Biomechanical evaluation of patellar- and hamstring tendon graft fixation for anterior cruciat ligament reconstruction using a poly-(D, L-lactide) interference screw

Anterior cruciate ligament (ACL) reconstruction using autologous hamstring tendons are being performed more frequently and satisfactory results have been reported. Advantages such as low donor site morbidity and ease of harvest as well as disadvantages like low initial construct stiffness have been described. Recently, it has been demonstrated that graft fixation close to the original ACL insertion sites increases anterior knee stability and graft isometry. Hamstring tendon fixation techniques using interference screws offer this possibility. To reduce the risk of graft laceration, a round threaded titanium interference screw (RCI) was developed. To improve initial fixation strength, fixation techniques for hamstring tendons with separate or attached tibial bone plugs were introduced. However, data on fixation strength do not yet exist. With respect to the proposed advantages of biodegradable implants, like undistorted magnetic resonance imaging, uncompromised revision surgery and a decreased potential of graft laceration during screw insertion, we performed pullout tests of round threaded biodegradable and round threaded titanium interference screw fixation of semitendinosus (ST) grafts with and without distally attached tibial bone plugs. Data were compared with bone-tendon-bone (BTB) graft fixation using biodegradable and conventional titanium interference screws. We used 56 proximal calf tibiae to compare maximum pullout force, screw insertion torque, and stiffness of fixation for biodegradable direct ST tendon and bone plug fixation (group I: without bone plug, group II: with bone plug) versus titanium interference screw fixation (group III: without bone plug, group IV: with bone plug). A round threaded biodegradable poly-(D, L-lactide) (Sysorb) and a round threaded titanium interference screw (RCI) were used. As a control calf bone-tendon-bone (BTB) grafts fixed with either poly-(D, L-lactide) (group V) or conventional titanium (group VI) interference screws were used. ST tendons were harvested either with or without their distally attached tibial bone plugs from human cadavers and were folded to a three-stranded graft. Specimen were loaded in a material testing machine with the applied load parallel to the long axis of the bone tunnel. Maximum pullout force of ST bone plug (group III: 717 N +/- 90, group IV: 602 N +/- 117) fixation was significantly higher than that of direct tendon (group I: 507 N +/- 93, group III: 419 N +/- 77) fixation. Maximum pullout force of biodegradable screw ST fixation was higher than that of titanium screw fixation in both settings. There was no significant difference in pullout force between biodegradable (713 N +/- 210) and titanium (822 N +/- 130) BTB graft fixation or between ST fixation with bone plug and biodegradable screw with BTB fixation. Pullout force of hamstring tendon interference screw fixation can be improved by using a biodegradable implant. In addition, initial pullout force can be greatly improved by harvesting the hamstring tendon graft with its distally attached tibial bone plug. This may be important, especially in improving tibial graft fixation. This study encourages further research in tendon-bone healing with direct interference screw fixation to confirm the potential of this advanced method.     

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.     

geneX骨水泥强化骨质疏松椎体椎弓根钉的生物力学研究

目的评估骨质疏松情况下geneX骨水泥强化椎弓根钉的固定强度。方法应用微量注射泵对30个新鲜小牛腰椎标本注射稀盐酸建立骨质疏松椎体模型。60个椎弓根分为四组:geneX骨水泥组,硫酸钙骨水泥(CSC)组,聚甲基丙烯酸甲酯骨水泥(PMMA)组,对照组。随机选择一侧注射2.5 ml骨水泥,然后置入螺钉;另一侧行正常螺钉固定对照,应用材料试验机进行轴向拔出力测试,记录各组的轴向最大拔出力和能量吸收值并进行比较。结果 geneX组与CSC组两组拔出力及能量吸收值比较,差异无统计学意义(P>0.05),两组均显著低于PMMA组(P<0.05),两组均显著高于对照组(P<0.05)。结论 geneX骨水泥强化椎弓根钉可显著提高椎弓根固定强度,geneX骨水泥可用作椎弓根强化螺钉的填充材料。     

