A Biomechanical Evaluation of Pull out Strength for Sacral Screw Fixation

Abstract

Objective To evaluate the effects of fatigue loading on the pull out strength of medial and lateral unicortical and bicortical sacral screws Methods Eleven fresh specimens of human sacrum were used in this study Bone mineral density (BMD) at the vertebral body and the ala were determined by peripheral quantitative computed tomography Seven millimeter compact CD sacral screws were inserted into the sacrum anteriomedially,anteriolaterally, unicortically and bicortically,and the insertion torque for each screw was measured Cyclic loading from 40 N to 400 N was applied to each screw at a frequency of 2 Hz up to 20 000 cycles Pull out tests were conducted after completion of the fatigue tests Results The average bone density at the S 1 body was 0 38 g/ml, and 0 24 g/ml at the S 1 ala The insertion torque and average pull out force following cyclic loading were significantly higher in bicortical fixation than that in unicortical fixation However, the pull out strength and insertion torque of medially oriented fixation was always higher than lateral fixation, regardless of whether the insertion was unicortical or bicortical The pull out force of unicortical and bicortical medial screw fixations following cyclic loading showed significant linear correlations with both the insertional torque and the bone mineral density of the S 1 body Conclusion In a group of young population, screw orientation(anterolateral or anteromedial) is more important in determining pull out strength than screw depth(unicortical or bicortical) following fatigue loading, anteromedially directed screws being significantly stronger than laterally placed screws Bone mineral density of the S 1 body and insertion torque are good preoperative and intraoperative indicators of screw pull out strength

目的 研究骶骨螺釘固定在承受周期性載荷后的拔出強度,評價前內、前外側和單、雙邊皮質螺釘固定的生物力學作用。方法 對11 具新鮮成人尸體的骶骨行定量CT(QCT)掃描以確定S1 椎體和骶骨翼的骨礦物質密度。CCD骶骨螺釘分別放置在骶骨的前內、前外側和單、雙邊皮質等四個位置上,測量螺釘固定時旋入的力矩。對螺釘施加40～400 N 的垂直周期性載荷,載荷頻率為2Hz,加載次數為20 000 次。然后行螺釘拔出試驗,獲得螺釘最大拔出力。結果 S1 椎體和骶骨翼的平均骨礦物質密度分別為0-38 g/ml 和0-24g/ml。雙邊皮質螺釘固定時的旋入力矩和最大拔出力均顯著大于單邊皮質螺釘。而無論是單邊或雙邊皮質固定, 內側螺釘固定的旋入力矩和最大拔出力都顯著高于外側螺釘。內側螺釘固定的最大拔出力與S1 椎體的骨礦物質密度和旋入力矩呈線性相關。結論對于年輕人群, 骶骨螺釘固定的方向對螺釘固定的最大拔出力的影響比螺釘插入深度的影響要大,前內側螺釘固定要明顯強于前外側螺釘固定。S1 椎體骨礦物質密度和螺釘旋入力矩可分別反映螺釘在術前和術中的固定強度