Biomechanics of high-grade spondylolisthesis with and without reduction.

The clinical advantages of reducing spondylolisthesis over fusion in situ have several intuitive reasons such as restore the spinal column into a more anatomic relationship and alignment. However, there is only little evidence in the literature supporting the theoretical advantages of reduction, and its effect on spinopelvic alignment remains poorly defined. In this study, a comprehensive finite element model was developed to analyze the biomechanics of the spine after spinal fusion at L5-S1 in both types of high-grade spondylolisthesis (balanced and unbalanced pelvis). The relevant clinical indices (i.e. spondylolisthesis grade and Dubousset lumbosacral angle), the displacement of L4-L5, pressure within the annulus and nucleus, and stress at L4-L5 were evaluated and compared. The model can well predict the changes of the important clinical indices during the surgery. For a balanced pelvis, the reduction has a minimal effect on the biomechanical conditions at the adjacent level during postsurgical activities. In the unbalanced case, reduction induced larger deformation in the lumbosacral region and a higher stress concentration at adjacent level. Whether such a stress concentration can lead to long-term disc degeneration is not known. The results provide additional information for the clinician considering reduction of high-grade spondylolisthesis.

The Benefits of Cement Augmentation of Pedicle Screw Fixation Are Increased in Osteoporotic Bone: A Finite Element Analysis.

STUDY DESIGN: Biomechanical study using a finite element model of a normal and osteoporotic lumbar vertebrae comparing resistance with axial pullout and bending forces on polymethylmethacrylate-augmented and non-augmented pedicle screws. OBJECTIVE: To compare the effect of cement augmentation of pedicle screw fixation in normal and osteoporotic bone with 2 different techniques of cement delivery. SUMMARY OF BACKGROUND DATA: Various clinical and biomechanical studies have addressed the benefits of cement augmentation of pedicle screws, but none have evaluated whether this effect is similar, magnified, or attenuated in osteoporotic bone compared with normal bone. In addition, no study has compared the biomechanical strength of augmented pedicle screws using cement delivery through the pedicle screw with delivery through a pilot hole. METHODS: This study was funded by a grant from DePuy Synthes Spine. Normal and osteoporotic lumbar vertebrae with pedicle screws were simulated. The models were tested for screw pullout strength with and without cement augmentation. Two methods of cement delivery were also tested. Both methods were tested using 1 and 2.5 cm3 volume of cement infiltrated in normal and osteoporotic bone. RESULTS: The increase in screw pullout force was proportionally greater in osteoporotic bone with equivalent volumes of cement delivered. The researchers found that 1 and 2.5 cm3 of cement infiltrated bone volume resulted in an increase in pullout force by about 50% and 120% in normal bone, and by about 64% and 156% in osteoporotic bone, respectively. The delivery method had only a minimal effect on pullout force when 2.5 cm3 of cement was injected (<4% difference). CONCLUSIONS: Cement augmentation increases the fixation strength of pedicle screws, and this effect is proportionately greater in osteoporotic bone. Cement delivery through fenestrated screws and delivery through a pilot hole result in comparable pullout strength at higher cement volumes.

The Use of Finite Element Models to Assist Understanding and Treatment For Scoliosis: A Review Paper.

INTRODUCTION: Scoliosis is a complex spinal deformity whose etiology is still unknown, and its treatment presents many challenges. Finite element modeling (FEM) is one of the analytical techniques that has been used to elucidate the mechanism of scoliosis and the effects of various treatments. METHODS: A literature review on the application of FEM in scoliosis evaluation and treatment has been undertaken. A literature search was performed in each of three major electronic databases (Google Scholar, Web of Science, and Ovid) using the key words "scoliosis" and "finite element methods/model". Articles using FEM and having a potential impact on clinical practice were included. RESULTS: A total of 132 abstracts were retrieved. The query returned 105 articles in which the abstracts appeared to correspond to this review's focus, and 85 papers were retained. The current state of the art of FEM related to the biomechanical analysis of scoliosis is discussed in 4 sections: the etiology of adolescent idiopathic scoliosis, brace treatment, instrumentation treatment, and sensitivity studies of FEM. The limitations of FEM and suggested future work are also discussed.

Effect of posterolateral disc replacement on kinematics and stress distribution in the lumbar spine: a finite element study.

