PURPOSE The cement augmentation of a conventional anterior screw fixation in type II odontoid process fractures for elderly patients significantly increased stiffness and load to failure under anterior-posterior load in comparison with non-augmented fixation. The amount and quality of bone cement are usually taken ad hoc in clinical practise. In this study, we wanted to clarify the role of bone cement amount and its quality to the stiffness of odontoid and vertebrae body junction. METHODS Finite-element method was used to achieve different scenarios of cement augmentation. For all models, an initial stiffness was calculated. Model (1) the intact vertebrae were virtually potted into a polymethylmethacrylate base via the posterior vertebral arches. A V-shaped punch was used for loading the odontoid in an anterior-posterior direction. (2) The odontoid fracture type IIa (Anderson-D'Alonzo classification) was achieved by virtual transverse osteotomy. Anterior screw fixation was virtually performed by putting self-drilling titanium alloy 3.5 mm diameter anterior cannulated lag screw with a 12 mm thread into the inspected vertebrae. A V-shaped punch was used for loading the odontoid in an anterior-posterior direction. The vertebrae body was assumed to be non-cemented and cemented with different volume. RESULTS The mean cement volume was lowest for body base filling with 0.47 ± 0.03 ml. The standard body filling corresponds to 0.95 ± 0.15 ml. The largest volume corresponds to 1.62 ± 0.12 ml in the presence of cement leakage. The initial stiffness of the intact C2 vertebrae was taken as the reference value. The mean initial stiffness for non-porous cement (E = 3000 MPa) increased linearly (R2 = 0.98). The lowest stiffness (123.3 ± 5.8 N/mm) was measured in the intact C2 vertebrae. However, the highest stiffness (165.2 ± 5.2 N/mm) was measured when cement leakage out of the odontoid peg occurred. The mean initial stiffness of the base-only cemented group was 147.2 ± 8.4 N/mm compared with 157.9 ± 6.6 N/mm for the base and body cemented group. This difference was statistically significant (p < 0.0061). The mean initial stiffness for porous cement (E = 500 MPa) remains constant. Therefore, there is no difference between cemented and non-cemented junction. This difference was not statistically significant (p < 0.18). CONCLUSION The present study showed that the low porous cement was able to significantly influence the stiffness of the augmented odontoid screw fixation in vitro, although further in vivo clinical studies should be undertaken. Our results suggest that only a small amount of non-porous cement is needed to restore stiffness at least to its pre-fracture level and this can be achieved with the injection of 0.7-1.2 ml of cement. These slides can be retrieved under Electronic Supplementary Material.

中文翻译：

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牙托钉骨折的水泥增强：水泥体积和分布对结构刚度的影响。

目的与老年患者相比，在老年患者的Ⅱ型齿状突骨折中，常规的前路螺钉固定术的水泥增强明显增加了其刚度和在失败后的载荷。骨水泥的量和质量通常在临床实践中是临时采取的。在这项研究中，我们想阐明骨水泥的量及其质量对齿状突和椎体交界处的硬度的作用。方法采用有限元方法来实现不同的水泥填充方案。对于所有模型，都计算了初始刚度。模型（1）将完整的椎骨实际上通过后椎弓根植入聚甲基丙烯酸甲酯基体中。使用V形冲头在前后方向上加载齿状突。（2）通过虚拟横向截骨术实现了IIa类齿状突骨折（Anderson-D'Alonzo分类）。通过将自钻钛合金，直径为3.5 mm的前空心拉力螺钉（带12 mm螺纹）放入被检查的椎骨中，实际上可以进行前螺钉固定。使用V形冲头在前后方向上加载齿状突。假定椎体为非骨水泥，骨水泥的体积不同。结果身体填充的平均水泥量最低，为0.47±0.03 ml。标准的身体填充量相当于0.95±0.15毫升。在水泥泄漏的情况下，最大体积为1.62±0.12 ml。完整的C2椎骨的初始刚度作为参考值。无孔水泥的平均初始刚度（E = 3000 MPa）线性增加（R2 = 0.98）。在完整的C2椎骨中测得的最低刚度（123.3±5.8 N / mm）。但是，当水泥从齿状钉中漏出时，测得的最高刚度（165.2±5.2 N / mm）。仅基础骨水泥组的平均初始刚度为147.2±8.4 N / mm，而基础骨水泥和身体骨水泥组的平均初始刚度为157.9±6.6 N / mm。这种差异具有统计学意义（p <0.0061）。多孔水泥的平均初始刚度（E = 500 MPa）保持恒定。因此，胶结和非胶结之间没有区别。该差异无统计学意义（p <0.18）。结论本研究表明，尽管应进行进一步的体内临床研究，但低孔隙水泥在体外能够显着影响增强齿状突螺钉固定的刚度。我们的结果表明，仅需少量无孔水泥即可将刚度恢复到至少其断裂前的水平，而这可以通过注入0.7-1.2 ml水泥来实现。这些幻灯片可以在电子补充材料下找到。