The effect of implant design and bone density on maximum torque and holding power for femoral neck fracture devices

BACKGROUND AND AIMS: Displacement of internally fixed femoral neck fractures due to implant migration is a common problem. This in vitro study was performed to compare maximum torque and holding power for five different implants. MATERIAL AND METHODS: Synthetic porous material with low, medium or high density was used to simulate cancellous bone. The tested implants included three conventional screws (AO, Olmed, Hansson), one screw with threads and a barb (Hybrid), and a pin with a hook (LIH). RESULTS: The Hansson screw provided higher maximum torque in low and medium density blocks when compared with the other implants (p < 0.0001) followed by LIH, Hybrid, Olmed, and AO. For high-density blocks there was no significant difference between Hansson and Hybrid screws, both with significantly higher torque than the other implants. The maximal pullout in low-density blocks differed significantly between all five implants with the Hansson screw providing the highest holding power. For medium and high-density blocks the conventional screws had significantly higher pullout resistance compared with the Hybrid and LIH. CONCLUSION: The in vitro model used seemed to provide reproducible and clinically relevant results. There was a good correlation between material density and holding power for all implants. Screws inserted without predrilling provided higher maximal torque while maximal pull out load seemed less affected by predrilling being used or not.     

Calcium phosphate cement augmentation of the femoral neck defect created after dynamic hip screw removal

OBJECTIVE: To determine the effect of reinforced calcium phosphate cement augmentation of the femoral neck defect created after dynamic hip screw removal in a cadaveric model. METHODS: The lag screws of dynamic hip screw implants were inserted and subsequently removed in 8 matched pairs of cadaveric, osteoporotic femurs to create a femoral neck defect. One of each pair had the defect augmented with osteoconductive calcium phosphate cement reinforced with poly(lactide-coglycolide) fibers (Norian Reinforced, Synthes, West Chester, PA), and the other defect was not augmented. Each specimen was first cyclically loaded with 750 N vertical loads applied for 1000 cycles to simulate early weightbearing, and then loaded to failure. RESULTS: Calcium phosphate cement augmentation of the lag screw defect significantly increased the mean femoral neck failure strength (4819 N) compared to specimens in which the defect was left untreated (3995 N) (P < 0.004). The mechanism of failure for each specimen was a fracture through the femoral neck. Regression analysis demonstrated that load to failure was directly related to the bone mineral density at Ward's triangle, and the impact of cement augmentation on failure strength was greatest for specimens with the lowest bone mineral density (correlation coefficient: -0.82, P < 0.0001). CONCLUSION: This study demonstrates that augmentation of the bony defect created by dynamic hip screw removal with reinforced calcium phosphate cement significantly improved the failure strength of the bone. Cement augmentation after hardware removal may decrease the risk of refracture and allow early weightbearing, especially in elderly patients with osteoporotic bone.     

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。     

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.     

Biomechanical comparison of anatomic trajectory pedicle screw versus injectable calcium sulfate graft-augmented pedicle screw for salvage in cadaveric thoracic bone

Many salvage options for failed thoracic pedicle screws exist including the use of a different trajectory or the augmentation of the screw with polymethylmethacrylate cement. Although polymethylmethacrylate immediately increases the construct stiffness and the pull-out strength, it may cause bone necrosis, toxin relaxation, and/or neural injury. On the other hand, calcium sulfate bone grafts have a high potential for biologic incorporation and no thermal damage effect. In the current study, polyaxial pedicle screws were first inserted with a straightforward approach on both sides in 17 fresh human cadaveric thoracic vertebrae. The maximal insertion torque for each screw was measured and then the pull-out strengths were recorded. Afterward, these pedicle screws were randomly assigned to be replaced either by graft augmentation or by anatomic trajectory technique for salvage. The graft-augmented screws were placed using the previous holes. The maximum insertional torque for each anatomic trajectory screw was measured. Finally, the pull-out strengths of the revision screws were recorded. The mean maximum insertional torque decreased with the anatomic trajectory salvage technique when compared with the straightforward approach, 0.23 versus 0.38 Nm, respectively (P=0.003). The anatomic trajectory revision resulted in decreased pull-out strength when compared with the pull-out strength of the straightforward technique, 297 versus 469 N, respectively (P=0.003).The calcium sulfate graft augmentation increased the pull-out strength when compared with the pull-out strength of the straightforward technique, 680 versus 477 N, respectively (P=0.017). The mean pull-out strength ratio of revised screw to original was 0.71 for anatomic trajectory and 1.8 for graft-augmented screws, a statistically significant difference (P=0.002).     

