新型可膨胀椎弓根螺钉在骨质疏松性腰椎融合与固定手术中的初步应用

目的 评价新型可膨胀椎弓根螺钉(Thunder钉)在骨质疏松性腰椎椎体中固定的可靠性及其临床疗效.方法 随访2006年8月至2007年12月,应用Thunder钉治疗合并骨质疏松症的各种腰椎疾患42例(236枚螺钉),其中腰椎退行性疾病24例,腰椎结核二期后路融合手术8例,腰椎骨质疏松性压缩骨折6例,腰椎翻修手术4例.所有病例术前均行骨密度检查,患者腰椎骨密度平均下降2.6个标准差.术后处理与其他腰椎内固定术后患者相同.术后1周、3个月、半年、1年及2年随访患者,摄X线片并进行CT扫描三维重建,了解椎弓根螺钉稳定性及脊柱融合情况.结果42例患者均获平均16(12～24)个月的随访.术后3、6、12及24个月的影像学检查显示,所有Thunder钉位置良好,无螺钉松动、断裂迹象,评分平均值分别为3.0、3.0、2.9和2.9.脊柱骨融合于术后随时间延长趋于完善,骨融合评分平均值分别为2.3、2.5、3.0、3.0.所有患者临床症状缓解,疗效满意.结论 Thunder钉具有良好的钉-骨界面结合,能够提高螺钉在骨质疏松椎体中的固定强度,为脊柱融合提供良好的基础,临床疗效满意.     

Improving the pullout strength of pedicle screws by screw coupling

The objective of this study was to determine the effect of pedicle screw coupling on the pullout strength of pedicle screws in the osteoporotic spine. The vertebral bone mineral density (BMD) of 33 cadaveric lumbar vertebrae were measured by quantitative computed tomography. Pedicle screws were inserted into each pedicle. The pullout strength and displacement of the screws, without coupling and with single or double couplers, were studied, and the relationship between pullout strength and BMD was analyzed. The average pullout strength of the pedicle screws without screw coupling was 909.3 +/- 188.6 N (n = 9), that coupled with a single coupler was 1,409.0 +/- 469.1 N (n = 9), and that with double couplers was 1,494.0 +/- 691.6 N (n = 9). The pullout strength of the screws coupled with single or double couplers was significantly greater than that of screws without couplers (p < 0.01); however, there was no significant difference between the groups of single and double couplers. The improvement of pullout strength by screw coupling was significant in a test group with BMD of more than 90 mg/ml (p < 0.01), but was not in the group with BMD less than 90 mg/ml (p = 0.55). These results suggest that the coupling of pedicle screws improves pullout strength; however, the effect tends to be less significant in severely osteoporotic spines.     

Efficacy of novel-concept pedicle screw fixation augmented with calcium phosphate cement in the osteoporotic spine

Pedicle screw instrumentation has become increasingly popular for rigid internal stabilization of the thoracolumbar spine. However, when pedicle screws are used in elderly osteoporotic patients, the screw–bone interface is stripped easily. Therefore, the risk of screw loosening and backing-out after surgery has increased. The purpose of this study was to evaluate the efficacy of the novel-concept pedicle screw fixation augmented with calcium phosphate cement (CPC) in the osteoporotic spine. The novel-concept screw has the same shape as the ordinary screw, but it is hollow and fabricated with 20 small holes (1.3mm in diameter) leading to the hollow part on the bottom of the thread. Fifteen embalmed cadaveric lumbar vertebrae were instrumented with two types of pedicle screw (the ordinary screw and the novel-concept screw) in each pedicle. Only the novel-concept screws were augmented with CPC after insertion. Seven days later, axial pull-out testing was performed at a crosshead speed of 10mm/min. The mean maximal pull-out strength of the ordinary screws was 258N, and that of the novel concept screws was 637N. These results suggest that the novel-concept screw augmented with CPC can be useful for pedicle screw fixation of the osteoporotic spine.     

