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PubMed1992

Freeze-substitution of chemically stabilized samples for biological field emission scanning electron microscopy.

The high resolution imaging capabilities of modern field emission scanning electron microscopes require adequately improved tissue preparation procedures to prevent the collapse of macromolecular structures and the extraction of molecules. A routine cryo-stabilization technique is described which utilizes chemical crosslinking and cryo-dehydration for mechanical and chemical stabilization of protein and lipid structures and increase of electrical conductivity of the sample. Thiocarbohydrazide (TCH) serves as a general mordant for osmium tetroxide crosslinking. However, extensive washing after all impregnation steps is necessary to dissolve unspecific osmium black precipitations at the sample surface. Collagen I aggregates showed increased stability against collapse after TCH osmification alone, whereas pulmonary surfactant liposomes require additional freeze-substitution in methanol and Freon 113 for stabilization during critical point drying. Environmental scanning electron microscopy (at water vapor pressures of 5-10 torr within the specimen chamber) was used to control, in the wet phase, the stabilization procedure at the level of chemical crosslinkage. It could be confirmed that tannic acid, often used to stabilize lipids, leads to artificial rearrangement of bilayered liposomes into compact presumable multilayered bodies, whereas the TCH osmification preserved liposome structures and their aggregates. The increase of electrical conductivity of sliced tissue was demonstrated on kidney. Support technologies for the cryo-stabilization procedures are described in detail, as well as simple routines for first stabilization trials with new samples. On pulmonary tissue, the excellent preservation of alveolar shape and fine structures of intermediate forms of surfactant are described.

Animals↗
PubMed1996

Perturbations of the denatured state ensemble: modeling their effects on protein stability and folding kinetics.

By considering the denatured state of a protein as an ensemble of conformations with varying numbers of sequence-specific interactions, the effects on stability, folding kinetics, and aggregation of perturbing these interactions can be predicted from changes in the molecular partition function. From general considerations, the following conclusions are drawn: (1) A perturbation that enhances a native interaction in denatured state conformations always increases the stability of the native state. (2) A perturbation that promotes a non-native interaction in the denatured state always decreases the stability of the native state. (3) A change in the denatured state ensemble can alter the kinetics of aggregation and folding. (4) The loss (or increase) in stability accompanying two mutations, each of which lowers (or raises) the free energy of the denatured state, will be less than the sum of the effects of the single mutations, except in cases where both mutations affect the same set of partially folded conformations. By modeling the denatured state as the ensemble of all non-native conformations of hydrophobic-polar (HP) chains configured on a square lattice, it can be shown that the stabilization obtained from enhancement of native interactions derives in large measure from the avoidance of non-native interactions in the D state. In addition, the kinetic effects of fixing single native contacts in the denatured state or imposing linear gradients in the HH contact probabilities are found, for some sequences, to significantly enhance the efficiency of folding by a simple hydrophobic zippering algorithm. Again, the dominant mechanism appears to be avoidance of non-native interactions. These results suggest stabilization of native interactions and imposition of gradients in the stability of local structure are two plausible mechanisms involving the denatured state that could play a role in the evolution of protein folding and stability.

Kinetics↗
PubMed2001

Coordination topology and stability for the native and binding conformers of chymotrypsin inhibitor 2.

We demonstrate that the stabilization of the binding region is accomplished at the expense of a loss in the stability of the rest of the protein. A novel molecular mechanics (MM) approach is introduced to distinguish residue stabilities of proteins in a given conformation. As an example, the relative stabilities of folded chymotrypsin inhibitor 2 (CI2) in unbound form, and CI2 in complex with subtilisin novo is investigated. The conformation of the molecule in the two states is almost identical, with an approximately 0.6-A root-mean-square deviation (RMSD) of the Calpha atoms. On binding, the packing density changes only at the binding loop. However, residue fluctuations in the rest of the protein are greatly altered solely due to those contacts, indicating the effective propagation of perturbation and the presence of remotely controlling residues. To quantify the interplay between packing density, packing order, residue fluctuations, and residue stability, we adopt an MM approach whereby small displacements are inserted at selected residues, followed by energy minimization; the displacement of each residue in response to such perturbations are organized in a perturbation-response matrix L. We define residue stability lambda(i) = summation operator((j)L(ij))/ summation operator((j) L(ji)) as the ratio of the amount of change to which the residue is amenable, to the ability of a given residue to induce change. We then define the free energy associated with residue stability, DeltaG(lambda) = -RT ln lambda. DeltaG(lambda) intrinsically selects the residues that are in the folding core. Upon complexation, the binding loop becomes more resistant to perturbation, in contrast to the alpha-helix that favors change. Although the two forms of CI2 are structurally similar, residue fluctuations differ vastly, and the stability of many residues is altered upon binding. The decrease in entropy introduced by binding is thus compensated by these changes.

