Biomechanical evaluation of kyphoplasty with calcium sulfate cement in a cadaveric osteoporotic vertebral compression fracture model.

BACKGROUND CONTEXT: Vertebral compression fractures can cause deformity, pain, and disability. Kyphoplasty involves percutaneous insertion of an inflatable balloon tamp into a fractured vertebra followed by injection of polymethylmethacrylate (PMMA) bone cement. PMMA has several disadvantages such as potential thermal necrosis and monomer toxicity. Calcium sulfate cement (CSC) is nontoxic, osteoconductive, and bioabsorbable. PURPOSE: To evaluate the biomechanical performance of CSC for kyphoplasty in cadaveric osteoporotic vertebral bodies. STUDY DESIGN: Destructive biomechanical tests using fresh cadaveric thoracolumbar vertebral bodies. METHODS: Thirty-three vertebral bodies (T9 to L4) from osteoporotic cadaveric spines were disarticulated, stripped of soft tissue, and measured for height and volume. Each vertebral body was compressed at 0.5 mm/s using a hinged plating system on a materials testing machine to create an anterior wedge fracture and reduce the anterior height by 25%. Pretreatment strength and stiffness were measured. Two KyphX inflatable balloon tamps were used to reexpand each vertebral body. After randomization, three groups were created: Group A-no cement; Group B-PMMA; Group C-calcium sulfate cement. Groups B and C were filled with the corresponding cement to 25% of the vertebral body volume. All vertebral bodies were then recompressed by 25% of the post-kyphoplasty anterior height to obtain posttreatment strength and stiffness. RESULTS: Treatment with PMMA restored vertebral strength to 127% of the intact level (4168.2 N+/-2288.7) and stiffness to 70% of the intact level (810.0 N/mm+/-380.6). Treatment with CSC restored strength to 108% of the intact level (3429.6 N+/-2440.7) and stiffness to 46% of the intact level (597.7 N/mm+/-317.5). CSC and PMMA were not significantly different for strength restoration (p=.4). Significantly greater strength restoration was obtained with either PMMA or CSC, compared with the control group (p=.003 and .03, respectively). Stiffness restoration tended to be greater with PMMA than for CSC, but this difference was not statistically significant (p=.1). Both cements had significantly greater stiffness when compared with the control group (p=.001 and p=.04, respectively). CONCLUSIONS: Use of CSC for kyphoplasty yields similar vertebral body strength and stiffness as compared with PMMA. It may be a useful alternative bone cement for kyphoplasty. Further studies are required to assess the bioabsorption of CSCs after kyphoplasty in vivo.     

Spinal loads after osteoporotic vertebral fractures treated by vertebroplasty or kyphoplasty

Vertebroplasty and kyphoplasty are routine treatments for compression fractures of vertebral bodies. A wedge-shaped compression fracture shifts the centre of gravity of the upper body anteriorly and generally, this shift can be compensated in the spine and in the hips. However, it is still unclear how a wedge-shaped compression fracture of a vertebra increases forces in the trunk muscle and the intradiscal pressure in the adjacent discs. A nonlinear finite element model of the lumbar spine was used to estimate the force in the trunk muscle, the intradiscal pressure and the stresses in the endplates in the intact spine, and after vertebroplasty and kyphoplasty treatment. In this study, kyphoplasty represents a treatment with nearly full fracture reduction and vertebroplasty one without restoration of kyphotic angle although in reality kyphoplasty does not guarantee fracture reduction. If no compensation of upper body shift is assumed, the force in the erector spine increases by about 200% for the vertebroplasty but by only 55% for the kyphoplasty compared to the intact spine. Intradiscal pressure increases by about 60 and 20% for the vertebroplasty and kyphoplasty, respectively. In contrast, with shift compensation of the upper body, the increase in muscle force is much lower and increase in intradiscal pressure is only about 20 and 7.5% for the vertebroplasty and kyphoplasty, respectively. Augmentation of the vertebral body with bone cement has a much smaller effect on intradiscal pressure. The increase in that case is only about 2.4% for the intact as well as for the fractured vertebra. Moreover, the effect of upper body shift after a wedge-shaped vertebral body fracture on intradiscal pressure and thus on spinal load is much more pronounced than that of stiffness increase due to cement infiltration. Maximum von Mises stress in the endplates of all lumbar vertebrae is also higher after kyphoplasty and vertebroplasty. Cement augmentation has only a minor effect on endplate stresses in the unfractured vertebrae. The advantages of kyphoplasty found in this study will be apparent only if nearly full fracture reduction is achieved. Otherwise, differences between kyphoplasty and vertebroplasty become small or vanish. Our results suggest that vertebral body fractures in the adjacent vertebrae after vertebroplasty or kyphoplasty are not induced by the elevated stiffness of the treated vertebra, but instead the anterior shift of the upper body is the dominating factor.     

