Da Liu

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.

Augmentation of pedicle screw stability with calcium sulfate cement in osteoporotic sheep: biomechanical and screw-bone interfacial evaluation.

Study Design Augmentation of pedicle screws with calcium sulfate cement (CSC) was performed in osteoporotic sheep. Biomechanical tests, micro-computed tomography (CT) analysis, and histological observation were performed. Objective To investigate the long-term biomechanical performance of pedicle screws augmented with CSC in vivo and evaluated the screw-bone interfacial bonding with micro-CT and histological techniques. Summary of Background Data There is little information on the long-term biomechanical performance and screw-bone interfacial bonding of pedicle screws augmented with CSC in osteoporosis in vivo. Methods Twelve months after ovariectomy, bilateral pedicles of lumbar vertebrae (L1 to L5) of 6 female sheep were fixed with pedicle screws. One pedicle of each vertebral body was treated with a screw augmented with CSC (CSC group) and the contralateral pedicle was treated with a screw without any augmentation (control group). Three months later, the sheep were killed and biomechanical tests, micro-CT analysis, and histological observation were conducted on the isolated specimen vertebrae. Results Twelve months after ovariectomy, animal model of osteoporosis was established successfully. Both the axial and vertical stabilities of the pedicle screws in CSC group were significantly enhanced compared with those in the control group (P<0.05). Micro-CT reconstruction and analysis showed that there were more bone trabeculae around the screws in CSC group compared with those in control group (P<0.05), and the bone trabeculae were significantly denser than those in control group (P<0.05). Histological observation showed that CSC was completely degradated and bone trabeculae around the screws in CSC group were more and denser than that in the control group. Bone trabeculae held the screws tightly without any interspaces between screw and bone, which formed strong bonding between bone and screw. Conclusions CSC can significantly improve screw-bone interfacial bonding and strengthen the long-term stability of pedicle screws in osteoporotic sheep. Augmentation with CSC may be a potentially useful method to increase the stability of pedicle screws in patients with osteoporosis.

Biomechanical Comparison of Expansive Pedicle Screw and Polymethylmethacrylate-augmented Pedicle Screw in Osteoporotic Synthetic Bone in Primary Implantation: An Experimental Study.

Study Design:Expansive pedicle screws (EPS) and polymethylmethacrylate-augmented pedicle screws (PMMA-PS) were inserted into osteoporotic synthetic bones, which were then tested by radiographic and biomechanical examinations. Objective:To compare the stability of EPS and PMMA-PS with that of a conventional pedicle screw (CPS) in an osteoporotic synthetic bone. Summary of Background Data:It is a significant challenge for orthopedic surgeons performing transpedicular fixation in the osteoporotic spine. Prior studies have suggested that both EPS and PMMA-PS can increase the screw stability effectively. However, there are no biomechanical comparisons of EPS and PMMA-PS, especially in primary spinal surgery in osteoporosis. Methods:Thirty osteoporotic synthetic bone blocks were divided into 3 groups randomly. A pilot hole was prepared in advance in all samples by the same method. Then, the CPS was inserted directly into the pilot hole in the CPS group; the hole in the PMMA-PS group was first filled with polymethylmethacrylate (PMMA; 2.5 mL) and then inserted with CPS, and the EPS was inserted directly into the blocks in the EPS group. Twenty-four hours later, x-ray and computed tomography examination and axial pullout tests were performed on all samples; the block destructions were then recorded, and the hole diameters were measured. Results:In the CPS group, the screw was surrounded directly by the synthetic bone without any other materials, whereas in the PMMA-PS group, the screw was totally wrapped up by PMMA, and the PMMA was evenly distributed in the synthetic bone around the screw, indicating obvious improvement of the local density around the track. In the EPS group, the anterior part of the EPS presented an obvious expansion in synthetic bone and formed an unguiform structure pressing the surrounding synthetic bone. Screw stabilities in both the PMMA-PS and the EPS groups were significantly enhanced compared with those in the CPS group, and the screw stability in the PMMA-PS group was significantly higher than that in the EPS group. After the pullout tests, the block destructions were the most severe in the PMMA-PS group and the lightest in the CPS group. Hole diameters in the PMMA-PS and the EPS groups were significantly larger than that in the CPS group, whereas the diameter of the hole in the PMMA-PS group was significantly greater than that in the EPS group. Conclusions:EPS can significantly increase the strength of screw fixation compared with CPS in osteoporotic synthetic bone. Although EPS shows a weaker fixation strength compared with PMMA-PS in the osteoporotic synthetic bone, it may still provide an alternative option to prevent screw loosening in the clinical treatment of osteoporosis.

