Quantitative analysis of participants
Thirty-four cases were followed-up with no peeling off, the follow-up time ranged from 9 to 26 months, mean 14 months, all were involved in the result analysis.

Baseline data of two groups (Table 1)

Fixation effect
No breakage of screw could be seen, only two cases with Jikei Ⅰ stage osteoporosis in the fracture group in whom non-cement method was used noted with screw loosening and partial loss of correction of reduction at 5-6 months after operation, but no other adverse effects were found. The loss of correction of reduction in the fracture group was 5%, the fusion rate was 100% in the fusion group. In the three cases of nerve injury, one case recovered from Frankel C preoperatively to Frankel D level postoperatively, the other two cases recovered from Frankel D preoperatively to Frankel E level postoperatively.

Typical case analysis
Typical case 1
Male, 61 years old, L1 fracture-dislocation with Jikei Ⅰ stage osteoporosis. He was treated with pedicle screws 12 days after trauma, and underwent decompression of laminectomy and intertransverse process graft fusion. The diameter of screw was 6.5 mm, placement of screw closed to the front cortex of vertebral body, combined with rigid connection between screw and rod, two cross-link devices, placement of pedicle screws with large angles in the horizontal and the sagittal planes were applied. The patient was followed-up for 12 months without screw loosening, the loss of reduction was 5% (Figure 1).

Typical case 2
Male, 67 years old, L5 spondylolisthesis with Jikei Ⅰ stage osteoporosis. The patient underwent pedicle screws and intervertebral body graft fusion. The diameter of screw was 6.5 mm, which was placed perforating the front cortex of vertebral body by less than 2 mm. The patient was followed-up for 15 months, and got fused fine without screw loosening (Figure 2).

Typical case 3
Female, 70 years old, L4, 5 degenerative lumbar instability with Jikei Ⅲ stage osteoporosis, who underwent pedicle screws and posterolateral fusion, polymethyl methacrylate was used to fill the screw path to improve the stability of screw. The patient was followed-up for 24 months, and exhibited well fusion without screw loosening (Figure 3).

[1] Zhou ZH, Cheng L, Yin QD. Research progress in stability of pedicle screw.Zhongguo Zuzhi Gongcheng Yanjiu yu Linchuang Kangfu. 2011;15(26):4872-4875. [2] Amendola L, Gasbarrini A, Fosco M, et al. Fenestrated pedicle screws for cement-augmented purchase in patients with bone softening: a review of 21 cases. J Orthop Traumatol. 2011;12(4):193-199. [3] Paré PE, Chappuis JL, Rampersaud R, et al. Biomechanical evaluation of a novel fenestrated pedicle screw augmented with bone cement in osteoporotic spines.Spine (Phila Pa 1976). 2011;36(18):E1210-E1214. [4] Soshi S, Shiba R, Kondo H, et al. An experimental study on transpedicular screw fixation in relation to osteoporosis of the lumbar spine.Spine (Phila Pa 1976). 1991;16(11):1335-1341. [5] Weinstein JN, Rydevik BL, Rauschning W. Anatomic and technical considerations of pedicle screw fixation. Clin Orthop Relat Res. 1992;(284):34-46. [6] Polly DW Jr, Orchowski JR, Ellenbogen RG. Revision pedicle screws. Bigger, longer shims--what is best. Spine (Phila Pa 1976). 1998;23(12):1374-1379. [7] Weng XS,Qiu GX,Zhao WD, et al. Biomechanical study of various pedicle screw revision techniques. Zhonghua Guke Zazhi. 2003;23(10):622-626. [8] Frankel BM, Jones T, Wang C. Segmental polymethylmethacrylate-augmented pedicle screw fixation in patients with bone softening caused by osteoporosis and metastatic tumor involvement: a clinical evaluation. Neurosurgery. 2007;61(3):531-537. [9] Yin QD, Zheng ZG, Cai JP.  Applied anatomy and mechanical test for placement of screw of pedicle-rib unit in the thoracic spine. Zhongguo Linchuang Jiepouxue Zazhi. 2005;23(5):538-539. [10] Moon BJ, Cho BY, Choi EY, et al. Polymethylmethacrylate- augmented screw fixation for stabilization of the osteoporotic spine: a three-year follow-up of 37 patients. J Korean Neurosurg Soc. 2009;46(4):305-311. [11] Becker S, Chavanne A, Spitaler R, et al. Assessment of different screw augmentation techniques and screw designs in osteoporotic spines. Eur Spine J. 2008;17(11): 1462-1469. [12] Bullmann V, Liljenqvist UR, Rödl R, et al. Pedicle screw augmentation from a biomechanical perspective. Orthopade. 2010;39(7):673-678. [13] Yin QD, Zheng ZG, Cai JP, et al. An in vitro biomechanical study of the relative stability of pedicle screw. Jiangsu Yiyao. 2005;31(2):119-121. [14] Ding Y, Ruan DK, Huang WH, et al. Biomechanical evaluation of cross-link device in pedicle screw systems. Zhongguo Linchuang Jiepouxue Zazhi. 2002;20(6): 466-468. [15] Zhang GL, Li N, Gong MQ, et al. Characteristics of pedicle screw fixation for thoracolumbar burst fractures among middle and old aged patients. Zhonghua Guke Zazhi. 2002; 22(12):727-731. [16] Heyde CE, Rohlmann A, Weber U, et al. Stabilization of the osteoporotic spine from a biomechanical viewpoint. Orthopade. 2010;39(4):407-416.