Comparison of unipedicular and bipedicular kyphoplasty on the stiffness and biomechanical balance of compression fractured vertebrae

Percutaneous kyphoplasty (PKP) has been used to treat osteoporotic vertebral compression fractures for over 10 years; however, clinically speaking it is still controversial as to whether the use of unipedicular PKP or bipedicular PKP is best. Our study aimed to compare the different effects of unipedicular PKP and bipedicular PKP on the stiffness of compression fractured vertebral bodies (VBs), as well as to assess how cement distribution affect the bilateral biomechanical balance of the VBs. During this study, 30 thoracic VBs were compressed, creating vertebral compression fracture models; then they were augmented by unipedicular (group A and B) PKP and bipedicular (group C) PKP. In group A (unipedicular PKP), the cement was injected into one side and the augmentation was limited to the same side of the VB. In group B (unipedicular PKP), the cement was injected at only one side but the augmentation extended across the midline and filled both sides of the VB. In group C (bipedicular PKP), the cement was injected into both sides and thus achieved the bilateral augmentation. For the unipedicular PKP, the amount of cement injected was 15% of the original VB volume; while in bipedicular PKP, the amount of cement injected was a total of 20% of the original VB volume (10% was injected into each side). Using a MTS-858, we examined three phases of the VBs (intact, pre-augmented, post-augmented), by applying loads axially to the total vertebra and bilateral sides of the vertebra for each of three cycles, respectively. The changes of force and displacement were then recorded and the stiffness of the total vertebra and bilateral sides of the vertebra were calculated. For the pre-augmentation stage, the total VB stiffness of groups A, B and C significantly decreased when the compression fracture models were established (P < 0.05). After the cement augmentation (the post-augmentation stage), both groups A and B, showed that the stiffness could be restored to the initial, intact state; however, in group C, the stiffness was significantly higher than the initial, intact state (P < 0.01). The stiffness of the augmented side of group A was significantly higher than the non-augmented side (P < 0.001). In groups B and C, no significant differences were observed in the stiffness between total VB and each individual side. Thus, we can conclude that both unipedicular PKP and bipedicular PKP significantly increase the total VB stiffness. Bipedicular PKP creates stiffness uniformly across both sides of the vertebrae, while unipedicular PKP, creates a biomechanical balance depending on the distribution of cement. If bone cement is augmented only on one side, the stiffness of non-augmented side will be significantly lower than the augmented side, which might lead to an imbalance of stress on the VB. However, when cement augmentation crosses the midline, stiffness of both sides increase comparatively and biomechanical balance is thus achieved.     

Effects of bone cement volume and distribution on vertebral stiffness after vertebroplasty.

STUDY DESIGN: The biomechanical behavior of a single lumbar vertebral body after various surgical treatments with acrylic vertebroplasty was parametrically studied using finite-element analysis. OBJECTIVES: To provide a theoretical framework for understanding and optimizing the biomechanics of vertebroplasty. Specifically, to investigate the effects of volume and distribution of bone cement on stiffness recovery of the vertebral body. SUMMARY OF BACKGROUND DATA: Vertebroplasty is a treatment that stabilizes a fractured vertebra by addition of bone cement. However, there is currently no information available on the optimal volume and distribution of the filler material in terms of stiffness recovery of the damaged vertebral body. METHODS: An experimentally calibrated, anatomically accurate finite-element model of an elderly L1 vertebral body was developed. Damage was simulated in each element based on empirical measurements in response to a uniform compressive load. After virtual vertebroplasty (bone cement filling range of 1-7 cm3) on the damaged model, the resulting compressive stiffness of the vertebral body was computed for various spatial distributions of the filling material and different loading conditions. RESULTS: Vertebral stiffness recovery after vertebroplasty was strongly influenced by the volume fraction of the implanted cement. Only a small amount of bone cement (14% fill or 3.5 cm3) was necessary to restore stiffness of the damaged vertebral body to the predamaged value. Use of a 30% fill increased stiffness by more than 50% compared with the predamaged value. Whereas the unipedicular distributions exhibited a comparative stiffness to the bipedicular or posterolateral cases, it showed a medial-lateral bending motion ("toggle") toward the untreated side when a uniform compressive pressure load was applied. CONCLUSION: Only a small amount of bone cement ( approximately 15% volume fraction) is needed to restore stiffness to predamage levels, and greater filling can result in substantial increase in stiffness well beyond the intact level. Such overfilling also renders the system more sensitive to the placement of the cement because asymmetric distributions with large fills can promote single-sided load transfer and thus toggle. These results suggest that large fill volumes may not be the most biomechanically optimal configuration, and an improvement might be achieved by use of lower cement volume with symmetric placement.     

