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PubMed1990

Postoperative lumbar spinal instability occurring or progressing secondary to laminectomy.

The manifestations and pathomechanism of postoperative lumbar spinal instability, occurring or progressing secondary to laminectomy, was clarified by means of functional radiographic analysis in a series of 46 patients over 40 years of age. The relation between instability and the clinical symptoms also is discussed. In patients under 60 years of age, instability at the operated level tended to appear in cases of wide laminectomy more often than in cases of partial laminectomy. Occurrence or progress of instability seems to be promoted by resection of the posterior spinal elements rather than the disc. It is further considered that the postoperative aggravation of clinical symptoms may be influenced not only by instability, but also by the other factors.

Age Factors↗
PubMed2003

Intraspinal synovial cyst communicating with the C1-C2 facet joints and subarachnoid space associated with rheumatoid atlantoaxial instability.

STUDY DESIGN: The first case of intraspinal synovial cyst communicating with the C1-C2 facet joints and subarachnoid space associated with rheumatoid atlantoaxial instability is presented. OBJECTIVES: To describe the diagnosis and successful treatment of a synovial cyst of the C1-C2 junction in a patient with rheumatoid arthritis and atlantoaxial instability. SUMMARY OF BACKGROUND DATA: Intraspinal synovial cysts of the C1-C2 junction are extremely rare. Neither association with rheumatoid atlantoaxial instability nor communication with the C1-C2 facet joints and the subarachnoid space has been previously reported in a synovial cyst of the C1-C2 junction. MATERIALS AND METHODS: The clinical and radiologic features of a 71-year-old woman with a symptomatic synovial cyst of the C1-C2 junction with rheumatoid atlantoaxial instability are detailed. Posterior atlantoaxial fusion alone was performed. RESULTS: Preoperative magnetic resonance images demonstrated a large cystic mass around the dens, compressing the spinal cord. The mass was of low signal intensity on T1-weighted images, was of high signal intensity on T2-weighted images, and was enhanced marginally with gadolinium-DTPA on T1-weighted images. Computed tomograms performed after myelography disclosed the cyst around the dens communicating with the subarachnoid space and the C1-C2 facet joints. Spontaneous regression of the cyst was identified on the magnetic resonance images 3 months after surgery. One year after surgery, myelopathic symptoms were improved. CONCLUSION: An intraspinal cyst communicating with the C1-C2 facet joints and subarachnoid space associated with rheumatoid atlantoaxial instability was reported. Computed tomograms after myelography were useful in delineating the relationships among the synovial cyst, facet joints, and the subarachnoid space. Spontaneous regression of the cyst and clinical improvement were achieved by C1-C2 posterior fusion alone.

Aged↗
PubMed2004

Effect of postoperative lumbar sagittal alignment on the development of adjacent instability.

OBJECTIVE: This study investigated the effect of postoperative lumbar sagittal alignment on the development of adjacent segment instability. METHODS: The patients were retrospectively divided into two groups (hypolordotic and hyperlordotic) using the lumbar lordosis on lateral lumbosacral view. Diagnosis of adjacent segment instability depended on the dynamic lateral views of the lumbosacral spine. RESULTS: Thirteen (21.7%) cases developed adjacent segment instability during the 6-year follow-up. The two groups had no significant difference in rates of developing adjacent segment instability. CONCLUSION: Restoring the lordosis of the lumbar curve during one motion segment fusion does not prevent the development of adjacent instability.

Aged↗
PubMed2002

Upper tract urothelial carcinoma: a clinicopathologic study including microsatellite instability analysis.

Urothelial carcinoma of the renal pelvis and ureter may develop as a manifestation of the hereditary nonpolyposis colorectal cancer syndrome that is characterized by mutations in a number of DNA mismatch repair genes and detectable as microsatellite instability. In this study, we examined microsatellite instability and the clinicopathologic features of urothelial carcinoma of the renal pelvis (n = 61) and ureter (n = 53) from 114 consecutive patients surgically treated from 1985-1992. Clinical data were obtained through chart review. Matched normal and tumor DNA was extracted from paraffin-embedded tissue, and a panel of six microsatellite loci was analyzed. The male-female ratio was 2.8:1 with a median age of 70 years (range, 28 to 92 y). Microsatellite analysis was successful in 67 tumors, and 21 (31.3%) patients had tumors that exhibited microsatellite instability. Patients with microsatellite-unstable tumors were significantly more likely to have additional nonurologic cancers (P =.015) including colorectal carcinoma (P =.001) compared with patients with tumors that did not exhibit microsatellite instability. In addition, patients with microsatellite-unstable tumors showed more colorectal cancers in their family (P =.026) and were more likely to have higher grade urothelial carcinoma of the upper tract (P =.028). Grade and stage, but not microsatellite status, were the strongest predictors of cancer-specific survival. This study found the highest frequency of microsatellite instability in upper urothelial tract carcinomas reported to date and highlights upper tract urothelial carcinoma as a marker of the hereditary nonpolyposis colorectal cancer syndrome in some patients. These findings reinforce the importance of obtaining cancer histories in patients with upper tract urothelial carcinoma to subsequently identify individuals with the hereditary nonpolyposis colorectal cancer syndrome and at-risk relatives for surveillance and management programs.

