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Biomedical subjects

Peter Kroll

Publications and source records attributed to Peter Kroll.

32 records · Page 2Linked to original sources
PubMed2003

Hafnium nitride with thorium phosphide structure: physical properties and an assessment of the Hf-N, Zr-N, and Ti-N phase diagrams at high pressures and temperatures.

The physical properties of the new cubic phase of Hf3N4 as well as of isomorphic Zr3N4 and Ti3N4 are studied using first-principles calculations. Hf3N4, Zr3N4, and Ti3N4 are semiconductors with band gaps of 1.8, 1.1, and 0.6 eV, respectively. The band structure is characterized by the simultaneous presence of steep and extremely flat bands. The calculated shear modulus G indicates that the cubic Hf3N4 will be harder than the mononitride HfN. At ambient conditions, the cubic modifications of M3N4 (M=Hf, Zr, Ti) are metastable with respect to orthorhombic M3N4 phases, but the orthorhombic phases of Hf3N4 and Zr3N4 are stable with respect to the mononitrides and nitrogen.

Journal Article↗
PubMed2002

Synthesis, structure determination, and quantum-chemical characterization of an alternate HgNCN polymorph.

A novel, possibly metastable form of HgNCN, designated HgNCN(II), is accessible under soft-chemical reaction conditions, and its existence has been established from combined X-ray/neutron Rietveld refinements (P2(1)/a, a = 6.8521(4) A, b = 6.9797(4) A, c = 5.5516(4) A, beta = 113.212(4) degrees ), vibrational spectroscopy investigations, and plane-wave DFT calculations utilizing pseudopotentials. In contrast to the known mercury carbodiimide HgNCN(I) with two practically identical N-C double bonds of 1.22 A, the true cyanamide HgNCN(II) is characterized by a short ("triple") N-C bond (1.12 A) and a long ("single") C-N bond (1.35 A); two Hg atoms coordinate the asymmetrically shaped NCN(2)(-) unit only on the side of the C-N "single" bond. Mercury carbodiimide HgNCN(I) and mercury cyanamide HgNCN(II) are thermochemically separated from each other by such a high energy barrier that thermal decomposition prevents structural interconversion. The structure-chemical relationship of the two HgNCN phases is discussed in terms of bond-stretch and linkage isomerism.

Journal Article↗
PubMed2002

A density functional study of phosphorus nitride P3N5: refined geometries, properties, and relative stability of alpha-P3N5 and gamma-P3N5 and a further possible high-pressure phase delta-P3N5 with kyanite-type structure.

The crystal structures and the enthalpy-pressure phase diagram of P(3)N(5) were investigated by using density functional methods. Applying both approximations to the electron exchange and correlation gives a consistent picture for the two known polymorphs, alpha-P(3)N(5) and gamma-P(3)N(5). The calculated zone-center phonon modes compare very well with the experimental results. They indicate low-frequency bending modes for two-coordinate N atoms of alpha-P(3)N(5), which are responsible for a C2/c --> C c structural modulation of alpha-P(3)N(5) at moderate pressures. alpha-P(3)N(5) transforms into gamma-P(3)N(5) at pressures of about 6 GPa. We propose gamma-P(3)N(5) transforms into a delta-P(3)N(5) with Kyanite-type structure at pressures exceeding 43 GPa. Upon quenching, this triclinic modification of P(3)N(5) is likely to distort into a more symmetric monoclinic structure.

Journal Article↗
PubMed2002

Effect of pars plana vitrectomy for proliferative diabetic vitreoretinopathy on preexisting diabetic maculopathy.

BACKGROUND: Vitrectomy is the treatment of choice for proliferative diabetic vitreoretinopathy with tractions and persistent vitreous hemorrhage. Since vitrectomy has recently been discussed as a possible therapy for diabetic maculopathy as well, we were especially interested in studying the change in diabetic maculopathy following surgery. For that purpose a grading system developed at our clinic was used. METHODS: In a retrospective study we evaluated fundus photographs and fluorescein angiograms of 33 eyes of 30 patients who had undergone vitrectomy for proliferative diabetic vitreoretinopathy at our clinic between 1990 and 1997. In all eyes diabetic maculopathy was present at the time of surgery. The grading was performed using preoperative images and images taken a median of 18 months postoperatively. RESULTS: Best corrected visual acuity increased by 3.7 lines on average. Intraretinal dot and spot hemorrhages, hard exudates, microaneurysms on fundus photos, and leakage and cysts on fluorescein angiograms decreased after surgery. The extent of the foveolar avascular zone and the extent of the perifoveolar ischemic area worsened, however. CONCLUSION: Vitrectomy seems to help diabetic eyes not only by removal of membranes, tractions, and vitreous hemorrhage; it seems to have a positive influence on the course of diabetic maculopathy as well. We suspect that the removal of the posterior vitreous membrane is one of the crucial factors in interrupting the disease process. From these results the indication for vitrectomy in diabetic patients may have to be reconsidered and extended to include diabetic maculopathy prior to the development of ischemia.

Adult↗
PubMed2002

[When is congenital hypertrophy of the retinal pigment epithelium (CHRPE) associated with the Gardner's syndrome? An overview with clinical examples].

