Stability of pedicle screws in comparison to anterior vertebral body screws after kyphoplasty augmentation

AIM: Aim of the study was to compare stability of pedicle screws and ventral implanted screws after insertion in soft or cured kyphoplasty cement. METHODS: Pedicle screws were inserted in a total of 40 thoracolumbar vertebrae of 10 different formalin-fixed human specimens: each 10 pedicle screws were implanted in soft (group 1) and cured cement (group 2), each 10 ventral screws were placed in soft (group 3) and cured (group 4) cement. Pedicle screws were then evaluated for biomechanical axial pullout resistance. RESULTS: Mean pull-out force was 452 N (range 60-1 125 N) in group 1, 367 N (range 112-840 N) in group 2, 364 N (range 65-875 N) in group 3 and 271 N (range 35-625 N) in group 4. CONCLUSION: Implantation of pedicle screws and ventral implanted screws in soft and cured kyphoplasty cement is a sufficient method. We achieved more stability with pedicle screws compared with ventral implanted screws in soft and cured cement. No significant difference was seen.     

Stability of anterior vertebral body screws after kyphoplasty augmentation

The goal of this cadaver study was to compare the stability of anterior vertebral body screws after implantation in soft or cured kyphoplasty cement. Anterior vertebral body screws were inserted in a total of 30 thoracolumbar vertebrae of ten different human specimens: ten screws were implanted in non-augmented vertebrae (group 1), ten screws were placed in soft cement (group 2), and ten screws were placed in cured cement (group 3). The screws were then tested for biomechanical axial pullout resistance. Mean axial pullout strength was 192 N (range: 10–430 N) in group 1, 364 N (range: 65–875 N) in group 2, and 271 N (range: 35–625 N) in group 3. The paired Student’s t-test demonstrated a significant difference between pullout strength of groups 1 and 2 (p= 0.0475). No significant difference was seen between pullout strength of groups 1 and 3 (p= 0.2646) and between groups 2 and 3 (p= 0.3863). We achieved a 1.9 times higher pullout strength with kyphoplasty augmentation of osteoporotic vertebrae compared with the pullout strength of non-augmented vertebrae. Implantation of anterior vertebral body screws in cured cement is a satisfactory method. With this method we found a 1.4 times higher pullout strength than non-augmented vertebrae.

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Résumé Le but de cette étude est de comparer la stabilité de vis implantées dans le corps vertébral de sujets cadavériques avec ou sans kyphoplastie par ciment. Méthode : 30 vertèbres thoraciques et lombaires sur dix sujets ont été ainsi implantées, 10 vis dans des vertèbres non traitées. Groupe 1 : 10 vis ont été placées dans du ciment mou et 10 vis (groupe 2) et 10 vis (groupe 3) dans du ciment dur. Les vis ont été évaluées sur le plan bio-mécanique et sur la résistance à l’arrachage. Résultat : la résistance à l’arrachage a été de 192 N dans le groupe 1, 1 364 N dans le groupe 2 et 271 N dans le groupe 3. Le test de Student a démontré une différence significative entre groupes 1 et 2 et les groupes 1 et 3, par contre, entre les groupes 2 et 3 il n’a pas été observé de différences significatives. Conclusion : la Kyphoplastie multiplie 1,9 fois la résistance des vertèbres ostéoportiques. Cette technique par inclusion de vis dans le corps vertébral, des vertèbres ainsi traitées est une méthode expérimentale suffisante.

     

Pullout resistance of conventional pedicle screw implantation in comparison to fluoroscopic computer-assisted technique

AIM: Aim of the study was to compare pullout resistance of pedicle screws after conventional and fluoroscopic computer-assisted implantation in the cadaveric thoracic and lumbar spine. METHODS: Pedicle screws were inserted in a total of 10 vertebrae of different human specimens: 10 screws were placed using conventional technique (group 1) and 10 screws were inserted with fluoroscopic computer-assisted system contralaterally (group 2). Then pedicle screws were evaluated for biomechanical axial pullout resistance. RESULTS: Mean pullout force was 232 N (range 60-600 N) in group 1 and 353 N (range 112-625 N) in group 2. The difference was significant (p=0,0425). CONCLUSION: Fluoroscopic navigated implantation of pedicle screws increases the pullout strength in thoracic and lumbar cadaveric spines as compared with conventional methods.     

