Background: 

A new locking-screw technology, the Variable Fixation Locking Screw (VFLS; Biomech Innovations), was developed with the aim of promoting secondary fracture-healing. The VFLS features a resorbable sleeve that progressively decreases its mechanical properties and mass during the fracture-healing time. In this study, we investigated whether the VFLS can provide rigid as well as progressive dynamic fixation.

Methods: 

The interfragmentary stability provided by the VFLS was tested in a simulated fracture-gap model and compared with that provided by standard locking or by a combination of both technologies under compression and torsional loading. Tests were performed with an intact sleeve (initial condition) and after its chemical dissolution. An optical measurement system was used to characterize interfragmentary movements.

Results: 

The axial stiffness did not differ significantly among groups in the initial condition. Sleeve resorption significantly decreased construct stiffness. The torsional stiffness of the samples instrumented with the VFLS was lower than that of the control group. The degradation of the sleeve resulted in a significant increase in axial displacement recorded at both the cis and trans cortices. In samples featuring combined technologies, this increase was about 12% to 20% at the trans cortex and about 50% to 60% at the cis cortex. In samples featuring VFLS technology only, this increase was about 20% to 37% at the trans cortex and about 70% to 125% at the cis cortex.

Conclusions: 

The initial stability offered by the VFLS is equivalent to that of standard locking-screw technology. The resorption of the degradable sleeve leads to effective and reproducible fracture-gap dynamization, progressively varying the way the fracture gap is strained and the magnitude of the strain.

Clinical Relevance: 

The VFLS provides rigid and progressive dynamic fixation in vitro. Such variable stability might have beneficial effects in terms of triggering and boosting secondary fracture-healing.

中文翻译：

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可变固定技术提供刚性以及渐进式动态固定：一种生物力学研究

背景： 

为了促进二次骨折愈合，开发了一种新的锁定螺钉技术，即可变固定锁定螺钉（VFLS； Biomech Innovations）。VFLS具有可吸收的套筒，可在骨折愈合期间逐渐降低其机械性能和质量。在这项研究中，我们调查了VFLS是否可以提供刚性以及渐进式动态固定。

方法： 

在模拟的缝隙模型中测试了VFLS提供的片段间稳定性，并将其与标准锁定或压缩和扭转载荷下两种技术的组合所提供的片段间稳定性进行了比较。使用完整的套筒（初始条件）及其化学溶解后进行测试。使用光学测量系统来表征片段间运动。

结果： 

在初始状态下，各组之间的轴向刚度没有显着差异。套筒吸收明显降低了结构刚度。装有VFLS的样品的扭转刚度低于对照组。套筒的退化导致在顺式和反式皮层记录的轴向位移显着增加。在采用联合技术的样本中，这种增加在反式皮层中约为12％至20％，而在顺式皮层中约为50％至60％。仅在采用VFLS技术的样品中，反式皮层的这种增加约为20％至37％，而顺式皮层的这种增加约为70％至125％。

结论： 

VFLS提供的初始稳定性等同于标准的锁紧螺钉技术。可降解套筒的吸收导致有效且可重复的断裂间隙动态化，从而逐步改变裂缝间隙的应变方式和应变幅度。

临床相关性： 

VFLS在体外提供刚性和渐进的动态固定。这种可变的稳定性可能在触发和促进二次骨折愈合方面具有有益的作用。