Adult↗
PubMed2000

Mechanism for the faraday instability in viscous liquids

This report presents a discussion of the wave number selection mechanism in the Faraday instability, which arises when standing waves form on the free surface of a liquid subject to vertical sinusoidal oscillation. The focus is on the case where the viscous effects are strong, i.e., when the wavelength is of the same order of magnitude as the boundary layer thickness at the free surface. We investigate the relationship between the Faraday instability and the Rayleigh-Taylor instability by performing linear stability calculations. In the deep water limit (wavelength << liquid depth), our results indicate that the preferred wave number in the Faraday instability is primarily determined through a Rayleigh-Taylor instability. In the case of shallow water (wavelength approximately liquid depth), the agreement between the Rayleigh-Taylor and Faraday wave numbers does not appear to be as good, probably due to the interaction between the oscillatory motion of the standing waves and the bottom boundary.

Journal Article↗
PubMed2001

Capillary instabilities in thin nematic liquid crystalline fibers.

A complete identification and characterization of three distinct capillary instabilities in nematic liquid crystal fibers is presented. Linear stability analysis of capillary instabilities in thin nematic liquid crystalline cylindrical fibers is performed by formulating and solving the governing nematocapillary equations. A representative axial nematic orientation texture is studied. The surface disturbance is expressed in normal modes, which include the azimuthal wavenumber m to take into account nonaxisymmetric modes of the disturbance. Capillary instabilities in nematic fibers reflect the anisotropic nature of liquid crystals, such as the orientation contribution to the surface elasticity and surface bending stresses. Surface gradients of bending stresses provide additional anisotropic contributions to the capillary pressure that may renormalize the classical displacement and curvature forces that exist in any fluid fiber. The exact nature (stabilizing and destabilizing) and magnitude of the renormalization of the displacement and curvature forces depend on the nematic orientation and the anisotropic contribution to the surface energy, and accordingly capillary instabilities may be axisymmetric or nonaxisymmetric, with finite or unbounded wavelengths. Thus, the classical fiber-to-droplet transformation is one of several possible instability pathways while others include surface fibrillation.

Journal Article↗
PubMed2001

Seeding of the forward Raman instability by ionization fronts and Raman backscatter.

The self-modulated laser wakefield accelerator utilizes the forward Raman instability to drive a large-amplitude plasma wave. The effectiveness of this process could depend on how it competes with other processes such as self-focusing and cavitation, or other instabilities such as filamentation and hosing. The relative timing between the various processes is dependent on the nature of the seed for each instability. Both ionization fronts and Raman backscatter are capable of seeding the forward Raman instability. This causes the forward Raman instability to emerge much earlier than would otherwise be expected.

Journal Article↗
PubMed2001

Anisotropy effects of Hele-Shaw cells on viscous fingering instability in dilute polymer solutions.

We study the effect of anisotropy of the Hele-Shaw cell on the viscous fingering instability in dilute polymer solutions with shear thinning. In isotropic cells, the tip-splitting instability is observed at the same pressure gradient for a fixed polymer concentration, whereas in anisotropic cells the side oscillation instability occurs at higher pressure gradient than the tip-splitting instability. Narrowing of the finger width in the isotropic cell is well correlated with the tip-splitting instability, whereas the finger width in the anisotropic cell is almost independent of the sample. The modified Darcy's law, where the constant viscosity is replaced by the shear thinning viscosity, gives good agreement with the experiments, irrespective of the cell and the fluid.

Journal Article↗
PubMed2002

Unified formulation for inhomogeneity-driven instabilities in the lower-hybrid range.