Biomechanical and histological evaluation of an expandable pedicle screw in osteoporotic spine in sheep

Transpedicular fixation can be challenging in the osteoporotic spine as reduced bone mineral density compromises the mechanical stability of the pedicle screw. Here, we sought to investigate the biomechanical and histological properties of stabilization of expandable pedicle screw (EPS) in the osteoporotic spine in sheep. EPSs and standard pedicle screws, SINO screws, were inserted on the vertebral bodies in four female ovariectomized sheep. Pull-out and cyclic bending resistance test were performed to compare the holding strength of these pedicle screws. High-resolution micro-computed tomography (CT) was performed for three-dimensional image reconstruction. We found that the EPSs provided a 59.6% increase in the pull-out strength over the SINO screws. Moreover, the EPSs withstood a greater number of cycles or load with less displacement before loosening. Micro-CT image reconstruction showed that the tissue mineral density, bone volume fraction, bone surface/bone volume ratio, trabecular thickness, and trabecular separation were significantly better in the expandable portion of the EPSs than those in the anterior portion of the SINO screws (P < 0.05). Furthermore, the trabecular architecture in the screw–bone interface was denser in the expandable portion of the EPS than that in the anterior portion of the SINO screw. Histologically, newly formed bone tissues grew into the center of EPS and were in close contact with the EPS. Our results show that the EPS demonstrates improved biomechanical and histological properties over the standard screw in the osteoporotic spine. The EPS may be of value in treating patients with osteoporosis and warrants further clinical studies.     

Cervical anterior transpedicular screw fixation (ATPS)—Part II. Accuracy of manual insertion and pull-out strength of ATPS

Reconstruction after multilevel decompression of the cervical spine, especially in the weakened osteoporotic, neoplastic or infectious spine often requires circumferential stabilization and fusion. To avoid the additional posterior surgery in these cases while increasing rigidity of anterior-only screw-plate constructs, the authors introduce the concept of anterior transpedicular screw (ATPS) fixation. We demonstrated its morphological feasibility as well as its indications in a previous study in Part I of our project. Consequently, the objectives of the current study were to assess the ex vivo accuracy of placing ATPS into the cervical vertebra as well as the biomechanical performance of ATPS in comparison to traditional vertebral body screws (VBS) in terms of pull-out strength (POS). Twenty-three ATPS were inserted alternately to two screws into the pedicles and vertebral bodies, respectively, of six cadaveric specimens from C3–T1. For insertion of ATPS, a manual fluoroscopically assisted technique was used. Pre- and post insertional CT-scans were used to assess accuracy of ATPS insertion in the axial and sagittal planes. A newly designed grading system and accuracy score were used to delineate accuracy of ATPS insertion. Following insertion of screws, 23 ATPS and 22 VBS were subjected to pull-out testing (POT). The bone mineral density (BMD) of each specimen was assessed prior to POT. Statistical analysis showed that the incidence of correctly placed screws and non-critical pedicles breaches in axial plane was 78.3%, and 95.7% in sagittal plane. Hence, according to our definition of “critical” pedicle breach that exposes neurovascular structures at risk, 21.7% (n = 5) of all ATPS inserted showed a critical pedicle breach in axial plane. Notably, no critical pedicle perforation occurred at the C6 to T1 levels. Pull-out testing of ATPS and VBS revealed that pull-out resistance of ATPS was 2.5-fold that of VBS. Mean POS of 23 ATPS with a mean BMD of 0.566 g/cm² and a mean osseus screw purchase of 27.2 mm was 467.8 N. In comparison, POS of 22 VBS screws with a mean BMD of 0.533 g/cm² and a mean osseus screw purchase of 16.0 mm was 181.6 N. The difference in ultimate pull-out strength between the ATPS and VBS group was significant (p < 0.000001). Also, accuracy of ATPS placement in axial plane was shown to be significantly correlated with POS. In contrast, there was no correlation between screw-length, BMD, or level of insertion and the POS of ATPS or VBS. The study demonstrated that the use of ATPS might be a new technique worthy of further investigation. The use of ATPS shows the potential to increase construct rigidity in terms of screw-plate pull-out resistance. It might diminish construct failures during anterior-only reconstructions of the highly unstable decompressed cervical spine. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