The current study aimed to compare the biomechanics of the L3-S1 spine segment treated either by fusion or total disc replacement (TDR) using the TRIUMPH(®) Lumbar Disc (Globus Medical, Audubon, PA). A validated three-dimensional, nonlinear finite element model (FEM) of L3-S1 was altered at L4/L5 by fusion and implantation of the TRIUMPH(®) Lumbar Disc. Under a hybrid testing protocol, the resultant range of motion (ROM), nucleus pressure at the adjacent levels, facet joint force, and anterior longitudinal ligament (ALL) force were analyzed. FEM predicted several changes in biomechanics when compared to the intact segment. The analyses suggest that posterolateral lumbar disc arthroplasty with the TRIUMPH(®) Lumbar Disc can preserve the mobility of the surgical level while not allowing excessive ROM and reducing segmental motion at the adjacent levels when compared to fusion. The current finite element model could be valuable for engineers and surgeons seeking to optimize TDR designs.

ß-Peptide coatings by surface-initiated polymerization.

Homopolymers of ß-lactams can be grown by surface-initiated polymerization. These surface-linked ß-peptides are living polymers with the potential to be utilized as tunable, protease-resistant interfaces in multiphase structural composites where the characteristics of the interface influence bulk properties.

The use of a transition rod may prevent proximal junctional kyphosis in the thoracic spine after scoliosis surgery: a finite element analysis.

STUDY DESIGN: Finite element analysis. OBJECTIVE: Via finite element analysis: (1) to demonstrate the abnormal forces present at the top of a scoliosis construct, (2) to demonstrate the importance of an intact interspinous and supraspinous ligament (ISL/SSL) complex, and (3) to evaluate a transition rod (a rod that has a short taper to a smaller diameter at one end) as an implant solution to diminish these pathomechanics, regardless of the integrity of the ISL/SSL complex. SUMMARY OF BACKGROUND DATA: The pathophysiology of increased nucleus pressure and increased angular displacement may contribute to proximal junctional kyphosis. Furthermore, high implant stress can be demonstrated at the upper end of the construct, possibly leading to the risk of implant failure. METHODS: A finite element model was constructed to simulate a thoracic spinal fusion. The model was altered to remove the ISL/SSL complex at the level above the construct. Then, the model was altered again by extending the construct one level superior with a transition rod. The angular displacement, the maximum pressure in the nucleus, and stress within the implant were extracted from computational results under 2 conditions: load control and displacement control. The testing was performed with both titanium and stainless steel implants. RESULTS: Pressure in the nucleus and angular displacement are all increased when the ISL/SSL complex is removed immediately above the instrumented levels, whereas the screw pullout force and maximum stress within the screw are decreased. The nucleus pressure increases by more than 50%. The angular displacement increases by 19% to 26%. This absence of the ISL/SSL complex simulates the clinical scenario that occurs when these structures are iatrogenically detached. Abnormal mechanics can be restored to normal level by extending the construct rostral one level with a transition rod. Furthermore, the elevated nucleus pressure and angular displacement noted even when the ISL/SSL complex is intact can be avoided with the use of a transition rod. Under the same bending moment (3 Nm), the nucleus pressure at the level immediately cephalad is up to 23% lower than the pressure in a standard construct. The angular displacement is 18% to 19% less than the standard construct. The maximum implant stress is also decreased by as much as 60%. CONCLUSION: Finite element modeling suggests that the pathomechanics at the proximal end of a scoliosis construct may be diminished by preserving the ISL/SSL complex and possibly completely eliminated with the use of rods with a diameter transition at the most proximal level.

Thermal pretreatment of silica composite filler materials.

Three different silica filler materials were thermally treated in order to effect dehydration, dehydroxylation, and rehydroxylation. Samples were characterized by thermogravimetry (TG), pycnometry, elemental analysis, and scanning electron microscopy (SEM). For all fillers, our results indicate incremental removal of silanol groups at higher heating temperatures and irreversible dehydroxylation at over 673 K. To remove the organic content and maintain adequate silanol density for subsequent silanization on Stöber-type silica, we suggest heating at 673 K followed by overnight boiling in water.

Mechanical properties of experimental dental composites containing a combination of mesoporous and nonporous spherical silica as fillers.