Biomechanical Phenomena↗
PubMed2004

Locating the stabilizing residues in (alpha/beta)8 barrel proteins based on hydrophobicity, long-range interactions, and sequence conservation.

In nature, 1 out of every 10 proteins has an (alpha/beta)(8) (TIM)-barrel fold, and in most cases, pairwise comparisons show no sequence similarity between them. Hence, delineating the key residues that induce very different sequences to share a common fold is important for understanding the folding and stability of TIM-barrel domains. In this work, we propose a new consensus approach for locating these stabilizing residues based on long-range interactions, hydrophobicity, and conservation of amino acid residues. We have identified 957 stabilizing residues in 63 proteins from a nonredundant set of 71 TIM-barrel domains. Most of these residues are located in the 8-stranded beta-sheet, with nearly one half of them oriented toward the interior of the barrel and the other half oriented toward the surrounding alpha-helices. Several stabilizing residues are found in the N- and C-terminal loops, whereas very few appear in the alpha-helices that surround the internal beta-sheet. Further, these 957 residues are placed in 434 stabilizing segments of various sizes, and each domain contains 1-10 of these segments. We found that 8 segments per domain is the most abundant one, and two thirds of the proteins have 7-9 stabilizing segments. Finally, we verified the identified residues with experimental temperature factors and found that these residues are among the ones with less mobility in the considered proteins. We suggest that our new protocol serves as a powerful tool to identify the stabilizing residues in TIM-barrel domains, which can be used as potential candidates for studying protein folding and stability by means of protein engineering experiments.

Conserved Sequence↗
PubMed2026

Finlay-Wilkinson random regression for yield and yield stability prediction in cereals.

Year-to-year climate variability poses a challenge for agriculture by increasing crop yield variability; therefore, there is a need to identify genotypes that can withstand these fluctuations. With the right selection criteria, genotypes with yield stability across variable environmental conditions can be selected. Methods such as Finlay-Wilkinson random regression (FWRR) may allow us to use sparse datasets-common in plant breeding pipelines-and incorporate genomic data to leverage phenotypic information from related genotypes to predict yield stability. Our objective was to examine how the number of environments and the variance among those environments affect stability predictions. We also integrate FWRR as a genomic prediction tool for characterizing yield stability, comparing it to the traditional genomic prediction models as a reference. We used three datasets: one highly unbalanced dataset for oats (Avena sativa L.) and two completely balanced datasets with different numbers of environments for barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.). We fit standard Finlay-Wilkinson (FW) and FWRR models to estimate grain yield and stability under various scenarios. We found that the estimated stability values obtained were similar using balanced datasets for FW or FWRR. FWRR also achieved moderate predictive ability for stability using unbalanced datasets under 10-fold cross-validation (CV1) with new genotypes. In terms of environmental representation, selecting the right set of environments for inclusion in the model was more important than adding more environments. Our results suggest the possibility of using FWRR to select stable genotypes earlier in line development, as well as to design resource-efficient stability-testing schemes.

Hordeum↗
PubMed1997

Effects of Asphaltene Aggregation in Model Heptane-Toluene Mixtures on Stability of Water-in-Oil Emulsions