聚甲基丙烯酸甲酯强化对骨质疏松椎弓根螺钉固定的生物力学作用

目的探讨聚甲基丙烯酸甲酯 (polymethylmethacrylate,PMMA)骨水泥强化椎弓根螺钉的方法和评价 PMMA强化骨质疏松椎弓根螺钉后的生物力学性质。方法 6具新鲜老年女性胸腰段骨质疏松脊柱标本 (T10～ L5),使用双能 X线骨密度吸收仪测试每个椎体的骨密度,随机取 16个椎体 (32侧椎弓根 ),一侧椎弓根拧入 CCD螺钉,测量最大旋入力偶矩后拔出螺钉作为正常对照组,用 PMMA骨水泥强化椎弓根螺钉作为修复固定组,行螺钉拔出试验;另一侧经导孔直接强化椎弓根螺钉后拔出作为强化固定组,记录三组螺钉的最大轴向拔出力。结果椎体平均骨密度为 (0.445± 0.019)g/cm2;螺钉最大旋入力偶矩为（ 0.525± 0.104） Nm;正常对照组螺钉最大轴向拔出力为 (271.5± 57.3)N;修复固定组为 (765.9± 130.7)N;强化固定组为 (845.7± 105.0)N。 PMMA骨水泥强化或修复骨质疏松椎弓根螺钉后最大抗压力明显高于强化前,差异有非常显著性意义 (P< 0.01)。结论 PMMA骨水泥强化骨质疏松椎弓根螺钉能显著增加螺钉在椎体内的稳固性。     

Pedicle screw fixation in the cervical spine

Pedicle screw fixation of the lower cervical spine is a new technique that provides an alternative to posterior lateral mass plating. Although biomechanical studies support the use of pedicle screws to reconstruct the cervical spine, placing screws into the small cervical pedicle poses a technical challenge. Penetration of the pedicle is the primary complication associated with screw insertion in the lower cervical spine. Pedicle screw fixation at the C2 and C7 pedicles in conjunction with use of plates for occipitocervical or cervicothoracic plating is becoming an accepted technique; however, pedicle screw fixation should not be routinely used at the C3-C6 levels. It may be indicated in patients who have osteoporotic bone or when rigid internal fixation cannot be achieved by conventional techniques.     

后凸成形骨水泥强化技术对骶骨钉固定强度的生物力学影响

目的评价后凸成形骨水泥(Polymethylmethacrylate,PMMA)强化技术对骨质疏松情况下骶骨钉固定强度的生物力学影响,为骶骨钉松动选择坚强的补救技术提供依据。方法11具新鲜骶骨标本用于实验,并采用DEXA评价标本骨密度。在同一骶骨标本上,依次建立非PMMA强化和PMMA强化骶骨钉的固定模型如下,A组:单皮质椎弓根钉;B组:双皮质椎弓根钉;C组:传统PMMA强化单皮质椎弓根钉;D组:后凸成形PMMA强化椎弓根钉;E组:后凸成形PMMA强化侧翼钉。在MTS试验机上对五种骶骨钉依次进行轴向拔出测试,记录最大拔出力并比较。结果11具标本的平均骨密度为0.71±0.08g/cm2。A组的螺钉拔出力(508N)显著低于其他4种固定组(P0.05)。B组的螺钉拔出力(685N)与E组(702N)无显著差异(P0.05),但是,两者的拔出力均显著低于C和D组(P0.05)。重要的是,D组(986N)的拔出力显著高于C组(846N)。结论在骨质疏松患者的骶骨固定中,双皮质骶骨椎弓根钉较单皮质具有显著的力学优势。骶骨椎弓根钉一旦发生松动,传统的和后凸成形PMMA强化技术均可成为补救手段,并且后凸成形PMMA强化骶骨椎弓根钉可获得最坚强的锚定。     

The insertional torque of a pedicle screw has a positive correlation with bone mineral density in posterior lumbar pedicle screw fixation