OBJECTIVES: Mesoporous fillers have been investigated for use in dental composites because of their potential for creating micromechanical filler/resin matrix interphase bonding. Such a micromechanical bonding could eliminate the need for the silane treatment of fillers for interfacial chemical bonding that is prone to hydrolysis in the oral environment. In the case of micromechanical bonding, dental polymer chains are threaded mechanically (like a "necklace") through nanosized channels in the fillers. METHODS: A combination of mesoporous silica, which was synthesized using the non-surfactant templating method, and nonporous spherical silica (500nm) was used to prepare experimental dental composites. The porous silica used in this study contained interconnected pores and channels as opposed to porous fillers containing surface pores. The compressive strength, compressive modulus, flexural modulus, and flexural strength of these composites were evaluated. RESULTS: The results showed that composites containing a combination of mesoporous and nonporous fillers have better mechanical properties than the composites having either of these fillers alone. SIGNIFICANCE: The results showed that a combination of mesoporous and nonporous materials can be used to prepare stronger dental materials that may resist hydrolysis and wear.

Light curable dental composites designed with colloidal crystal reinforcement.

OBJECTIVES: Methods to prepare dental composites with a periodic filler arrangement were developed following a strategy of colloidal crystallization. The aims of this study were to determine the influence of suspension medium, silane treatment and amine additive on colloidal particle redispersion and subsequent ordering, and to evaluate the effect of filler ordering on mechanical properties of composites. METHODS: Dry monodisperse silica particles (spherical, approximately 500-nm diameter) were redispersed in selected solvents and monomers (e.g. triethyleneglycol dimethacrylate, TEGDMA) to form sediments or dispersions with ordered particle arrangements. Ordering was evaluated by microscopy and mechanical properties of the composites were measured using compression tests (n=6). RESULTS: A face-centered cubic packed structure could form in both the sediment from silica dispersions in polar solvents and stable dispersions in TEGDMA. Dimethylaminoethyl methacrylate (DMAEMA) was found to disrupt an ordered structure when non-silanized silica particles were used. Silanization with 3-methacryloxypropyl trimethoxysilane (MPS) promoted filler ordering. Standard compression tests on composites containing 60wt% silica in TEGDMA with or without DMAEMA indicated that DMAEMA had a clearly significant effect (p<0.05) on failure strain, compressive strength, and toughness, and a marginally significant effect on modulus (p=0.12). SIGNIFICANCE: Significant increases in compressive strength (16%), failure strain (71%), and toughness (135%) were observed for composites with ordered filler compared to non-ordered composites.

Finite element analysis of the effect of an interphase on toughening of a particle reinforced polymer composite.

A numerical method was used to study the interaction between a crack and the filler phase in a particle-reinforced polymer composite. The simulation was achieved by implementing a progressive damage-and-failure material model and element-removal technique through finite element analysis, providing a framework for the quantitative prediction of the deformation and fracture response of the composite. The effect of an interphase on composite toughness was also studied. Results show that a thin and high strength interphase results in efficient stress transfer between particle and matrix and causes the crack to deflect and propagate within the matrix. Alternatively, a thick and low strength interphase results in crack propagation within the interphase layer, and crack blunting. Further analysis of the effect of volume fraction and particle-particle interactions on fracture toughness as well as prediction of the fracture toughness can also be achieved within this framework.

Oxidation of titanium, RGD peptide attachment, and matrix mineralization rat bone marrow stromal cells.

The aim of this study was to compare the efficacy of attachment of arginine-glycine-aspartic acid (RGD) peptide to titanium surfaces oxidized by different methods. Titanium surfaces were treated as follows: (1) treatment A: passivation in nitric acid, (2) treatment B: heated in air at 400 degrees C for 1 hour, (3) treatment C: immersed in 8.8 M H2O2/0.1 M HCl at 80 degrees C for 30 minutes, and (4) treatment D: treated as in treatment C and then heated at 400 degrees C for 1 hour. RGD was attached to titanium samples treated as in treatments A through D. The quantity of attached RGD was determined by an enzyme-linked immunoabsorbent assay. Mineralization of a rat bone marrow stromal cell (RMSC) culture on the titanium surfaces after 21 days was determined y atomic absorption spectroscopy. The treatments were ranked according to quantity of RGD attached as C, A, B, and D. Twenty-one days after RMSC culture, the degree of mineralization was significantly higher for treatment C than for treatments A, B, and D and for controls. The efficacy of RGD attachment varies with the oxidation treatment given to titanium. Oxidation in H2O2/0.1 M HCl at 80 degrees C provided the best overall surface for RGD attachment as well as calcified matrix formation of RMSCs.

