Department of Mechanical Engineering, University of California Berkeley, Etcheverry Hall, Berkeley, CA 94720.
Department of Orthopaedic Surgery, University of California, Davis, Davis Medical Center, Sacramento, CA 95817.
J Biomech Eng. 2021 Aug 1;143(8). doi: 10.1115/1.4050538.
Painful herniated discs are treated surgically by removing extruded nucleus pulposus (NP) material (nucleotomy). NP removal through enzymatic digestion is also commonly performed to initiate degenerative changes to study potential biological repair strategies. Experimental and computational studies have shown a decrease in disc stiffness with nucleotomy under single loading modalities, such as compression-only or bending-only loading. However, studies that apply more physiologically relevant loading conditions, such as compression in combination with bending or torsion, have shown contradicting results. We used a previously validated bone-disc-bone finite element model (Control) to create a Nucleotomy model to evaluate the effect of dual loading conditions (compression with torsion or bending) on intradiscal deformations. While disc joint stiffness decreased with nucleotomy under single loading conditions, as commonly reported in the literature, dual loading resulted in an increase in bending stiffness. More specifically, dual loading resulted in a 40% increase in bending stiffness under flexion and extension and a 25% increase in stiffness under lateral bending. The increase in bending stiffness was due to an increase and shift in compressive stress, where peak stresses migrated from the NP-annulus interface to the outer annulus. In contrast, the decrease in torsional stiffness was due to greater fiber reorientation during compression. In general, large radial strains were observed with nucleotomy, suggesting an increased risk for delamination or degenerative remodeling. In conclusion, the effect of nucleotomy on disc mechanics depends on the type and complexity of applied loads.
疼痛性椎间盘突出症通过手术切除突出的髓核（NP）物质（髓核切除术）来治疗。通过酶消化去除 NP 也是常见的做法，以启动退行性变化，研究潜在的生物修复策略。实验和计算研究表明，在单一加载模式下，如仅压缩或仅弯曲加载下，髓核切除术后椎间盘刚度降低。然而，应用更符合生理的加载条件的研究，如压缩与弯曲或扭转的组合，得出了相互矛盾的结果。我们使用了先前经过验证的骨-椎间盘-骨有限元模型（Control）来创建 Nucleotomy 模型，以评估双加载条件（压缩与扭转或弯曲）对椎间盘内变形的影响。虽然在单一加载条件下，如文献中常见的那样，椎间盘关节刚度随髓核切除术而降低，但双加载导致弯曲刚度增加。更具体地说，双加载导致在屈伸时弯曲刚度增加了 40%，在侧屈时增加了 25%。弯曲刚度的增加是由于压缩时压应力的增加和转移，峰值应力从 NP-环界面迁移到外环。相比之下，扭转刚度的降低是由于压缩时纤维的更大重新定向。总的来说，髓核切除术后观察到较大的径向应变，表明分层或退行性重塑的风险增加。总之，髓核切除术对椎间盘力学的影响取决于施加的负荷的类型和复杂性。