Hamstring tendon fixation using interference screws: a biomechanical study in calf tibial bone

It has recently been shown that graft fixation close to the ACL insertion site is optimal in order to increase anterior knee stability. Hamstring tendon fixation using interference screws offers this possibility and a round threaded titanium interference screw has been previously developed. The use of a round threaded biodegradable interference screw may be equivalent. In addition, to increase initial fixation strength, graft harvest with a distally attached bone plug may be advantageous, but biomechanical data do not exist. This study compares the initial pullout force, stiffness of fixation, and failure modes of three strand semitendinosus grafts in 36 proximal calf tibiae using either biodegradable poly-(D,L-lactide) (Sysorb; Sulzer Orthopaedics Ltd, Munsingen, Switzerland) or round threaded titanium (RCI; Smith & Nephew DonJoy, Carlsbad, CA) interference screws, harvested either without (biodegradable: group I, titanium III) or with (biodegradable: group II, titanium: group IV) attached tibial bone plugs. Maximum pullout force in group I (507 ± 93 N) was significantly higher than in group III (419 ± 77 N). Pullout force of bone plug fixation was significantly higher than that of direct tendon fixation (717 ± 90 N in group II and 602 ± 117 N in group IV). Pullout force of biodegradable fixation was significantly higher in both settings. These results indicate that initial pullout force of hamstring-tendon graft interference screw fixation can be increased by using a biodegradable interference screw. In addition, initial pullout force of hamstring-tendon graft fixation with an interference screw can be greatly increased by harvesting the graft with its distally attached tibial bone plug.Arthroscopy 1998 Jan-Feb;14(1):29-37     

Evaluation of calcium sulfate paste for augmentation of lumbar pedicle screw pullout strength.

BACKGROUND CONTEXT: Many authors have evaluated the components responsible for ultimate pullout strength of pedicle screws. In these studies, one important variable has been the screw fixation. Because pedicle screw fixation has increased in popularity over recent years, so has the need for augmentation in difficult situations. Polymethylmethacrylate (PMMA) has been established as the gold standard in terms of strength of fixation but has the potential for severe complications when applied in spine surgery. Calcium sulfate is an alternative to PMMA, because it lacks the exothermic reaction, is potentially osteoconductive and is resorbed by the body in 30 to 60 days. PURPOSE: To determine the strength of a new calcium sulfate cement in terms of pedicle screw augmentation. The purpose was to evaluate calcium sulfate versus PMMA in terms of pullout strength. PMMA was considered the gold standard in terms of strength for this experiment. STUDY DESIGN: Lumbar vertebrae implanted with pedicle screws were subjected to axial pullout tests. The force required to cause implant failure was measured and compared for three methods of fixation. OUTCOME MEASURES: Force to failure (Newtons) for each pedicle test was recorded and analyzed with results from similarly augmented pedicles. METHODS: Lumbar vertebrae were harvested from four cadavers and implanted with pedicle screws. These screws were either placed in native bone or augmented with either calcium sulfate paste or PMMA. In those pedicles that had augmentation, the material was permitted to set for a minimum of 24 hours. Axial pullout tests were then performed using an MTS (Materials Testing System Corporation, Minneapolis, MN) testing machine. The screws were pulled out over a distance of up to 6 mm at 0.25 mm/sec. This rate and distance ensured failure in each case. The load to failure was recorded for each pedicle. RESULTS: Calcium sulfate augmentation improved pedicle screw pullout strength significantly when compared with native bone (p=.0003). This represented an average increase of 167% over the native bone. Likewise, PMMA improved the pullout strength significantly over native bone (p<.0001) for an average increase of 199%. There were no significant differences between the calcium sulfate and PMMA augmentation (p=.0791), although the PMMA averaged 119% of the strength of calcium sulfate. CONCLUSIONS: Although PMMA is considered the gold standard in terms of fixation strength, its use around the spine has been limited because of concern over complications. The calcium sulfate bone paste tested in this study demonstrated strength similar to PMMA without the major risks to the spinal cord. In addition, the calcium sulfate paste allows for possible osteoconduction to aid in spinal fusion. Further study is needed to determine the applicability of this calcium sulfate paste in the clinical setting of spine surgery.     

