS组和EPS组的EAV分别为1.47±0.51J、3.09±0.93J和2.46±0.69J,PMMA-PS组和EPS组均显著高于CPS组(P<0.001,P=0.005),而PMMA-PS组和EPS组之间的差异无统计学意义(P=0.067)。结论:EPS可显著提高骨质疏松腰椎内椎弓根螺钉的稳定性,达到了与传统PMMA强化椎弓根螺钉接近的固定强度,具有良好的临床应用前景。

Objectives： To compare the biomechanical characteristics of expansive pedicle screw （EPS） and polymethylmethaerylate-augmented pedicle serew（PMMA-PS） in osteoporotie cadaverie lumbar vertebrae. Methods： A total of sixteen vertebrae（L1-L4） was obtained from four fresh-frozen human eadaveric 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 resuhing 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 posi- tions 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.