Background

Clarifying the effect of each parameter of screw design on its fixation strength is critical in the development of any type of screw. The purpose of this study was to clarify the relationship between the thread depth and fixation strength of metal screws for cancellous bone.

Methods

Nine types of custom-made screws with the only changed variable being the thread depth were manufactured. Other elements were fixed at a major diameter of 4.5 mm, a thread region length of 15 mm, a pitch of 1.6 mm, and a thread width of 0.20 mm. The pull-out strength and insertion torque of each screw were measured for each of two foam-block densities (10 or 20 pcf). The correlation between the thread depth of the screw and the mechanical findings were investigated with single regression analysis.

Results

Regardless of the foam-block density, the pull-out strength significantly increased as the thread depth increased from 0.1 mm to 0.4 mm; after that, the increase was more gradual (p < 0.01, respectively). The relationship between the thread depth and insertion torque was similar. In addition, the insertion torque tended to be more strongly affected by screw depth than the pull-out strength (2.6 times at 20 pcf and 1.4 times at 10 pcf).

Conclusions

The pull-out strength of 4.5-mm-diameter metal screws in a cancellous bone model was found to be biphasic, although linearly correlated with the change in screw depth in both phases. The boundary of the correlation was 0.4 mm regardless of the density of the bone model, with the effect of screw depth on pull-out strength beyond that being small in comparison.

中文翻译：

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松质骨螺钉螺纹深度与固定强度的关系

背景

阐明螺钉设计的每个参数对其固定强度的影响对于任何类型螺钉的开发都至关重要。本研究的目的是阐明松质骨金属螺钉螺纹深度与固定强度之间的关系。

方法

制造了九种类型的定制螺钉，唯一改变的变量是螺纹深度。其他元件以4.5mm的大直径、15mm的螺纹区域长度、1.6mm的螺距和0.20mm的螺纹宽度固定。针对两种泡沫块密度（10 或 20 pcf）中的每一种，测量每个螺钉的拔出强度和插入扭矩。通过单一回归分析研究了螺钉的螺纹深度与机械结果之间的相关性。

结果

无论泡沫块密度如何，随着螺纹深度从0.1毫米增加到0.4毫米，拉拔强度显着增加；此后，增加更加缓慢（p < 0.01，分别）。螺纹深度和插入扭矩之间的关系类似。此外，与拔出强度相比，插入扭矩受螺钉深度的影响更大（20 pcf 时为 2.6 倍，10 pcf 时为 1.4 倍）。

结论

松质骨模型中 4.5 毫米直径金属螺钉的拔出强度被发现是双相的，尽管与两个相中螺钉深度的变化呈线性相关。无论骨模型的密度如何，相关性的边界均为 0.4 毫米，相比之下，螺钉深度对拔出强度的影响较小。