Zi-xiang Wu

Biomechanical evaluation of fixation strength of conventional and expansive pedicle screws with or without calcium based cement augmentation

BACKGROUND The expansive pedicle screw was originally developed to be installed in the bone of compromised quality, but there are some concerns whether it can provide enough fixation strength in the spine with osteoporosis or severe osteoporosis. METHODS Twelve fresh human cadaver spines were stratified into four levels: normal, osteopenia, osteoporosis and severe osteoporosis. The vertebra was bilaterally instrumented with pedicle screws according to four protocols, including conventional pedicle screw without augmentation, expansive pedicle screw without augmentation, conventional screw with augmentation and expansive screw with augmentation. Screw pullout tests were conducted. FINDINGS Given the same specimen, the fixation strength of expansive screw was significantly higher than that of the conventional screw. When the same type of screw was used, the fixation strength of the calcium based cement augmented group was stronger than that of the non-augmented group. The pullout strength and stiffness of the expansive screw, augmented conventional screw and augmented expansive screw groups at the osteoporotic level were comparable to those of the conventional pedicle screw group at the osteopenic level. However, under the severely osteoporotic bone environment, the pullout strength of pedicle screw with whatever placement protocol was significantly lower than that of the conventional screw group at the osteopenic level. INTERPRETATION Our results demonstrate that (i) the expansive pedicle screw appears feasible and safe in either osteopenic or osteoporotic spine; (ii) calcium based cement augmentation can offer improved initial fixation strength of pedicle screws.; and (iii) no screw placement protocol we examined is efficacious in the bone at the severely osteoporotic level.

Effect of ovariectomy on BMD, micro-architecture and biomechanics of cortical and cancellous bones in a sheep model.

Osteoporotic/osteopenia fractures occur most frequently in trabeculae-rich skeletal sites. The purpose of this study was to use a high-resolution micro-computed tomography (micro-CT) and dual energy X-ray absorptionmeter (DEXA) to investigate the changes in micro-architecture and bone mineral density (BMD) in a sheep model resulted from ovariectomy (OVX). Biomechanical tests were performed to evaluate the strength of the trabecular bone. Twenty adult sheeps were randomly divided into three groups: sham group (n=8), group 1 (n=4) and group 2 (n=8). In groups 1 and 2, all sheep were ovariectomized (OVX); in the sham group, the ovaries were located and the oviducts were ligated. In all animals, BMD for lumbar spine was obtained during the surgical procedure. BMD at the spine, femoral neck and femoral condyle was determined 6 months (group 1) and 12 months (group 2) post-OVX. Lumbar spines and femora were obtained and underwent BMD scan, micro-CT analysis. Compressive mechanical properties were determined from biopsies of vertebral bodies and femoral condyles. BMD, micro-architectural parameters and mechanical properties of cancellous bone did not decrease significantly at 6 months post-OVX. Twelve months after OVX, BMD, micro-architectural parameters and mechanical properties decreased significantly. The results of linear regression analyses showed that trabecular thickness (Tb.Th) (r=0.945, R2=0.886) and bone volume fraction (BV/TV) (r=0.783, R2=0.586) had strong (R2>0.5) correlation to compression stress. In OVX sheep, changes in the structural parameters of trabecular bone are comparable to the human situation during osteoporosis was induced. The sheep model presented seems to meet the criteria for an osteopenia model for fracture treatment with respect to morphometric and mechanical properties. But the duration of OVX must be longer than 12 months to ensure the animal model can be established successfully.

Effects of glucocorticoid on BMD, micro-architecture and biomechanics of cancellous and cortical bone mass in OVX rabbits.

