U.S. Food and Drug Administration, 10903 New Hampshire Ave, Silver Spring, MD, 20993, USA; University of Maryland, College Park, College Park, MD 20742.
U.S. Food and Drug Administration, 10903 New Hampshire Ave, Silver Spring, MD, 20993, USA.
Spine J. 2019 Apr;19(4):744-754. doi: 10.1016/j.spinee.2018.09.006. Epub 2018 Sep 13.
Growing rod constructs are an important contribution in the treatment of children with early onset scoliosis even though these devices experience high rates of rod fracture. The mechanical performance of traditional, distraction-based dual growing rod constructs is not well understood, and mechanical models for predicting device performance are limited.
Two mechanical models were developed and used to determine the mechanical performance of various growing rod configurations by increasing construct complexity.
STUDY DESIGN/SETTING: Mechanical bench testing and finite element (FE) analysis.
Static and dynamic compression bending tests were based on an ASTM F1717 method modified to accommodate dual growing rod constructs. Six construct configurations were tested, mechanical properties were recorded, and statistical analyses were performed to determine significant differences between groups: (1) no connectors (rods only), (2) side-by-side connectors, (3) side-by-side connectors plus 4 crosslinks, (4) (40-mm long tandem connectors, (5) 80-mm long tandem connectors, and (6) 80-mm long tandem connectors plus 4 crosslinks. FE analysis was used to predict the stress distribution within the constructs.
The static results indicated greater stiffness, yield load, and peak load as the axial connector length increased (side-by-side to 40 mm tandem to 80 mm tandem). The dynamic results showed similar cycles to failure for side-by-side and tandem connector (40 and 80 mm) construct configurations without crosslinks. Crosslinks shifted the location of rod fracture observed experimentally and significantly reduced the fatigue life of the construct. The flexibility of the construct decreased significantly as the axial connector length increased. FE predictions were highly consistent with the experimentally measured values and provided information on stress distribution within the rod for comparison to experimental fracture locations.
This is the first study to evaluate mechanical performance of various configurations of pediatric growing rod constructs using preclinical models. The current study is consistent with a previous retrieval study in that rigid constructs lacking flexibility (ie, higher stiffness and lower displacement), such as those with 80-mm tandem connectors and multiple crosslinks, demonstrated decreased mechanical performance as shown through both experimental and computational models. Additionally, the experimental and computational findings suggest that surgeons should strategically consider the number of interconnecting components and subsequent stress concentrations along the posterior side of the rod. For example, changing the placement of crosslinks to low stress regions of the construct or not using crosslinks in the construct are options.
尽管生长棒装置的断裂发生率较高，但在治疗早发性脊柱侧凸的儿童中，生长棒装置仍是一项重要的贡献。传统的、基于分离的双生长棒装置的机械性能尚不清楚，用于预测装置性能的机械模型也有限。
通过增加装置复杂性，开发并使用两种机械模型来确定各种生长棒装置的机械性能。
研究设计/设置：力学台架试验和有限元（FE）分析。
静态和动态压缩弯曲试验基于经改良的 ASTM F1717 方法，以适应双生长棒装置。测试了 6 种装置构型，记录了力学性能，并进行了统计学分析，以确定各组之间的显著差异：（1）无连接器（仅棒）、（2）并排连接器、（3）并排连接器加 4 个交叉连接器、（4）（40mm 长的串联连接器、（5）80mm 长的串联连接器和（6）80mm 长的串联连接器加 4 个交叉连接器。FE 分析用于预测装置内的应力分布。
静态结果表明，随着轴向连接器长度的增加（从并排连接器到 40mm 串联连接器到 80mm 串联连接器），刚度、屈服载荷和峰值载荷均增加。动态结果显示，没有交叉连接器的并排连接器和串联连接器（40mm 和 80mm）装置构型的失效循环相似。交叉连接器改变了实验中观察到的棒断裂的位置，并显著降低了装置的疲劳寿命。随着轴向连接器长度的增加，装置的灵活性显著降低。FE 预测与实验测量值高度一致，并提供了杆内的应力分布信息，以便与实验断裂位置进行比较。
这是第一项使用临床前模型评估各种儿童生长棒装置构型的机械性能的研究。本研究与之前的检索研究一致，即缺乏灵活性的刚性装置（即更高的刚度和更低的位移），例如具有 80mm 串联连接器和多个交叉连接器的装置，其力学性能降低，这通过实验和计算模型都得到了证明。此外，实验和计算结果表明，外科医生应战略性地考虑连接组件的数量和随后的杆后侧面的应力集中。例如，将交叉连接器的位置更改为装置的低应力区域或在装置中不使用交叉连接器是一种选择。