Screw Fixation in Cancellous Osteoporotic Bone - First in Vitro Results with a Novel Augmentation Material

Background: Screw fixation during internal fixation of osteoporotic fractures can be difficult due to severe reduction of bone mass and structure in these patients. Material and Methods: The use of PMMA-bone cement to anchor implants in patients in a "combined osteosynthesis" has empirically been developed. The irreversibility of this methodology is currently limiting it to exceptional use in salvage cases of pathological fractures. A new degradable screw augmentation material was therefore developed on the basis of alkylene bis(oligolactoyl)methacrylates. The mechanical properties of this degradable polymer were tested by pull-out and torque tests in augmenting cancellous stainless steel screws in non-osteoporotic bovine cancellous bone blocks and in polypropylene blocks. In these tests, augmentation was improved significantly by the material. Results: The maximum pull-out force obtained showed an 256% increase compared to the bovine standard system. Screw torques measured were up to 400% higher than this standard for the new material. The in vitro degradation kinetics of the new polymer were tested in a standardized system showing an almost linear weight loss of the material of 2-3% per week between weeks 2 and 16. Conclusion: For further material development improved in vivo and in vitro models must be developed. They should include either considerable overdrilling in normal cancellous bone or human osteoporotic bone.     

Vertebroplasty with self-locking hexagonal metal implants shows comparable primary and secondary stiffness to PMMA cement augmentation techniques in a biomechanical vertebral compression fracture model

With the growing incidence of vertebral compression fractures in elderly patients having a fair overall health condition, minimal-invasive treatment techniques are getting in focus of surgical therapy. Cement augmentation is widely performed and its complications and mechanical limitations are well described. Implants avoiding the side effects of cement augmentation while reaching the same level of stability would be desirable. The primary and secondary stability of a new augmentation method with self-locking hexagonal metal implants were investigated and compared with the performance of established augmentation options. 18 fresh-frozen human spinal specimens (Th12–L2/L3–L5) were tested with pure moments of 7.5 Nm in a six-degree-of-freedom spine simulator to investigate primary and secondary stability of three augmentation techniques: (1) vertebroplasty, (2) PMMA filled cavity and (3) hexagonal metal implants. An increasing three-step cyclic loading model was included. Elastic displacement and height loss under loading did not show significant differences between the three test groups. Investigation of primary and secondary stability evenly demonstrated comparable results for all techniques indicating an insufficiency to stabilise the fracture with higher load cycles. The newly introduced method for augmentation with the metal implant Spine Pearls achieved comparable results to bone cement based techniques in a biomechanical in vitro study. Midterm and longterm reduction preservation and ingrowth of the implants have to be proven in further studies.     

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。     

Norian SRS cement compared with conventional fixation in distal radial fractures. A randomized study