Sandwich Vertebral Fracture in the Study of Adjacent-level Fracture After Vertebral Cement Augmentation

The literature is inconclusive on the development of adjacent-level vertebral fracture after initial cement augmentation. A preliminary hypotheses is that cement injection exaggerates force transmission to the adjacent vertebral bodies, thereby predisposing those levels to future fractures. A sandwich vertebra is an intact vertebral body located between 2 previously cemented vertebrae. The purpose of this study was to determine whether the risk of adjacent-level fracture increased due to load shift after a cement injection procedure. The authors retrospectively investigated the rate of adjacent-level fracture after sandwiching compared with conservative treatment and determined the potential causative factors of sandwich vertebral fracture. Age, sex, weight, height, body mass index, follow-up period, and location of sandwich level (T10-L2 or nonT10-L2 junction) were assessed. Surgical variables, including surgical procedure (vertebroplasty or balloon kyphoplasty), surgical approach (through uni- or bilateral pedicle), volume of cement injected into the painful vertebrae, cement leakage into the intervertebral disk, cumulative number of treated levels, and pre- and postoperative kyphotic angulation of the sandwich region, were also analyzed. Nine of 42 sandwiched levels developed fatigue fractures, whereas 11 of 71 patients treated with conservative therapy sustained new vertebral fractures adjacent to the treated levels. Only preoperative kyphotic angulation was the variable positively associated with sandwich vertebral fracture at follow-up (P=.021). Although subjected to double load shifts, the sandwich vertebra was not prone to structural failure. Thus, cement augmentation protocol does not increase the incidence of adjacent vertebral fracture.     

Organ culture stability of the intervertebral disc: Rat versus rabbit

There is a need to develop mechanically active culture systems to better understand the role of mechanical stresses in intervertebral disc (IVD) degeneration. Motion segment cultures that preserve the native IVD structure and adjacent vertebral bodies are preferred as model systems, but rapid ex vivo tissue degeneration limits their usefulness. The stability of rat and rabbit IVDs is of particular interest, as their small size makes them otherwise suitable for motion segment culture. The goal of this study was to determine if there are substantial differences in the susceptibility of rat and rabbit IVDs to culture‐induced degeneration. Lumbar IVD motion segments were harvested from young adult male Sprague–Dawley rats and New Zealand White rabbits and cultured under standard conditions for 14 days. Biochemical assays and safranin‐O histology showed that while glycosaminoglycan (GAG) loss was minimal in rabbit IVDs, it was progressive and severe in rat IVDs. In the rat IVD, GAG loss was concomitant with the loss of notochordal cells and the migration of endplate (EP) cells into the nucleus pulposus (NP). None of these changes were evident in the rabbit IVDs. Compared to rabbit IVDs, rat IVDs also showed increased matrix metalloproteinase‐3 (MMP‐3) and sharply decreased collagen type I and II collagen expression. Together these data indicated that the rabbit IVD was dramatically more stable than the rat IVD, which showed culture‐related degenerative changes. Based on these findings we conclude that the rabbit motion segments are a superior model for mechanobiologic studies. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 838–846, 2013     

Biomechanical evaluation of kyphoplasty and vertebroplasty with calcium phosphate cement in a simulated osteoporotic compression fracture

