Ming-xuan Gao

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.

Surgical Treatment of Osteoporotic Thoracolumbar Compressive Fractures with Open Vertebral Cement Augmentation of Expandable Pedicle Screw Fixation: A Biomechanical Study and a 2-Year Follow-up of 20 Patients

BACKGROUND The incidence of screw loosening increases significantly in elderly patients with severe osteoporosis. Open vertebral cement augmentation of expandable pedicle screw fixation may improve fixation strength in the osteoporotic vertebrae. MATERIALS AND METHODS Twenty cadaveric vertebrae (L1-L5) were harvested from six osteoporotic lumbar spines. Axial pullout tests were performed to compare the maximum pullout strength (Fmax) of four methods: 1. Conventional pedicle screws (CPS), 2. Expandable pedicle screws (EPS), 3. Cement augmentation of CPS (cemented-CPS), 4. Cement augmentation of EPS (cemented-EPS). Thirty-six consecutive patients with single-vertebral osteoporotic compressive fractures received posterior decompression and spinal fusion with cemented-CPS (16 cases) or cemented-EPS (20 cases). Plain film and/or CT scan were conducted to evaluate the spinal fusion and fixation effectiveness. RESULTS The Fmax and energy absorption of cemented-EPS were significantly greater than three control groups. The mean BMD in the severe osteoporosis group was significantly lower than that in the osteoporosis group (t = 2.04, P = 0.036). In the osteoporosis group, cemented-EPS improved the Fmax by 43% and 21% over CPS and cemented-CPS group. In the severe osteoporosis group, cemented-EPS increased the Fmax by 59%, 22%, and 26% over CPS, EPS, and cemented-CPS, respectively. The clinical results showed that all patients suffered from severe osteoporosis. Six months after operation, the JOA and VAS scores in cemented-EPS group improved from 11.4 ± 2.6 and 7.0 ± 1.4 mm to 24.9 ± 1.6 and 2.1 ± 1.3 mm, respectively. No screw loosening occurred in the cemented-EPS group and spinal fusion was achieved. In the cemented-CPS group, four screws loosened (4.2%) according to the radiolucency. Six months after operation, the JOA and VAS scores improved from 13.1 ± 1.9 and 7.6 ± 1.5 mm to 22.8 ± 2.2 and 2.5 ± 1.6 mm, respectively. No cement leaked into the spinal canal in both groups. CONCLUSIONS Cemented-EPS could increase fixation strength biomechanically. It could reduce the risks of screw loosening in patients with severe osteoporosis, requiring instrumented arthrodesis.

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.

Locally Administrated Perindopril Improves Healing in an Ovariectomized Rat Tibial Osteotomy Model

Angiotensin-converting enzyme inhibitors are widely prescribed to regulate blood pressure. High doses of orally administered perindopril have previously been shown to improve fracture healing in a mouse femur fracture model. In this study, perindopril was administered directly to the fracture area with the goal of stimulating fracture repair. Three months after being ovariectomized (OVX), tibial fractures were produced in Sprague–Dawley rats and subsequently stabilized with intramedullary wires. Perindopril (0.4 mg/kg/day) was injected locally at the fractured site for a treatment period of 7 days. Vehicle reagent was used as a control. Callus quality was evaluated at 2 and 4 weeks post-fracture. Compared with the vehicle group, perindopril treatment significantly increased bone formation, increased biomechanical strength, and improved microstructural parameters of the callus. Newly woven bone was arranged more tightly and regularly at 4 weeks post-fracture. The ultimate load increased by 66.1 and 76.9% (p<0.01), and the bone volume over total volume (BV/TV) increased by 29.9% and 24.3% (p<0.01) at 2 and 4 weeks post-fracture, respectively. These findings suggest that local treatment with perindopril could promote fracture healing in ovariectomized rats.

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.

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.

Improving fixation strength of pedicle screw by microarc oxidation treatment: an experimental study of osteoporotic spine in sheep.

Failure of fixation caused by loosening of pedicle screws in osteoporosis is a problem in spinal surgery. We compared the in vivo fixation strength between pedicle screws treated with microarc oxidation (MAO) and untreated screws in an osteoporotic model of ovariectomized sheep. The MAO treated and untreated screws were placed in lumbar vertebral bodies. After 3 months of implantation, biomechanical tests, micro‐CT analysis, and histological observations were conducted to examine the performance of the two groups. At time 0, no significant difference was found between the two groups in biomechanical tests (p > 0.05); 3 months later, higher pull‐out strength and load with less displacement were detected in the MAO‐treated group (p < 0.05). Micro‐CT analysis showed that the tissue mineral density, bone volume fraction, trabecular thickness, and trabecular number in the MAO‐treated group were all higher than those in untreated group (p < 0.05), and trabecular spacing was smaller (p < 0.05). Histologically, the bone–implant interface in the MAO‐treated group was better than that in untreated group (p < 0.05). In conclusion, pedicle screws with a bioactive surface treated by MAO can improve screw fixation strength in osteoporotic spines in sheep.

Expandable pedicle screw trajectory in cadaveric lumbar vertebra: an evaluation using microcomputed tomography.

Study Design An in-vitro cadaver-based measurement using microcomputed tomography. Objectives To evaluate the safety of removal of expandable pedicle screws (EPSs) in human vertebral bodies after a long (6 mo) insertion time. Summary of Background Data Transpedicular fixation is commonly used for posterior fixation of the spine. Osteoporotic patients are in high risk of internal fixation failure. EPS is an available device for increasing transpedical fixation in the osteoporotic spine. However, it is unclear that how much the diameter of the expanding screw tip increases in the human vertebral body, and no reports have assessed the potential for fracture of the pedicle when the EPS is removed. Therefore, the safety of removal of EPS needs to be evaluated. Methods In this study, 6.50 mm EPSs were inserted into cadaveric lumbar vertebrae. The maximum diameters of expansion, the maximum diameters of the EPS trajectories, and the diameters of the EPS trajectories in the pedicles were measured using microcomputed tomography imaging and 3-dimensional reconstruction. Regions of interest of the same size adjacent to the EPS in the pedicle and in the vertebral body were reconstructed and analyzed using the same thresholds (1000). Bone volume divided by total volume (BV/TV) in these regions of interest was determined. Results The maximum diameter of expansion of the EPSs in the vertebral body was 7.63 mm, a 17.38% increase from the original diameter. The maximum diameter of the EPS trajectory and the diameter of the EPS trajectory in the pedicle were 7.56 mm and 7.09 mm, respectively, increases of 16.31% and 9.08%. The differences were statistically significant (P<0.05). In the pedicle, there was a negative correlation between the diameter of the EPS trajectory and the BV/TV along the EPS trajectory (r=−0.88; P=0.002). In the vertebral body, a negative correlation occurred between the maximum expansion diameter of the EPS and vertebral BV/TV (r=−0.85; P=0.004). Conclusions The EPS could be reliably removed after expansion without fracturing the pedicle. Furthermore, the BV/TV had an important influence on the degree of expansion of EPS and whether or not pedicle fracture occurs.