Department of Orthopaedic Surgery, Laboratory for Experimental Orthopaedics, CAPHRI, Maastricht University Medical Centre, Maastricht, the Netherlands; Department of Biomedical Engineering, Orthopaedic Biomechanics, Eindhoven University of Technology, Eindhoven, the Netherlands.
Spine-Unit, University Hospital of Valladolid, Valladolid, Spain.
Spine J. 2021 Mar;21(3):528-537. doi: 10.1016/j.spinee.2020.09.010. Epub 2020 Sep 30.
Manual contouring of spinal rods is often required intraoperatively for proper alignment of the rods within the pedicle screw heads. Residual misalignments are frequently reduced by using dedicated reduction devices. The forces exerted by these devices, however, are uncontrolled and may lead to excessive reaction forces. As a consequence, screw pullout might be provoked and surrounding tissue may experience unfavorable biomechanical loads. The corresponding loads and induced tissue deformations are however not well identified. Additionally, whether the forced reduction alters the biomechanical behavior of the lumbar spine during physiological movements postoperatively, remains unexplored.
To predict whether the reduction of misaligned posterior instrumentation might result in clinical complications directly after reduction and during a subsequent physiological flexion movement.
Finite element analysis.
A patient-specific, total lumbar (L1-S1) spine finite element model was available from previous research. The model consists of poro-elastic intervertebral discs with Pfirrmann grade-dependent material parameters, with linear elastic bone tissue with stiffness values related to the local bone density, and with the seven major ligaments per spinal motion segment described as nonlinear materials. Titanium instrumentation was implemented in this model to simulate a L4, L5, and S1 posterolateral fusion. Next, coronal and sagittal misalignments of 6 mm each were introduced between the rod and the screw head at L4. These misalignments were computationally reduced and a physiological flexion movement of 15° was prescribed. Non-instrumented and well-aligned instrumented models were used as control groups.
Pulling forces up to 1.0 kN were required to correct the induced misalignments of 6 mm. These forces affected the posture of the total lumbar spine, as motion segments were predicted to rotate up to 3 degrees and rotations propagated proximally to and even affect the L1-2 level. The facet contact pressures in the corrected misaligned models were asymmetrical suggesting non-physiological joint loading in the misaligned models. In addition, the discs and vertebrae experienced abnormally high forces as a result of the correction procedure. These effects were more pronounced after a 15° flexion movement following forced reduction.
The results of this study indicate that the correction of misaligned posterior instrumentation can result in high forces at the screws consistent with those reported to cause screw pullout, and may cause high-tissue strains in adjacent and downstream spinal segments.
Proper alignment of spinal posterior instrumentation may reduce clinical complications secondary to unfavorable biomechanics.
术中通常需要手动调整脊柱杆以使其在椎弓根螺钉头内正确对齐。通过使用专用的复位装置，可以减少残余的错位。然而，这些装置施加的力是不受控制的，可能会导致过大的反作用力。结果，可能会引发螺钉拔出，并且周围组织可能会承受不利的生物力学载荷。但是，尚未很好地确定相应的载荷和引起的组织变形。此外，术后生理运动期间，强制复位是否会改变腰椎的生物力学行为，这仍然是未知的。
预测纠正错位的后路器械是否会导致术后即刻复位和随后的生理弯曲运动时出现临床并发症。
有限元分析。
从先前的研究中获得了特定于患者的全腰椎（L1-S1）脊柱有限元模型。该模型由具有 Pfirrmann 分级依赖性材料参数的多孔弹性椎间盘组成，具有与局部骨密度相关的线性弹性骨组织，以及描述为非线性材料的每个脊柱运动节段的七个主要韧带。在该模型中实现了钛制器械，以模拟 L4、L5 和 S1 后外侧融合。接下来，在 L4 处将棒和螺钉头之间的冠状面和矢状面错位各引入 6mm。通过计算来纠正这些错位，并施加 15°的生理弯曲运动。未器械化和器械化良好的模型用作对照组。
要纠正 6mm 的诱导错位，需要施加高达 1.0kN 的拉力。这些力影响了整个腰椎的姿势，因为预测运动节段会旋转多达 3 度，并且旋转会向近端传播，甚至会影响到 L1-2 水平。在纠正后的错位模型中，关节面接触压力不对称，表明错位模型中的关节受力不正常。此外，由于校正过程，椎间盘和椎体承受异常高的力。在强制复位后的 15°弯曲运动后，这些影响更为明显。
本研究结果表明，纠正错位的后路器械可能会导致与螺钉拔出相关的高螺钉力，并可能导致相邻和下游脊柱节段的高组织应变。
适当的脊柱后路器械对齐可以减少因生物力学不良而引起的临床并发症。