The incidence of osteoporosis continues to increase with progressively aging populations. The purpose of this study was to detect the effects of glucocorticoid (GC) treatment on bone mineral density (BMD), biomechanical strength and micro-architecture in cancellous and cortical bone in ovariectomized (OVX) rabbits. Twenty adult female New Zealand white rabbits were randomly divided into three groups. The OVX-GC group (n=8) received a bilateral ovariectomy first and then daily GC treatment (methylprednisolone sodium succinate, 1mg/kg/day) for 4 weeks beginning 2 weeks after ovariectomy treatment. The OVX group (n=4) received a bilateral ovariectomy without GC treatment. The sham group (n=8) only received the sham operation. BMD was determined prior to and 6 weeks after the operation in the spine. Six weeks after the operation, the animals were sacrificed, and cancellous bone specimens were harvested from the femoral condyle and lumbar vertebrae. Cortical bone specimens were obtained from the femoral midshaft. The femoral specimens were scanned for apparent BMD. All specimens were tested mechanically and analyzed by microcompute tomography (micro-CT). In cancellous bone, GC treatment resulted in significant decreases in BMD, bone biomechanical strength and micro-architecture parameters in lumbar vertebrae. Similar trends in BMD and micro-architectural changes were also observed in the femoral condyle in the OVX-GC group compared with the sham group. However, there was no significant decline in any parameter in either lumbar vertebrae or femoral condyle in the OVX group. Similarly, no significant difference was found in any parameter in cortical bone among the three groups. Thus, the 4-week GC treatment in OVX rabbits could result in a significant bone loss in cancellous bone but not in cortical bone. This model is comparable to the osteoporosis-related changes in humans. OVX alone was not sufficient to induce osteoporosis.

Staged-injection procedure to prevent cement leakage during vertebroplasty: an in vitro study.

Study Design. Fibrin sealant (FS) combined with or without growth factor was used to improve the micro-architectural and biomechanical properties of vertebral body in osteoporotic ovine spine. Objective. To analyze the treatment effects of bovine bone morphogenetic protein (bBMP) combined with FS on osteopenic ovine vertebral architecture, bone mineral density, and biomechanics in vivo. Summary of Background Data. Vertebroplasty and kyphoplasty were used to treat spinal osteoporosis. They can increase strength of vertebrae physically. However, each has specific disadvantages. bBMP could rapidly increasing bone formation and suppressing bone resorption, but little is known about its effect on ovariectomized-induced osteoporosis. Methods. Six sheep underwent ovariectomy and were placed on a low-calcium diet. Twelve months later, according to Ladin square design, L4–L6 vertebrae in all sheep were randomly assigned to 3 treatment groups: A (30 mg bBMP/1.5 mL FS), B (30 mg bBMP) and C (1.5 mL FS). All materials were injected into the assigned vertebra transpedicularly. Animals were killed 3 months after injection, and bone mineral density (BMD), biomechanics, and histomorphometry were assessed. Analysis of variance was used to determine effects of bBMP/FS (&agr; = 0.05). Results. The BMD in Group 1 was 17.1% and 14.7% higher than that in Group 2 and Group 3, respectively. The micro-CT reconstruction analysis showed that the density and connectivity of trabecular bone in bBMP/FS treated vertebrae were higher than those in control groups. The mechanical properties (yield stress, ultimate stress, energy absorption, bone modulus) of the vertebrae were also significantly higher. In this study, local bBMP/FS treatment showed a positive trend in improving BMD, histomorphometric parameters, and mechanical strength of osteoporotic vertebra. Slow release of bBMP using FS appeared to be an effective method of protein delivery. Conclusion. The use of bBMP/FS in the treatment of vertebral osteoporosis in an attempt to enhance bone strength merits further study.

Impact of leg lengthening on viscoelastic properties of the deep fascia.

BackgroundDespite the morphological alterations of the deep fascia subjected to leg lengthening have been investigated in cellular and extracellular aspects, the impact of leg lengthening on viscoelastic properties of the deep fascia remains largely unknown. This study aimed to address the changes of viscoelastic properties of the deep fascia during leg lengthening using uniaxial tensile test.MethodsAnimal model of leg lengthening was established in New Zealand white rabbits. Distraction was initiated at a rate of 1 mm/day and 2 mm/day in two steps, and preceded until increases of 10% and 20% in the initial length of tibia had been achieved. The deep fascia specimens of 30 mm × 10 mm were clamped with the Instron 1122 tensile tester at room temperature with a constant tensile rate of 5 mm/min. After 5 load-download tensile tests had been performed, the specimens were elongated until rupture. The load-displacement curves were automatically generated.ResultsThe normal deep fascia showed typical viscoelastic rule of collagenous tissues. Each experimental group of the deep fascia after leg lengthening kept the properties. The curves of the deep fascia at a rate of 1 mm/day with 20% increase in tibia length were the closest to those of normal deep fascia. The ultimate tension strength and the strain at rupture on average of normal deep fascia were 2.69 N (8.97 mN/mm2) and 14.11%, respectively. The increases in ultimate tension strength and strain at rupture of the deep fascia after leg lengthening were statistically significant.ConclusionThe deep fascia subjected to leg lengthening exhibits viscoelastic properties as collagenous tissues without lengthening other than increased strain and strength. Notwithstanding different lengthening schemes result in varied viscoelastic properties changes, the most comparable viscoelastic properties to be demonstrated are under the scheme of a distraction rate of 1 mm/day and 20% increase in tibia length.

