三种不同长度全螺纹松质骨螺钉最大轴向拔出强度的测试

目的探讨螺钉长度对全螺纹松质骨螺钉轴向最大拔出强度（Axial maximum pullout strength）的影响。方法分三种螺钉长度组，每组八个螺钉，总共24个全螺纹松质骨螺钉。三具福尔马林液防腐固定尸体脊柱段标本（T10-L5），并行X-线（X-rays）检查和应用美国Norland公司的XR-36型双能X线吸收骨密度仪Dual-energy X-rays absorptiometry bone（DEXA）densitometer]测试椎体骨密度（BMD）。根据骨密度值选出12个椎体，并根据骨密度分成四个骨密度段。在每个骨密度段内，每个椎体中随机植入2个不同长度的螺钉固定在材料实验机（Material testing system machine）上分别进行轴向拔出强度的测试。结果A组（18mm）和B（20mm）组的拔出强度之间无显著性差异（P=0．407），B组（20mm）和C组（24mm）的拔出强度之间无显著性差异（P=0．092），A组（18mm）和C组（24mm）的拔出强度之间有显著性差异（P＜0．05）。结论本实验数据分析24mm长的全螺纹松质骨螺钉比18mm长的螺钉具有更强的握持力（Holding power）。螺钉轴向最大拔出强度与螺钉长度明显相关，在临床中，为提高螺钉的最大拔出强度尽可能应用长的螺钉。

The Axial Maximum Pull out Strength Test of Full Thread Cancellous Bone Screws with Different Length

Objective To investigate the influence of screw length(18,20,and 24 mm) on the axial pullout strength of full thread cancellous screws.Methods A total of 24 screws were implanted and tested. The screws were divided into three groups according to the length(18 mm,20 mm,24 mm) . Each group with eight screws was implanted and tested. Three formalin cadaveric spine specimens(T10-L5)were radiographed to exclude fracture and tumour ,and their bone mineral density were measured with NORLAND-XR36DEXA.Twelve vertebral bodies were selected and were divided into four groups according to the bone mineral density. Two screws of difference length were randomly implanted in each vertebral body within each group of different bone mineral density, and their axial pullout strength were individually tested on the MTS(material testing system)machine.Results There was no significant difference in pullout strength between group A(18 mm) and group B(20 mm)P=0.407).Also, there was no significant difference between group B(20 mm) and group C(24 mm) (P=0.092). A significant difference in pullout strength existed between group A(18 mm) and group C(24 mm)(P<0.05).A significant correlation existed between pullout strength and bone mineral density. A significant correlation existed between screw length and pullout strength. Conclusion The data of this study indicate that the long screws with 24 mm have greater holding power than the screws with 18 mm do. The length of full thread cancellous screw significantly influences the pullout strength of screw. It would be appear that the longest screw feasible should always be used clinically for maximum pullout strength.