影响椎弓根螺钉拔出力的相关因素

Factors for pedicle screw extraction force

椎弓根螺钉的松动和轴向脱出是导致脊柱内固定失败的主要原因,螺钉的松动、脱出与螺钉的拔出力有关,螺钉的拔出力与椎弓根螺钉直径、螺纹设计、骨密度及螺钉的操作技术有关。拔出强度随外径的增加而增加,螺钉越长,拔出强度越大。椎弓根螺钉的稳定性在骨质疏松时明显降低,对于骨质疏松患者,单纯增加螺钉直径较为困难,一般需进一步强化椎弓根螺钉的稳定性,聚甲基丙烯酸甲酯骨水泥强化骨质疏松椎弓根螺钉能显著增加螺钉的稳定性和抗屈强度,但由于骨水泥聚合产生高热可导致组织损伤,体内长期留置会产生毒性和致癌作用,具有良好生物相容性的新型添加材料,如羟基磷灰石骨水泥、磷酸钙骨水泥等将逐渐替代聚甲基丙烯酸甲酯骨水泥。同时螺钉置入过程中准确的进钉点、进钉方向和置入位置可以减少因螺钉重新拧入而导致的把持力下降,提高螺钉的拔出强度和稳定性。

Pedicle screw loosening and axial dislocation are main reasons for spinal internal fixation failure. Screw loosening and dislocation is caused by screw extraction force, and the screw withdrawl force is related to screw diameter, screw thread design, bone density, and operation skills. Withdrawl intensity is increased with screw diameter and length. Pedicle screw stability can be decreased by osteoporosis. For osteoporotic patients, it is difficult to increase screw diameter alone, so polymethyl methacrylate bone cement is used to enhance screw stability and anti-flexion intensity. However, polymerization heat may induce tissue injury, and long-term stay may produce toxicity and exhibit cancerogenesis. Novel additive materials with good biocompatibility such as hydroxyapatite and calcium phosphate bone cement are gradually replacing polymethyl methacrylate bone cement. In addition, screw entrance point, direction and exact location during screw implantation can decrease pullout strength reducation caused by screw re-twisting and improve screw extraction intensity and stability.