Ti2448 pedicle screw system augmentation for posterior lumbar interbody fusion.

Study Design. A finite element analysis was used. Objective. To evaluate the feasibility of using the Ti-24Nb-4Zr-7.9Sn (Ti2448) pedicle screw system to augment single-level posterior lumbar interbody fusion (PLIF). Summary of Background Data. The Ti-6Al-4V pedicle screw system increases the risk of adjacent disc degeneration and stress-shielding effect due to enormous rigidity. A titanium alloy with much lower elastic modulus, Ti2448, may help to resolve the complications. Methods. A finite element model of intact L3–S1 was established and then validated. Single-level PLIF at L4–L5 with or without a supplementary titanium-alloy pedicle screw system was simulated. A pure moment of 7.6 Nm and a 400 N preload was applied to the finite element model of PLIF, PLIF with the Ti-6Al-4V screw system, and PLIF with the Ti2448 screw system in flexion, extension, axial rotation, and lateral bending. Results. The axial displacement at the fusion level decreased to 64%, 72%, 84%, and 92% of screw-free status in flexion, extension, axial rotation, and lateral bending, respectively, after augmentation of the Ti2448 screw system, which was 1% to 3% lower than the performance of the Ti-6Al-4V system. The angular displacement at the fusion level with the Ti2448 system was similar to that of the Ti-6Al-4V system, only 2% lower in flexion. Compared with the Ti-6Al-4V system, the Ti2448 system suppressed the increase of intradiscal pressures at the upper adjacent disc in all bending directions, but only in extension and axial rotation at the lower adjacent disc; the maximum stress experienced by cages and screws was higher in all bending directions when augmented with the Ti2448 system. Conclusion. Using the Ti2448 screw system is suggested for augmenting single-level PLIF because it induces less disc intradiscal pressure at adjacent levels and the stress-shielding effect at implant-bone surface with stabilization performance compared with the Ti-6Al-4V screw system. Level of Evidence: N/A

Staged-Injection Procedure to Prevent Cement Leakage During Vertebroplasty: An In Vitro Study

BACKGROUND Epidural leaks or canal intrusion are often found in vertebroplasty, which can lead to several complications. To the best of our knowledge, this is the first report on using two-stage injection procedure in vertebroplasty to reduce cement leakage rate. MATERIALS AND METHODS Fifty cadaveric vertebrae (L1-L5) were harvested from 10 osteoporotic lumbar spines. The age of the female donors ranged from 60 to 72 y; the bone mineral density (BMD) of the vertebrae ranged from 0.226 to 0.631 g/cm(2). Polymethylmethacrylate (PMMA) was injected into vertebral bodies by either novel two-stage injection (1 mL cement was injected at first stage and another 4 mL cement was injected at second stage after the cement solidified) or standard conventional injection procedure. After injection, all vertebral bodies (VBs) were checked for cement leakage into spinal canal by vision and fluoroscopy. Cement leakage rates were recorded. RESULTS The two-stage injection procedure resulted in a significant decrease in the epidural or canal leakage rate (P = 0.032). In addition, the paravertebral leakage rate was significantly higher in two-stage injection procedure group (48%) than in conventional injection procedure group (16%). However, no significant difference was found for the total cement leakage rate between the two groups. CONCLUSION Compared with conventional procedure, the two-stage injection procedure in vertebroplasty decreased incidence of epidural and canal leaks. The first-stage injecting PMMA can be a barrier to the epidural extravasation of cement during vertebroplasty. Further in vivo and clinical researches are needed to evaluate the new procedure.