 Kyphoplasty and vertebroplasty with polymethylmethacrylate (PMMA) have been used for the treatment of osteoporotic vertebral compression fractures. We performed kyphoplasty and vertebroplasty with α-tricalcium phosphate cement (CPC) and PMMA to compare the biomechanical properties. Thirty osteoporotic vertebrae were harvested from nine embalmed cadavers. We randomized the vertebrae into four treatment groups: (1) kyphoplasty with CPC; (2) kyphoplasty with PMMA; (3) vertebroplasty with CPC; and (4) vertebroplasty with PMMA. Prior to injecting the cement, all vertebrae were compressed to determine their initial strength and stiffness. They were then recompressed to determine their augmented strength and stiffness. Although the augmented strength was greater than the initial strength in all groups, there was no significant difference between the two bone cements for either kyphoplasty or vertebroplasty. The augmented stiffness was significantly less than the initial stiffness in the kyphoplasty groups, but the difference between the two cements did not reach significance. In the vertebroplasty groups, the augmented stiffness was not significantly different from the initial stiffness. There was no significant difference between the two bone cements for either procedure when cement volume and restoration of anterior height were assessed. We concluded that kyphoplasty and vertebroplasty with CPC were viable treatment alternatives to PMMA for osteoporotic vertebral compression fractures. Received: July 18, 2002 / Accepted: November 6, 2002 Offprint requests to: S. Tomita     

An analytical investigation of the mechanics of spinal instrumentation

Three-dimensional nonlinear finite element models of the intact L4-5 one motion segment/two-vertebrae and L3-5 two motion segments/three-vertebrae were developed using computed tomography (CT) films. The finite element mesh of the L4-5 motion segment model was modified to simulate bilateral decompression surgery. The mesh was further altered to achieve stabilization, using an interbody bone graft and a set of Steffee plates and screws. The model behavior of the intact specimen in all loading modes and of the stabilized model in compression, flexion, and extension modes were studied. The stresses in the cancellous bone region were found to decrease. The interbody bone graft, due to an overall decrease in stresses in the bone below the screw, transmits about 80% of the axial load as compared with 96% transmitted by an intact disc in an intact model. Thus, the use of a fixation device induces a stress shielding effect in the vertebral body. The results indicate that although the bone graft transmits lesser loads than the intact disc, it is active in transmitting loads. The presence of low stresses in the cancellous bone region and high localized stresses in the cortical pedicle region surrounding the screw, compared with the intact case, suggests that the screws are likely to become loose over time. The use of an interbody bone graft alone or in combination with any existing fixation device also induces higher stresses at the adjacent levels. This may be responsible for the adverse iatrogenic effects seen clinically.     

经双侧椎弓根通道椎体后凸成形术治疗胸腰椎椎体转移性肿瘤

目的探讨经皮双侧椎弓根通道椎体后凸成形术在胸腰椎椎体转移性肿瘤治疗中的临床疗效和安全性.方法：对17例胸腰椎椎体转移性肿瘤患者共25椎节行经皮双侧椎弓根通道球囊扩张,骨水泥注入.对患者疼痛程度及功能障碍情况分别采用视觉模拟评分和功能障碍指数于治疗前后进行评估.对骨水泥渗漏情况,各例术后以正侧位X线平片判断骨水泥渗漏为X线平片渗漏率,以注射节段CT薄层扫描评判骨水泥渗漏为CT渗漏率,进行统计计算.结果：所有患者均顺利完成手术,无-例症状性骨水泥渗漏发生.术后胸腰背痛缓解明显.VAS 评分术前为（7.2±1.1）分、术后3 d内为（2.3±0.9）分、术后3个月为（2.5±1.1）分.ODI 评分术前为（70.1±1.1）分、术后3 d内为（33.1±1.2）分、术后3个月为（35.2±1.4）分.计算得骨水泥X线平片渗漏率为32%,CT渗漏率为38%.结论：经皮椎体后凸成形术可有效缓解椎体转移性肿瘤患者腰背部疼痛,改善患者日常生活功能;双侧穿刺通道可降低骨水泥渗漏率及减少症状性骨水泥渗漏发生.     