As part of an ongoing investigation into the stability of water-in-crude oil emulsions, model oils have been utilized to further probe the effects of crude solvency as well as specific resin-asphaltene interactions on emulsion stability. These model oils were constructed by dissolving varying amounts of resins and/or asphaltenes in a mixture of heptane and toluene. The resins and asphaltenes used in this study were isolated from four different crude types-Arab Berri (AB), Arab Heavy (AH), Alaska North Slope (ANS), and San Joaquin Valley (SJV)-and characterized in a previous study using heptane precipitation of the asphaltenes followed by an extrographic separation of the resins from silica gel. Asphaltenes dissolved in heptol at concentrations of just 0.5% were shown to generate emulsions which were even more stable than those generated from their respective whole crude oils. Some types of resins (e.g., from AH and SJV) also demonstrated an ability to stabilize emulsions although these resin-stabilized emulsions were considerably less stable than those prepared with asphaltenes. The primary factors governing the stability of these model emulsions were the aromaticity of the crude medium (as controlled by the heptane:toluene ratio), the concentration of asphaltenes, and the availability of solvating resins in the oil (i.e., the resin/asphaltene or R/A ratio). The model emulsions were the most stable when the crude medium was 30-40% toluene and in many cases at small R/A ratios (i.e., R/A </=1). This strongly supports the theory that asphaltenes are the most effective in stabilizing emulsions when they are near the point of incipient precipitation. The types of resins and asphaltenes used to construct these model oils also played a role in determining the resultant emulsion stability which indicates the importance of specific resin-asphaltene interactions. The interfacially active components that stabilized these model systems were the most polar and/or condensed portions of the resin and asphaltene fractions as determined by elemental and neutron activation analyses. All of these results point to the significance of the solubility state of the asphaltenes in determining the emulsifying potential of these crude oils. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1998

Polymer Latex Stability Modification by Exposure to Hydrophobic Solvents.

The stability of latex particles toward coagulation in the presence of salt is modified by swelling the latex with toluene and chloroform vapors. Short-term stability was determined by turbidimetric titrations, and the long-term stability was evaluated by adding latex and salt solutions, allowing the mixture to age for 24 or 48 h and determining the characteristics of the supernatant and of the sediment. Nine different latexes were examined, with variable results: in some cases, both apolar solvents stabilize the latex; in other cases, increased stability is induced by only one of the solvents, either toluene or chloroform. There is also coherence, but not a strict correlation, between the solvent effects on short- and long-term stability. For instance, in the case of a core-and-shell styrene-butyl methacrylate latex, chloroform has a small stabilizing effect in the titration experiment, but it prevents the formation of a coagulated latex sediment even 48 h after mixing latex and salt. Two hypotheses are discussed to account for these observations: (i) swelling solvents decrease the particles ability to dissipate the collision kinetic energy, so that particles collide but without joining each other; (ii) the solvents induce the release of trapped charged groups from the particle interior to the interface, enhancing the usual (electrostatic, steric, hydration) stability factors. Copyright 1998 Academic Press.

Journal Article↗
PubMed1998

Anomalous Colloidal Stability of Latex-Protein Systems.

An experimental study on the colloidal stability of latex-F(ab')2 and latex-IgG systems is described. The stability domains were obtained using a low-angle scattering technique to measure the rates of aggregate formation and plotting the stability ratio as a function of electrolyte concentration. The protein-coated particles present an anomalous stability at high ionic strength when the classical theory predicts aggregation. This observed deviation from DLVO behavior appears for electrolyte concentrations above some critical bulk concentration called critical stabilization concentration (csc). As we have suggested in previous publications, the existence of an additional short-range repulsive "hydration force" can explain this anomalous stability. In order to resolve more fully if this anomalous stability at high ionic strength is due to the hydration forces, the effects of pH, cation and anion type, temperature, and polyethylene glycol (a dehydrating polymer) on the experimental stability are investigated. Finally, the immunoreactivity of an anti-CRP F(ab')2-latex conjugate is studied in reaction buffers with high ionic strength. Copyright 1998 Academic Press.

Journal Article↗
PubMed1997

Stabilization centers in proteins: identification, characterization and predictions.

Methods are presented to locate residues, stabilization center elements, which are expected to stabilize protein structures by preventing their decay with their cooperative long range interactions. Artificial neural network-based algorithms were developed to predict these residues from the primary structure of single proteins and from the amino acid sequences of homologous proteins. The prediction accuracy using only single sequence information is 65%, but the incorporation of evolutionary information in the form of multiple alignments and conservation scores raises the efficiency by 3%. The composition, relative accessibility, number and type of interactions, conservation and the X-ray thermal factor of the identified stabilization center residues are different, not only from the whole data set but from the rest of the long range interacting residues as well. The most frequent stabilization center residues are usually found at buried positions and have a hydrophobic or aromatic side-chain, but some polar or charged residues also play an important role in the stabilization. The stabilization centers show significant difference in the composition and in the type of linked secondary structural elements compared with the rest of the residues. The performed structural and sequential conservation analysis showed the higher conservation of stabilization centers over protein families. The relation of the proposed stabilization centers to folding nuclei is also discussed.