In patients with osteoporosis there is always a strong possibility that pedicle screws will loosen. This makes it difficult to select the appropriate osteoporotic patient for a spinal fusion. The purpose of this study was to determine the correlation between bone mineral density (BMD) and the magnitude of torque required to insert a pedicle screw. To accomplish this, 181 patients with degenerative disease of the lumbar spine were studied prospectively. Each underwent dual-energy x-ray absorptiometry (DEXA) and intra-operative measurement of the torque required to insert each pedicle screw. The levels of torque generated in patients with osteoporosis and osteopenia were significantly lower than those achieved in normal patients. Positive correlations were observed between BMD and T-value at the instrumented lumbar vertebrae, mean BMD and mean T-value of the lumbar vertebrae, and mean BMD and mean T-value of the proximal femur. The predictive torque (Nm) generated during pedicle screw insertion was [-0.127 + 1.62 × (BMD at the corresponding lumbar vertebrae)], as measured by linear regression analysis. The positive correlation between BMD and the maximum torque required to insert a pedicle screw suggests that pre-operative assessment of BMD may be useful in determining the ultimate strength of fixation of a device, as well as the number of levels that need to be fixed with pedicle screws in patients who are suspected of having osteoporosis.     

Influence of bone mineral density on pedicle screw fixation : a study of pedicle screw fixation augmenting posterior lumbar interbody fusion in elderly patients

BACKGROUND CONTEXT: Some biomechanical studies have demonstrated that bone mineral density of the lumbar spine (BMD) affects the stability of pedicle screws in vitro. PURPOSE: To investigate influence of BMD on loosening and related failure of pedicle screws in vivo. STUDY DESIGN/SETTING: A clinical study of 52 patients who underwent pedicle screw fixation augmenting posterior lumbar interbody fusion (PLIF). PATIENT SAMPLE: There were 13 men and 39 women, with an average age of 63 years (range, 45-76 years) at the time of operation. The mean follow-up period was 2.8 years (range, 2-6 years). OUTCOME MEASURES: Relationship between BMD, screw loosening, and its related failures were statistically analyzed. METHODS: BMD was measured by the dual energy X-ray absorptiometry (DEXA) method. Radiographic assessments were done by the first author and independently by another orthopedist who was not informed of the values of BMD. RESULTS: The mean BMD of all patients was 0.879 +/- 0.215 (mean +/- S.D.) g/cm2. The mean BMD in patients with and without screw loosening was 0.720 +/- 0.078 g/cm2 (n=11) and 0.922 +/- 0.221 g/cm2 (n=41). There was a significant difference between the mean BMD of patients with and without screw loosening (P<.01). The mean BMD of patients with "union," "nonunion" and "undetermined union" was 0.934 +/- 0.210 g/cm2 (n=40), 0.674 +/- 0.104 g/cm2 (n=4) and 0.710 +/- 0.116 g/cm2 (n=8), respectively. The mean BMD of patients with "union" was significantly greater than those with "nonunion" and "undetermined union" (P<.05). CONCLUSION: It could be concluded that BMD has a close relation with the stability of pedicle screws in vivo, and BMD value below 0.674 +/- 0.104 g/cm2 suggests a potential increased risk of "nonunion" when pedicle screw fixation is performed in conjunction with PLIF.     