BACKGROUND: A prospective, randomized multicenter study was conducted to evaluate closed reduction and immobilization with and without Norian SRS (Skeletal Repair System) cement in the management of distal radial fractures. Norian SRS is a calcium-phosphate bone cement that is injectable, hardens in situ, and cures by a crystallization reaction to form dahllite, a carbonated apatite equivalent to bone mineral. METHODS: A total of 323 patients with a distal radial fracture were randomized to treatment with or without Norian SRS cement. Stratification factors included fracture type (intra-articular or extra-articular), hand dominance, bone density, and the surgeon's preferred conventional treatment (cast or external fixator). The subjects receiving Norian SRS underwent a closed reduction followed by injection of the cement percutaneously or through a limited open approach. Wrist motion, beginning two weeks postoperatively, was encouraged. Control subjects, who had not received a Norian SRS injection, underwent closed reduction and application of a cast or external fixator for six to eight weeks. Supplemental Kirschner wires were used in specific instances in both groups. Patients were followed clinically and radiographically at one, two, four, and between six and eight weeks and at three, six, and twelve months. Patients rated pain and the function of the hand with use of a visual analog scale. Quality of life was assessed with use of the Short Form-36 (SF-36) health status questionnaire. Complications were recorded. RESULTS: Significant clinical differences were seen at six to eight weeks postoperatively, with better grip strength, wrist range of motion, digital motion, use of the hand, and social and emotional function, and less swelling in the patients treated with Norian SRS than in the control group (p < 0.05). By three months, these differences had normalized except for digital motion, which remained significantly better in the group treated with Norian SRS (p = 0.015). At one year, no clinical differences were detected. Radiographically, the average change in ulnar variance was greater in the patients treated with Norian SRS (+2.0 mm) than in the control group (+1.4 mm) (p < 0.02). No differences were seen in the total number of complications, including loss of reduction. The infection rate, however, was significantly higher (p < 0.001) in the control group (16.7%) than in the group treated with Norian SRS (2.5%) and the infections were always related to external fixator pins or Kirschner wires. Four patients with intra-articular extravasation of cement were identified; no sequelae were observed at twenty-four months. Cement was seen in extraosseous locations in 112 (70%) of the SRS-treated patients; loss of reduction was highest in this subgroup (37%). The extraosseous material had disappeared in eighty-three of the 112 patients by twelve months. CONCLUSIONS: Our results indicate that fixation of a distal radial fracture with Norian SRS cement may allow for accelerated rehabilitation. A limited open approach and supplemental fixation with Kirschner wires are recommended. Additional or alternate fixation is necessary for complex articular fractures.     

Tibial plateau fracture--biodegradable bonecement-augmentation

Between October 1996 and January 1999,29 patients (f:16,m:13,age: 22-86) with fractures of the lateral tibial plateau were operated on arthroscopic,fluoroscopic control or were treated with open reduction and internal fixation. 15 of them were retrospective and 14 prospective analysed.The metaphyseal defect after elevation of the depressed fragment was augmented in 11 cases with autologous spongeous bone grafting,in 9 cases with biodegradable bone cement (Norian SRS).Augmentation was unnecessary in 9 cases. The results according to the Lysholm score and the radiological results were good or excellent.Concerning the kind of augmentation no difference was noted. In the Norian SRS-group the duration of postoperative treatment was shorter than in the other group.The duration of partial weight bearing was shorter too.The results of the present study suggest that an injectable calcium phosphate cement may be a competent material for augmentation in lateral tibial plateau fractures because of the application form and the initial high mechanical stability.     

Screw Fixation in Cancellous Osteoporotic Bone – First in Vitro Results with a Novel Augmentation Material

Background: Screw fixation during internal fixation of osteoporotic fractures can be difficult due to severe reduction of bone mass and structure in these patients. Material and Methods: The use of PMMA-bone cement to anchor implants in patients in a “combined osteosynthesis” has empirically been developed. The irreversibility of this methodology is currently limiting it to exceptional use in salvage cases of pathological fractures. A new degradable screw augmentation material was therefore developed on the basis of alkylene bis(oligolactoyl)methacrylates. The mechanical properties of this degradable polymer were tested by pull-out and torque tests in augmenting cancellous stainless steel screws in non-osteoporotic bovine cancellous bone blocks and in polypropylene blocks. In these tests, augmentation was improved significantly by the material. Results: The maximum pull-out force obtained showed an 256% increase compared to the bovine standard system. Screw torques measured were up to 400% higher than this standard for the new material. The in vitro degradation kinetics of the new polymer were tested in a standardized system showing an almost linear weight loss of the material of 2–3% per week between weeks 2 and 16. Conclusion: For further material development improved in vivo and in vitro models must be developed. They should include either considerable overdrilling in normal cancellous bone or human osteoporotic bone.     