Algorithms↗
PubMed1998

Electrostatic contributions to the stability of halophilic proteins.

Examination of the first crystal structures of proteins from a halophilic organism suggests that an abundance of acidic residues distributed over the protein surface is a key determinant of adaptation to high-salt conditions. Although one extant theory suggests that acidic residues are favored because of their superior water-binding capacity, it is clear that extensive repulsive electrostatic interactions will also be present in such proteins at physiological pH. To investigate the magnitude and importance of such electrostatic interactions, we conducted a theoretical analysis of their contributions to the salt and pH-dependence of stability of two halophilic proteins. Our approach centers on use of the Poisson-Boltzmann equation of classical electrostatics, applied at an atomic level of detail to crystal structures of the proteins. We first show that in using the method, it is important to account for the fact that the dielectric constant of water decreases at high salt concentrations, in order to reproduce experimental changes in pKa values of small acids and bases. We then conduct a comparison of salt and pH effects on the stability of 2Fe-2S ferredoxins from the halophile Haloarcula marismortui and the non-halophile anabaena. In both proteins, substantial upward shifts in pKa accompany protein folding, though shifts are considerably larger, on average, in the halophile. Upward shifts for basic residues occur because of favorable salt-bridge interactions, whilst upward shifts for acidic residues result from unfavorable electrostatic interactions with other acidic groups. Our calculations suggest that at pH 7 the stability of the halophilic protein is decreased by 18.2 kcal/mol on lowering the salt concentration from 5 M to 100 mM, a result that is in line with the fact that halophilic proteins generally unfold at low salt concentrations. For comparison, the non-halophilic ferredoxin is calculated to be destabilized by only 5.1 kcal/mol over the same range. Analysis of the pH stability curve suggests that lowering the pH should increase the intrinsic stability of the halophilic protein at low salt concentrations, although in practice this is not observed because of aggregation effects. We report the results of a similar analysis carried out on the tetrameric malate dehydrogenase from H. marismortui. In this case, we investigated the salt and pH dependence of the various monomer-monomer interactions present in the tetramer. All monomer-monomer interactions are found to make substantial contributions to the salt-dependence of stability of the tetramer. Excellent agreement is obtained between our calculated results for the stability of the tetramer and experimental results. In particular, the finding that at 4 M NaCl, the tetramer is stable only between pH 4.8 and 10 is accurately reproduced. Taken together, our results suggest that repulsive electrostatic interactions between acidic residues are a major factor in the destabilization of halophilic proteins in low-salt conditions, and that these interactions remain destabilizing even at high salt concentrations. As a consequence, the role of acidic residues in halophilic proteins may be more to prevent aggregation than to make a positive contribution to intrinsic protein stability.

Archaeal Proteins↗
PubMed2000

Effects of side-chain characteristics on stability and oligomerization state of a de novo-designed model coiled-coil: 20 amino acid substitutions in position "d".