Strength comparison of allogenic bone screws, bioabsorbable screws, and stainless steel screw fixation.

Allogenic bone screws are new to the fixation market and have yet to be tested against current fixation materials. An in vitro comparison of the same sizes of stainless steel, bioabsorbable, and allogenic bone screws was undertaken to assess screw resistance to the forces of bending, pullout, and shear. Using aluminum plates to support the screws, forces up to 1000 Newtons were applied to six to eight samples of each type of screw. During each test, stainless steel screws withstood the maximum force that could be exerted by the testing apparatus without failing (bending, 113.9 +/- 11.8 N mean +/- SE; pullout 999.1 +/- 33.7 N; and shear, 997.5 +/- 108.8 N). In each test, compared to bioabsorbable screws, allogenic bone screws failed faster (pullout, allogenic: 12.4 +/- 1.1 seconds vs. bioabsorbable, 120.6 +/- 13.8 seconds; p = .001; bending, allogenic: 53.4 +/- 4.8 seconds vs. bioabsorbable, 201.9 +/- 11.1 seconds; p = .001; shear, allogenic 13.5 +/- 1.4 seconds vs. bioabsorbable, 43.8 +/- 0.9 seconds; p = .001) under equivalent (pullout: bioabsorbable, 385.0 +/- 18.4 N vs. allogenic, 401.0 +/- 35.9 N; p = .001) or lower (bending, allogenic: 4.7 +/- 0.2 N vs. bioabsorbable, 11.0 +/- 0.9 N; p = .675; shear, allogenic: 312.1 +/- 15.5 N vs. bioabsorbable 680.9 +/- 8.5 N; p = .001) loads, and in a highly variable fashion. Overall, the bioabsorbable screws withstood the forces of bending, pullout, and shear better than the allogenic screws, and stainless steel screws outperformed both bioabsorbable and allogenic screws. Despite these results, allogenic screws could still be useful in compliant patients who would benefit from their osteoconductive properties.     

The use of an injectable, biodegradable calcium phosphate bone substitute for the prophylactic augmentation of osteoporotic vertebrae and the management of vertebral compression fractures.

STUDY DESIGN: A biomechanical study comparing two materials for augmentation of osteoporotic vertebral bodies and vertebral bodies after compression fracture. OBJECTIVES: To compare an injected, biodegradable calcium phosphate bone substitute with injected polymethylmethacrylate bone cement for strengthening osteoporotic vertebral bodies and improving the integrity of vertebral compression fractures. SUMMARY OF BACKGROUND DATA: Injection of polymethylmethacrylate bone cement into fractured vertebral bodies has been used clinically. However, there is concern about thermal damage to the neural elements during polymerization of the polymethylmethacrylate bone cement as well as its negative effects on bone remodeling. Biodegradable calcium phosphate bone substitutes have been studied for enhancement of fixation in fractured vertebrae. METHODS: Forty fresh osteoporotic thoracolumbar vertebrae were used for two separate parts of this study: 1) injection into osteoporotic vertebrae: intact control (n = 8), calcium phosphate (n = 8), and polymethylmethacrylate bone cement (n = 8) groups. Each specimen then was loaded in anterior compression until failure; 2) injection into postfractured vertebrae: calcium phosphate (n = 8) and polymethylmethacrylate bone cement (n = 8) groups. Before and after injection, the specimens were radiographed in the lateral projection to determine changes in vertebral body height and then loaded to failure in anterior bending. RESULTS: For intact osteoporotic vertebrae, the average fracture strength was 527 +/- 43 N (stiffness, 84 +/- 11 N/mm), 1063 +/- 127 N (stiffness, 157 +/- 21 N/mm) for the group injected with calcium phosphate, and 1036 +/- 100 N (stiffness, 156 +/- 8 N/mm) for the group injected with polymethylmethacrylate bone cement. The fracture strength and stiffness in the calcium phosphate bone substitute group and those in the polymethylmethacrylate bone cement group were similar and significantly stronger than those in intact control group (P < 0.05). For the compression fracture study, anterior vertebral height was increased 58.5 +/- 4.6% in the group injected with calcium phosphate and 58.0 +/- 6.5% in the group injected with polymethylmethacrylate bone cement as compared with preinjection fracture heights. No significant difference between the two groups was found in anterior vertebral height, fracture strength, or stiffness. CONCLUSION: This study demonstrated that the injection of a biodegradable calcium phosphate bone substitute to strengthen osteoporotic vertebral bodies or improve vertebral compression fractures might provide an alternative to the use of polymethylmethacrylate bone cement.     