膨胀式椎弓根螺钉结合骨水泥强化钉道在严重骨质疏松脊柱手术中的应用

目的 探讨膨胀式椎弓根螺钉结合骨水泥强化钉道在治疗严骨质疏松性脊柱内固定手术中的早期疗效。方法 自2006年10月至2008年10月对20例需行内固定治疗同时合并严重骨质疏松的腰椎疾病患者采用膨胀式椎弓根螺钉结合骨水泥强化钉道进行脊柱后路稳定手术,其中男9例,女11例;年龄43～73岁,平均59岁。分别于术后1周、3个月、6个月、12个月及24个月摄动力位X线片及CT扫描,了解螺钉稳定性、骨水泥分布及脊柱融合情况。术前及术后3个月时应用日本矫形外科协会(JOA)不腰痛评分及视觉模拟评分(VAS)评价疗效。结果 本组共置人膨胀式椎弓根螺钉168枚。所有患者术后获12～ 38个月（平均26个月）随访。术前及术后3个月JOA评分平均分别为(11.4±2.6)分和(24.9±1.6)分,差异有统计学意义(t= 19.776,P=0.000);术前及术后3个月VAS评分分别为(7.0±1.4)分和(2.1±1.3)分,差异有统计学意义(t=11.470,P=0.000)。螺钉及骨水泥在椎体内位置稳定,周围骨小梁致密,未见明显透光带,无内固定松动移位迹象;后外侧融合节段植骨愈合良好,椎体间或椎板、关节突及棘突旁有连续性骨小梁形成,连接上下椎体。临床症状明显缓解,无早期及晚期感染等并发症发生,无临床复发。结论 对于合并严重骨质疏松的脊柱后路内固定手术,膨胀式椎弓根螺钉结合骨水泥强化钉道的方法能够极大地提高螺钉固定的稳定性。     

Clinical evaluation and computed tomography scan analysis of screw tracts after percutaneous insertion of pedicle screws in the lumbar spine

STUDY DESIGN: An examination of the accuracy of percutaneous pedicle screw placement in the lumbar spine. Using computed tomography scan analysis after implant removal, the screw tracts could be analyzed regarding the degree and direction of screw dislocation. OBJECTIVES: To investigate the misplacement rate and related clinical complications of percutaneous pedicle screw insertion in the lumbar spine. SUMMARY OF BACKGROUND DATA: The feasibility of the external fixation test has been investigated in several studies. Although pedicle screw misplacement has been reported as one of the main complications, there are no reliable data on the misplacement rate for this difficult surgical procedure. METHODS: In this study, 51 consecutive patients with suspected segmental instability were investigated after external transpedicular screw insertion for the external fixation test. Computed tomography scans of all instrumented pedicles from L2 to S1 were performed after screw removal. The screw tracts were analyzed, and the direction and degree of the pedicle violations were noted. In addition, the screw and pedicle angles were measured. RESULTS: Of 408 percutaneously inserted pedicle screws, only 27 screws (6.6%) were misplaced. There were 19 medial pedicle violations, 6 lateral cortical defects, and only 1 cranial and 1 caudal displacement. With respect to the spinal level, S1 showed the highest misplacement rate, with 11 screw dislocations (12%). After surgery, found two nerve root injuries were found. Only one of the injuries (L4) was related to the malposition of a screw. CONCLUSIONS: This study has shown that percutaneous insertion of pedicle screws in the lumbar spine is a safe and reliable technique. Despite the low misplacement rate of only 6.6%, it should be kept in mind that the surgical procedure is technically demanding and should be performed only by experienced spine surgeons.     

Pullout strength of misplaced pedicle screws in the thoracic and lumbar vertebrae - A cadaveric study

Background:

The objective of this cadaveric study was to analyze the effects of iatrogenic pedicle perforations from screw misplacement on the mean pullout strength of lower thoracic and lumbar pedicle screws. We also investigated the effect of bone mineral density (BMD), diameter of pedicle screws, and the region of spine on the pullout strength of pedicle screws.

Materials and Methods:

Sixty fresh human cadaveric vertebrae (D10–L2) were harvested. Dual-energy X-ray absorptiometry (DEXA) scan of vertebrae was done for BMD. Titanium pedicle screws of different diameters (5.2 and 6.2 mm) were inserted in the thoracic and lumbar segments after dividing the specimens into three groups: a) standard pedicle screw (no cortical perforation); b) screw with medial cortical perforation; and c) screw with lateral cortical perforation. Finally, pullout load of pedicle screws was recorded using INSTRON Universal Testing Machine.

Results:

Compared with standard placement, medially misplaced screws had 9.4% greater mean pullout strength and laterally misplaced screws had 47.3% lesser mean pullout strength. The pullout strength of the 6.2 mm pedicle screws was 33% greater than that of the 5.2 mm pedicle screws. The pullout load of pedicle screws in lumbar vertebra was 13.9% greater than that in the thoracic vertebra (P = 0.105), but it was not statistically significant. There was no significant difference between pullout loads of vertebra with different BMD (P = 0.901).