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.     

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.     

Pedicle screw augmentation with bone cement enforced Vicryl mesh

Achieving sufficient mechanical purchase of pedicle screws in osteoporotic or previously instrumented bone is technically and biologically challenging. Techniques using different kinds of pedicle screws or methods of cement augmentation have been used to address this challenge, but are associated with difficult revisions and complications. The purpose of this biomechanical trial was to investigate the use of biocompatible textile materials in combination with bone cement to augment pullout strength of pedicle screws while reducing the risk of cement extrusion. Pedicle screws (6/40 mm) were either augmented with standard bone‐cement (Palacos LV + G) in one group (BC, n = 13) or with bone‐cement enforced by Vicryl mesh in another group (BCVM, n = 13) in osteoporosis‐like saw bone blocks. Pullout testing was subsequently performed. In a second experimental phase, similar experiments were performed using human cadaveric lumbar vertebrae (n = 10). In osteoporosis‐like saw bone blocks, a mean screw pullout force of 350 N (±125) was significantly higher with the Bone cement (BC) compared to bone‐cement enforced by Vicryl mesh (BCVM) technique with 240 N (±64) (p = 0.030). In human cadaveric lumbar vertebrae the mean screw pullout force was 784 ± 366 N with BC and not statistically different to BCVM with 757 ± 303 N (p = 0.836). Importantly, cement extrusion was only observed in the BC group (40%) and never with the BCVM technique. In vitro textile reinforcement of bone cement for pedicle screw augmentation successfully reduced cement extrusion compared to conventionally delivered bone cement. The mechanical strength of textile delivered cement constructs was more reproducible than standard cementing. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:212–216, 2018.     

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.     

Pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine

While the biomechanical properties of pedicle screws have proven to be superior in the lumbar spine, little is known concerning pullout strength of pedicle screws in comparison to hooks in the thoracic spine. In vitro biomechanical pullout testing was performed to evaluate the axial pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine with regard to surgical correction techniques in scoliosis. Nine human cadaveric thoracic spines were harvested and disarticulated. To simulate a typical posterior segmental scoliosis instrumentation, standard pedicle hooks were used between T4 and T8 and supralaminar hooks between T9 and T12 and tested against pedicle screws. The pedicle screws were loaded strictly longitudinal to their axis; the hooks were loaded perpendicular to the intended rod direction. In total, 90 pullout tests were performed. Average pullout strength of the pedicle screws was significantly higher than in the hook group (T4-T8: 531 N versus 321 N, T9-T12: 807 N versus 600 N, p < 0.05). Both screw diameter and the bone mineral density (BMD) had significant influence on the pullout strength in the screw group. For scoliosis correction, pedicle screws might be beneficial, especially for rigid thoracic curves, since they are significantly more resistant to axial pullout than both pedicle and laminar hooks.     

Evaluation of cyanoacrylate augmentation of transpedicular screw pullout strength

PURPOSE: Pedicle screw fixation of osteoporotic bone in the elderly is a challenge. Various augmentation methods have been studied by many authors. Although polymethylmethacrylate (PMMA) augmentation is believed to be a standard method, its usage is fraught with complications. Butyl-2-cyanoacrylate is an alternative to PMMA as it is bioresorbable, biocompatible, inexpensive, and noninfective. The objective of the current study was to determine the pullout strength of the pedicle screws when butyl-2-cyanoacrylate is used for augmentation. METHODS: Fresh calf lumbar vertebrae were obtained from male calves weighing 100-120 kg and implanted with pedicle screws. The screws were placed in native, unaugmented bone (group 1), butyl-2-cyanoacrylate-augmented bone (group 2), and PMMA-augmented bone (group 3). Axial pullout tests were done by an Instron 4411 universal testing machine. Statistical analysis was performed using the SPSS 9.0 for Windows program. Paired samples t test was used, and P < 0.05 was considered significant. RESULTS: The mean bone mineral density of the vertebrae was 1.6 +/- 0.1 g/cm2. The mean pullout strengths were 1.55 +/- 0.23 kN for group 1, 1.62 +/- 0.42 kN for group 2, and 2.55 +/- 0.22 kN for group 3. There was no statistically significant difference between groups 1 and 2. PMMA augmentation increased the pullout strength significantly when compared with butyl-2-cyanoacrylate augmentation and native bone (P = 0.002 and P = 0.001, respectively). CONCLUSIONS: The results of this study show that butyl-2-cyanoacrylate has no contribution to the augmentation of pedicle screw fixation in a calf model when compared with native bone or PMMA augmentation. Further studies are required to evaluate the effectiveness of butyl-2-cyanoacrylate in osteoporotic specimens and under cyclic loading in calf vertebra and animal and cadaver models before dispensing with its utility as an augmentation method in the clinical setting.     