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.     

Assessment of different screw augmentation techniques and screw designs in osteoporotic spines

This is an experimental study on human cadaver spines. The objective of this study is to compare the pullout forces between three screw augmentation methods and two different screw designs. Surgical interventions of patients with osteoporosis increase following the epidemiological development. Biomechanically the pedicle provides the strongest screw fixation in healthy bone, whereas in osteoporosis all areas of the vertebra are affected by the disease. This explains the high screw failure rates in those patients. Therefore PMMA augmentation of screws is often mandatory. This study involved investigation of the pullout forces of augmented transpedicular screws in five human lumbar spines (L1–L4). Each spine was treated with four different methods: non-augmented unperforated (solid) screw, perforated screw with vertebroplasty augmentation, solid screw with vertebroplasty augmentation and solid screw with balloon kyphoplasty augmentation. Screws were augmented with Polymethylmethacrylate (PMMA). The pullout forces were measured for each treatment with an Instron testing device. The bone mineral density was measured for each vertebra with Micro-CT. The statistical analysis was performed with a two-sided independent student t test. Forty screws (10 per group and level) were inserted. The vertebroplasty-augmented screws showed a significant higher pullout force (mean 918.5 N, P = 0.001) than control (mean 51 N), the balloon kyphoplasty group did not improve the pullout force significantly (mean 781 N, P > 0.05). However, leakage occurred in some cases treated with perforated screws. All spines showed osteoporosis on Micro-CT. Vertebroplasty-augmented screws, augmentation of perforated screws and balloon kyphoplasty augmented screws show higher pullout resistance than non-augmented screws. Significant higher pullout forces were only reached in the vertebroplasty augmented vertebra. The perforated screw design led to epidural leakage due to the position of the perforation in the screw. The position of the most proximal perforation is critical, depending on screw design and proper insertion depth. Nevertheless, using a properly designed perforated screw will facilitate augmentation and instrumentation in osteoporotic spines.     

Mechanical evaluation of fracture fixation augmented with tricalcium phosphate bone cement in a porous osteoporotic cancellous bone model

OBJECTIVE: The purpose of this study was to examine the effects of resorbable bone cement on screw and plate-screw fracture fixation in a porous osteoporotic bone model. METHODS: Experiment 1: Screw pullout strength was assessed for 4 sets of 4.5-mm cortical screws inserted into a synthetic osteoporotic cancellous bone model, including screws inserted without cement augmentation (control), screws augmented with tricalcium phosphate (TCP) bone cement (Norian SRS; Synthes USA, Paoli, PA), and screws augmented with polymethylmethacrylate. Experiment 2: The effects of cement augmentation on plate-screw fixation strength were examined by performing cantilever bending tests on 4 sets of 8 plate-screw constructions, including nonaugmented and TCP-augmented standard and locked screw-plate constructions in a similar bone model. RESULTS: Experiment 1: Cement augmentation with both TCP and polymethylmethacrylate increased screw pullout strength from a porous osteoporotic cancellous bone model by about 4-fold (P < 0.05), and there was no significant difference between the 2 cements (P > 0.1). Experiment 2: Fixation strength was 1.5 times higher for locked plates compared with standard plates when neither was augmented with cement (P = 0.07). Cement augmentation with TCP improved fixation strength by 3.6 times for a standard plate-screw construction (P < 0.05) and 3.3 times for a locked plate-screw construction (P < 0.05). The most stable construction was the TCP-augmented locked plate, in which a 5-fold increase was observed compared with that of standard plates without TCP (P < 0.05). CONCLUSIONS: This study indicates augmenting screws with TCP cement during osteosynthesis improves fixation strength in an osteoporotic cancellous bone model. CLINICAL RELEVANCE:: In fracture situations in which osteoporotic bone makes screw and screw-plate fixation tenuous, screw augmentation with TCP cement should be considered as adjunct treatment.     