Conclusion:

The mean pullout strength was less with lateral misplaced pedicle screws while medial misplaced pedicle screw had more pullout strength. The pullout load of 6.2 mm screws was greater than that of 5.2 mm pedicle screws. No significant correlation was found between bone mineral densities and the pullout strength of vertebra. Similarly, the pullout load of screw placed in thoracic and lumbar vertebrae was not significantly different.     

Biomechanical evaluation of pedicle screws versus pedicle and laminar hooks in the thoracic spine.

BACKGROUND CONTEXT: Pedicle screws have been shown to be superior to hooks in the lumbar spine, but few studies have addressed their use in the thoracic spine. PURPOSE: The objective of this study was to biomechanically evaluate the pullout strength of pedicle screws in the thoracic spine and compare them to laminar hooks. STUDY DESING/SETTING: Twelve vertebrae (T1-T12) were harvested from each of five embalmed human cadavers (n=60). The age of the donors averaged 83+8.5 years. After bone mineral density had been measured in the vertebrae (mean=0.47 g/cm(3)), spines were disarticulated. Some pedicles were damaged during disarticulation or preparation for testing, so that 100 out of a possible 120 pullout tests were performed. METHODS: Each vertebra was secured using a custom-made jig, and a posteriorly directed force was applied to either the screw or the claw. Constructs were ramped to failure at 3 mm/min using a Mini Bionix II materials testing machine (MTS, Eden Prairie, MN). RESULTS: Pedicle claws had an average pullout strength of 577 N, whereas the pullout strength of pedicle screws averaged 309 N. Hooks installed using the claw method in the thoracic spine had an overwhelming advantage in pullout strength versus pedicle screws. Even in extremely osteoporotic bone, the claw withstood 88% greater pullout load. CONCLUSION: The results of this study indicate that hooks should be considered when supplemental instrumentation is required in thoracic vertebrae, especially in osteoporotic bone.     

椎弓根螺钉对幼犬腰椎发育的影响

目的通过动物实验观察椎弓根螺钉置入对腰椎形态发育的影响。方法取16只2月龄幼犬，随机分为三组。A组为空白对照组（3只），自L1-7做后路切开显露，暴露目标椎的椎板部分，仅造成对目标椎椎板的软组织剥离损伤。B组为对照组（5只），暴露目标椎椎板后，将螺钉穿过椎弓根，造成对目标椎椎弓根及神经弓中心软骨的隧道破坏后再将螺钉取出，螺钉不留置。C组为实验组（8只），暴露目标椎椎板后，将螺钉穿过椎弓根，造成对目标椎椎弓根及神经弓中心软骨的隧道破坏后不取出，螺钉留置于目标椎椎弓根内。实验的目标椎设计为L2、L4、L6，目标椎的上下两个相邻椎体的相应测量指标的平均值设计为其自身对照。CT测量术后当日及术后3个月的腰椎形态学指标，通过配对t检验比较各组目标椎在不同时期的形态学指标测量值与其自身对照值的差异情况，从而判断椎弓根螺钉的置入对脊椎形态发育的影响效果。结果C组目标椎的椎管面积、横径、纵径、椎弓根长度与相邻脊椎平均值比较差异有统计学意义（P〈0．05），而A组、B组与相邻脊椎平均值比较差异无统计学意义（P〉0．05）。各组椎体横径、纵径、高度与相邻脊椎平均值比较差异无统计学意义（P〉0．05）。结论椎弓根螺钉固定可影响幼犬腰椎椎管及椎弓的发育，可能导致医原性椎管狭窄而对于椎体发育无明确影响。仅破坏神经弓中心软骨联合而不留置螺钉对腰椎发育无明显的影响。     

Biomechanical evaluation of a double-threaded pedicle screw in elderly vertebrae

We sought to test the hypothesis that a pedicle screw that has two parallel threads of different heights throughout the full length of the screw could increase both bone purchase and pull-out strength compared with a standard single-threaded screw of similar dimensions. A single-threaded pedicle screw and a double-threaded pedicle screw were respectively placed into the paired pedicles of 21 vertebral bodies. The screws were then pulled out of the pedicles, and output parameters were measured. Although insertional torque was, on average, 14.5% higher (p = 0.039) for the single-threaded screw, maximum pull-out strength (p = 0.12), energy-to-failure (p = 0.39), and stiffness (p = 0.54) were not statistically different for the two screw types. It is concluded that a second, smaller inner thread on a double-threaded pedicle screw does not translate into either increased bone purchase or higher pull-out strengths.     

