The effect of screw length and bone cement augmentation on the fixation strength of iliac screws: a biomechanical study.
METHODS
Comparison of the biomechanical fixation strengths offered by 3 iliac screw fixation techniques: short screw, short screw augmented with cement, and long screw.


OBJECTIVE
Evaluate the effect of screw length and bone cement augmentation on the fixation strength of iliac screw upon fatigue loading.


BACKGROUND
Iliac screws have been used in treating spinal disorders such as spinal deformity, spondylolisthesis, and sacral tumor. In clinical practices, both short screws and long screws are being used. It has been reported that short iliac screws have a higher rate of loosening. Therefore, short iliac screws are being used with bone cement augmentation to improve fixation. To date, no biomechanical study has compared the strengths of these 3 different iliac screw fixation techniques.


METHODS
Fresh, frozen human cadaveric pelvis specimens (n = 18, 12 males, 6 females, average age 61 y) were used. Bone density was measured to characterize bone quality. The specimens were randomly divided into 2 groups. In group 1 (n = 8), short screws of 7.0-mm diameter and 70 + or - 4 mm length (as the length of exceeding over ischial notch) and long screw of 7.0-mm diameter and 120 + or - 4 mm length were placed on either side of the pelvis (left and right). In group 2 (n = 10), short iliac screws were placed after augmentation with polymethyl methacrylate bone cement on 1 side of the pelvis and long iliac screw were placed on the other side (left and right). Cyclic loading ranging from 20 to 200 N was applied to each screw at a frequency of 2 Hz up to 5000 cycles. Pullout tests were then conducted at the rate of 5 mm/min after the fatigue test, and the maximum pullout strength for each screw was recorded and analyzed.


RESULTS
The maximum pullout strength of the long screw and short screw groups after fatigue conditioning were 2386 + or - 1470 and 833 + or - 681 N respectively. Significant difference was found between the 2 groups (P < 0.05). The short iliac screw had a higher loosening rate. The pullout force of the short screw fixation with augmentation and the long screw fixation after cyclic loading were 2436 + or - 915 and 2529 + or - 1055 N, respectively. No significant difference was found between the 2 groups (P > 0.05).


CONCLUSIONS
Short iliac screws are susceptible to loosening after cyclic loading. Bone cement augmentation of short screws has demonstrated a significant increase in the fixation strength of short screws to an extent similar to that of long iliac screws. Thus, short iliac screw fixation after augmentation with bone cement will be a viable clinical option for spino-pelvic reconstruction.
Zheng
Zhao-Min
ZM
Department of Spine Surgery, the First Affiliated Hospital of Sun Yat-sen University, Guangzhou 510080, China. zhengzm1@163.com
Zhang
Kui-Bo
KB
Zhang
Jia-Fang
JF
Yu
Bin-Sheng
BS
Liu
Hui
H
Zhuang
Xin-Ming
XM
D001696:Biomechanical Phenomena / Q000502:physiology; D001843:Bone Cements / Q000592:standards* / Q000627:therapeutic use; D015519:Bone Density / Q000502:physiology; D001863:Bone Screws / Q000592:standards*; D002102:Cadaver; D004868:Equipment Failure; D019544:Equipment Failure Analysis / Q000379:methods; D005260:Female; D006801:Humans; D007085:Ilium / Q000033:anatomy & histology / Q000502:physiology / Q000601:surgery*; D007593:Joint Instability / Q000601:surgery; D008159:Lumbar Vertebrae / Q000601:surgery; D008297:Male; D008875:Middle Aged; D019904:Polymethyl Methacrylate / Q000592:standards / Q000627:therapeutic use; D011183:Postoperative Complications / Q000209:etiology / Q000503:physiopathology / Q000517:prevention & control*; D019651:Plastic Surgery Procedures / Q000295:instrumentation / Q000379:methods; D013121:Spinal Curvatures / Q000601:surgery; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*; D013314:Stress, Mechanical; D016474:Weight-Bearing / Q000502:physiology
D001843:Bone Cements; D019904:Polymethyl Methacrylate