Finite element study on the amount of injection cement during the pedicle screw augmentation.

Study Design: A finite element analysis of the screw pullout procedure for the osteoporotic cancellous bone using screw-bone unit model without cortical layer. Objective: The objective is to determine the region of effect (RoE) during the screw pullout procedure and predict the proper amount of injection cement (AIC) in screw augmentation. Summary of Background Data: For the osteoporotic spine, the AIC is a critical factor for the augmentation screw performance and leakage risk. There are few studies on the proper AIC in literature. Methods: Three finite element models were established, 2 screw-foam models were used for validation study, and 1 screw-bone model was used for investigation of RoE and AIC. The simulations of screw pullout were conducted. A velocity loading of 0.01 mm/s with a maximum displacement of 2.7 mm was applied on the screw. For the validation, the screw-foam models with 2 different densities were used for comparison of pullout force with those published experimental data. After validation, the screw-bone model was used to investigate the RoE and predict the proper AIC during screw augmentation in spine surgery. Results: In validation, the predicted pullout strengths were 2028.8 N for high-density foam model and 607 N for low-density foam model, respectively. They were in good agreement with those of the published experiment. In the screw-bone model, the simulations demonstrated that the RoE changed with the displacement of screw and reached the maximum when the displacement of screw was 1.8 mm. Similar trend was found for the AIC with the displacement. The proper AIC was 2.6 mL when the displacement of screw was 1.8 mm in this study. Conclusions: The RoE and proper AIC for augmentation were evaluated in the osteoporotic spine. This information could provide practical reference for screw augmentation in spinal decompression and instrumentation in the spine surgery.

A new method of partial screw augmentation in sheep vertebrae in vitro: biomechanical and interfacial evaluation.

Study Design Partial screw augmentation was carried out by the injection of polymethylmethacrylate (PMMA) through the novel tap into the bone tissue in sheep vertebrae in vitro. Biomechanical tests and microcomputerized tomography analysis were done. Objective To evaluate the stability of the screw and the interface between the bone and the screw in partial screw augmentation. Summary of Background Data Augmentation of the pedicle screw with PMMA improves screw stability significantly. However, it results in the full envelopment of the screw by PMMA and the formation of a full “screw-PMMA-bone” interface, which may increase the difficulties in screw removal. Therefore, it is urgent to develop a novel method to augment screw fixation, which avoids full wrapping of PMMA around the screw and facilitates screw removal. We designed a novel tap with an inner hollow and the multiple lateral apertures. Methods A total of 78 fresh adult sheep lumbar vertebrae were randomly divided into 3 groups. No screw augmentation was performed in the control group. The pilot hole was filled with PMMA in the full screw augmentation group, whereas PMMA was injected through the tap into the bone tissue in the partial screw augmentation group. Pedicle screws were ultimately inserted into all the holes. Axial pullout tests, cyclic bending tests, and microcomputerized tomography analysis were carried out 24 hours after screw fixation. Results Both partial screw augmentation and full screw augmentation caused significant increase in pedicle screw stability compared with the control group in both the axial pullout tests and the cyclic bending resistance tests. Partial screw augmentation resulted in the formation of a partial “screw-bone” interface and a partial screw-PMMA-bone interface, which seems like 3 anchors or roots extending and fixing in the bone tissue in a cross section of the interface. It was obviously different from the complete screw-PMMA-bone interface with a complete PMMA mantle around the screw in the full screw augmentation group. Conclusions Our proof-of-concept study showed that partial screw augmentation could enhance pedicle screw fixation strength, avoided the full surrounding of the PMMA around the screw, and generated a partial screw-PMMA-bone and a partial screw-bone interface. Partial screw augmentation may be a potential method to augment pedicle screw stability.

Reciprocal inhibition between miR-26a and NF-κB regulates obesity-related chronic inflammation in chondrocytes.

miR-26a suppresses proinflammatory cytokine production via inactivating NF-κB, whereas NF-κB inhibits miR-26a production through binding to miR-26a promoter. We identified a reciprocal inhibition between miR-26a and NF-κB in obesity-related chondrocytes, providing a potential mechanism linking obesity to osteoarthritis.

Comparison of expansive pedicle screw and polymethylmethacrylate-augmented pedicle screw in osteoporotic sheep lumbar vertebrae: biomechanical and interfacial evaluations.