We describe the de novo design and biophysical characterization of a model coiled-coil protein in which we have systematically substituted 20 different amino acid residues in the central "d" position. The model protein consists of two identical 38 residue polypeptide chains covalently linked at their N termini via a disulfide bridge. The hydrophobic core contained Val and Ile residues at positions "a" and Leu residues at positions "d". This core allowed for the formation of both two-stranded and three-stranded coiled-coils in benign buffer, depending on the substitution at position "d". The structure of each analog was analyzed by CD spectroscopy and their relative stability determined by chemical denaturation using GdnHCI (all analogs denatured from the two-stranded state). The oligomeric state(s) was determined by high-performance size-exclusion chromatography and sedimentation equilibrium analysis in benign medium. Our results showed a thermodynamic stability order (in order of decreasing stability) of: Leu, Met, Ile, Tyr, Phe, Val, Gln, Ala, Trp, Asn, His, Thr, Lys, Ser, Asp, Glu, Arg, Orn, and Gly. The Pro analog prevented coiled-coil formation. The overall stability range was 7.4 kcal/mol from the lowest to the highest analog, indicating the importance of the hydrophobic core and the dramatic effect a single substitution in the core can have upon the stability of the protein fold. In general, the side-chain contribution to the level of stability correlated with side-chain hydrophobicity. Molecular modelling studies, however, showed that packing effects could explain deviations from a direct correlation. In regards to oligomerization state, eight analogs demonstrated the ability to populate exclusively one oligomerization state in benign buffer (0.1 M KCl, 0.05 M K(2)PO(4)(pH 7)). Ile and Val (the beta-branched residues) induced the three-stranded oligomerization state, whereas Tyr, Lys, Arg, Orn, Glu and Asp induced the two-stranded state. Asn, Gln, Ser, Ala, Gly, Phe, Leu, Met and Trp analogs were indiscriminate and populated two-stranded and three-stranded states. Comparison of these results with similar substitutions in position "a" highlights the positional effects of individual residues in defining the stability and numbers of polypeptide chains occurring in a coiled-coil structure. Overall, these results in conjunction with other work now generate a relative thermodynamic stability scale for 19 naturally occurring amino acid residues in either an "a" or "d" position of a two-stranded coiled-coil. Thus, these results will aid in the de novo design of new coiled-coil structures, a better understanding of their structure/function relationships and the design of algorithms to predict the presence of coiled-coils within native protein sequences.

Algorithms↗
PubMed2000

Structural stability and domain organization of colicin E1.

Thermodynamic properties, stability, and structure of the toxin-like molecule colicin E1 were analyzed by differential scanning calorimetry and circular dichroism to determine the number of structurally independent domains, and the interdomain interactions necessary for colicin import into the Escherichia coli cell. Analysis of denaturation profiles of the 522 residue colicin E1, together with fragments of 342 and 178 residues that contain subsets of the domains, showed three stable cooperative blocks that differ in thermal stability and correspond to three major functional domains of the colicin: (i) the COOH-terminal channel-forming (C) domain with the highest thermal stability; (ii) the BtuB receptor binding (R) domain; and (iii) the N-terminal translocation (T) domain that has the smallest stabilization enthalpy and thermal stability. Interdomain interactions were described in which T-R interactions stabilize R, and T-C and R-C interactions stabilize R and T, but destabilize C. The R and T domains behaved in a similar way as a function of pH and ionic strength. Interacting extended helices of the R domain, possibly a coiled-coil, were implied by: (i) the very high (>90%) alpha-helical content of the R domain, (ii) cooperative decreases in alpha-helical content near the T(tr) of thermal denaturation of the R domain; (iii) a large denaturation enthalpy, implying extensive H-bond and van der Waals interactions. The R domain was inferred, from the extended network of interacting helices, large DeltaH, and steep temperature dependence of its stabilization energy to have a dominant role in determining the conformation of other domains. It is proposed that cellular import starts with the R domain binding to the BtuB receptor, followed by unfolding of the R domain coiled-coil and thereby of the T domain, which then interacts with the TolC receptor-translocator.

Amino Acid Sequence↗
PubMed1982

Stabilization of human posture during induced oscillations of the body.

Displacements of the center of foot pressure, the hip and the head were recorded in six subjects standing on a platform, sinusoidally tilting in pitch (anterior-posterior). Stimulus frequencies ranged between 0.01 and 1 Hz. Stimulus amplitudes were 2, 4 and 6 degrees. With eyes open the displacements were minimal at 0.3 Hz. With eyes closed, however, induced sway was maximal at this frequency. The apparent lack of visual stabilization at the lowest frequency (0.01 Hz) might be attributed to a subthreshold velocity of the retinal image motion induced by the swaying body. A similar absence of visual stabilization of 1 Hz is assumed to indicate the limit of the working range of visual stabilization of posture. Independent of stimulus amplitude a phase lead of about 90 degrees was found at 0.01 Hz. This decreased with increasing frequency up to a phase lag of 100 degrees at the highest frequency (1 Hz). Head stabilization was generally more effective than hip stabilization. EMG recordings from the leg muscles suggest that with eyes closed the center of force is mainly stabilized by leg muscle activity, while with eyes open this stabilization is best, when vision allows for stabilization of body posture by intersegmental movements between head, trunk and legs.

Biomechanical Phenomena↗
PubMed1995

Extracellular ascorbate stabilization as a result of transplasma electron transfer in Saccharomyces cerevisiae.