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.     

Biomechanical evaluation of calcium phosphate cement-augmented fixation of unstable intertrochanteric fractures

OBJECTIVE: The purpose of this study was to evaluate the mechanical effects of using an injectable calcium phosphate cement, Norian Skeletal Replacement System (SRS), which is replaced by the native remodeling process, to augment sliding hip screw fixation of unstable intertrochanteric fractures in a cadaver model. DESIGN: Ten matched pairs of human cadaver femora were used. One randomly selected femur from each pair was designated as experimental, whereas the contralateral femur served as the control. SETTING: Testing was performed in the Orthopaedic Research Laboratories, The University of Michigan, Ann Arbor, Michigan, U.S.A. PATIENTS/PARTICIPANTS: Femora with no radiographic evidence of pathology and with below-normal bone mineral density in the neck region were accepted into the study. INTERVENTION: Three-part, intertrochanteric fractures were repaired by using a sliding hip screw and sideplate, without replacement of the posteromedial fragment. In the experimental side of each pair, SRS was used to grout the hip screw and to fill the posteromedial defect. MAIN OUTCOME MEASUREMENTS: Femora were subjected to simulated single-leg stance loading to approximately one body weight in a servohydraulic testing machine. Measurements of stiffness, medial bone surface strain, hip screw displacement, and sideplate strain were made in fractured femora. Stiffness and medial bone surface strain baseline data were obtained in the intact bone before fracture creation as well. RESULTS: Augmenting fixation with Norian SRS increased the fracture construct stiffness and minimized sliding hip screw displacement. It also maintained medial bone surface strain closer to the intact state and lowered sideplate strain relative to controls. However, there was no difference in the load to failure between SRS-augmented and control femora. CONCLUSIONS: SRS augmentation of unstable, intertrochanteric fractures significantly improved overall stability, facilitated load transfer across the fracture, and decreased both shortening of the proximal femur and stress on the sliding hip screw.     

髂骨短钉骨水泥强化与长钉的固定强度比较

目的 评价两种髂骨钉在承受周期性载荷后的固定强度,探讨聚甲基丙烯酸甲酯(poly-met hylmethacrylate,PMMA)强化髂骨短钉固定的生物力学作用.方法 对11具成人尸体标本(男7具、女4具;年龄41～78岁,平均60.7岁)骨盆标本,行双能X线吸收法(DEXA)扫描以确定骨矿物质密度(bone mineral density,BMD),平均值为(0.68±0.17) g/cm2.实验采用配对设计,随机选择一侧髂骨置入直径为7.0 mm、长度为(138±4) mm的髂骨长钉,另一侧置入PMMA强化的直径为7.0 mm、长度为(70±2) mm的髂骨短钉(短钉定义为60～80 mm,长钉定义为100～140 mm), PMMA的注入量为2～5 ml(平均3.6 ml).随后对螺钉施加30～300 N的垂直周期性载荷,载荷频率为2 Hz,加载次数为2000次,最后以5 mm/min的速度进行两种螺钉拔出试验,获得螺钉的最大拔出力.结果 髂骨长钉和髂骨短钉PMMA强化后的初始刚度分别为(249±101) N/mm和(253±122) N/mm.加载周期载荷后,两者的终末刚度分别为(214±92) N/mm和(229±120) N/mm.加载周期载荷后,短钉强化和长钉的最大拔出力分别为(2529±1055)N和(2436±915)N,两者之间差异无统计学意义.两种固定方式的最大拔出力与骨密度呈线性关系.结论 PMMA强化髂骨短钉应用于腰骶骨盆重建是可行的,对于骨质疏松的患者应尽量采用此方法固定.     

