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Biomedical subjects

W K Kwok

Publications and source records attributed to W K Kwok.

17 recordsLinked to original sources
PubMed2006

Electrical stimulation and biofeedback exercise of pelvic floor muscle for children with faecal incontinence after surgery for anorectal malformation.

We report our experience of electrical stimulation and biofeedback exercise of pelvic floor muscle for children with faecal incontinence after surgery for anorectal malformation (ARM). Electrical stimulation and biofeedback exercise of pelvic floor muscle were performed on children with post-operative faecal soiling following repair of intermediate or high type ARM. Children under the age of 5 years or with learning difficulties were excluded. They had 6 months supervised programme in the Department of Physiotherapy followed by 6 months home based programme. Bowel management including toilet training, dietary advice, medications and enemas were started before the pelvic floor muscle exercise and continued throughout the programme. Soiling frequency rank, Rintala continence score, sphincter muscle electromyography (EMG) and anorectal manometry were assessed before and after the programme. Wilcoxon signed rank test was performed for statistical analysis. From March 2001 to May 2006, 17 children were referred to the programme. Twelve patients (M:F = 10:2; age = 5-17 years) completed the programme. There was a trend of improvement in Rintala score at sixth month (p = 0.206) and at the end of programme (p = 0.061). Faecal soiling was significantly improved at sixth month (p = 0.01) and at the end of the programme (p = 0.004). Mean sphincter muscle EMG before treatment was 1.699 microV. Mean EMG at sixth month and after the programme was 3.308 microV (p = 0.034) and 3.309 microV (p = 0.002) respectively. After the programme, there was a mean increase in anal sphincter squeeze pressure of 29.9 mmHg (p = 0.007). Electrical stimulation and biofeedback exercise of pelvic floor muscle is an effective adjunct for the treatment of faecal incontinence in children following surgery for anorectal malformation.

Adolescent↗
PubMed2006

Minimal access surgery for sigmoid volvulus in children.

A 12-year-old boy presented with large-bowel obstruction due to sigmoid volvulus. Temporary relief was achieved with rectal tube decompression. Elective laparoscopic-assisted sigmoid colectomy was performed. Post-operative recovery was uneventful. The patient remained well with no recurrence after 4 years of follow-up. Laparoscopic-assisted sigmoid colectomy may be the procedure of choice for selected children with sigmoid volvulus.

Child↗
PubMed2005

Laparoscopic mobilization of testicular vessels: an adjunctive step in orchidopexy for impalpable and redo undescended testis in children.

The testicular position after conventional inguinal orchidopexy for canalicular, "peeping" and redo undescended testes may not be satisfactory despite retroperitoneal dissection. Laparoscopy allows extensive mobilization of testicular vessels to gain additional length. We review our experience of using laparoscopic mobilization of testicular vessels (LMTV) in orchidopexy for these difficult undescended testes. From January 2003 to May 2004, LMTV was performed in 18 boys. The testicular vessels were mobilized from just proximal to the internal inguinal ring to the level of caecum and sigmoid colon respectively. Fifteen patients had clinically impalpable testes. Diagnostic laparoscopy revealed 13 "peeping" and 2 canalicular testes. LMTV was performed together with inguinal orchidopexy. There were three cases of redo orchidopexies because of unfavourable testicular position after previous surgery. LMTV was performed following inguinal dissection. The median follow-up period is 11.7 months. Sixteen testes are located at the base of scrotum, and two at mid-scrotum. The size is normal in 17 testes, whereas 1 testis is smaller than the contralateral one. LMTV is a safe and efficient adjunctive step in orchidopexy for impalpable and redo undescended testes.

Child↗
PubMed2005

Direct observation of geometrical phase transitions in mesoscopic superconductors by scanning tunneling microscopy.

Using scanning tunneling microscopy, we mapped the distribution of the local density of states in a single crystal superconductor heterostructure with an array of submicron normal metal islands. We observe the coexistence of strongly interacting multiquanta vortex lattice with interstitial Abrikosov vortices. The newly formed composite magnetic flux structure undergoes a series of phase transitions between different topological configuration states. The vortex configuration states are strongly dependent on the number of flux quanta and the nanoscale confinement architecture of the mesoscopic superconductor. Here, we present images of vortex phase transitions due to confinement effects when the number of magnetic flux quanta in the system changes. The vortex dynamics in these systems could serve as a model for behavior of confined many-body systems when the number of particles changes.

Journal Article↗
PubMed2005

Nanowires and nanoribbons of charge-density-wave conductor NbSe3.

We report synthesis of nanowires and nanoribbons of the charge-density-wave conductor NbSe(3) through direct reaction of Nb and Se powders. The transverse dimension of the obtained nanostructures, as identified with scanning/transmission electron microscopy, ranges from 20 to 700 nm. X-ray and selected area electron diffraction analyses indicate that these nanowires and nanoribbons are single crystalline. Four-probe resistivity measurements confirm the expected charge-density-wave transitions, and furthermore, we find significant enhancement in the depinning threshold fields, which we attribute to a confinement effect.

Crystallization↗
PubMed2003

Crystal-lattice coupling to the vortex-melting transition in YBa(2)Cu(3)O(7-delta).

Distinct discontinuities in the thermal expansion of the crystal lattice are observed at the melting transition of the vortex lattice in a naturally untwinned reversible YBa(2)Cu3O(7-delta) single crystal using high-resolution dilatometry. This coupling between the vortex transition and the crystal lattice demonstrates that the crystal lattice is more than a mere host for the vortices, and it is attributed to a strong pressure dependence of the superconducting transition temperature and thus to the condensation energy at the vortex-melting temperature.