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。     

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.     

Bioabsorbable screw fixation in coracoclavicular ligament reconstruction

The purpose of this study is to evaluate the biomechanical properties of stainless steel and bioabsorbable screw fixation of the clavicle to the base of the coracoid. Seven matched pairs of fresh frozen shoulders were prepared by removing all soft tissue except the acromioclavicular and coracoclavicular ligament complexes. The shoulders were randomly selected and fixed with 4.5-mm stainless steel (SS) screws, while contralateral shoulders were fixed with 4.5-mm poly L-lactic acid polymer (PLLA) screws. Pullout strength, stiffness, and elongation to failure were measured using an Instron Mechanical Tester (Model 4202). The average pullout strength of 720.6 +/- 244.9 N of the metal screws was not statistically different (p = 0.089) from that of the biodegradable screws of 580.4 +/- 188.6 N. The pullout strengths exerted by both these screws exceeded the reported strength (500 N) of the intact coracoclavicular complex indicating adequate initial pullout strength for coracoclavicular fixation.     

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。     

Pullout strength of pedicle screws with cement augmentation in severe osteoporosis: A comparative study between cannulated screws with cement injection and solid screws with cement pre-filling

Background  

Pedicle screws with PMMA cement augmentation have been shown to significantly improve the fixation strength in a severely osteoporotic spine. However, the efficacy of screw fixation for different cement augmentation techniques, namely solid screws with retrograde cement pre-filling versus cannulated screws with cement injection through perforation, remains unknown. This study aimed to determine the difference in pullout strength between conical and cylindrical screws based on the aforementioned cement augmentation techniques. The potential loss of fixation upon partial screw removal after screw insertion was also examined.     

Mechanical and structural characteristics of the new BONE-LOK cortical-cancellous internal fixation device

The purpose of this study was to evaluate the structural and mechanical characteristics of a new and unique titanium cortical-cancellous helical compression anchor with BONE-LOK (Triage Medical, Inc., Irvine, CA) technology for compressive internal fixation of fractures and osteotomies. This device provides fixation through the use of a distal helical anchor and a proximal retentive collar that are united by an axially movable pin (U.S. and international patents issued and pending). The helical compression anchor (2.7-mm diameter) was compared with 3.0-mm diameter titanium cancellous screws (Synthes, Paoli, PA) for pullout strength and compression in 7# and 12# synthetic rigid polyurethane foam (simulated bone matrix), and for 3-point bending stiffness. The following results (mean +/- standard deviation) were obtained: foam block pullout strength in 12# foam: 2.7-mm helical compression anchor 70 +/- 2.0 N and 3.0-mm titanium cancellous screws 37 +/- 11 N; in 7# foam: 2.7-mm helical compression anchor 33 +/- 3 N and 3.0-mm titanium cancellous screws 31 +/- 12 N. Three-point bending stiffness, 2.7-mm helical compression anchor 988 +/- 68 N/mm and 3.0-mm titanium cancellous screws 845 +/- 88 N/mm. Compression strength testing in 12# foam: 2.7-mm helical compression anchor 70.8 +/- 4.8 N and 3.0-mm titanium cancellous screws 23.0 +/- 3.1 N, in 7# foam: 2.7-mm helical compression anchor 42.6 +/- 3.2 N and 3.0-mm titanium cancellous screws 10.4 +/- 0.9 N. Results showed greater pullout strength, 3-point bending stiffness, and compression strength for the 2.7-mm helical compression anchor as compared with the 3.0-mm titanium cancellous screws in these testing models. This difference represents a distinct advantage in the new device that warrants further in vivo testing.     