Background It was reported that expansive pedicle screw (EPS) and polymethylmethacrylate-augmented pedicle screw (PMMA-PS) could be used to increase screw stability in osteoporosis. However, there are no studies comparing the two kinds of screws in vivo. Thus, we aimed to compare biomechanical and interfacial performances of EPS and PMMA-PS in osteoporotic sheep spine. Methodology/Principal Findings After successful induction of osteoporotic sheep, lumbar vertebrae in each sheep were randomly divided into three groups. The conventional pedicle screw (CPS) was inserted directly into vertebrae in CPS group; PMMA was injected prior to insertion of CPS in PMMA-PS group; and the EPS was inserted in EPS group. Sheep were killed and biomechanical tests, micro-CT analysis and histological observation were performed at both 6 and 12 weeks post-operation. At 6-week and 12-week, screw stabilities in EPS and PMMA-PS groups were significantly higher than that in CPS group, but there were no significant differences between EPS and PMMA-PS groups at two study periods. The screw stability in EPS group at 12-week was significantly higher than that at 6-week. The bone trabeculae around the expanding anterior part of EPS were more and denser than that in CPS group at 6-week and 12-week. PMMA was found without any degradation and absorption forming non-biological “screw-PMMA-bone” interface in PMMA-PS group, however, more and more bone trabeculae surrounded anterior part of EPS improving local bone quality and formed biological “screw-bone” interface. Conclusions/Significance EPS can markedly enhance screw stability with a similar effect to the traditional method of screw augmentation with PMMA in initial surgery in osteoporosis. EPS can form better biological interface between screw and bone than PMMA-PS. In addition, EPS have no risk of thermal injury, leakage and compression caused by PMMA. We propose EPS has a great application potential in augmentation of screw stability in osteoporosis in clinic.

A Study of Low Elastic Modulus Expandable Pedicle Screws in Osteoporotic Sheep

Study Design Low elastic modulus expandable pedicle screw (L-EPS) was inserted into osteoporotic sheep. Biomechanical tests, micro-CT analysis and histologic observation were performed. Objective To find out whether the L-EPS can further improve fixation strength compared to the expandable pedicle screws (EPS) in osteoporotic sheep spine. The screw-bone interface is also detected by micro-computed tomography (CT) and histologic techniques. Summary of Background Data There are some studies on EPS in vivo; however, no earlier study has focused on the elastic modulus of EPS, especially in osteoporosis. Methods Twelve months after ovariectomy, bilateral pedicles of lumbar vertebrae (L1 to L5) of 4 female sheep were fixed with EPSs. The L-EPS and EPS were randomly placed into each pedicle and then were expanded. Six months later, the sheep were sacrificed and biomechanical tests, micro-CT analysis, and histologic observation were conducted on the isolated specimen vertebrae. Results Twelve months after ovariectomy, animal model of osteoporosis was established successfully. The axial pull-out strength in L-EPS group was significantly enhanced compared with that in the EPS group (P < 0.05). Micro-CT reconstruction and analysis showed that there was more bone around the L-EPS group compared with those in EPS group (P < 0.05), meanwhile the more homogeneous bone formation distribution around the screws was found in the L-EPS group. Histologic observation showed that newly formed bone extended along the expandable fissures and grew into the center of EPS; meanwhile, the more direct contact and the less fibrous tissue on the screw-bone interface were observed in the L-EPS group. Conclusions The L-EPS can further improve the biomechanical fixation strength of EPS in the osteoporotic sheep. The screw elastic matching with surrounding bone is helpful to distribute stress uniformly, relieve the stress shielding effect, and strengthen the screw-bone interface. Although the experience with the L-EPS is very limited and preliminary, results to date indicate that it is of value in treating patients with osteoporosis and warrants further study.

MiR-132 regulates osteogenic differentiation via downregulating Sirtuin1 in a peroxisome proliferator-activated receptor β/δ-dependent manner.