The presence of yeast cells in the incubation medium prevents the oxidation of ascrobate catalyzed by copper ions. Ethanol increases ascorbate retention. Pyrazole, an alcohol dehydrogenase inhibitor, prevents ascorbate stabilization by cells. Chelation of copper ions does not account for stabilization, since oxidation rates with broken or boiled cells or conditioned media are similar to control rates in the absence of cells. Protoplast integrity is needed to reach optimal values of stabilization. Chloroquine, a known inhibitor of plasma membrane redox systems, inhibits the ascorbate stabilization, the inhibition being partially reversed by coenzyme Q6. Chloroquine does not inhibit ferricyanide reduction. Growth of yeast in iron-deficient media to increase ferric ion reductase activity also increases the stabilization. In conclusion, extracellular ascorbate stabilization by yeast cells can reflect a coenzyme Q dependent transplasmalemma electron transfer which uses NADH as electron donor. Iron deficiency increases the ascorbate stabilization but the transmembrane ferricyanide reduction system can act independently of ascorbate stabilization.

Ascorbic Acid↗
PubMed2002

[Stabilizing effect and sintering tendency of 3 different cages and bone cement for fusion of cervical vertebrae segments].

Important requirement for spinal fusion devices for segment are that they provide sufficient stability and guarantee a low subsidence risk. An important requirement for spinal fusion devices for segments are that they provide sufficient stability and guarantee a low subsidence risk. Therefore, in the following in vitro study, the stabilizing effect and subsidence tendency of cervical fusion cages and bone cement were investigated during cyclic loading. The WING cages (Medinorm AG) and BAK cages (Spinetec) made of titanium, the carbon fiber reinforced PEEK cage from Acromed (DePuy Acromed), and bone cement (PMMA, Sulzer) were tested. Twenty-four human cervical spine specimens were first tested intact with a standardized flexibility test (+/- 2.5 Nm). Then the implants were inserted and the primary stability determined. For the simulation of the postoperative loading of the cervical spine a cyclic loading protocol with 700 loading cycles was performed. In this test pure moments +/- 2.0 Nm in 9 different loading directions in randomized order were applied together with a 50 N preload to simulate the weight of the head. The subsidence and "long term stability" was measured after 50, 100, 200, 300, 500, and 700 cycles. All implants had a stabilizing effect in all directions most obviously in lateral bending. Here the range of motion was between 20.9% (AcroMed Cage), and 62% (BAK Cage) with respect to the intact specimen (100%). In laterial bending, flexion, and axial rotation the AcroMed cage stabilized the most followed by the bone cement, WING and BAK Cage. In extension the specimens treated with bone cement were the most stable. After 700 loading cycles the specimens with the BAK cage lost 1.6 mm in height, with the WING Cage 0.8 mm, with the Acromed 0.7 mm, and with the bone cement 0.5 mm. Two Acromed Cages dislocated during the long term testing. Cages have the potential to stabilize as effectively as bone cement. A smaller contact area, however, causes a higher subsidence risk compared to bone cement but increases the fusion area, thus increasing the chance of obtaining bony fusion.

Biomechanical Phenomena↗
PubMed2006

A direct comparison of local dynamic stability during unperturbed standing and walking.

Standing and walking are very different tasks. It might be reasonable, therefore, to assume that the mechanisms used to maintain the stability of standing and walking should be quite different. However, many studies have shown that postural stability measures can generally predict risk of falls, even though most falls occur during locomotor tasks and not during postural tasks. This suggests that there is at least some commonality among the mechanisms governing the control of both standing and walking. The present study was conducted to determine whether the postural stability either is or is not directly related to locomotor stability. Twenty healthy adults, age 18-73 years, walked on a motorized treadmill at their preferred walking speed for three trials of 5 min. They also stood on a force plate for three trials of 5 min. Both tasks were performed without imposing any additional external perturbations. The motion of each subject's trunk segment was recorded and described using a multi-dimensional state space defined in the same manner for both tasks. Local dynamic stability was quantified from the mean divergence over time of locally perturbed trajectories in state space, which was parameterized as a double exponential process. Divergence parameters were compared to determine the relationship between local dynamic stability during standing and walking. Standing and walking exhibited local dynamic stability properties that were significantly different (P<0.001) and not correlated (P>0.1). Divergence parameters were also compared to traditional center of pressure (COP) measures obtained from standing trials. COP measures were significantly correlated to local divergence parameters for standing, but not to those for walking. This study provides direct evidence that the mechanisms governing standing and walking stability are significantly different.