Biomechanical study of pedicle screw fixation in severely osteoporotic bone.

BACKGROUND CONTEXT: Obtaining adequate purchase with standard pedicle screw techniques remains a challenge in poor quality bone. The development of alternate insertion techniques and screw designs was prompted by recognition of potential fixation complications. An expandable pedicle screw design has been shown to significantly improve fixation compared to a conventional screw in poor quality bone. PURPOSE: The purpose of this study was to determine if polymethylmethacrylate (PMMA) bone cement augmentation of an expandable pedicle screw can further improve fixation strength compared to the expandable screw alone in severely osteoporotic bone. A technique for cement insertion into the pedicle by means of the cannulated central portion of the expandable screw is also described. STUDY DESIGN: The axial pullout strength, stiffness and energy absorbed of cemented and noncemented expandable pedicle screws was determined in cadaveric vertebrae. METHODS: Twenty-one fresh unembalmed vertebrae from the thoracolumbar spine were used. Radiographs and bone mineral density measurements (BMD) were used to characterize bone quality. Paired cemented and noncemented pedicle screw axial pullout strength was determined through mechanical testing. Mechanical pullout strength, stiffness and energy to failure was correlated with BMD. RESULTS: Overall, there was a 250% increase in mean pullout strength with the cemented expandable screw compared with a noncemented expandable screw including a greater than twofold increase in pullout strength in the most severely osteoporotic bone. The mean stiffness and energy absorbed to failure was also significantly increased. A cemented conventional screw achieved a pullout strength similar to the noncemented expandable screw. CONCLUSIONS: PMMA cement augmentation of the expandable pedicle screw may be a viable clinical option for achieving fixation in severely osteoporotic bone.     

Cervical pedicle screws vs. lateral mass screws: uniplanar fatigue analysis and residual pullout strengths

BACKGROUND CONTEXT: Although successful clinical use of cervical pedicle screws has been reported, anatomical studies have shown the possibility for serious iatrogenic injury. However, there are only a limited number of reports on the biomechanical properties of these screws which evaluate the potential benefits of their application. PURPOSE: To investigate if the pull-out strengths after cyclic uniplanar loading of cervical pedicle screws are superior to lateral mass screws. STUDY DESIGN: An in vitro biomechanical study. METHODS: Twenty fresh-frozen disarticulated human vertebrae (C3-C7) were randomized to receive both a 3.5 mm cervical pedicle screw and lateral mass screw. The screws were cyclically loaded 200 times in the sagittal plane. The amount of displacement was recorded every 50 cycles. After cyclical loading, the screws were pulled and tensile load to failure was recorded. Bone density was measured in each specimen and maximum screw insertion torque was recorded for each screw. RESULTS: During loading the two screw types showed similar stability initially, however the lateral mass screws rapidly loosened compared to the pedicle screws. The rate of loosening in the lateral mass screws was widely variable, while the performance of the pedicle screws was very consistent. The pullout strengths were significantly higher for the cervical pedicle screws (1214 N vs. 332 N) and 40% failed by fracture of the pedicle rather than screw pullout. Pedicle screw pullout strengths correlated with both screw insertion torque and specimen bone density. CONCLUSIONS: Cervical pedicle screws demonstrated a significantly lower rate of loosening at the bone-screw interface, as well as higher strength after fatigue testing. These biomechanical strengths may justify their use in certain limited clinical applications.     