Accuracy and safety of free-hand pedicle screw fixation in age less than 10 years

Background:

Pedicle screws are being used commonly in the treatment of various spinal disorders. However, use of pedicle screws in the pediatric population is not routinely recommended because of the risk of complications. The present study was to evaluate the safety of pedicle screws placed in children aged less than 10 years with spinal deformities and to determine the accuracy and complication (early and late) of pedicle screw placement using the postoperative computed tomography (CT) scans.

Materials and Methods:

Thirty one patients (11 males and 20 females) who underwent 261 pedicle screw fixations (177 in thoracic vertebrae and 84 in lumbar vertebrae) for a variety of pediatric spinal deformities at a single institution were included in the study. The average age of patients was 7 years and 10 months. These patients underwent postoperative CT scan which was assessed by two independent observers (spine surgeons) not involved in the treatment.

Results:

Breach rate was 5.4% (14/261 screws) for all pedicles. Of the 177 screws placed in the thoracic spine, 13 (7.3%) had breached the pedicle, that is 92.7% of the screws were accurately placed within pedicles. Seven screws (4%) had breached the medial pedicle wall, 4 screws (2.3%) had breached the lateral pedicle wall and 2 screws (1.1%) had breached the superior or inferior pedicle wall respectively. Of the 84 screws placed in the lumbar spine, 83 (98.8%) screws were accurately placed within the pedicle. Only 1 screw (1.2%) was found to be laterally displaced. In addition, the breach rate was found to be 4.2% (11/261 screws) with respect to the vertebral bodies. No neurological, vascular or visceral complications were encountered.

Conclusions:

The accuracy of pedicle screw placement in pedicles and vertebral bodies were 94.6% and 95.8% respectively and there was no complication related to screw placement noted until the last followup. These results suggest that free-hand pedicle screw fixation can be safely used in patients younger than 10 years to treat a variety of spinal disorders.     

Comparative biomechanical analysis of an improved novel pedicle screw with sheath and bone cement

STUDY DESIGN: A human cadaveric biomechanical study of fixation strength of an improved novel pedicle screw (NPS) with cement and a conventional screw. OBJECTIVE: To clarify whether the NPS has adequate fixation strength without leakage in vertebrae with low bone quality. SUMMARY OF BACKGROUND DATA: The fixation strength of pedicle screws decreases in frail spines of elderly osteoporotic patients. Augmentation of screw fixation with bone cement must be balanced against increased difficulty of screw removal and risk of cement leakage. We developed the NPS consisting of an internal screw and an outer sheath to mitigate the disadvantages of cement augmentation. METHODS: The T12 and L1 vertebrae obtained from 18 formalin preserved cadavers (11 males and 7 females; mean age, 82.7 y) were used. The mean bone mineral density was 0.39 +/- 0.14 g/cm2. The NPS was inserted into one pedicle of each vertebra and the control screw, a Compact CD2 screw, was inserted into the contralateral pedicle. Both screws were 6mm in diameter and 40 mm in length. Pull-out tests were performed at a crosshead speed of 10 mm/min. Cyclic loading tests were performed with a maximum 250 N load at 2 Hz until 30,000 cycles. RESULTS: Cement leakage did not occur in any of the specimens tested. The mean maximum force at pull-out was 760 +/- 344 N for the NPS and 346 +/- 172N for the control screw (P < 0.01). Loosening of 50% of the screws was observed after 17,000 cycles of the NPS and after 30 cycles of the control screw. The hazard ratio of loosening was 19.6 (95% confidence interval 19.3-19.9) (P < 0.001). CONCLUSIONS: The NPS showed a significantly higher mechanical strength than the control screw in both pull-out tests and cyclic loading tests. The NPS showed more than adequate strength without cement leakage.     

