| The femoral neck fracture is a clinically common skeletal injury,accounting for about3.58%of all fractures and50%of the proximal femurfractures. Nonunion and avascular necrosis of femoral head are the mainproblems in treatment of femoral neck fracture. Up to now, considerableprogress has been made in the treatment of femoral neck fracture, but theguidelines for its management continue to be unsolved and are still evolving.We performed early intervention in femoral neck fracture with the method offree iliac bone graft combined with three cannulated screws fixation, andachieved satisfying clinical result. Based on the limited clinical experience, itwas demonstrated that this method had obvious advantages of simpleoperation, little truma, etc. Meanwhile, one stage bone grafting had reliablestability and improved the rate of fracture healing, helped to prevent theforward avascular necrosis of femoral head. Therefore, this method may be auseful option for the treatment of femoral neck fracture. However, the boneharvesting operation usually caused big surgical trauma and complications. Toavoid the disadvantages of the bone harvesting operation, we introduced anovel metal stent for early intervention in femoral neck fracture to replacebone graft. The novel metal stent is made of magnesium which can graduallydegrade in vivo and induce new bone formation around. In order to betterevaluate the degradation behavior and biomechanical properties of the novelmetal stent and to provide a theoretical basis for its further optimization andclinical application, the degradation behavior of MAO-Mg in simulated bodyfluid was studied systematically, the effects of magnesium ions on the mainbiological actions of osteoblast in vitro and the corresponding mechanismwere preliminarily discussed and disclosed. Additionally, the three-dimensional finite element model of the novel metal stent used for earlyintervention of femoral neck fracture was established to analyze the stresspatterns during a normal gait cycle, and compared with the method using bonegraft for early intervention of femoral neck fracture.Part1Study of degradation behavior and cytocompatibility for a novelMAO-treated magnesium materialObjective： The degradation behavior of MAO-Mg was studiedsystematically by immersion in simulated body fluid and electrochemical test,and the mechanical performance of MAO-Mg exposed in simulated body fluidwas investigated by tensile test. In addition, the cell toxicity of MAO-Mgduring the corrosion process was evaluated.Methods：Commercially available pure magnesium and MAO-coated Mgwere used in this study. All the samples were first weighed and recordedbefore the test. Immersion test was performed in a standard simulated bodyfluid (SBF) at a pH value of7.4and the temperature was maintained at37±0.5℃. The ratio of surface area to solution volume was1cm2:30ml. Sampleswere removed after6h,1,3,7,14and30days of immersion, rinsed withdistilled water and dried at room temperature. The surface morphology andcomposition of degradation products for the samples were analyzed byscanning electronic microscope (SEM), energy dispersive spectrometer (EDS)and X-ray diffraction analysis (XRD). The degradation rate, magnesium lossrate and the induced changes in pH were examined. Tension tests were carriedout with a universal testing machine to measure the tensile strength ofdifferent samples over time. In addition, electrochemical measurements wereperformed using a three electrode system, potentiodynamic polarization curvesand electrochemical impedance spectroscopy (EIS) of samples were measured,respectively. Lastly, Saos-2cells were cultured in Dulbecco’s modifiedEagle’s medium (DMEM), supplemented with15%fetal bovine serum (FBS).The effects on osteoblast proliferation and viability in the extraction mediumof the MAO-treated and untreated samples were measured by the MTT assay, and the osteoblastic apoptosis in different media was observed by fluorescentstaining. SAS software (version9.1) was used for the statistical analysis.Statistical significance was assigned to P＜0.05.Results：The corrosion rate of MAO-coated Mg in the SBF was muchlower than that of uncoated Mg, and MAO-coated Mg had lower influence onthe pH value variation of the SBF compared with uncoated Mg, the pH roserapidly from7.4to8.7and basically stabilized in the first72h, while the pHfor pure Mg reached9.1at last. The main components of the surface corrosionproducts on these metal samples were magnesium hydroxide [Mg(OH)2] andhydroxyapatite. Moreover, the MAO treatment slowed down the loss ofmechanical properties of the magnesium metal and maintained the strength ofthe sample at about90%after30days of immersion. The impedancespectroscopy and potentiodynamic polarization curves had also demonstratedthat the corrosion resistance ability of the MAO-coated Mg sample was higherthan that of uncoated magnesium. The cell toxicity experiments showed thattwo kinds of magnesium metal samples had not cytotoxic, and reached a levelof biosafety suitable for the cellular applications.Conclusion：The MAO coating could enhance the corrosion resistance oftreated Mg metal and help to keep its original mechanical properties. TheMAO-coated Mg material had good cytocompatibility, and the presence ofhydroxyapatite in the surface corrosion products could improve itsbiocompatibility and bone-binding capacity.Part2Research on the effects of magnesium ions on the biologicalbehaviors of osteoblast and the corresponding mechanismObjective：The effects of magnesium ions on the main biological actionsof osteoblast in vitro were were investigated and the correspondingmechanism were preliminarily discussed and disclosed.Methods：Saos-2cells were cultured in Dulbecco’s modified Eagle’smedium (DMEM), supplemented with15%fetal bovine serum (FBS). Theosteoblast was stimulated with MgSO4at a concentration of1,2,3mmol/L and then the effects of different treatment on osteoblast proliferation weremeasured by the MTT assay, and the osteoblastic apoptosis in different factorwas observed by fluorescent staining. The main biological characterizations ofosteoblast including ALP activation and BGP synthesis stimulated by differentfactors were surveyed with ELISA assay. Additionally, the expressions ofRunx2and Fas of the osteoblast under the stimulation of magnesium ions at1,2,3mmol/L were investigated by ELISA assay. SAS software (version9.1)was used for