Mechanical evaluation of a carbonated apatite cement in the fixation of unstable intertrochanteric fractures

We created three-part unstable intertrochanteric fractures in 6 pairs of aged, osteopenic, human, cadaveric femora. Fractures were reduced and fixed with a Dynamic Hip Screw (DHS) (Synthes, Paoli, PA). Two test groups were evaluated: 1. Fixation with DHS, and 2. Fixation with a DHS and calcium phosphate bone cement (Norian SRS (Skeletal Repair System)) augmentation of the fracture line and posteromedial calcar region of the proximal femur. Each femur was loaded to 1,650 N (2.5 body weight) for 10,000 cycles to simulate postoperative load transmission across the fracture construct during normal gait. The load was further increased successively by one body weight for another 10,000 cycles until failure. We evaluated fixation by measuring the amount of sliding of the lag screw of the DHS (shortening) and stiffness of the overall fracture construct (stability). SRS cement-augmented specimens had less shortening (1 mm versus 17 mm) and twice the initial construct stiffness compared to control specimens.     

Factors affecting the interface of cervical spine facet screws placed in the technique by Roy-Camille et al.

The objective of the study was to investigate the influence of bone cement, length of burr hole and bone density on pullout force and insertional screw torque of cervical spine facet screws. Both facets of 24 human cervical vertebrae were scanned for bone mineral density (BMD) and assigned to two groups for measuring of insertional screw torque and pullout strength. Maximal insertional screw torque was measured and removal of the screws was performed in displacement control (0.25 mm/s) without bone cement (PMMA), with 0.1 ml of PMMA and with the burr hole completely filled with PMMA. Screw torque was 59.1 N cm (±25.7 N cm), pullout force was 382.8 N (±140.5 N) without PMMA. Injection of 0.1 ml PMMA did not change significantly both screw torque (p=0.73) and pullout (p=0.129). Filling of the burr holes with PMMA increased significantly both screw torque (p<0.0001) and pullout force (p=0.028) when compared with injection of 0.1 ml of PMMA. A positive, moderate correlation was seen between BMD and screw torque before (r=0.501; p=0.097) and after filling with PMMA (r=0.514; p=0.088), BMD and pullout force before (r=0.441; p=0.152) and after complete filling with PMMA (r=0.673; p=0.047). The PMMA does increase both screw torque (p<0.0001) and pullout force (p=0.028) of facet screws significantly if the burr hole is filled with PMMA completely when compared with injection of 0.1 ml PMMA. Bone mineral density of the cervical facets moderately correlates with peak insertional torque and pullout force. This is true for a facet without PMMA and for a facet filled with PMMA. The length of the burr hole seems to be less important.     

不同椎弓根螺钉固定及骨水泥强化方法在骨质疏松骶骨上锚定强度的比较

目的:通过比较不同椎弓根螺钉固定及骨水泥强化方法在骨质疏松骶骨上的锚定强度,探讨骶骨椎弓根螺钉松动后的理想补救技术.方法:应用11具成人新鲜骶骨标本,经骨密度测试确认为骨质疏松后,在同一骶骨标本上,依次建立5种骶骨螺钉固定模型,A组,单皮质椎弓根螺钉固定(左侧):B组,双皮质椎弓根螺钉固定(右侧);C组,PMMA钉道强化后单皮质椎弓根螺钉固定(建立于A组螺钉拔出后);D组,PMMA钉道强化后侧翼钉固定(右侧);E组,后凸成形技术支持下的PMMA强化后侧翼钉固定(左侧).应用MTS材料测试机进行轴向拔出测试,记录各种骶骨螺钉固定技术的最大拔出力并进行比较.结果:11具标本的骨密度为0.55～0.79g,cm~2,平均0.71±0.08g/cm~2.A～E组最大拔出力分别为508±128N、685±126N、846±230N、543±121N和702±144N.A组与D组间无显著性差异(P>0.05),且均显著低于B、C和E组(P<0.05);B组与E组间无显著性差异(P>0.05),但两组的拔出力均显著低于C组(P<0.05).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更高的锚定强度.骶骨椎弓根钉一旦发生松动,PMMA钉道强化和后凸成形技术支持下的PMMA强化后的侧翼钉固定均可成为理想的补救手段.     

