Wadhwa Rishi Kumar, Thakur Jai Deep, Khan Imad Saeed, James Jeremy, Ahmed Osama, Zhang Shihao, Henderson Benjamin, Ogden Alan, Guthikonda Bharat, Nanda Anil
Department of Neurosurgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
Department of Orthopedic Surgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
World Neurosurg. 2015 Mar;83(3):368-75. doi: 10.1016/j.wneu.2014.04.065. Epub 2014 May 2.
Lumbar pedicle screws are placed for internal fixation and help to enhance bony fusion. Optimal screws are medially directed, should be parallel or pointing to the superior endplate, and penetrate 50%-80% of the vertebral body. "Nonparallel" pedicle screws can be inadvertently placed within the confines of the pedicle and vertebral body but are sometimes replaced to obtain a more acceptable postoperative image. A nonparallel (suboptimal) screw is one that is located within the pedicle and body and does not violate bone; however, it is not parallel to the superior endplate. These "cored-out" grooves left in the bone from the initial tap and screw placement may compromise the integrity of the bone and the construct.
Dual-energy x-ray absorptiometry scans and L4-5 laminectomies were performed on 6 fresh-frozen cadaveric lumbar spines. We placed 2 optimal pedicle screws in L4, 1 optimal screw in L5, and 1 suboptimal screw in L5 (construct A). Axial rotation, flexion/extension, and lateral bending were tested. The suboptimal screw was repositioned in an optimal trajectory and retested (construct B). Pullout strength was performed on optimal and revised L5 pedicle screws.
The mean axial rotation stiffness was 1.31 N-m/degrees ± 0.22 in construct A and 1.19 N-m/degrees ± 0.17 in construct B (P = 0.023; 95% CI [CI], 0.20-0.02). The mean lateral bending stiffness was 0.015 N/mm ± 0.002 in construct A and 0.016 N/mm ± 0.002 in construct B (P = 0.3; 95% CI, 0.0008-0.001). The mean flexion/extension stiffness was 0.0139 N/mm ± 0.002 in construct A and 0.0126 N/mm ± 0.002 in construct B (P = 0.01; 95% CI, 0.002-0.0004). Axial rotation and flexion/extension stiffness were significantly different between the 2 groups. The mean pullout strength was significantly higher in the nonrevised parallel screw group compared with the reimplanted parallel screw group (906.93 N ± 271.17 vs. 608.32 N ± 207.23, P = 0.031). Dual-energy x-ray absorptiometry imaging demonstrated 4 osteopenic and 2 osteoporotic specimens, although differences in bone mineral density did not play a significant role in assessing either the biomechanical parameters or the pullout strength.
Great care is warranted in the initial placement of lumbar pedicle screws. Revising a nonparallel screw placement decreases pullout strength and alters biomechanical movements (axial rotation and flexion/extension) in patients with decreased bone mineral density. If a screw is inadvertently placed nonparallel to the endplate but is within the confines of the pedicle and vertebral body with adequate bone purchase, it should not be revised and rather be left in its place.
腰椎椎弓根螺钉用于内固定，有助于促进骨融合。理想的螺钉应向内指向，与上终板平行或指向其上，且穿透椎体的50%-80%。“不平行”的椎弓根螺钉可能在无意中放置于椎弓根和椎体内，但有时会被更换以获得更理想的术后影像。不平行（欠佳）的螺钉位于椎弓根和椎体内且未破坏骨质，但其与上终板不平行。初始攻丝和螺钉置入后在骨内留下的这些“掏空”沟槽可能会损害骨和内固定结构的完整性。
对6具新鲜冷冻的尸体腰椎进行双能X线吸收测定扫描和L4-5椎板切除术。我们在L4置入2枚理想的椎弓根螺钉，在L5置入1枚理想螺钉和1枚欠佳螺钉（结构A）。测试轴向旋转、屈伸和侧方弯曲。将欠佳螺钉重新调整至理想轨迹并重新测试（结构B）。对理想的和调整后的L5椎弓根螺钉进行拔出强度测试。
结构A的平均轴向旋转刚度为1.31 N·m/度±0.22，结构B为1.19 N·m/度±0.17（P = 0.023；95%可信区间[CI]，0.20 - 0.02）。结构A的平均侧方弯曲刚度为0.015 N/mm±0.002，结构B为0.016 N/mm±0.002（P = 0.3；95% CI，0.0008 - 0.001）。结构A的平均屈伸刚度为0.0139 N/mm±0.002，结构B为0.0126 N/mm±0.002（P = 0.01；95% CI，0.002 - 0.0004）。两组之间轴向旋转和屈伸刚度有显著差异。未调整的平行螺钉组的平均拔出强度显著高于重新置入的平行螺钉组（906.93 N±271.17对608.32 N±207.23，P = 0.031）。双能X线吸收测定成像显示4个骨质减少和2个骨质疏松标本，尽管骨密度差异在评估生物力学参数或拔出强度方面未起显著作用。
腰椎椎弓根螺钉的初始置入需格外小心。在骨密度降低的患者中，调整不平行的螺钉置入会降低拔出强度并改变生物力学运动（轴向旋转和屈伸）。如果螺钉无意中与终板不平行，但位于椎弓根和椎体内且有足够的骨质把持力，则不应调整，应留在原位。
