A modified Edwards instrumentation for unstable fractures of the thoracolumbar spine: an in vitro biomechanical evaluation

Abstract

Objective To evaluat the instant spinal stability and stability following cyclic loading using modified Edwards instrumentation (MEI) for unstable thoracolumbar spine and to compare the spinal stability using Compact Cotrel Dubousset instrumentation (CCD). Methods Twelve thoracolumbar spine specimens(T9-L 5), obtained from fresh human cadavers, were divided into two groups following the production of wedged fracture over the body of L 1. MEI or CCD was applied to the two groups to restore spinal stability. A cyclic load with axial compression (?500 to ?50 N) and axial rotation (10 degrees) was applied to the specimens at rate of 0.5 Hz up to 1 500 cycles. Segmental instability tests were performed for the specimens in intact, injured, fixation and following cyclic loading. The instability test applied pure moments in flexion/extension, left/right lateral bending and left/right axial rotation to the specimen. The maximum moment was 10 N·m in each case. Ranges of motion (ROMs) were measured using stereophotogrammetry. Results 1)The compressive fractures at L 1 vertebral body resulted in unstability at thoracolumbar segment in all six directions. 2) T 12-L 2 segmental ROMs were smaller than those of the control in flexion, extension and lateral bending, but larger in axial rotation following MEI fixation. The ROMs increased in all directions following fatigue, especially became significantly larger in lateral bending. 3)The segmental ROMs decreased significantly in all six directions following CCD fixation, and even following fatigue except in right lateral bending. 4)ROMs with CCD fixation were significantly smaller than MEI fixation in left axial rotation, and in lateral bending and axial rotation following fatigue. Conclusion MEI provides instant stability in flexion, extension and lateral bending, but it does not reconstruct spinal stability in axial rotation. MEI fixation becomes unstable in lateral bending following cyclic loading. CCD is stronger biomechanically than MEI in lateral bending and axial rotation, either for instant stability or stability following cyclic loading. Orthosis or cast should be applied to the patient with MEI fixation to strengthen spinal stability.

目的評價改進型Edwards器械modifiedEdwardsinstrumentationMEI固定不穩定胸腰椎骨折即刻和承受周期性載荷后的穩定性,并與CCD器械相比較。方法在12具新鮮成人胸腰椎標本T9～L5的L1椎體上制備楔形骨折模型,分為兩組,分別采用MEI和CCD內固定器恢復脊柱的穩定性。對固定后的脊柱施加0.5Hz的疲勞載荷共1500次,然后評價正常、損傷、固定和周期性加載后胸腰椎節段進行前屈/后伸、左/右側彎和左/右旋轉等運動時的穩定性,經脊柱三維運動測量系統測出脊柱節段的運動范圍。結果1L1椎體壓縮骨折后,胸腰椎在6個運動方向上均出現不穩。2MEI固定后,T12～L2節段前屈、后伸和側彎運動的范圍均小于對照組,但軸向旋轉運動范圍大于對照組;周期性加載后胸腰椎節段在所有方向上的運動范圍均有所增加,以右側彎最為明顯P<0.01。3CCD固定后,T12～L2節段在6個方向上的運動范圍均顯著減小;與對照組相比,周期性加載后胸腰椎節段的運動范圍也顯著減小均P<0.05,但右側彎除外。4CCD固定后,T12～L2節段左旋、疲勞后側彎及旋轉運動范圍明顯小于MEI固定組P<0.01或P<0.05。結論MEI固定可為脊柱前屈、后伸和側彎運動提供即刻穩定性,但不能恢復旋轉運動的穩定性;在周期性加載后,胸腰椎節段側彎運動的穩定性喪失。而經CCD固定后,側彎和旋轉運