Pullout strength of pedicle screws using cadaveric vertebrae with or without artificial demineralization

OBJECTIVESTo evaluate the differences in the pullout strength and displacement of pedicle screws in cadaveric thoracolumbar vertebrae with or without artificial demineralization.METHODSFive human lumbar and five thoracic vertebrae from one cadaver were divided into two hemivertebrae. The left-side specimens were included in the simulated osteopenic model group and the right-side bones in a control group. In the model group, we immersed each specimen in HCl (1 N) solution for 40 minutes. We measured bone mineral density (BMD) using dual-energy X-ray absorptiometry and quantitative computerized tomography. We inserted polyaxial pedicle screws into the 20 pedicles of the cadaveric lumbar and thoracic spine after measuring the BMD of the 2 hemivertebrae of each specimen. We measured the pullout strength and displacement of the screws before failure in each specimen using an Instron system.RESULTSThe average pullout strength of the simulated osteopenic model group was 76% that of the control group. In the control and model groups, the pullout strength was 1678.87±358.96 N and 1283.83±341.97 N, respectively, and the displacement was 2.07±0.34 mm and 2.65±0.50 mm, respectively (p<.05). We detected positive correlations between pullout strength and BMD in the control group and observed a negative correlation between displacement and BMD in the model group.CONCLUSIONSBy providing an anatomically symmetric counterpart, the human cadaveric model with or without demineralization can be used as a test bed for pullout tests of the spine. In the simulated osteopenic model group, pullout strength was significantly decreased compared with the untreated control group.CLINICAL SIGNIFICANCEDecreased bone mineral density may significantly reduce the pullout strength of a pedicle screw, even though the range is osteopenic rather than osoteoporotic.     

Mechanical performance of cylindrical and dual core pedicle screws in calf and human vertebrae

Introduction Failure of pedicle screws by loosening and back out remains a significant clinical problem. Pedicle screw fixation is determined by bone mineral density, pedicle morphology and screw design. The objective of this study was to compare the holding strength of newly developed dual core pedicle screws having a cylindrical design in terms of outer diameter and two cylindrical inner core regions connected by a conical transition with conventional cylindrical pedicle screws.Materials and methods Fifty bovine lumbar vertebrae and 40 human lumbar vertebrae were used. Five different screws were tested in nine experimental “settings” and ten specimens each. The screws were tested for cranial displacement and pullout strength before and after 5,000 cycles of cranio-caudal loading. The tests included a setting with fully inserted and 4 mm backed out screws. For statistical analysis the incomplete balanced block design was used.Results Cyclic loading led to a decrease of pullout force between 24 and 31% and a 9% increase of displacement. The cylindrical screw designs were affected more than the dual core designs. The pullout force of cylindrical screws was smaller than of dual core screws. Even in a backed out condition dual core screws showed a significantly smaller displacement than cylindrical screws.Conclusion Pedicle screws with the dual core design provide good anchorage in the vertebra.     

可注射磷酸钙骨水泥对不同骨质椎弓根螺钉的强化作用

目的评价不同骨质疏松程度条件下,可注射性磷酸钙骨水泥对椎弓根螺钉稳定性强化作用,为其应用于合并有骨质疏松症的患者脊柱手术提供力学理论基础。方法采用新鲜尸体脊柱标本,根据骨密度检测结果,按临床诊断标准分成骨质正常、骨量减少、骨质疏松和重度骨质疏松四个水平;然后,每个骨密度水平,分直接置入椎弓根螺钉（对照组）和用可注射性磷酸钙骨水泥强化钉道后置入椎弓根螺钉（钉道强化组）,各12枚,进行螺钉轴向拔出实验,测定最大拔出力、刚度和能量吸收值三项指标,进行组间的对比分析。结果骨密度水平从正常下降到重度疏松程度,最大拔出力、刚度、能量吸收值均随之下降,同种置钉方法组间存在显著性差异（P〈0.05）。骨质疏松条件下钉道强化组最大拔出力、刚度、能量吸收值与骨量减少条件下对照组的比较,两者无显著性差异（P〉0.05）;但是,重度骨质疏松条件下钉道强化组的最大拔出力、刚度、能量吸收值均显著性低于骨量减少条件下对照组的（P〈0.05）。结论可注射性磷酸钙骨水泥强化钉道后可以提高椎弓根螺钉的稳定性,尤其是骨质疏松条件下经钉道强化后可以达到需要固定强度。     

Biomechanical comparison of pedicle screws versus spinous process screws in C2 vertebra: A cadaveric study

Background:

Biomechanical studies have shown C2 pedicle screw to be the most robust in insertional torque and pullout strength. However, C2 pedicle screw placement is still technically challenging. Smaller C2 pedicles or medial localization of the vertebral artery may preclude safe C2 pedicle screw placement in some patients. The purpose of this study was to compare the pullout strength of spinous process screws with pedicle screws in the C2.