椎体后凸成形术中灌注不同凝固状态骨水泥对骨质疏松性椎体压缩性骨折绵羊椎体生物力学的影响

目的 探讨椎体后凸成形术中灌注不同凝固状态骨水泥对骨质疏松性椎体压缩性骨折(OVCF)绵羊椎体强度和刚度的影响.方法 选取成年绵羊8只,获得L1～5椎体40个,随机分为4组,每组10个.采用3％稀盐酸浸泡和双侧椎弓根微泵灌注法制作骨质疏松椎体模型,再将骨质疏松椎体置于衡翼生物力学机上并压缩其高度的1/4制作压缩骨折椎体模型.制备骨水泥灌注通道后,用球囊经双侧椎弓根复位骨折椎体,在C形臂X线机透视下,对照组(A组)不灌注骨水泥,其余各组分别在聚甲基丙烯酸甲酯(PMMA)骨水泥粉液混合后2 min(骨水泥稀薄期,B组)、4 min(骨水泥黏稠期,C组)、6 min(骨水泥凝固期,D组)灌注骨水泥.室温放置24 h待骨水泥凝固.分别于术前和术后测量各组椎体的强度和刚度.结果 术前4组椎体强度和刚度组间比较,差异均无统计学意义(P>0.05).A组术后椎体强度低于术前,B、C、D组术后椎体强度均高于术前,差异有统计学意义(P<0.05).4组术后椎体刚度均低于术前,差异有统计学意义(P<0.05).B、C、D组术后椎体强度和刚度均高于A组,差异有统计学意义(P<0.05);B、C组术后椎体强度和刚度均高于D组,差异有统计学意义(P<0.05);B组和C组术后椎体强度和刚度差异无统计学意义(P>0.05).结论 OVCF绵羊采用椎体后凸成形术治疗,注入稀薄期和黏稠期骨水泥的椎体强度和刚度均高于注入凝固期骨水泥的椎体.注入不同凝固状态骨水泥均可增强椎体强度,但椎体刚度均恢复不到未骨折时期状态.     

PKP术中不同比例灌注骨水泥椎体生物力学分析

目的 利用乙二胺四乙酸二钠（EDTA-Na2）脱钙法制备离体椎体骨质疏松模型,在万能材料试验机上垂直压缩制成椎体压缩骨折模型,行椎体后凸成形术（percutaneous kyphoplasty,PKP）后再进行生物力学实验,分析按照球囊扩张容积的不同比例灌注骨水泥后椎体的生物力学性能变化,为临床治疗提供参考性资料.方法 选取新鲜成年猪胸腰段椎体36个,甲醛浸泡24 h,再以EDTA-Na2脱钙20 d,制成骨质疏松椎体模型,随机分成A、B、C、D4组,每组9个椎体,检测各组椎体骨矿密度,并依次放在万能材料试验机上行垂直压缩,测出椎体最大压缩强度及压缩刚度,记录各组椎体压缩前、后的椎体前缘高度.之后将各组分别进行聚甲基丙烯酸甲酯灌注椎体成形,按照球囊扩张容积的不同比例灌注聚甲基丙烯酸甲酯,记录球囊扩张容积、球囊压力、成型后椎体前缘高度及骨水泥渗漏椎体个数.之后进行第二次加载测试,按初始方法压缩椎体,记录此时椎体最大压缩强度和刚度.结果 PKP前四组间的椎体前缘高度、骨密度、椎体最大压缩强度及压缩刚度均无统计学意义（P＞0.05）;各组在PKP前、后的椎体最大压缩强度和压缩刚度的比较有统计学意义（P＜0.01）;PKP后A、B组间的椎体最大压缩强度和刚度比较无统计学意义（P＞0.05）,A、B两组分别与C、D组间的椎体最大压缩强度和刚度的比较有统计学意义（P＜0.01）;PKP后四组间椎体高度恢复值比较无统计学意义（P＞0.05）;A、B两组的骨水泥渗漏率为0,C、D两组的骨水泥渗漏率分别为22.22%及44.44%.结论 PKP中按照球囊扩张容积的0.8～1倍灌注骨水泥即可有效恢复骨质疏松性压缩骨折椎体的生物力学性能,又能减少骨水泥的渗漏;PKP中按照球囊扩张容积的0.8～1倍灌注骨水泥可以部分恢复压缩椎体的高度,其与高比例灌注组无明显差别;球囊扩张容积可以作为PKP中骨水泥灌注剂量的参考指标.     

