四种骶骨钉固定技术的生物力学比较

目的通过骨质疏松骶骨标本对4种骶骨椎弓根钉（双皮质、三皮质、标准骨水泥强化和终板下骨水泥强化）载荷后的下沉位移进行比较。方法取自11具新鲜骨质疏松尸体的骶骨标本用于实验。采用DEXA测定骨密度后在同一骶骨的左右侧随机置入直径7mm的双皮质和三皮质骶骨椎弓根钉。使用MTS试验机对螺钉进行30～250N压力加载2000次后取出螺钉。钉道内注入骨水泥（polymethylmethacrylate，PMMA），将比双皮质或三皮质固定短5mm的螺钉再次置人（分别定义为标准和终板下PMMA强化），并重复上述加载。记录加载后螺钉的下沉位移，进行比较。结果11具标本的骨密度为0．55～0．78g／cm^2，平均0．7lg／cm^2。三皮质和标准PMMA强化椎弓根钉间的下沉位移差异无统计学意义，此2种固定技术的下沉位移显著低于双皮质固定。终板下PMMA强化椎弓根钉的下沉位移显著低于其他固定技术。结论在骨质疏松状态下PMMA强化可显著提高骶骨钉一骨界面的结合强度。在上述4种骶骨固定技术中终板下PMMA强化椎弓根钉可获得最坚强的锚定。

Biomechanical comparison of four sacral pedicle screw fixation techniques

Objective To compare the subsidence displacement following cyclic loading among 4 sacral pedicle screw fixations of bicortical, tricortical, standard polymethylmethacrylate (PMMA) augmentation and sub-endplate PMMA augmentation in osteoporotic condition. Methods For this study, 11 fresh osteoporotic cadaver sacra were harvested, and bone mineral density was measured with dual energy X-ray absorptiometry. A 7 mm diameter monoaxial bicortical or tricortical pedicle screw (S1) was randomly assigned by side (left vs. right) and placed. The 2 screws which followed 2000 cyclic compression with 30-250 N pressure were removed. The screw tracts were filled up with PMMA, then, screws 5 mm shorter than the bicortical or tricortical fixation were reinserted (defined as standard and sub-endplate PMMA augmented sacral screw fixations, respectively). The PMMA augmented screws were then retested as before. Screw subsidence displacement following 2000 cyclic loading was measured and compared. Results The average bone mineral density of 11 specimens was 0.71 g/cm², ranged from 0.65 to 0.78 g/cm². No significant difference of subsidence displacement was detected between tricortical and standard PMMA augmented screws, however, the 2 fixations exhibited markedly less subsidence than bicortical screw. Sub-endplate PMMA augmented screw showed the least subsidence among all the screws.Conclusion PMMA augmentation can increase the bonding strength of sacral screw-bone interface, and the sub-endplate PMMA augmented sacral screw could obtain the most stability among the 4 fixation techniques in osteoporotic condition.