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The application of pedicle screws in patients with osteoporosis often meets with high incidence of screw loosening and loss of correction, and thus osteoporosis is considered as the relative contraindication for pedicle screws. With the increasing of posterior spinal fixation surgery performed in degenerative spine and elderly patients, which accompanied by some degree of osteoporosis, how to improve the stability of pedicle screw and make the relative contraindication to become the relative adaptation has become a clinical research subject. There are more experimental studies of improving stability of pedicle screw, but fewer reports of clinical applications, especially the method of screws augmented with bone cement to fill the screw path presents some complications or risk, so it is carried out later and less in clinic[1-3]. Application of methods of improving absolute or relative stability of pedicle screw for different degree of osteoporosis, and get good results.

Design
Application research.

Time and setting
The experiment was completed in the Department of Orthopedics, the Second Affiliated Hospital of Soochow University, Wuxi Traditional Chinese Medicine Hospital and Wuxi No. 9 People’s Hospital from January 1999 to December 2007.

Subjects
There were 34 cases, 20 males and 14 females; aged 40-71 years, averaged 57 years old. Primary osteoporosis found in four cases and secondary osteoporosis found in 30 cases. According to Jikei classification^{[1, 4]}, early stage osteoporosis occurred in eight cases, Ⅰ stage osteoporosis in 11 cases, Ⅱ stage osteoporosis in eight cases, Ⅲ stage osteoporosis in seven cases. There were 14 cases of thoracolumbar fracture (fracture group) and 20 cases of osteopathy, the latter included lumbar spondylolisthesis in nine cases, lumbar spinal canal stenosis in eight cases, degenerative lumbar instability in three cases. Transpedicular vertebral body bone graft performed in eight cases and spinal posterolateral graft performed in five cases in the fracture group. Spinal posterolateral or intervertebral body graft fusion performed in the 20 cases of osteopathy and five cases of fracture (fusion group). There were three cases of nerve injury in the fracture group (Frankel C grade in one case, D grade in two cases).

Diagnostic criteria
According to Jikei^{[1, 4]} osteoporosis grading scale, the osteoporosis could be divided into early stage: the number of bone trabeculae was normal, bone density was decreased and bone trabeculae became thinning; Ⅰ stage: transverse trabeculae was decreased, vertical trabeculae and endplate prominent; Ⅱ stage: transverse trabeculae was further decreased, vertical trabeculae also decreased; Ⅲ stage: transverse trabeculae was nearly disappeared, vertical trabecular bone was not clear and its shape like a meadow.

Products of thoracic and lumbar spine in titanium alloy AF, screw rod system and the lifting screw system from Jiangsu Jinlu Group Medical Instrument Limited Company and Jiangsu Zhangjiagang Jinfeng Ideal Medical Instrument Limited Company.

Methods
Methods for application
For 19 cases of osteoporosis in early stage and Ⅰ stage osteoporosis, longer and large size of pedicle screws were used (6.0 mm-7.0 mm screw diameter, closed to or perforated the front cortex of vertebral body by 2 mm or less) to improve the stability of screw. At the same time, combined with rod system with ankylosing collection between screw and rod with two cross-link devices, placement of pedicle screws with large angles in the horizontal and the sagittal planes were applied to improve the relative stability of screw. For 15 cases of osteoporosis in Ⅱ stage and Ⅲ stage osteoporosis, bone cement was used to fill the screw path to improve the absolute stability of screw, which including polymethyl methacrylate in 12 cases and hydroxyapatite cement in three cases. After drilling the hole and verifying that the screw path not perforating the pedicle, then while it was in paste state injected 2-3 mL of bone cement into the vertebral body and screw path in each side, at last placed screw into pedicle.