Journal Article↗
PubMed2003

Periodic and disordered structures in a modulated gas-driven granular layer.

Experiments with a thin gas-fluidized granular layer revealed a sequence of well-defined transitions as the amplitude and frequency of the gas flow modulation are varied. The observed patterns include subharmonic squares and stripes, quasiperiodic and disordered structures. The wavelength of subharmonic patterns increases with the mean flow rate and decreases with the modulation frequency.

Journal Article↗
PubMed2002

Two-band superconductivity in MgB2.

The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at Delta(1)=2.3 meV and Delta(2)=7.1 meV at 4.2 K. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multiband system. Temperature evolution of the tunneling spectra follows the BCS scenario [Phys. Rev. Lett. 3, 552 (1959)]] with both gaps vanishing at the bulk T(c). The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [Phys. Rev. Lett. 87, 087005 (2001)]].

Journal Article↗
PubMed2001

An unusual phase transition to a second liquid vortex phase in the superconductor YBa2Cu3O7.

A magnetic field penetrates a superconductor through an array of 'vortices', each of which carries one quantum of flux that is surrounded by a circulating supercurrent. In this vortex state, the resistivity is determined by the dynamical properties of the vortex 'matter'. For the high-temperature copper oxide superconductors (see ref.1 for a theoretical review), the vortex phase can be a 'solid', in which the vortices are pinned, but the solid can 'melt' into a 'liquid' phase, in which their mobility gives rise to a finite resistance. (This melting phenomenon is also believed to occur in conventional superconductors, but in an experimentally inaccessible part of the phase diagram.) For the case of YBa2Cu3O7, there are indications of the existence of a critical point, at which the character of the melting changes. But neither the thermodynamic nature of the melting, nor the phase diagram in the vicinity of the critical point, has been well established. Here we report measurements of specific heat and magnetization that determine the phase diagram in this material to 26 T, well above the critical point. Our results reveal the presence of a reversible second-order transition above the critical point. An unusual feature of this transition-namely, that the high-temperature phase is the less symmetric in the sense of the Landau theory-is in accord with theoretical predictions of a transition to a second vortex-liquid phase.

Journal Article↗
PubMed2001

Scanning tunneling spectroscopy in MgB2.

We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39 K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter gamma. The value of the superconducting gap is 5 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair breaking as seen in the conductance spectra in fields up to 6 T.

Journal Article↗
PubMed2000

Experimental evidence for the vortex glass phase in untwinned, proton irradiated YBa2Cu3O7-delta.

We report on magnetoresistance measurements of the effects of 9 MeV proton irradiation on a clean, untwinned single crystal of YBa2Cu3O7-delta. For the first time, evidence for a vortex glass transition is detected in an untwinned single crystal of YBa2Cu3O7-delta with induced pointlike disorder, in which the first order vortex melting transition is completely suppressed after proton irradiation. Our results suggest that a sufficiently high pinning disorder is required in order for the vortex glass phase to be observed.

Journal Article↗
PubMed1999

Testicular volume of boys after inguinal herniotomy: combined clinical and radiological follow-up.

A total of 173 boys aged 10 to 179 months with previous unilateral inguinal herniotomy were called back for follow-up. Clinical and ultrasound examinations of the scrotum were performed. The interval between operation and follow-up was 6 to 123 months (mean 31.68 months). One boy (0.58%) had a more than 50% and 10 (5.8%) had a more than 25% decrease in testicular volume on the operated side when compared with the non-operated side.

Adolescent↗
PubMed1999

A male newborn with colonic atresia and total colonic aganglionosis.

A case of colonic atresia (CA) and aganglionosis is presented, which is probably the tenth in the English literature. The boy presented on day 3 of life with delayed passage of meconium, milk intolerance, and progressive abdominal distension. A barium enema study was complicated by barium peritonitis. Emergency peritoneal lavage was done and the intestinal obstruction was relieved by a proximal defunctioning ileostomy; type III CA (Grosfeld classification) was identified. The proximal atretic end was the blind-ending caecum and the distal atretic end commenced in the splenic area as a microcolon extending to the pelvis and replacing the normal colon. Total colonic aganglionosis (TCA) was confirmed by biopsies. This is the first case in the literature of TCA with CA documented before any attempts at colonic resection and anastomosis were made. This association highlights the recommendation for biopsy of the micro-colon and rectum when Type III CA (Grosfeld classification) is encountered in a newborn. The infant underwent a cholecystectomy and Duhamel-Martin operation at 14 months that was complicated by abdominal wound dehiscence; the ileostomy was closed at 22 months of age.

Colectomy↗
PubMed1983

Effects of perilymph viscosity on low-frequency intracochlear pressures and the cochlear input impedance of the cat.

Cochlear model calculations are shown to be in reasonable agreement with recent low-frequency measurements of intracochlear pressures and the cochlear input impedance of the cat [V. Nedzelnitsky, J. Acoust. Soc. Am. 68, 1676-1689 (1980); T. J. Lynch, III, V. Nedzelnitsky, and W. T. Peake, J. Acoust. Soc. Am. 72, 108-130 (1982)]. Included in the cochlear model are perilymph viscosity, the measured variation of the area of the scala vestibuli with distance from the stapes [P. Dallos, J. Acoust. Soc. Am. 48, 489-499 (1970)], and finite impedance of the round window membrane. The WKB approximation and its extension to the low-frequency region is used in order to exhibit explicitly the dependence of the model results on the cochlear parameters.

Acoustic Impedance Tests↗