Increasing bending strength and pullout strength in conical pedicle screws: biomechanical tests and finite element analyses

STUDY DESIGN: Comparative in vitro biomechanical study and finite element analysis. OBJECTIVES: To investigate the bending strength and pullout strength of conical pedicle screws, as compared with conventional cylindrical screws. SUMMARY OF BACKGROUND DATA: Transpedicle screw fixation, the gold standard of spinal fixation, is threatened by screw failure. Conical screws can resist screw breakage and loosening. However, biomechanical studies of bending strength have been lacking, and the results of pullout studies have varied widely. METHODS: Ten types of pedicle screws with different patterns of core tapering and core diameter were specially manufactured with good control of all other design factors. The stiffness, yielding strength, and fatigue life of the pedicle screws were assessed by cantilever bending tests using high-molecular-weight polyethylene. The pullout strength was assessed by pullout tests using polyurethane foam. Concurrently, 3-dimensional finite element models simulating these mechanical tests were created, and the results were correlated to those of the mechanical tests. RESULTS: In bending tests, conical screws had substantially higher stiffness, yielding strength, and fatigue life than cylindrical screws (P<0.01), especially when there was no step at the thread-shank junction. In pullout tests, pullout strength was higher in screws with a conical core and smaller core diameter and also in situations with higher foam density (P<0.01). In finite element analysis, the maximal deflection and maximal tensile stress were closely related to yielding strength (r=-0.91) and fatigue life (r=-0.95), respectively, in the bending analyses. The total reaction force was closely related to the pullout strength in pullout analyses (r=0.84 and 0.91 for different foam densities). CONCLUSIONS: Conical screws effectively increased the bending strength and pullout strength simultaneously. The finite element analyses reliably predicted the results of the mechanical tests.     

Screw orientation and plate type (variable- vs. fixed-angle) effect strength of fixation for in vitro biomechanical testing of the Synthes CSLP

BACKGROUND CONTEXT: Two common justifications for orienting cervical screws in an angled direction are to increase pullout strength and to allow use of longer screws. This concept is widely taught and has guided implant design. Fixed- versus variable-angle systems may offer strength advantages. Despite these teachings, there is a paucity of supporting biomechanical evidence. The purpose of our study is to test the influence of screw orientation and plate design on the maximum screw pullout force. PURPOSE: This study evaluates the effect of screw orientation and plate type (fixed- vs. variable-angle) on screw pullout strength. STUDY DESIGN: Anterior cervical plates of both a fixed- and variable-angle CSLP, were tested for peak pullout strength in a direct plate pullout model using polyurethane foam bone, which models osteoporotic bone. METHODS: Self-tapping, locking screws (4.0x14mm and 4.0x16mm) were used. Screws were oriented in the fixed-angle plate in the standard fashion. In the variable plate, screws were instrumented in three different orientations. Biomechanical testing was performed on an Instron DynaMight 8841 servohydraulic testing machine, measuring maximum pullout force under a displacement-controlled pullout rate of 1mm/min. Five samples were tested per group. RESULTS: When all screws were placed 90 degrees to the plate, there was a significantly increased peak pullout strength (412.8+/-22.2N) compared with when all screws were placed 12 degrees "up and in" (376.2+/-24.3N, p less than or equal to .03). When the 90 degrees construct was reproduced using 14-mm screws and compared with 16-mm screws oriented 12 degrees "all up and in," there was still significantly higher pullout strength with the all 90 degrees construct (434.2+/-28.7N vs. 376.2+/-24.3N, p less than or equal to .009). The fixed-angle plate had a significantly decreased peak pullout strength (288.2+/-15.7N) compared with the variable-angle plate (416.6+/-12.6N) (p less than .00001) when the screws were placed in the same orientation. Overall, the variable-angle plate, regardless of the orientation of screws, had a significantly greater pullout strength than the fixed-angle plate (p less than .001). CONCLUSIONS: In this system, a variable-angle plate has greater pullout strength than a fixed-angle plate, regardless of the orientation of screws. With the variable-angle plate, a construct of all screws 12 degrees "up and in" is the weakest configuration. We found that with the 90 degrees construct, both 16- and 14-mm screws performed significantly better than 16-mm convergent screws. These findings are remarkable because they contradict the current doctrine. This may be a function of plate-dependent factors and should not be applied universally to all plate systems. Further study of screw orientation in additional plating systems is warranted.     