骨水泥在骨质疏松性骨折椎体内分布状态与生物力学性能的关系

 目的 研究骨水泥在腰椎骨质疏松性骨折椎体内不同区域分布状态的生物力学特性,为经皮椎体后凸成形术(percutaneous kyphoplasty,PKP)临床应用提供理论依据。方法 取12具福尔马林固定的老年尸体腰椎标本(包括L1~L5),共筛选49个椎体。对各椎体标本施加轴向压力负载,测量各椎体的原始强度和刚度,并建立椎体压缩骨折模型。按临床PKP手术操作要求根据不同的椎体分区灌注骨水泥,分为对照组和6个实验组,每组7个椎体。测量每组的最大压缩强度和刚度。结果 PKP术后各实验组最大压缩强度较初始强度均明显增强。单侧前2/3灌注组和单侧后2/3灌注组比较,单侧全灌注组、双侧前2/3灌注组和双侧后2/3灌注组比较差异均无统计学意义;椎体最大压缩强度双侧全灌注组>单侧全灌注组、双侧前2/3灌注组和双侧后2/3灌注组>单侧前2/3灌注组和单侧后2/3灌注组。PKP术后双侧全灌注组椎体刚度和初始刚度比较差异无统计学意义,其余各组度均明显小于初始刚度。单侧前2/3灌注组、单侧后2/3灌注组和单侧全灌注组比较,D和双侧后2/3灌注组比较差异无统计学意义;双侧全灌注组>双侧前2/3灌注组和双侧后2/3灌注组> 单侧前2/3灌注组、单侧后2/3灌注组和单侧全灌注组。结论 骨水泥分布在骨质疏松性骨折椎体的不同区域,其生物力学性能存在差异,骨水泥在椎体双侧分布较单侧分布可以获得更好的生物力学效应。骨水泥均匀分布于椎体前2/3区域是较为理想的分布状态,但仍需临床进一步验证。     

Biomechanics of adjacent segments after a multilevel cervical corpectomy using anterior,posterior, and combined anterior-posterior instrumentation techniques: a finite element model study

Background contextAdjacent segment degeneration (ASD) after cervical fusion is a clinical concern. Despite previous studies documenting the biomechanical effects of multilevel cervical fusion on segments immediately superior and inferior to the operative segments, the pathogenesis of the initiation of degeneration progression in neighboring segments is still poorly understood.PurposeTo test the hypothesis that changes in range of motion, disc stresses, and facet loads would be highest at the superior adjacent segment (C3–C4) after anterior C4–C7 corpectomy and fusion and that these changes would be the least in anterior fixation and the greatest in posterior or combined anterior-posterior instrumentation techniques.Study designA finite element (FE) analysis of adjacent vertebral segment biomechanics after a two-level corpectomy fusion with three different fixation techniques (anterior, posterior, and combined anterior-posterior).MethodsA previously validated three-dimensional FE model of an intact C3–T1 segment was used. From this intact model, three additional instrumentation models were constructed using anterior (rigid screw-plate), posterior (rigid screw-rod), and combined anterior-posterior fixation techniques after a C4–C7 corpectomy and fusion. Motion patterns, disc stresses, and posterior facet loads at the levels cephalad and caudal to the fusion were assessed.ResultsRange of motion, disc stresses, and posterior facet loads increased at the adjacent segments. Use of posterior fixation, whether alone or in combination with anterior fixation, infers higher changes in segmental motion, disc stresses, and posterior facet loads at adjacent segments compared with the use of anterior fixation alone. The superior C3–C4 motion was most affected during lateral bending and the inferior C7–T1 motion was most affected during flexion, whereas both superior C3–C4 and inferior C7–T1 motions were least affected during extension. However, disc stresses and facet loads were most affected during extension. Hence, it is speculated that the most remodeling changes in discs and facets might be related to the least changes in extension motion.ConclusionsBiomechanical factors such as increased mechanical demand and motion that have been associated with the development of ASD progression are highest in the segment immediately superior to the fusion. These changes are even more pronounced when the fixation technique involves the addition of posterior instrumentation, thereby supporting the hypothesis of the present study. Increased degrees of stiffening of the fused segments not only may lead to degenerative changes in the disc but may also predispose the segments to premature facet degeneration. Over subsequent time period, any remaining construct micro-motion is further eliminated with fusion of the posterior facet joints and the remaining regions in the disc space also filled in with bone, which eventually results in a circumferential type of fusion. After a circumferential fusion, authors, however, speculate that the role of instrumentation in ASD progression might not be significant. In fact, sufficient evidence to support this speculation is still lacking in the literature.     

