Loyola University Stritch School of Medicine, Maywood, IL, USA.
Spine (Phila Pa 1976). 2011 Aug 1;36(17):1359-66. doi: 10.1097/BRS.0b013e318222d4ad.
In vitro biomechanical study.
To characterize cervical total disc replacement (TDR) kinematics above two-level fusion, and to determine the effect of fusion alignment on TDR response.
Cervical TDR may be a promising alternative for a symptomatic adjacent level after prior multilevel cervical fusion. However, little is known about the TDR kinematics in this setting.
Eight human cadaveric cervical spines (C2-T1, age: 59 ± 8.6 years) were tested intact, after simulated two-level fusion (C4-C6) in lordotic alignment and then in straight alignment, and after C3-C4 TDR above the C4-C6 fusion in lordotic and straight alignments. Fusion was simulated using an external fixator apparatus, allowing easy adjustment of C4-C6 fusion alignment, and restoration to intact state upon disassembly. Specimens were tested in flexion-extension using hybrid testing protocols.
The external fixator device significantly reduced range of motion (ROM) at C4-C6 to 2.0 ± 0.6°, a reduction of 89 ± 3.0% (P < 0.05). Removal of the fusion construct restored the motion response of the spinal segments to their intact state. The C3-C4 TDR resulted in less motion as compared to the intact segment when the disc prosthesis was implanted either as a stand-alone procedure or above a two-level fusion. The decrease in motion of C3-C4 TDR was significant for both lordotic and straight fusions across C4-C6 (P < 0.05). Flexion and extension moments needed to bring the cervical spine to similar C2 motion endpoints significantly increased for the TDR above a two-level fusion compared to TDR alone (P < 0.05). Lordotic fusion required significantly greater flexion moment, whereas straight fusion required significantly greater extension moment (P < 0.05).
TDR placed adjacent to a two-level fusion is subjected to a more challenging biomechanical environment as compared to a stand-alone TDR. An artificial disc used in such a clinical scenario should be able to accommodate the increased moment loads without causing impingement of its endplates or undue wear during the expected life of the prosthesis.
体外生物力学研究。
描述颈椎全椎间盘置换（TDR）在双节段融合上方的运动学特征，并确定融合对线对 TDR 反应的影响。
颈椎 TDR 可能是先前多节段颈椎融合后有症状的相邻节段的一种有前途的替代方法。然而，对于这种情况下的 TDR 运动学知之甚少。
8 个人体颈椎标本（C2-T1，年龄：59±8.6 岁）分别进行了完整、模拟双节段（C4-C6）融合（在前凸对线）和直对线、C3-C4 TDR 在 C4-C6 融合上方（在前凸和直对线）的测试。融合采用外部固定器装置模拟，便于调整 C4-C6 融合对线，并在拆卸后恢复到完整状态。标本在屈伸试验中采用混合测试方案进行测试。
外部固定器装置使 C4-C6 的活动范围显著减少到 2.0±0.6°，减少了 89±3.0%（P<0.05）。去除融合结构使脊柱节段的运动反应恢复到完整状态。与完整节段相比，当椎间盘假体作为独立手术或在双节段融合上方植入时，C3-C4 TDR 的运动幅度较小。在 C4-C6 处，无论是前凸还是直对线，C3-C4 TDR 的运动减少在两种融合方式下均有统计学意义（P<0.05）。与单独的 TDR 相比，在双节段融合上方放置 TDR 时，使颈椎达到类似 C2 运动终点所需的屈伸力矩显著增加（P<0.05）。前凸融合需要更大的弯曲力矩，而直融合需要更大的伸展力矩（P<0.05）。
与独立的 TDR 相比，放置在双节段融合相邻处的 TDR 面临更具挑战性的生物力学环境。在这种临床情况下使用的人工椎间盘应该能够适应增加的力矩负荷，而不会在假体的预期使用寿命内导致终板撞击或过度磨损。