the statistical analysis on experimental data. Statisticalsignificance was assigned to P＜0.05.Results：The proliferation index (OD value) of the osteoblasts exposed tomagnesium ions at the concentrations of1,2and3mmol/L for24,48and72hours were all significantly higher than those of the blank control group(F=44.77，Р＜0.0001), and the OD value was positively associated with theconcentration of magnesium ions and the time of exposure. The result offluorescent staining showed the osteoblast had good activity under the actionof magnesium ions. In addition, the ALP activity and osteocalcin level ofosteoblasts treated with magnesium ions were stimulated in thedose-dependent and time-dependent manner. Moreover, the expression ofRunx2of the osteoblast under the stimulation of magnesium ions increasedsignificantly, and the expression quantity was positively with theconcentrations of magnesium ions. But the expression of Fas treated withmagnesium ions were significantly lower than that of normal control group(F=16.97，Р＜0.0001), and its expression quantity was inversely associatedwith the concentration of magnesium ions.Conclusion：The magnesium ions could induce the proliferation of theosteoblasts, and stimulate their ALP activity and osteocalcin synthesis, themechanism may be associated with the regulation of expressions of Runx2andFas. The magnesium ions could promote the differentiation and mature of theosteoblast by stimulating the expression of Runx2, on the other hand, itinhibited the osteoblast apoptosis by decreasing the expression of Fas. Part3The establishment of three-dimensional finite element model of thenovel metal stent used for early intervention of femoral neckfracture and the stress analysis during a normal gait cycleObjective：To establish the three-dimensional finite element model of thenovel metal stent used for early intervention of femoral neck fracture, toanalyze the stress patterns during a normal gait cycle, in order to help us tobetter evaluate the design and biomechanical properties of the novel metalstent and to direct its clinical application.Methods：A normal adult male volunteer was selected for the test. Hiship and proximal femurs were scanned by CT to obtain the two-dimensionalcomputer tomography image data, and reconstruct the three-dimensionalmodel of femur using Mimics10.01software. And then, the model wasimported to the Ansys13.0software and meshed to establish thethree-dimensional finite element model of the proximal femur, furthersimulated and established the three-dimensional finite element model of thenovel metal stent used for early intervention of femoral neck fracture. Next,the boundary and loading condition were set to perform the biomechanicalanalysis on these models. SAS software (version9.1) was used for thestatistical analysis on data. Statistical significance was assigned to P＜0.05.Results：Firstly, the precise and effective finite element models of theproximal femur and the novel metal stent were established in this study. Andthen the further analysis demonstrated that the model of femoral neck facturefixation had the similar trend for Von Mises stress to the model of normalfemur during a normal gait cycle, and the values of stress and displacementbetween the two models had no significant difference. It was also found thatthe magnitudes of the axial displacement in the model of femoral neck facturefixation increased with the hip joint load, and the maximal displacement waspresented in the second phase of the normal gait cycle.Conclusion：It was showed that the finite element models of the proximalfemur established in this study was objective and effective, and which laid the foundation for the further related studies. In addition, the method of the novelmetal stent implantation combined with three cannulated screws for earlyintervention of femoral neck fracture could not only recover the stressconduction of femoral neck but also better resist compression, torsion, andshear and increase the stability of fracture. Thus this study had a certainguiding significance to the clinical application of the novel metal stent andprovided a theoretical basis for its further optimization.Part4The comparative analysis of a novel metal stent against bone graftused for early intervention of femoral neck fracture. A study offinite element analysisObjective：The purpose of this study was to evaluate the stress patterns ofthe novel metal stent and bone graft used in the early intervention of femoralneck fracture, and to compare with the method of three cannulated screwsfixation commonly used in clinic. Moreover, the implantation angle of thenovel metal stent or bone graft was analyzed. The study contributed tocomprehensively understanding on biomechanical properties of the novelmethod and providing a theoretical guidance for its further clinical application.Methods：Finite element models of the novel metal stent and bone graftused in the early intervention of femoral neck fracture were created accordingto the same way mentioned in the third test. The implantation angle of thenovel metal stent or bone graft was set95°and127°, respectively. Moreover,the finite element models of three cannulated screws fixation for femoral neckfracture was also established. The boundary condition and two loads of426Nand2158N were set to perform the biomechanical analysis on these models.SAS software (version9.1) was used for the statistical analysis on data.Statistical significance was assigned to P＜0.05.Results：The finite element analysis demonstrated that the levels of stressand displacement of femoral neck had no significant difference among threekinds of models. In case of the novel metal stent or bone graft, the change ofthe implantation angle from95°to127°did not affect on the stress of femoral neck. And the stress and the axial displacement between the model ofthe novel metal stent and the model of bone graft had also no significantdifference.Conclusion：It was found that the novel metal stent could also be usedfor the early intervention of femoral neck fracture with good stability offracture compared to using bone graft. The implantation angle of the metalstent could be adjusted as necessary. Based on results of this study and its owninherent advantages, it was concluded that the novel metal stent could be areliable option for the early intervention of femoral neck fracture. |
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