A local dispersion relation that describes inhomogeneity-driven instabilities in the lower-hybrid range is derived following a procedure that correctly describes energy exchange between waves and particles in inhomogeneous media, correcting some inherent ambiguities associated with the standard formalism found in the literature. Numerical solutions of this improved dispersion relation show that it constitutes a unified formulation for the instabilities in the lower-hybrid range, describing the so-called modified two-stream instability, excited by the ion cross-field drift, including the ion Weibel instability, and also describing the lower-hybrid drift instability, which is due to inhomogeneity effects on the electron population.

Journal Article↗
PubMed2003

Parametric instabilities of circularly polarized Alfvén waves in a relativistic electron-positron plasma.

Dispersion relation and nonlinear evolution of the parametric instabilities of circularly polarized Alfvén waves in a relativistic electron-positron plasma are investigated by theoretical and numerical approaches. In the nonrelativistic limit, when 2omega(2)(p)>omega20, the characteristics of the instabilities are similar to those in an electron-ion plasma, except that the modulational instability takes place only if v(2)(s)<omega(2)(0)/k(2)(0), where omega(p) and v(s) denote the plasma frequency and the acoustic speed, and omega(0), k(0) indicate the frequency and the wave number of the parent wave. On the other hand, when 2omega(2)(p)<omega20, two new types of instabilities emerge between the parallel or antiparallel propagating Alfvén-like waves and the parallel propagating Langmuir-like wave. The weakly relativistic effect is discussed for all the instabilities. The one-dimensional full particle simulation and bicoherence analysis of the simulation result suggest that successive decay via the interaction between the parallel propagating Langmuir-like wave and antiparallel propagating Alfvén-like wave can efficiently generate a continuum of low frequency electromagnetic waves, which can interact with energetic particles.

Journal Article↗
PubMed2004

Long-wavelength thermocapillary instability with the Soret effect.

We study the onset of Marangoni instability of the quiescent equilibrium in a binary liquid layer with a nondeformable interface in the presence of the Soret effect. Linear stability analysis shows that both monotonic and oscillatory long-wavelength instabilities are possible depending on the value of the Soret number chi. Sets of long-wavelength nonlinear evolution equations are derived for both types of instability. Bifurcation analyses reveal that in the regime of monotonic instability square patterns bifurcate supercritically and they are preferred in competition with roll patterns. Hexagonal patterns bifurcate transcritically and the condition for the emergence of steady stable hexagonal patterns is derived. In the case of oscillatory instability, traveling and standing waves are found to bifurcate supercritically in the narrow range of the Soret parameter and traveling waves are found to become the selected type of flow.

Journal Article↗
PubMed2004

Absolute instabilities in a high-order-mode gyrotron traveling-wave amplifier.

The absolute instability is a subject of considerable physics interest as well as a major source of self-oscillations in the gyrotron traveling-wave amplifier (gyro-TWT). We present a theoretical study of the absolute instabilities in a TE01 mode, fundamental cyclotron harmonic gyro-TWT with distributed wall losses. In this high-order-mode circuit, absolute instabilities arise in a variety of ways, including overdrive of the operating mode, fundamental cyclotron harmonic interactions with lower-order modes, and second cyclotron harmonic interaction with a higher-order mode. The distributed losses, on the other hand, provide an effective means for their stabilization. The combined configuration thus allows a rich display of absolute instability behavior together with the demonstration of its control. We begin with a study of the field profiles of absolute instabilities, which exhibit a range of characteristics depending in large measure upon the sign and magnitude of the synchronous value of the propagation constant. These profiles in turn explain the sensitivity of oscillation thresholds to the beam and circuit parameters. A general recipe for oscillation stabilization has resulted from these studies and its significance to the current TE01 -mode, 94-GHz gyro-TWT experiment at UC Davis is discussed.

Journal Article↗
PubMed2005

Effect of inertia on the insoluble-surfactant instability of a shear flow.

We study, for the case of the two-layer plane Couette flow, the effects of inertia on the recently found instability due to insoluble surfactants. The insoluble-surfactant instability takes place even when inertia is absent provided an interface or a free surface under a nonzero shear is laden with an insoluble surfactant. Considering a normal mode of the streamwise wave number alpha , the perturbation theory we construct is good for any alpha provided the Reynolds number is correspondingly small. Inertia is responsible for some notable effects, including the appearance of new regions of instability and stability. For long--and only for long--waves, the following growth-rate additivity property for the inertia and interfacial instabilities holds: the growth rate corresponding to some nonzero values of the Marangoni number M and the Reynolds number Re is the sum of two contributions, one corresponding to the same value of M but zero Re, and the other corresponding to the same (nonzero) Re but zero M. This violation of the additivity property is in contrast to the case of a surfactantless Couette flow where this property holds for all wave numbers. Thus these results provide a counterexample to a conjecture that this additivity property is a universal principle. Among other results, when the thinner layer is the less viscous one, there is a nonzero critical Marangoni number Mc for the onset of instability; this (long-wave) threshold Mc grows from zero with the Reynolds number. Also, varying the ratio of viscosities through certain characteristic values leads to changes in the topology of marginal-stability curves.