骨质疏松椎体增强后对相邻椎体生物力学影响的有限元研究

目的利用骨质疏松腰椎三维有限元模型,探讨椎体成形术中骨水泥的量、不同分布及骨水泥向椎间隙渗漏等对邻近椎体生物力学的影响。方法选取老年男性正常人体脊柱标本一具,范围为L_(4．5),建立L_(4．5)的三维脊柱功能单位的有限元模型。模拟骨水泥在椎体内的不同分布特点,观察不同压力方向时,相邻椎体终板的应力变化。结果不同骨水泥的量对邻近椎体生物力学的影响不大,但骨水泥分布不均匀和骨水泥在椎间隙的渗漏可导致邻近椎体的终板应力增加。结论椎体成形术后骨水泥分布不均匀和骨水泥渗漏到椎间隙可引起邻近椎体终板应力的集中,这可能是邻近椎体骨折的原因。进行椎体成形手术时建议骨水泥均匀分布,并避免渗漏到椎间隙。     

Biomechanical behavior of novel composite PMMA‐CaP bone cements in an anatomically accurate cadaveric vertebroplasty model

Vertebral compression fractures are caused by many factors including trauma and osteoporosis. Osteoporosis induced fractures are a result of loss in bone mass and quality that weaken the vertebral body. Vertebroplasty and kyphoplasty, involving cement augmentation of fractured vertebrae, show promise in restoring vertebral mechanical properties. Some complications however, are reported due to the performance characteristics of commercially available bone cements. In this study, the biomechanical performance characteristics of two novel composite (PMMA‐CaP) bone cements were studied using an anatomically accurate human cadaveric vertebroplasty model. The study involves mechanical testing on two functional cadaveric spinal unit (2FSU) segments which include monotonic compression and cyclical fatigue tests, treatment by direct cement injection, and microscopic visualization of sectioned vertebrae. The 2FSU segments were fractured, treated, and mechanically tested to investigate the stability provided by two novel bone cements; using readily available commercial acrylic cement as a control. Segment height and stiffness were tracked during the study to establish biomechanical performance. The 2FSU segments were successfully stabilized with all three cement groups. Stiffness values were restored to initial levels following fatigue loading. Cement interdigitation was observed with all cement groups. This study demonstrates efficient reinforcement of the fractured vertebrae through stiffness restoration. The pre‐mixed composite cements were comparable to the commercial cement in their performance and interdigitative ability, thus holding promise for future clinical use. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2067–2074, 2017.

     

The influence of cancellous bone density on load sharing in human lumbar spine: a comparison between an intact and a surgically altered motion segment

The aim of the current study is twofold: first, to compare load sharing in compression between an intact and a surgically repaired lumbar spine motion segment L3/4 using a biomechanically validated finite element approach; second, to analyse the influence of bone mineral density on load sharing. Six cadaveric human lumbar spine segments (three segments L2/3 and three segments L4/5) were taken from fresh human cadavers. The intact segments were tested under axial compression of 600 N, first without preload and then following instrumented stabilisation. These results were compared to a finite element model simulating the effect of identical force on the intact segments and the segments with constructs. The predictions of both the intact and the surgically altered finite element model were always within one standard deviation of the mean stiffness as analysed by the biomechanical study. Thus, the finite element model was used to analyse load sharing under compression in an intact and a surgically repaired human lumbar spine segment model, using a variety of E moduli for cancellous bone of the vertebral bodies. In both the intact and the surgically altered model, 89% of the applied load passed through the vertebral bodies and the disc if an E modulus of 25 MPa was used for cancellous bone density. Using 10 MPa--representing soft, osteoporotic bone--this percentage decreased, but it increased using 100 MPa in both the intact and the altered segment. Thus, it is concluded that reconstruction of both the disc and the posterior elements with the implants used in the study recreates the ability of the spine to act as a load-sharing construction in compression. The similarity in load sharing between normal and instrumented spines appears to depend on assumed bone density, and it may also depend on applied load and loading history.     

Preliminary results with modified techniques of balloon kyphoplasty for vertebra plana, traumatic fractures and neoplasms

Percutaneous vertebroplasty and balloon kyphoplasty are less invasive treatment options than open surgery for patients with vertebral compression fractures. With balloon kyphoplasty, the injection of bone cement is preceded by inflation and removal of bone tamps (balloons) inside the fractured vertebral body. This allows for the creation of a void, where viscous cement is delivered resulting in a lower risk for cement leakage than with vertebroplasty. Another advantage of the balloon inflation is the potential to correct the deformity and restore sagittal alignment. The percutaneous techniques normally require intact pedicles and intact posterior elements. We found that modifying the technique made it suitable for the management of vertebra plana, traumatic fractures, and neoplasms. Our study documents the different modified techniques and the clinical results obtained within the first 21 patients.     

