Augmentation and restoration of pedicle screw fixation with polymethylmethacrylate: an in vitro biomechanical study

Abstract

Objective To evaluate pedicle screw fixation strength following augmentation and restoration with polymethylmethacrylate (PMMA). Methods Compact CD (CCD) screws of 6.0 mm diameter was implanted into the pedicle unilaterally. And a pilot hole was created using 3.5 mm drill contralaterally on eight fresh human lower lumbar vertebrae (L 3-5).Neither the screws nor the drillings penetrated the anterior cortex of the vertebral bodies. The screws were pulled out from the vertebrae at 5 mm/min. The screw track and the pilot hole were filled with 3-5 ml of PMMA. Then, the screws were implanted bilaterally, and the PMMA was allowed to harden for 1 hour. Morphology of PMMA around the screws was observed by section of the vertebrae along the screw. Results Pullout force of pedicle screw fixation in normal vertebrae was (825.5±254.7) N and (1 711.1±488.8)N following augmentation with PMMA. Restoration of a pullout pedicle screw fixation with PMMA was able to salvage pullout force to (1 832.4±497.4) N. The pullout forces following augmentation and restoration were significantly greater than that in normal vertebrae. Morphology of PMMA around the screws was related to the viscosity of PMMA during injection. No PMMA was found in vertebral canal or surface of pedicle. Conclusion Augmentation and restoration with PMMA offered a significantly increase of strength for pedicle screw fixation, and could be used clinically in case of failure of pedicle screw fixation.

目的 評價聚甲基丙烯酸甲酯(polymethylmethacrylate ,PMMA)強化和修復椎弓根螺釘固定的生物力學效果。方法8具成人新鮮腰椎骨L3～5,隨機選擇一側椎弓根放置直徑為6.0mm的CCD(compactCD)椎弓根螺釘 ,另一側以直徑為3.5mm的鉆頭導孔 ,均不穿透椎體前皮質。在材料試驗機上進行椎弓根螺釘拔出實驗 ,拔出速率為5mm/min。然后沿椎弓根孔道注入PMMA粉和水按體積1∶2混合的PMMA強化修復材料3～5ml,雙側椎弓根分別使用與前相同的CCD椎弓根螺釘 ,室溫下1h后再行前述拔出實驗。沿椎弓根螺釘長度方向鋸開標本 ,觀察PMMA在螺釘孔道中的分布范圍。結果CCD椎弓根螺釘在正常椎弓根內的最大拔出力為 (825.5±254.7)N ;在經強化后的正常椎弓根內和在修復失效的椎弓根內 ,CCD椎弓根螺釘最大拔出力分別為 (1711.1±488.8)N和 (1832.4±497.4)N ,高于正常椎弓根螺釘的最大拔出力 ,差異有顯著性意義(P<0.01)。PMMA在螺釘周圍的分布與其注入時的粘稠度有關 ,未見椎管內或椎弓根外有PMMA分布。結論沿椎弓根孔道注入PMMA可顯著增加椎弓根螺釘的拔出力 ,適用于螺釘松動和拔出的修復固定