Evaluation criteria
Loss of correction: loss of correction of reduction for fracture was evaluated using percentage of anterior vertebral body height, and loss of correction for spondylolisthesis using percentage of vertebral body length on lateral X-ray. Bone fusion with Suk standard to judge: fused in the bone graft area judged as there was a clear continuity of trabeculae, the changes of intervertebral angle in hyperextension and hyperflexion lateral X-ray should be less than 4°; non fused judged as there was no continuity of trabeculae in the fusion zone, gap was visible and the changes of intervertebral angle in hyperextension and hyperflexion lateral X-ray should be more than 4°. If X-ray was difficult to judge, CT examination was performed.

Factors closely related with the stability of pedicle screw are the quality of screw placement and the connection between screw-bone interface and screw. Among these factors, the key is screw-bone interface. It is generally believed that bone mineral density is positively correlated with the stability of screw. With the increasing of screw length and diameter, the contact area in the screw-bone interface increases, the screw stability increases correspondingly. Weinstein et al ^[5] study showed that 60% of fixation strength of pedicle screw lies in the pedicle, and the cancellous bone of vertebral body increased by 15%-20%, close to but not perforate the front cortex and increased by 16%, perforate the front cortex increased by 20%-25%. Sacrum is the weakest part for pedicle screw in thoracolumbar spine, as in which the channel length is short, only about 30-35 mm; if perforating the front cortex, the fixation strength will increased by 60%. Therefore, perforating the front cortex (especially in sacrum) is an effective way to improve stability of screw, but do not forget to control the perforation of cortical less than 2 mm, so as not to damage the blood vessels and other important tissues. Studies also showed that screw diameter and length synergy with each other. When increase 2 mm in diameter and 10 mm in length, compared with that increase 1 mm in diameter and 5mm in length, there is very significant difference in the increasing of the stability of screw in the former^[6-7]. As cortical and cancellous bones are non-homogeneity, stability of screw close to cortex of pedicle is better than that of screw completely located within cancellous bone[8-9]. Yin et al [9-10] showed that when the pedicle screw placed close to the cortex even if very small part of perforation of pedicle cortex, the pullout strength and torque of screw have not been reduced especially in osteoporosis, but more than those of screw completely located within cancellous bone. Using a large size of screw causes pedicle expansion, as long as the insertion direction is accurate, nerve damage caused by pedicle perforation could be avoided. Therefore, application of thicker and longer screws is clinically safe and convenient way to improve stability of screw^[5-6].

For severe osteoporosis spine, the most reliable method to improve the stability of screw is to use strengthening agent in the screw path, of which polymethyl methacrylate has the best effect. Experimental studies have shown that polymethyl methacrylate can immediately increase the pullout strength of screw by 95%, adding pressure to raise 196%^[10-12]. Soshi et al ^[4] reported that the pullout strength doubled in the osteoporosis spine using bone cement to fill the screw path. However, the application of polymethyl methacrylate has some disadvantages such as: aggregation produces high heat, wrong injection or overflow that will injury the nerve tissue, long-term retention may have some toxic and carcinogenic risks, difficult for further removing, without bone conduction, a layer of connective organization grows between bone and metal screw, which under long-term load can produce relaxation and lead to bone resorption and pedicle fracture^[10]. Human artificial chromosome, calcium phosphate cement, composite bio-ceramic bone cement and other new enhancers are blessed with good biocompatibility and bone conductivity, slow degradation in vivo, non-toxication, does not produce high heat. In these areas, they are better than polymethyl methacrylate, but their effect in improving the stability of screw is poor than polymethyl methacrylate. The main complication of new strengthening agent is leakage out of screw path and might cause nerve tissue damage^{[7, 10, 12]}. There was no neurovascular injury or nerve damage in this paper. We understood that probe must be used to verify that there is not rupture of pedicle wall before application of strengthening agent, furthermore, while it is in paste state inject bone cement into vertebral body and screw path, as well as the pressure injected is not too high. In this way to use strengthening agent was yet relatively safe.