医用骨水泥对腰椎椎弓根钉固定的影响

目的探讨医用骨水泥对椎弓根钉固定强度的影响。方法24个新鲜冰冻尸体的腰椎椎体作为实验对象,随机分为3组,第1组为单纯钻孔组,第2组丝锥攻丝组,第3组为扩大钻孔组。每组8个椎体,每个椎体自身对照,两侧的固定方式一样,只是一侧在固定螺钉前加用骨水泥,另一侧不加骨水泥作为对照。固定后分别在万能机上进行轴向拉力测试,比较二者的差异。结果在第1组单纯钻孔组中,骨水泥固定侧拉力范围平均(1988±289)N,对照侧平均(1242±264)N,二者比较有显著差异(P<0001)。第2组丝锥攻丝组骨水泥固定侧拉力范围平均(4553±945)N,对照侧为平均(1303±138)N,二者比较有显著差异(P<0001)。第3组扩大钻孔组骨水泥固定侧拉力范围平均(4653±675)N,对照侧平均1213N,二者比较有显著差异(P<0001)。3组骨水泥固定侧的结果相比,第1组单纯钻孔组与第2组丝锥攻丝组、第3组扩大钻孔组之间有明显差异(P<0001),而第2组与第3组之间无差异(P>005)。结论骨水泥对椎弓根螺钉的固定有明显的增强作用。对于骨质疏松的患者或手术中椎弓根钉固定失败,或二次手术重新固定时,都可以采取骨水泥进行加强固定的方法,且强度可靠。     

骨水泥修复颈椎前路椎体螺钉钉道的生物力学研究

目的 评价聚甲基丙烯酸甲酯(Polymethylmethacrylate,PMMA)修复颈椎前路椎体螺钉钉道的生物力学效果。方法 防腐成人颈椎标本(C_3~7)共30个,分3组:正常骨密度组(A组)、骨质疏松组(B组),骨质疏松修复组(C组),各组10个标本,测试疲劳前后的最大轴向拔出力。A、B对照组椎体二侧钉孔均放置4 mm的椎体螺钉,随机选择一侧行即时最大拔出力实验,另一侧在疲劳实验(2 Hz,20 000次)后行最大拔出力实验。C组(实验前需进行螺钉拔松实验)注入PMMA 0.6~1.0 mL,并拧入螺钉,随机选择一侧进行即时和疲劳后最大拔出力实验。结果 A、B、C组即时拔出力明显高于疲劳后拔出力,差异具有统计学意义(P< 0.05),C组疲劳后与A组即时拔出力比较,差异无统计学意义(P >0.05),但高于B组即时拔出力,差异具有统计学意义(P< 0.05)。即时拔出力C组 >A组 >B组,差异均有统计学意义(P< 0.05)。疲劳后拔出力C组 >A组 >B组,差异均有统计学意义(P< 0.05)。PMMA注射后未发现钉道渗漏。结论 PMMA钉道修复能够明显增强椎体螺钉的即时稳定性和抗疲劳能力, 并且安全有效,适用于螺钉松动和拔出的修复固定。