Effect of disc degeneration at one level on the adjacent level in axial mode

Nonlinear three-dimensional finite element models of a ligamentous two motion segments spine specimen (L3-L4-L5) were developed to investigate the effects of disc degeneration, simulated at the L4-L5 level, on the biomechanical behavior of the adjacent intact L3-L4 motion segment. The disc degeneration was simulated by removing the hydrostatic capabilities of the nucleus and making the L4-L5 disc stiffer than a normal disc. The results of the degenerated model were compared with the predictions for a model in which the L4-L5 disc was left intact. The loads on the facets decreased, and intradiscal pressure in the intact L3-L4 disc increased as a result of disc degeneration compared with the intact model. The predicted increase in the intradiscal pressure and the associated increase in the disc bulge in the posterior region over time may trigger the degenerative process at the L3-L4 motion segment. This is in accordance with the Wolff's law; living tissue responds to chronic changes in stresses and strains. The limitations of the present two motion segments model and the potentials of multisegmental models are discussed.     

Should we fear cement leakage during kyphoplasty in percutaneous traumatic spine surgery? A single experience with 76 consecutive cases

ObjectiveAlthough kyphoplasty is widely used to repair osteoporotic and pathologic vertebral fractures, balloon kyphoplasty and vertebral body stenting are new treatment options in cases of traumatic spinal injury. To our knowledge, there are no literature data on the incidence of cement leakage whereas these two percutaneous techniques are commonly used to repair non-pathologic fractures. The aim of this study was to evaluate and compare the clinical characteristics and the incidence of cement leakage associated with balloon kyphoplasty and vertebral body stenting in the percutaneous treatment of traumatic spinal injury.MethodsA series of 76 consecutive kyphoplasties (50 with vertebral body stenting and 26 balloon kyphoplasties) were retrospectively reviewed. Preoperative and postoperative computed tomography scans were analyzed in order to detect cement leakage and grade it as minor, moderate or major.ResultsThe overall leakage rate was 50%. None of the leakages gave rise to clinical symptoms. Although balloon kyphoplasty and vertebral body stenting did not differ in terms of the leakage rate, the latter technique was associated with a lower leakage volume. The Magerl type, fracture level and use of concomitant osteosynthesis did not appear to significantly influence the leakage rate.ConclusionVertebral body stenting can reduce the amount of cement leakage due to a better cohesion of the bone fragments after kyphosis correction and maintenance.     

Kyphoplasty for the treatment of incomplete osteoporotic burst fractures

Kyphoplasty has become a standard procedure in the treatment of painful osteoporotic compression fractures. According to current guidelines, involvement of the posterior wall of the vertebral body is a relative contraindication. From February 2002 until January 2008, 97 patients with at least one AO classification A 3.1 fracture were treated by kyphoplasty. There was a structured follow-up for the medium-term evaluation of the patients’ outcome. Ninety-seven patients (68 of whom were females and 29 of whom were males) with involvement of the vertebra’s posterior margin averaging 76.1 ± 12.36 (59–98) years were treated by kyphoplasty. The fractures of 75 patients were caused by falls from little height, 5 patients had suffered traffic accidents and in the case of 17 patients, no type of trauma was remembered. According to the AO classification, there were 109 A 3.1.1 and one A3.1.3 injuries. Prior to surgery, all patients were neurologically without pathological findings. Seventy-nine fractures were accompanied by a narrowing of the spinal canal [average of 15% (10–40)]. Overall, 134 vertebras were treated by Balloon kyphoplasty (81 × 1 segment, 22 × 2 segments, 3 × 3 segments). In 47.4% of the patients, cement leakage was observed after surgery. All patients with cement extravasation, however, were clinically unremarkable. Using the visual analog scale, patients stated that prior to surgery their pain averaged 8.1, whereas after surgery it significantly decreased and averaged 1.6 (p < 0.001). In geriatric patients with osteoporotic vertebral fractures with partial inclusion of the posterior wall of the vertebral body, kyphoplasty is an effective procedure with few complications.