Journal Article↗
PubMed2005

Comment on "linear instability of magnetic Taylor-Couette flow with Hall effect".

In the paper we comment on [Rudiger and Shalybkov (RS), Phys. Rev. E 69, 016303 (2004)], the instability of the Taylor-Couette flow interacting with a homogeneous background field subject to the Hall effect is studied. We correct a falsely generalizing interpretation of results presented there which could be taken to disprove the existence of the Hall-drift-induced magnetic instability described in Rheinhardt and Geppert, Phys. Rev. Lett. 88, 101103 (2002). It is shown that, in contrast to what is suggested by RS, no additional shear flow is necessary to enable such an instability with a nonpotential magnetic background field, whereas for a curl-free one it is. In the latter case, the instabilities found in RS in situations where neither a hydrodynamic nor a magnetorotational instability exists are demonstrated to be most likely magnetic instead of magnetohydrodynamic. Further, some minor inaccuracies are clarified.

Journal Article↗
PubMed2005

Rayleigh-Taylor instability in elastic solids.

We present an analytical model for the Rayleigh-Taylor instability that allows for an approximate but still very accurate and appealing description of the instability physics in the linear regime. The model is based on the second law of Newton and it has been developed with the aim of dealing with the instability of accelerated elastic solids. It yields the asymptotic instability growth rate but also describes the initial transient phase determined by the initial conditions. We have applied the model to solid/solid and solid/fluid interfaces with arbitrary Atwood numbers. The results are in excellent agreement with previous models that yield exact solutions but which are of more limited validity. Our model allows for including more complex physics. In particular, the present approach is expected to lead to a more general theory of the instability that would allow for describing the transition to the plastic regime.

Journal Article↗
PubMed2006

Upstream ionization instability associated with a current-free double layer.

A low frequency instability has been observed using various electrostatic probes in a low-pressure expanding helicon plasma. The instability is associated with the presence of a current-free double layer (DL). The frequency of the instability increases linearly with the potential drop of the DL, and simultaneous measurements show their coexistence. A theory for an upstream ionization instability has been developed, which shows that electrons accelerated through the DL increase the ionization upstream and are responsible for the observed instability. The theory is in good agreement with the experimental results.

Journal Article↗
PubMed1983

Lignocaine test and detrusor instability.

Detrusor instability in infravesical obstruction is well recognised and often resolves after surgery. The aetiology is not clearly understood but is probably due to increased vesical sensory input. Since intravesical lignocaine has been proved to have an effect (although shortlived) on detrusor instability in some patients, we have used it in order to differentiate between idiopathic detrusor instability and detrusor instability caused by infravesical obstruction. Our study showed that in patients with a positive lignocaine test pre-operatively, the detrusor instability and micturition disorder disappeared after operation, and in patients with a negative lignocaine test, it persisted. The differences were statistically significant (P less than 0.5).

Aged↗
PubMed1996

Ultrasound: a noninvasive screening test for detrusor instability.

OBJECTIVE: To determine whether transvaginal ultrasound measurement of bladder wall thickness can be used as a screening test for detrusor instability in women with urinary symptoms. DESIGN: A blinded prospective study. SETTING: A London teaching hospital. PARTICIPANTS: One hundred and eight-four symptomatic women presenting to a urodynamic clinic. MAIN OUTCOME MEASURE: The detection of detrusor instability by means of videocystourethrography (VCU) and ambulatory urodynamics in women with a mean bladder wall thickness of greater than 5 mm measured by transvaginal ultrasound. RESULTS: One hundred and eight women had a mean bladder wall thickness of greater than 5 mm. Ninety-four percent (102) of these women had detrusor instability either when undergoing VCU or ambulatory urodynamics. Seventeen women had a bladder wall thickness of less than 3.5 mm of whom three were found to have detrusor instability on VCU. CONCLUSION: The measurement of a mean bladder wall thickness greater than 5 mm with transvaginal ultrasound is a sensitive screening method for diagnosing detrusor instability in symptomatic women without outflow obstruction.

Adult↗