Adolescent↗
PubMed2004

Effect of constrained posterior screw and rod systems for primary stability: biomechanical in vitro comparison of various instrumentations in a single-level corpectomy model.

Cervical corpectomy is a frequently used technique for a wide variety of spinal disorders. The most commonly used approach is anterior, either with or without plating. The results for single-level corpectomy are better than in multilevel procedures. Nevertheless, hardware- or graft-related complications are observed. In the past, constrained implant systems were developed and showed encouraging stability, especially for posterior screw and rod systems in the lumbar spine. In the cervical spine, few reports about the primary stability of constrained systems exist. Therefore, in the present study we evaluated the primary stability of posterior screw and rod systems, constrained and non-constrained, in comparison with anterior plating and circumferential instrumentations in a non-destructive set-up, by loading six human cadaver cervical spines with pure moments in a spine tester. Range of motion and neutral zone were measured for lateral bending, flexion/extension and axial rotation. The testing sequence consisted of: (1) stable testing; (2) testing after destabilization and cage insertion; (3a) additional non-constrained screw and rod system with lateral mass screws, (3b) with pedicle screws instead of lateral mass screws; (4a) constrained screw and rod system with lateral mass screws, (4b) with pedicle screws instead of lateral mass screws; (5) 360 degrees set-up; (6) anterior plate. The stability of the anterior plate was comparable to that of the non-constrained system, except for lateral bending. The primary stability of the non-constrained system could be enhanced by the use of pedicle screws, in contrast to the constrained system, for which a higher primary stability was still found in axial rotation and flexion/extension. For the constrained system, the achievable higher stability could obviate the need to use pedicle screws in low instabilities. Another benefit could be fewer hardware-related complications, higher fusion rate, larger range of instabilities to be treated by one implant system, less restrictive postoperative treatment and possibly better clinical outcome. From a biomechanical standpoint, in regard to primary stability the constrained systems, therefore, seem to be beneficial. Whether this leads to differences in clinical outcome has to be evaluated in clinical trials.

Aged↗
PubMed1999

Lumbar spine stability can be augmented with an abdominal belt and/or increased intra-abdominal pressure.

The increased intra-abdominal pressure (IAP) commonly observed when the spine is loaded during physical activities is hypothesized to increase lumbar spine stability. The mechanical stability of the lumbar spine is an important consideration in low back injury prevention and rehabilitation strategies. This study examined the effects of raised IAP and an abdominal belt on lumbar spine stability. Two hypotheses were tested: (1) An increase in IAP leads to increased lumbar spine stability, (2) Wearing an abdominal belt increases spine stability. Ten volunteers were placed in a semi-seated position in a jig that restricted hip motion leaving the upper torso free to move in any direction. The determination of lumbar spine stability was accomplished by measuring the instantaneous trunk stiffness in response to a sudden load release. The quick release method was applied in isometric trunk flexion, extension, and lateral bending. Activity of 12 major trunk muscles was monitored with electromyography and the IAP was measured with an intra-gastric pressure transducer. A two-factor repeated measures design was used (P < 0.05), in which the spine stability was evaluated under combinations of the following two factors: belt or no belt and three levels of IAP (0, 40, and 80% of maximum). The belt and raised IAP increased trunk stiffness in all directions, but the results in extension lacked statistical significance. In flexion, trunk stiffness increased by 21% and 42% due to 40% and 80% IAP levels respectively; in lateral bending, trunk stiffness increased by 16% and 30%. The belt added between 9% and 57% to the trunk stiffness depending on the IAP level and the direction of exertion. In all three directions, the EMG activity of all 12 trunk muscles increased significantly due to the elevated IAP. The belt had no effect on the activity of any of the muscles with the exception of the thoracic erector spinae in extension and the lumbar erector spinae in flexion, whose activities decreased. The results indicate that both wearing an abdominal belt and raised IAP can each independently, or in combination, increase lumbar spine stability. However, the benefits of the belt must be interpreted with caution in the context of the decreased activation of a few trunk extensor muscles.

Abdomen↗