MicroRNAs (miRNAs) play significant roles in multiple diseases by regulating the expression of their target genes. Type 2 diabetes mellitus (T2DM) is a chronic endocrine and metabolic disease with complex mechanisms. T2DM can result in diabetic osteoporosis (DO), which is characterized by bone loss, decreased bone mineral density and increased bone fractures. The promotion of osteogenic differentiation of osteoblasts is an effective way to treat osteoporosis. In the present study, high glucose (HG) and free fatty acids (FFA) were employed to mimic T2DM in MC3T3-E1 cells. To induce osteogenic differentiation, MC3T3-E1 cells were cultured in osteogenic medium. The results showed that osteogenic differentiation was significantly suppressed by HG and FFA. We found that miR-132 expression was significantly upregulated and much higher in HG-FFA-induced cells than other selected miRNAs, indicating that miR-132 might play an important role in DO. Furthermore, overexpression of miR-132 markedly inhibited the expression of key markers of osteogenic differentiation and alkaline phosphatase (ALP) activity. Reciprocally, inhibition of miR-132 restored osteogenic differentiation, even under treatment with HG-FFA. We also showed that Sirtuin 1 (Sirt1) was one of the target genes of miR-132, whose expression was controlled by miR-132. Ectopic expression of Sirt1 reversed the decrease in osteogenic differentiation caused by miR-132 and HG-FFA. These results demonstrated the direct role of miR-132 in suppressing osteogenic differentiation through downregulating Sirt1. Moreover, we demonstrated that peroxisome proliferator-activated receptor β/δ (PPARβ/δ) was a downstream molecule of Sirt1, and its knockout by PPARβ/δ siRNA significantly abolished the promotive effects of Sirt1 on osteogenic differentiation, indicating that Sirt1 functioned in a PPARβ/δ-dependent manner. Taken together, we provide crucial evidence that miR-132 plays a key role in regulating osteogenic differentiation through Sirt1 in a PPARβ/δ-dependent manner, indicating that miR-132 and Sirt1-PPARβ/δ may act as potential therapeutic targets for T2DM-induced osteoporosis.

Biomechanical comparison of pedicle screw augmented with different volumes of polymethylmethacrylate in osteoporotic and severely osteoporotic cadaveric lumbar vertebrae: an experimental study.

BACKGROUND CONTEXT Polymethylmethacrylate (PMMA) is widely used for pedicle screw augmentation in osteoporosis. Intriguingly, there have been no biomechanical comparisons of the stability of pedicle screws augmented with different volumes of PMMA or studies of the relationship between screw stability and volume of PMMA, especially in different degrees of osteoporosis. PURPOSE The purposes of the study reported here were to compare screw stability by different volumes of PMMA augmentation, to analyze the relationship between screw stability and PMMA volume, and to make a preliminary determination of the optimum volume of PMMA augmentation for different degrees of osteoporosis. STUDY DESIGN This study is a biomechanical comparison of pedicle screws augmented with various volumes of PMMA in cadaveric lumbar vertebrae. METHODS Thirty-six pedicles from 18 osteoporotic lumbar vertebrae were randomly divided into groups A0 through A5, and 36 pedicles from 18 severely osteoporotic lumbar vertebrae were randomly divided into groups B0 through B5. A different volume of PMMA was injected into each one of groups A0 through A5 (0, 0.5, 1.0, 1.5, 2.0, and 2.5 mL, respectively) and into each one of groups B0 through B5 (0, 1.0, 1.5, 2.0, 2.5, and 3.0 mL, respectively), and then pedicle screws were inserted in all vertebrae. After complete solidification of the PMMA, we examined pedicle X-rays, performed axial pullout tests, and determined the maximum axial pullout strength (Fmax) for all samples. RESULTS No PMMA was found around the screws in groups A0 and B0. In groups A1 to A5 and B1 to B5, screws were wrapped by gradually increasing amounts of PMMA. There was no PMMA leakage or screw malpositioning in any samples. The Fmax in groups A1 through A5 increased by 32.40%, 64.42%, 116.02%, 174.07%, and 207.42%, respectively, compared with that in group A0. There were no significant differences in Fmax between groups A0 and A1, A1 and A2, A2 and A3, A3 and A4, and A4 and A5 (p>.05), but there were significant differences in Fmax between any other two groups (p<.05). The Fmax in groups B1 through B5 increased by 23.48%, 48.40%, 106.60%, 134.73%, and 210.04%, respectively, compared with that in group B0. There were no significant differences in Fmax between groups B0 and B1, B0 and B2, B1 and B2, B2 and B3, B3 and B4 (p>.05), but there were significant differences in Fmax between any other two groups (p<.05). There was a significant positive correlation between Fmax and volume of PMMA in both osteoporotic and severely osteoporotic lumbar vertebrae (p<.05). CONCLUSIONS Polymethylmethacrylate can significantly enhance stability of pedicle screws in both osteoporotic and severely osteoporotic lumbar vertebrae. There is a significant positive correlation between screw stability and volume of PMMA. Within a certain range, nevertheless, increasing the volume of PMMA does not significantly improve screw stability. We suggest that 1.5 and 3 mL, respectively, are the volumes of injected PMMA that will optimize pedicle screw stability in osteoporotic and severely osteoporotic lumbar vertebrae.