Mechanical evaluation of a carbonated apatite cement in the fixation of unstable intertrochanteric fractures

We created three-part unstable intertrochanteric fractures in 6 pairs of aged, osteopenic, human, cadaveric femora. Fractures were reduced and fixed with a Dynamic Hip Screw (DHS) (Synthes, Paoli, PA). Two test groups were evaluated: 1. Fixation with a DHS, and 2. Fixation with a DHS and calcium phosphate bone cement (Norian SRS (Skeletal Repair System)) augmentation of the fracture line and posteromedial calcar region of the proximal femur. Each femur was loaded to 1,650 N (2.5 body weight) for 10,000 cycles to simulate postoperative load transmission across the fracture construct during normal gait. The load was further increased successively by one body weight for another 10,000 cycles until failure. We evaluated fixation by measuring the amount of sliding of the lag screw of the DHS (shortening) and stiffness of the overall fracture construct (stability). SRS cement-augmented specimens had less shortening (1 mm versus 17 mm) and twice the initial construct stiffness compared to control specimens.     

可灌注骨水泥椎弓根螺钉的生物力学研究

目的 探讨可灌注骨水泥椎弓根螺钉(novel perfusional pedicle screw,NPPS)在骨质疏松椎体内的生物力学稳定性.方法 选取平均年龄73岁的完整湿润脊柱标本(T11～L5)6具,共42个椎体,平均骨密度为(0.696±0.14)g/cm2.所有椎体均任取一侧椎弓根置入可灌注螺钉后,使用配套的骨水泥推杆和灌注筒向椎体内灌注骨水泥2ml,对侧椎弓根置入常规螺钉,作为对照组.随机选取3个椎体,剖开椎体观察骨水泥分布情况.余39个椎体随机分成3组,每组13个,分别行最大轴向拔出力、最大旋出力矩、周期抗屈试验.另取10个可灌注螺钉和10个对照组螺钉行三点弯曲实验.结果 所有椎体均没有观察到骨水泥渗漏.可灌注螺钉的最大轴向拔出力为(760±178)N,对照组为(355±87)N;可灌注螺钉的最大旋出力矩为(1.347±0.377)N·m,对照组为(0.488±0.205)N·m.4枚(4/13,占30.8%)可灌注螺钉发生松动,其平均载荷为(150±46)N;未松动的螺钉中,松动位移半均(0.661±0.289)mm,对照组中所有的螺钉在最大负荷介于50～200 N时开始松动(位移>2.000 mm),平均载荷(104±35)N.可灌注螺钉的平均极限弯曲载荷为(3082±144)N,对照组螺钉为(3357±263)N.结论 可灌注椎弓根螺钉,结合骨水泥推杆和灌注筒能有效控制骨水泥渗漏,明显增强椎弓根螺钉在骨质疏松椎体内的稳定性.     

骨水泥强化正常骨质动力髋螺钉固定的生物力学研究

目的选取正常骨密度的股骨转子间骨折标本,研究骨水泥强化对动力髋螺钉(dynamic hip screw,DHS)固定的生物力学影响。方法取成年男性防腐尸体双侧股骨上段标本24对48侧,制备A2型股骨转子间骨折DHS固定模型。右侧标本行骨水泥强化DHS固定,为强化组;左侧行DHS常规固定,为对照组。两组标本进行弯曲强度实验及扭转强度实验。结果强化组最大负荷及最大扭矩分别为3852.1602±143.6031N、15.5±2.6Nm,对照组为3702.9667±133.8601N、14.7±3.4Nm;两组比较生物力学差异均无统计学意义(P>0.05)。结论正常骨密度的股骨转子间骨折行骨水泥强化,对DHS固定强度及骨折整体稳定性无显著影响。