Materials and Methods:

Eight fresh human cadaveric cervical spine specimens (C2) were harvested and subsequently frozen to −20°C. After being thawed to room temperature, each specimen was debrided of remaining soft tissue and labeled. A customs jig as used to clamp each specimen for screw insertion firmly. Screws were inserted into the vertebral body pairs on each side using one of two methods. The pedicle screws were inserted in usual manner as in previous biomechanical studies. The starting point for spinous process screw insertion was located at the junction of the lamina and the spinous process and the direction of the screw was about 0° caudally in the sagittal plane and about 0° medially in the axial plane. Each vertebrae was held in a customs jig, which was attached to material testing machine (Material Testing System Inc., Changchun, China). A coupling device that fit around the head of the screw was used to pull out each screw at a loading rate of 2 mm/min. The uniaxial load to failure was recorded in Newton''st dependent test (for paired samples) was used to test for significance.

Results:

The mean load to failure was 387 N for the special protection scheme and 465 N for the protection scheme without significant difference (t = −0.862, P = 0.403). In all but three instances (38%), the spinous process pullout values exceeded the values for the pedicle screws. The working distances for the spinous process screws was little shorter than pedicle screws in each C2 specimen.

Conclusion:

Spinous process screws provide comparable pullout strength to pedicle screws of the C2. Spinous process screws may provide an alternative to pedicle screws fixation, especially with unusual anatomy or stripped screws.     

Biomechanical study on pullout strength of thoracic extrapedicular screw fixation

Objective:To identify the biomechanical feasibility of the thoracic extrapedicular approach to the placement of screws. Methods:Five fresh adult cadaveric thoracic spine from T1 to T8 were harvested. The screw was inserted either by pedicular approach or extrapedicular approach. The result was observed and the pullout strength by pedicular screw approach and extrapedicular screw approach via sagittal axis of the vertebrale was measured and compared statistically. Results:In thoracic pedicular approach, the pullout strength of pedicle screw was 1001.23 N±220 N (288.2-1561.7 N) and that of thoracic extrapedicular screw approach was 827.01 N±260 N when screw was inserted into the vertebrae through transverse process,and 954.25 N±254 N when screw was inserted into the vertebrae through the lateral cortex of the pedicle. Compared with pedicular group, the pullout strength in extrapedicular group was decreased by 4.7% inserted through transverse process (P>0.05) and by 17.3% inserted through the lateral cortex (P<0.05). The mean pullout strength by extrapedicular approach was decreased by 11.04% as compared with pedicular approach (P<0.05). Conclusions:It is feasible biomechanically to use extrapedicular screw technique to insert pedicular screws in the thoracic spine when it is hard to insert by pedicular approach.     

Hydroxyapatite composite resin cement augmentation of pedicle screw fixation

Pedicle screw stability is poor in osteopenic vertebrae attributable, in part, to low screw-bone interface strength. The current authors examined cement augmentation using a low curing temperature hydroxyapatite and bis-phenol-A glycidol methacrylate-based composite resin. This cement may stiffen the screw-bone interface and reduce the harmful effects associated with polymethylmethacrylate regarding temperature and toxic monomer. Thirty-five lumbar vertebrae from human cadavers were instrumented with pedicle screws, with one pedicle previously injected with cement and the other as the control. Caudocephalad toggling of +/- 1 mm for 1600 cycles was applied to the pedicle screws, and the resulting forces supported by the implant-bone interface were captured by a load cell. A curve was constructed from the peak caudal load for each cycle and three mechanical measures parameterized this curve: (1) initial load; (2) rate of load decay during the first 400 cycles; and (3) final load. The initial load increased by 16% as a result of cement augmentation, the final load increased by 65%, and the rate of load decay decreased by 59%. Cement augmentation of pedicle screws increased the stiffness and stability of the screw-bone interface.     

