Impact of Different Screw Designs on Durability of Fracture Fixation: In Vitro Study with Cyclic Loading of Scaphoid Bones

Purpose
The use of new headless compression screws (HCSs) for scaphoid fixation is growing, but the nonunion rate has remained constant. The aim of this study was to compare the stability of fixation resulting from four modern HCSs using a simulated fracture model to determine the optimal screw design(s).
Methods
We tested 40 fresh-frozen cadaver scaphoids treated with the Acumed Acutrak 2 mini (AA), the KLS Martin HBS2 midi (MH), the Stryker TwinFix (ST) and the Synthes HCS 3.0 with a long thread (SH). The bones with simulated fractures and implanted screws were loaded uniaxially into flexion for 2000 cycles with a constant bending moment of 800 Nmm. The angulation of the fracture fragments was measured continuously. Data were assessed statistically using the univariate ANOVA test and linear regression analysis, and the significance level was set at p < 0.05.
Results
The median angulation of bone fragments φ allowed by each screw was 0.89° for AA, 1.12° for ST, 1.44° for SH and 2.36° for MH. With regards to linear regression, the most reliable curve was achieved by MH, with a coefficient of determination of R2 = 0.827. This was followed by AA (R2 = 0.354), SH (R2 = 0.247) and ST (R2 = 0.019). Data assessed using an adapted ANOVA model showed no statistically significant difference (p = 0.291) between the screws.
Conclusions
The continuous development of HCSs has resulted in very comparable implants, and thus, at this time, other factors, such as surgeons’ experience, ease of handling and price, should be taken into consideration.

研究目的
新型无头加压螺钉（headless compression screws, HCSs）用于舟骨固定的临床应用日益增多，但骨不连率始终维持恒定。本研究旨在通过模拟骨折模型，对比4款现代无头加压螺钉的固定稳定性，以确定最优的螺钉设计方案。

研究方法
本研究共纳入40例新鲜冷冻尸体舟骨标本，分别采用Acumed Acutrak 2 mini（AA）、KLS Martin HBS2 midi（MH）、Stryker TwinFix（ST）以及Synthes长螺纹HCS 3.0（SH）进行固定。将构建了模拟骨折并植入螺钉的舟骨标本以800牛·毫米的恒定弯矩进行单轴向屈曲加载，循环加载2000次，持续监测骨折块的成角变化。采用单因素方差分析与线性回归分析对数据进行统计学评估，设定显著性水平为p < 0.05。

研究结果
各螺钉对应的骨折块中位成角分别为：AA为0.89°、ST为1.12°、SH为1.44°、MH为2.36°。线性回归分析结果显示，MH的拟合曲线可靠性最高，决定系数R²=0.827；其后依次为AA（R²=0.354）、SH（R²=0.247）与ST（R²=0.019）。经改良方差模型分析，四款螺钉的固定稳定性未呈现统计学显著性差异（p=0.291）。

研究结论
无头加压螺钉的持续研发已使各款植入物的性能高度趋同，因此现阶段临床选择时，应综合考量术者操作经验、手术操作便捷性与产品价格等其他因素。