Nagaraja Srinidhi, Palepu Vivek
US Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics, Silver Spring, Maryland, USA.
Int J Spine Surg. 2017 Jun 28;11(3):20. doi: 10.14444/4020. eCollection 2017.
Screw loosening is a well-known adverse event in traditional spinal fusion instrumentation. This phenomenon may hinder segmental stability of the spine leading to bony non-union. In recent years numerous lumbar integrated fixation cages (IFC) have been introduced that offer a low profile alternative to a standard cage with an anterior plate (AP+C). The fixation approach for IFCs is different than a traditional anterior approach; therefore, it is unclear whether IFCs may loosen from the surrounding bone over time. The purpose of this study was to quantify screw loosening of IFC devices compared to AP+C implants under fatigue loading using micro-CT and image processing techniques.
L2-3 and L4-5 functional spinal units (FSUs) were obtained from nine human lumbar spines. These FSUs were then reconstructed with either AP+C or IFC implants designed to attach to vertebral bodies using four screws (two top and two bottom for AP+C; two medial and two lateral for IFC). The reconstructed specimens were fatigued in flexion-extension load of ±3 Nm at 1Hz for first 5,000 cycles and it was increased to ±5 Nm until 20,000 cycles. After removing screws to prevent image artifact, micro-CT scans were performed on all FSUs post-fatigue. These images were post-processed to calculate three-dimensional volumes around screw holes created due to damage at the screw-implant interface.
IFC screws had significantly greater (p=0.008) screw hole volumes compared to AP+C screws after fatigue testing. This increased screw hole volume for IFC devices was mainly due to loosening in medial screws. Medial screws had significantly greater (p<0.003) screw hole volumes compared to lateral IFC screws and all AP+C screws. There was no difference (p>0.888) between the screw hole volumes of lateral IFC, top AP+C, and bottom AP+C screws.
This study elucidated screw-loosening mechanisms in integrated fixation cages under simulated physiological loading. In particular, spatial differences in fixation was observed for IFC screws across the vertebra where medial screws loosened at a greater frequency compared to lateral screws post-fatigue. This novel technique may also be used to quantitatively investigate screw fixation post-fatigue testing in a variety of spinal devices.
螺钉松动是传统脊柱融合内固定术中一种广为人知的不良事件。这种现象可能会妨碍脊柱的节段稳定性，导致骨不连。近年来，许多腰椎一体化固定融合器（IFC）被引入，它为带有前路钢板（AP+C）的标准融合器提供了一种低外形的替代方案。IFC的固定方式与传统前路方法不同；因此，尚不清楚IFC是否会随着时间推移从周围骨质松动。本研究的目的是使用微型计算机断层扫描（micro-CT）和图像处理技术，在疲劳载荷下，对IFC装置与AP+C植入物的螺钉松动情况进行量化比较。
从9具人类腰椎获取L2-3和L4-5功能脊柱单元（FSU）。然后用AP+C或IFC植入物对这些FSU进行重建，这些植入物设计为使用4枚螺钉（AP+C为上方2枚和下方2枚；IFC为内侧2枚和外侧2枚）连接椎体。重建后的标本在1Hz频率下进行±3 Nm的屈伸载荷疲劳试验，前5000个循环，之后增加到±5 Nm直至20000个循环。在移除螺钉以防止图像伪影后，对所有疲劳试验后的FSU进行微型计算机断层扫描。对这些图像进行后处理，以计算由于螺钉-植入物界面损伤而在螺钉孔周围形成的三维体积。
疲劳试验后，IFC螺钉的螺钉孔体积相比AP+C螺钉显著更大（p=0.008）。IFC装置螺钉孔体积增加主要是由于内侧螺钉松动。内侧螺钉的螺钉孔体积相比外侧IFC螺钉和所有AP+C螺钉显著更大（p<0.003）。外侧IFC螺钉、上方AP+C螺钉和下方AP+C螺钉的螺钉孔体积之间无差异（p>0.888）。
本研究阐明了一体化固定融合器在模拟生理载荷下的螺钉松动机制。特别是，观察到IFC螺钉在整个椎体上的固定存在空间差异，疲劳试验后内侧螺钉相比外侧螺钉松动频率更高。这种新技术还可用于定量研究各种脊柱装置疲劳试验后的螺钉固定情况。