The effect on the extracellular matrix of the deep fascia in response to leg lengthening

BackgroundWhereas the alterations of diverse tissues in cellular and molecular levels have been investigated during leg lengthening via microscopy and biochemical studies, little is known about the response of deep fascia. This study aims to investigate the changes of the extracellular matrix in deep fascia in response to leg lengthening.MethodsAnimal model of leg lengthening was established in New Zealand white rabbits. Distraction was initiated at a rate of 1 mm/day and 2 mm/day in two steps, and preceded until increases of 10% and 20% in the initial length of tibia had been achieved. Alcian blue stain and picrosirius-polarization method were used for the study of the extracellular matrix of deep fascia samples. Leica DM LA image analysis system was used to investigate the quantitative changes of collagen type I and III.ResultsAlcian blue stain showed that glycosaminoglycans of fascia of each group were composed of chondroitin sulphate and heparin sulphate, but not of keratan sulphate. Under the polarization microscopy, the fascia consisted mainly of collagen type I. After leg lengthening, the percentage of collagen type III increased. The most similar collagen composition of the fascia to that of the normal fascia was detected at a 20% increase in tibia length achieved via a distraction rate of 1 mm/d.ConclusionThe changes in collagen distribution and composition occur in deep fascia during leg lengthening. Although different lengthening schemes resulted in varied matrix changes, the most comparable collagen composition to be demonstrated under the scheme of a distraction rate of 1 mm/day and 20% increase in tibia length. Efficient fascia regeneration is initiated only in certain combinations of the leg load parameters including appropriate intensity and duration time, e.g., either low density distraction that persist a relatively short time or high distraction rates.

Local Treatment of Osteoporotic Sheep Vertebral Body With Calcium Sulfate for Decreasing the Potential Fracture Risk: Microstructural and Biomechanical Evaluations.

Study Design:In this study, calcium sulfate (CS) was injected through pedicle into the osteoporotic vertebral body in vivo in sheep, and micro–computed tomography analysis, histologic observation, and biomechanical test were performed. Objective:To investigate the improvement on microstructure and biomechanical performance of lumbar vertebrae augmented with CS in osteoporotic sheep. Summary of Background Data:The present treatments for osteoporosis relies on systemic medications intended to increase the bone mineral density (BMD). Although effective, these time-consuming medications provide little protection from fracture in the “early period” after initiation of therapy. In this regard, the strategy of local treatment is to target specific areas of the skeletal system that are prone to osteoporotic fractures. However, there is little or no research focused on local treatment of osteoporotic vertebrae with CS. Methods:Eight female sheep were induced to osteoporosis with bilateral ovariectomy and methylprednisolone administration for 12 months. After successful establishment of an osteoporotic model, lumbar vertebrae (L1–L4) in every sheep were randomly divided into 2 groups: CS group and control group (2 vertebrae in each group in every sheep). CS was injected into the vertebral body transpedicularly in the CS group and no treatments were performed in the control group. Three months later, all sheep were killed and all L1–L4 vertebrae were harvested. Thereafter, microstructure and biomechanical performance of the cancellous bone of the vertebral body were assessed through micro–computed tomography analysis, histologic observation, and biomechanical test, respectively. Results:After a 12-month induction with ovariectomy and methylprednisolone administration, the mean BMD of the sheep lumbar vertebrae significantly decreased (>25%) compared with the value before induction, which demonstrated successful establishment of osteoporosis. Three months after injection of CS, CS was completely degraded without any remains in bone tissue and the quality of bone tissue (amount and density of the bone tissue) in the CS group was significantly higher than that in the control group. The ultimate load, stiffness, and energy absorption in the CS group were all significantly higher than those in the control group. Conclusions:The preliminary data suggest that local injection of CS can significantly improve the amount, density, and biomechanical performance of the bone trabeculae in osteoporotic vertebra. The local injection of CS could also be used as a new method to improve the physical microstructure and augment the mechanical properties in “high-risk” vertebral bodies, decreasing the potential fracture risk of patients with osteoporosis. The strict inclusion and exclusion criteria should be performed before treatment.