Enhancement of bone cement or replacement with thicker and longer screws can increase stability of screw that is the change of absolute value of stability of screw. On the other hand, the role of pedicle screw fixation does not rely solely on a single screw to work, but through connection between screw and rod or plate, that means the whole set of instruments plays a regular role as a whole. Therefore, the upper and the lower screws, the left and the right screws with different connectors, placement of screws in different directions will affect the stability of screws in the device, which named as the change of relative value of stability of screw by Yin et al ^[13]. Biomechanical tests show that the application of screws with rigid collection between screw and rod with two cross-link devices, screw placement with large angles for the upper and the lower screws in the sagittal plane and the left and the right screws in the horizontal plane, respectively, as well as more fixing segments, will help to improve the relative value of stability of screw in osteoporosis spine ^[13-14]. As the rod system with rigid connection between screw and rod, and two cross-link devices put the apparatus together into one solid whole, just like screws in a locking plate, which have a good anchor force and pullout strength, meanwhile also have an increase in torque and fatigue resistance stability of apparatus. Screws placed in large angles in the sagittal plane and horizontal plane increase the shear strength of screw, which gives a triangular stability. Krag and Yin et al ^[13] named them as “toe-nailing effect” in the horizontal and the sagittal planes, respectively. Zhang et al ^[15] reported that application of thicker and longer screws and a larger horizontal angle of screws placement can improve the stability of screws in less osteoporosis spine and achieve good clinical results.

Polly and other studies^[6,8] have shown that the same screw inserted at the first time has the greatest stability, but spin out and then spin in again, the screw torque will drop 34% than the initial. Therefore, in patients with osteoporosis, the screw must be placed in a one-time, do not spin out and replace. Drilling a small hole and placement with a needle to ensure accurate location under C-arm X-ray machine, then placement with screw in the surgery, that is the right way.

Soshi and other studies^{[1, 4]} showed that for Jikei early and Ⅰ stage osteoporosis spine, an increase of 20% in diameter of screw can double the screw pullout resistance, whereas for Ⅱ and Ⅲ stage osteoporosis spine, the effect of simply increasing the diameter of screw on the increasing of screw pullout resistance is not very obvious, thus other methods must be used in this situation. Instability such as screw loosening and partial loss of correction of reduction in two cases with Jikei Ⅰ stage osteoporosis, in whom non-cement augmented screw was used after 5 to 6 months in this group, that showed the effect of this method was limited and there were still defects, the same method should not be used in patients with Jikei Ⅱ and Ⅲ stage osteoporosis. In general, the authors agreed with views that for Jikei early and Ⅰ stage osteoporosis spine using thicker and longer screws to increase the absolute stability of screw is safe and convenient, combined with rigid collection between screw and rod with two cross-link devices, and placement of pedicle screw with large angles in the horizontal and sagital planes were applied to improve the relative stability of screw. However, for Jikei Ⅱ and Ⅲ stage osteoporosis spine, more reliable method of bone cement must be used to improve the absolute stability of screw^[6,16]. According to these principles, we used different methods to improve the stability of screw in 34 cases with osteoporosis, the overall results showed that the methods were safe and effective. Resently, expandable and cannulate fenestrated pedicle screws in osteoporotic patients have achieved good fixation effect, it is a promising research subject^{[2-3, 16]}.

对于轻度骨质疏松患者，采用较粗和较长螺钉，同时结合采用螺钉与连接棒强直性连接，且有两根横向连接装置，左右、上下椎弓根螺钉分别在水平面和矢状面以较大成角置入螺钉等方法提高螺钉固定相对稳定性;对于较严重骨质疏松患者，采用钉道注入骨水泥方法提高螺钉固定绝对稳定性。

对提高椎弓根螺钉固定强度的研究，目前基本集中在实验研究阶段，临床开展应用相对较少，尤其是钉道使用骨水泥加强，有一定并发症或风险存在，所以开展较少、较晚。特殊螺钉(如膨胀螺钉、空心开侧孔螺钉或涂层的生物活性钛钉)提高椎弓根螺钉固定强度的效果良好，有很好的应用前景，是将来研究热点。本文是临床应用研究，针对不同骨质疏松程度采用提高螺钉固定绝对或相对稳定性的方法，结果表明，可以减少骨质疏松患者螺钉松动、复位矫正丢失等并发症，无不良并发症发生，优良率高，处同类研究中较高水平。

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