Silane treatment effects on glass/resin interfacial shear strengths.

OBJECTIVE: Methacrylic resin-based dental composites normally use a bifunctional silane coupling agent with an intermediary carbon connecting segment to provide the interfacial phase that holds together the organic polymer matrix with the reinforcing inorganic phase. In this study, fiber pull-out tests were used to measure the interfacial bond strength at the fiber-matrix interface. METHODS: Glass fibers (approximately 30 microm diameter, 8 x 10 (-2)m length, MoSci) were silanated using various concentrations (1, 5 and 10%) of either 3-methacryloxypropyl-trimethoxysilane (MPS) or glycidoxypropyltrimethoxy-silane (GPS) in acetone (99.8%). Rubber (poly(butadiene/acrylonitrile), amine terminated, M(w) 5500) molecules were also attached to the fiber surface via GPS molecules. The resin was comprised of a 60/40 mixture of Bis-phenol-A bis-(2-hydroxypropyl)-methacrylate (BisGMA) and tri (ethylene glycol) dimethacrylate (TEGDMA). A bead of resin approximately 2-4 x 10(-3)m in embedded length was placed on the treated fibers and light cured. The load required to pull the fiber out of the resin was converted to shear bond strength. RESULTS: Interfacial shear strengths were greater for silanated specimens compared with unsilanated, and for MPS compared with GPS. The same set of samples soaked in 50:50 (v/v) mixtures of ethanol and distilled water for a period of 1 month showed a decrease in properties. SIGNIFICANCE: A positive correlation was found between the amount of silane on the filler surface and the property loss after soaking. Rubber treatment provided improvement in interfacial strength. 5% MPS samples had the highest strength both in soaked as well as unsoaked samples.

Miniaturized nebulization catheters: a new approach for delivery of defined aerosol doses to the rat lung.

A nebulization catheter technique (AeroProbe) was adapted and evaluated as a new approach for pulmonary delivery of defined aerosol doses to the rat lung. The lung distribution profile was evaluated by dosing Evans blue and Nile blue dye, respectively, to isolated and perfused rat lungs (IPL) and to the lungs of anesthetized and tracheal-intubated rats. The intratracheal aerosol dosing was synchronized with the inspiration of the lungs. Immediately after dosing, the lungs were dissected into upper- and lower trachea, bronchi, and parenchyma. The dye was then extracted from the tissue samples to determine the regional distribution and the recovery of the aerosol dose in the lungs. The droplet-size distribution and the weight of the delivered aerosol dose were analyzed with laser diffraction and gravimetric analysis respectively. The recovery of the delivered dose was high, 99 +/- 12 and 105 +/- 1%, respectively, in the in vivo administrations and IPL-experiments. The lung distribution profile after aerosol dosing to anesthetized rats was mainly tracheobronchial. Only 12 +/- 4% of the dose was recovered in the lung parenchyma. However, after aerosol dosing to the IPL, 38 +/- 11% of the dose was distributed to the lung parenchyma. At the settings used, the nebulization catheter aerosolized 1-2 microL of liquid per puff using 1-1.5 mL of air. The droplet-size distribution of the generated aerosols was broad (2-8% <3 microm; 10% <4-7 microm; 50% <10-15 microm; and 90% <20-40 microm). The nebulization catheter technique provides a complement to existing methodology for pulmonary drug delivery in small animals. With this new technique, defined aerosol doses can be delivered into the lungs of rats with no need for aerosol dosimetry.

The role of adsorbed endotoxin in particle-induced stimulation of cytokine release.