颈椎经关节椎弓根螺钉固定与标准椎弓根螺钉固定的生物力学比较

目的 比较颈椎经关节椎弓根螺钉固定和标准椎弓根螺钉固定的拔出强度.方法 取10具新鲜尸体颈椎标本(C_3～T_1),游离成三个颈椎运动节段(C_(3,4),C_(5,6),C_7T_1).在椎体两侧随机进行经关节椎弓根螺钉固定或标准椎弓根螺钉固定,置入直径3.5 mm皮质骨螺钉.经关节椎弓根螺钉固定以上位椎骨侧块外下象限中点为进钉点,在直视椎弓根下,螺钉在冠状面内倾约45°、矢状面尾倾约50°.由上位椎骨下关节突经关节突关节、下位椎骨的椎弓根,进入下位椎骨的椎体内.标准椎弓根螺钉固定以侧块外上象限中点为进钉点,在直视椎弓根下,螺钉方向参考CT测量结果 ,尽量与椎弓根倾斜角度保持一致,在横断面上内倾约45°、矢状面上螺钉指向椎体的上1/3.在生物力学试验机上行拔出强度试验,比较两种螺钉固定的最大轴向拔出力.结果 颈椎经关节椎弓根螺钉固定平均最大轴向拨出力为(694±42)N,标准椎弓根螺钉固定为(670±36)N,两者比较差异有统计学意义(P<0.05).结论 颈椎后路经关节椎弓根螺钉固定的拔出强度大干标准椎弓根螺钉固定,从生物力学强度方面考虑经关节椎弓根螺钉固定可以作为标准椎弓根螺钉固定的一种补充方法.     

Augmentation of anterior vertebral body screw fixation by an injectable, biodegradable calcium phosphate bone substitute.

STUDY DESIGN: A biomechanical study to evaluate the effects of a biodegradable calcium phosphate (Ca-P) bone substitute on the fixation strength and bending rigidity of vertebral body screws. OBJECTIVES: To determine if an injectable, biodegradable Ca-P bone substitute provides significant augmentation of anterior vertebral screw fixation in the osteoporotic spine. SUMMARY OF BACKGROUND DATA: Polymethylmethacrylate (PMMA) augmented screws have been used clinically; however, there is concern about thermal damage to the neural elements during polymerization of the PMMA as well as its negative effects on bone remodeling. Injectable, biodegradable Ca-P bone substitutes have shown enhanced fixation of pedicle screws. METHODS: Sixteen fresh cadaveric thoracolumbar vertebrae were randomly divided into two groups: control (no augmentation) (n = 8) and Ca-P bone substitute augmentation (n = 8) groups. Bone-screw fixation rigidity in bending was determined initially and after 10(5) cycles, followed by pullout testing of the screw to failure to determine pullout strength and stiffness. RESULTS: The bone-screw bending rigidity for the Ca-P bone substitute group was significantly greater than the control group, initially (58%) and after cyclic loading (125%). The pullout strength for Ca-P bone substitute group (1848 +/- 166 N) was significantly greater than the control group (665 +/- 92 N) (P < 0.01). Stiffness in pullout for the Ca-P bone substitute groups (399 +/- 69 N/mm) was significantly higher than the control group (210 +/- 51 N/mm) (P < 0.01). CONCLUSION: This study demonstrated that augmentation of anterior vertebral body screw fixation with a biodegradable Ca-P bone substitute is a potential alternative to the use of PMMA cement.     

The ipsilateral lamina–pedicle angle: can it be used to guide pedicle screw placement in the sub-axial cervical spine?

Pedicle screws in the sub-axial spine are infrequently used because of concerns over their safety and difficulty in placement, despite their superior pullout strength. In the sub-axial cervical vertebrae, we have observed that the lamina appears to project at right angles to the ipsilateral pedicle axis. The aim of this investigation was to confirm the lamina orientation as a reliable landmark for pedicle screw placement. 80 digital cervical spine CT were analysed. The angle formed by the ipsilateral outer lamina cortex to the pedicle axis was recorded. A total of 398 vertebrae were analysed from patients with a mean age of 39.5 years (range 18–78). Average axial lamina–pedicle angle ranged from 96.6° at C3 to 87.2° at C7 in males, and from 95.6° to 87.5° in females. The angle formed by the posterior cortex of the lamina and the ipsilateral pedicle shows a high level of consistency for sub-axial cervical vertebrae ranging from 96° at C3 to 87° at C7. Although the angle is not exactly 90° at all levels as hypothesised, the orientation of the lamina, nevertheless, forms a useful reference plane for insertion of pedicle screws in the sub-axial cervical spine.