Parameters of screws.

BackgroundLateral mass screw (LMS) is a more widely adopted method for posterior cervical spine fixation than the cervical pedicle screw (CPS). Despite its lower pullout strength, the insertions of LMS are more reproducible and have a lower risk. CPS insertion is a technically demanding procedure due to the small pedicle channel. Thus, CPS insertion has a high risk of pedicle wall perforation, resulting in neurovascular injury. For these reasons, surgeons may avoid CPS insertion despite its benefit of greater biomechanical strength. Therefore, an improvement in the CPS design is needed to avoid this catastrophic complication.ObjectivesTo develop a new design of CPS, aiming to decrease pedicle wall perforation, while maintaining the biomechanical properties comparable to those of standard CPS.Materials & methodsTo reduce the risk of pedicle wall perforation, a novel CPS design should be configured in tapered shape, with a tapering screw pitch and thread diameter with a self-tapping thread. A bilayer bone finite element model representing the cortical and cancellous bone of the cervical spine pedicle was used for pullout strength test. According to our CT-based study of cervical pedicle anatomy in a normal population, the final CPS was created according to the parameters that yielded the best biomechanical strength according to finite element studies. The safety of CPS insertion, in terms of pedicle wall penetration, was assessed in 3D-printed cervical spine models of C3-C7. The pullout test was subsequently performed in a tri-layer sawbones foam model to compare the novel CPS, convention CPS, and lateral mass screw.ResultsThe final screw design was a taper configuration with core diameter from 2.5 to 2.0 mm, thread diameter from 4.0 to 2.5 mm and pitch length from 1.0 to 1.25 mm. A total of 60 screws (30 conventional CPS screw and 30 Novel CPS screw) were tested in 6 3D cervical spine models. No case of pedicle wall perforation were found in the novel-design CPS group. In the conventional CPS group, 8 pedicle wall perforations were encountered, which was a statistically significant difference (p = 0.002). The novel CPS screw design and conventional CPS screw yielded pullout strengths of 449.7 N and 495.0 N, respectively, which showed no statistical difference. The LMS screw yielded a pullout strength of 168.3 N, showing statistically less strength compared with the 2 types of CPS screws.ConclusionsThe proposed novel CPS could decrease pedicle wall perforation and enhance the safety of screw insertion. Its pullout strength is comparable to that of a 3.5-mm standard CPS and superior to that of a 3.5-mm lateral mass screw.背景：侧块螺钉（Lateral mass screw, LMS）是目前临床应用更为广泛的颈椎后路固定术式，相较于颈椎椎弓根螺钉（cervical pedicle screw, CPS）。尽管其抗拔出强度更低，但LMS的置入操作可重复性更强，且风险更低。由于颈椎椎弓根通道尺寸微小，CPS置入手术对操作技术要求极高，因此极易发生椎弓根壁穿孔风险，进而引发神经血管损伤。基于上述原因，尽管CPS具备更优异的生物力学性能优势，外科医师仍常规避该术式。因此，亟需对CPS的设计进行优化，以避免此类灾难性并发症。 研究目的：开发一款新型CPS设计，旨在降低椎弓根壁穿孔风险的同时，保留与标准CPS相当的生物力学性能。 材料与方法：为降低椎弓根壁穿孔风险，新型CPS设计应采用锥形构型，搭配渐缩的螺距与螺纹直径，并具备自攻螺纹特性。本研究采用模拟颈椎椎弓根皮质骨与松质骨的双层骨有限元模型（finite element model）进行抗拔出强度测试。基于我们针对正常人群颈椎椎弓根解剖结构的CT研究，最终依据有限元分析得出的最优生物力学参数，完成了新型CPS的设计建模。随后，在3D打印的C3~C7颈椎模型中评估CPS置入的安全性（以椎弓根壁穿透情况为评价指标）。最后，在三层Sawbones泡沫模型中开展拔出试验，对比新型CPS、传统CPS与侧块螺钉的性能差异。 结果：最终的螺钉设计为锥形构型，其芯径范围为2.5~2.0 mm，螺纹直径范围为4.0~2.5 mm，螺距范围为1.0~1.25 mm。本研究共在6个3D打印颈椎模型中置入60枚螺钉（30枚传统CPS螺钉与30枚新型CPS螺钉）。新型CPS组未出现任何椎弓根壁穿孔病例；而传统CPS组共发生8例椎弓根壁穿孔，两组差异具有统计学意义（p=0.002）。新型CPS与传统CPS的抗拔出强度分别为449.7 N与495.0 N，二者无统计学差异。侧块螺钉的抗拔出强度为168.3 N，显著低于两种CPS螺钉，差异具有统计学意义。 结论：本研究提出的新型CPS设计可有效降低椎弓根壁穿孔风险，提升螺钉置入的安全性。其抗拔出性能与3.5 mm标准CPS相当，且优于3.5 mm侧块螺钉。