Accuracy of fluoroscopically assisted laser targeting of the cadaveric thoracic and lumbar spine to place transpedicular screws

A simple and inexpensive method was developed to obtain a coaxial view of the pedicles to assist with screw insertion. The authors evaluated the accuracy of this device to place transpedicular vertebral screws in a human adult cadaver model. A dual radiation targeting system, a laser targeting system for fluoroscopically guided procedures, was developed to provide an accurate surface entry point and angle of approach to radiographic landmarks. After fluoroscopic cross-hair target localization of the coaxial view of the pedicle, X-ray radiation is turned off and the laser beam allows the surgeon to guide the screw through the pedicle. Nine cadaver spines were removed and mounted. Three surgeons, inexperienced in the technique of pedicle screw placement, fitted instruments to 184 pedicles between L5 and T5. A total of 83 lumbar and 101 thoracic pedicles underwent screw placement. After specimen dissection, the degree and location of any screw perforation were measured by direct inspection. Three screws perforated a pedicle, for an error rate of 1.6%. Two lumbar screws (2.4% error) and one thoracic screw (1% error) perforated the pedicle. No screw was more than 1 mm outside the pedicle. Five other screws, four in the thoracic spine and one in the lumbar spine (error rate of 2.7%) were directed too far laterally and perforated the lateral vertebral body. This low rate of pedicle wall cortical perforation by inexperienced surgeons compares favorably with much higher pedicle perforation rates by experienced surgeons when no imaging was used. In conclusion, this in vitro model using a dual radiation targeting system assisted with the accurate placement of transpedicular vertebral screws with minimal radiation exposure.     

Characteristics of immediate and fatigue strength of a dual-threaded pedicle screw in cadaveric spines

Background contextNovel dual-threaded screws are configured with overlapping (doubled) threads only in the proximal shaft to improve proximal cortical fixation.PurposeTests were run to determine whether dual-threaded pedicle screws improve pullout resistance and increase fatigue endurance compared with standard pedicle screws.Study design/settingIn vitro strength and fatigue tests were performed in human cadaveric vertebrae and in polyurethane foam test blocks.Patient sampleSeventeen cadaveric lumbar vertebrae (14 pedicles) and 40 test sites in foam blocks were tested.Outcome measuresMeasures for comparison between standard and dual-threaded screws were bone mineral density (BMD), screw insertion torque, ultimate pullout force, peak load at cyclic failure, and pedicular side of first cyclic failure.MethodsFor each vertebral sample, dual-threaded screws were inserted in one pedicle and single-threaded screws were inserted in the opposite pedicle while recording insertion torque. In seven vertebrae, axial pullout tests were performed. In 10 vertebrae, orthogonal loads were cycled at increasing peak values until toggle exceeded threshold for failure. Insertion torque and pullout force were also recorded for screws placed in foam blocks representing healthy or osteoporotic bone porosity.ResultsIn bone, screw insertion torque was 183% greater with dual-threaded than with standard screws (p<.001). Standard screws pulled out at 93% of the force required to pull out dual-threaded screws (p=.42). Of 10 screws, five reached toggle failure first on the standard screw side, two screws failed first on the dual-threaded side, and three screws failed on both sides during the same round of cycling. In the high-porosity foam, screw insertion torque was 60% greater with the dual-threaded screw than with the standard screw (p=.005), but 14% less with the low-porosity foam (p=.07). Pullout force was 19% less with the dual-threaded screw than with the standard screw in the high-porosity foam (p=.115), but 6% greater with the dual-threaded screw in the low-porosity foam (p=.156).ConclusionsAlthough dual-threaded screws required higher insertion torque than standard screws in bone and low density foam, dual-threaded and standard pedicle screws exhibited equivalent axial pullout and cyclic fatigue endurance. Unlike single-threaded screws, the mechanical performance of dual-threaded screws in bone was relatively independent of BMD. In foam, the mechanical performance of both types of screws was highly dependent on porosity.