Numerous in vitro models have demonstrated the capacity of wear particles to stimulate the release of soluble pro-inflammatory products with the ability to induce local bone resorption. Recent observations have demonstrated that binding of lipopolysaccharide (LPS) to particulate wear debris can significantly modulate the pattern of cell response in the in vitro models. These findings raise concerns over the possible role of LPS in the pathogenesis of aseptic loosening after total joint replacements, and also indicates the importance of controlling for possible confounding effects of LPS contamination in the in vitro models used to study the reactive nature of wear debris. Our studies were undertaken to rigorously analyze the effects of particle-associated LPS on cell responses and to assess the efficacy of different treatment protocols to inactivate LPS associated with different particulate materials. Particles of cobalt-chrome alloy, titanium-6-aluminum-4-vanadium, titanium nitride and silica were pretreated with LPS and exposed to multiple treatment protocols. When cells were treated with "as-received" particles prepared by washing in ethanol, small amounts of TNF-alpha, IL-1beta. and IL-1alpha were detected. In contrast, all particle species pretreated with LPS produced marked increases in TNF-alpha, IL-1alpha, and IL-1beta release, as well as upregulation of corresponding mRNA levels even after ethanol washing. Boiling the LPS-pretreated particles in 1% acetic acid or autoclaving and baking the particles also markedly reduced and in some instances abolished the effect of the LPS-pretreatment. This indicates that LPS binds to the surface of particles of diverse composition and that the bound LPS is biologically active. Treatment protocols to inactivate particle-associated LPS demonstrated significant differences in efficacy. When the most rigorous treatments were utilized, essentially all LPS activity could be eliminated. Particles treated with these methods retained some capacity to stimulate cytokine release, but activities were markedly reduced. These results provide further evidence indicating that LPS contamination of particulate materials can markedly enhance their biological activity. This potential confounding effect needs to be carefully monitored and controlled in the in vitro model systems used to evaluate wear particles. Furthermore, the presence of particle-associated endotoxin at the bone-implant interface in vivo could markedly enhance the adverse biological activity of particulate wear debris.

The role of surface functional groups in calcium phosphate nucleation on titanium foil: a self-assembled monolayer technique.

Surface functional groups play important roles in nucleating calcium phosphate deposition on surgical titanium implants. In this study, various functional groups were introduced onto the surface of commercially pure titanium foils using a self-assembled monolayer (SAM) technique. An organic silane, 7-oct-1-enyltrichlorosilane (OETS) was used and -OH, -PO4H2, -COOH groups were derived from its unsaturated double bond. Ti foils were first oxidized in concentrated H2SO4/H2O2. ESCA and contact angle measurements were used to characterize the SAM surfaces and confirm the presence of various functional groups. A fast calcium phosphate deposition experiment was carried out by mixing Ca2+- and (PO4)(3-)-containing solutions in the presence of the surface-modified Ti samples at pH 7.4 at room temperature in order to verify the nucleating abilities of these functional groups. SEM, Raman spectroscopy, XRD and ATR-FTIR results showed that poorly crystallized hydroxyapatite (HA) can be deposited on the SAM surfaces with -PO4H2 and -COOH functional groups, but not onto the SAM with -CH=CH2 and -OH. -PO4H2 exhibited a stronger nucleating ability than that of -COOH. The oxidized Ti sample also showed some calcium phosphate deposition but to a lesser extent as compared to SAM surfaces with -PO4H2 and -COOH. The pre-deposited HA can rapidly induce biomimetic apatite layer formation after immersion in 1.5 SBF for 18 h regardless of the amount of pre-deposited HA. The results suggested that the pre-deposition of HA onto these functionalized SAM surfaces might be an effective and fast way to prepare biomimetic apatite coatings on surgical implants.

Strength and fatigue of polyacid-modified restorative materials (compomers).

The purpose of this study was to compare the fatigue behavior of a hybrid composite, four compomers, and two viscous glass ionomers after short- and long-term soaking in distilled water. Bars with dimensions of 30 mm x 2 mm x 2 mm were formed in stainless steel molds, finished with 600 grit SiC, then soaked for either 24 h or one year, and tested in 3-point flexure at stressing rates between 0.001 and 2500 MPa s(-1). Data were plotted as fracture stress vs. stressing rate, and the exponent N in the power law for crack growth rate was computed from the slope of these plots. All compomer and resin composite materials tested exhibited subcritical crack growth (i.e. a reduction in strength with a decrease in stressing rate). Soaking lowered the slope for all materials, that is, increased the crack propagation rate. F2000 exhibited an increase in fracture strength while Hytac and Compoglass exhibited a decrease following long-term soaking in distilled water. Crack-growth exponents obtained from these fatigue data were used to estimate the stresses which would result in a five-year lifetime for these materials.