Congenital hypertrophy of the retinal pigment epithelium (CHRPE) can be associated with Gardner's syndrome (GS). During childhood multiple adenomatose polyps develop in the colon ascendens (familiar-autosomal Polyposis coli [FAP]), and always become malignant until the age of 35. The tumour-suppressive FAP-gen was identified at the long arm of chromosome 5 (5q21). Ophthalmic funduscopy is very important. Patients with more than 3 CHRPE in one eye or a bilateral CHRPE, as well as patients with a positive family history and one unilateral solid CHRPE require further gastroenterological evaluation.

Adenomatous Polyposis Coli Protein↗
PubMed2002

[Epidemiology of diabetes-induced blindness -- a review].

BACKGROUND: In the declaration of St. Vincent of 1989 the World Health Organization (WHO) and the International Diabetes Federation (IDF) aimed at a reduction of diabetes-induced blindness in Europe by one third within five years. Despite that, proliferative diabetic vitreoretinopathy (PDVR) and diabetic macular edema (DME) continue to be the leading causes for diabetes-induced legally defined blindness in working-age adults in western industrialized nations. MATERIAL AND METHODS: An analysis of literature of recent years is performed to give an overview over prevalence and incidence of diabetic blindness in Western industrialised countries. Data from literature is compared to new data collected in the German State of Hessen. RESULTS: Both our own data and data from literature show a constant to slightly decreasing prevalence and incidence of diabetes-related blindness. CONCLUSIONS: The aims of the St. Vincent declaration of 1989 have not been attained so far. Diabetes-related ocular diseases continue to be treated only after onset of symptoms. Effective screening programs for prevention, which would entail tremendous saving of Public Health expenditure, are an exception.

Adult↗
PubMedDate not supplied

[The predictive value of a classification for proliferative diabetic vitreoretinopathy].

BACKGROUND: Kroll's classification of proliferative diabetic vitreoretinopathy (PDVR) defines stage A (vitreoretinal proliferations without retinal detachment), stage B (partially detached retina not involving the macula), stage C (partially detached macula) and stage D (completely detached macula). The purpose of this study was to analyse the prognostic value of Kroll's classification in respect of the postoperative results of vitreoretinal surgery based on our group of patients. PATIENTS AND METHODS: The charts of 563 patients who underwent vitrectomy because of PDVR between 1990 and 1997 were examined retrospectively. Postoperative visual acuity, possible risk factors, frequencies of silicone oil tamponade and revitrectomies were related to the preoperative staging of PDVR. The influence of possible predictive factors on the postoperative visual outcome were evaluated using multivariate logistic regression analysis. RESULTS: After vitreoretinal surgery mean postoperative visual acuity was significantly better in stage A compared to stage C (p < 0.01) or D (p < 0.0001). In 179 out of 563 eyes (31.7 %) revitrectomy (including silicone oil removal) was required and in 51 eyes (9.1 %) more than one revitrectomy was performed. Silicone oil tamponade was used in 22 out of 253 eyes (8.7 %) classified as stage A, in 27 out of 201 eyes (13.4 %) of stage B, in 17 out of 78 eyes (21.8 %) of stage C and in 10 of 31 eyes (32.3 %) of stage D. Postoperative increase of visual acuity of more than 3 lines was significantly less frequent in stage B (p < 0.014), C (p < 0.039) and D (p < 0.001) compared to stage A. CONCLUSION: Kroll's classification for PDVR has a high prognostic value for the postoperative visual outcome and level of surgical risk management. Thus patients with good prognosis can be identified easily, facilitating the decision for surgery.

Adult↗
PubMedDate not supplied

The role of the vitreous in retinal detachment surgery with external buckling.

PURPOSE: To prove the role of the vitreous in retinal detachment surgery with scleral buckling procedures, we retrospectively investigated the charts of patients who were operated on for retinal detachment by scleral buckling between January 1995 and June 1997. METHODS: Out of a total of 718 cases, 41 retinal detachments occurred in previously vitrectomized eyes. The buckling procedure consisted of silicone sponge explants in 513 cases (254 radial, 259 circumferential) and of encircling elements in 202 cases. Three eyes received a temporary balloon. The minimal follow-up was 3 months. RESULTS: After primary surgery in the 677 nonvitrectomized eyes, the retina was attached in 482 eyes (71.2%), after reoperation in 627 eyes (92.6%). With up to 3 reoperations (including vitrectomy with gas or silicone oil tamponade in 79 eyes), the final success rate was 98.7%. The corresponding reattachment rates in the 41 vitrectomized eyes were 82.9% after primary surgery and 97.6% after reoperation. Repeated surgery in 6 eyes consisted of successful internal tamponade by silicone oil. The reattachment rates after primary buckle procedure are of most interest to the role of the vitreous: the highest success rate (82.9%) was achieved in the vitrectomized eyes versus 71.2% in the nonvitrectomized eyes. The difference is statistically not significant. CONCLUSIONS: We therefore conclude that the absence of the vitreous has no adverse effect on the results of buckling procedures for retinal detachment. The choice of the surgical technique in treating retinal detachments in vitrectomized eyes should only determined by retinal findings such as the number and extent of breaks or the presence and stage of proliferative vitreoretinopathy.

Humans↗
PubMedDate not supplied

Is membrane extraction in cases of exudative age-related macular degeneration still up-to-date? A 4-year résumé.

BACKGROUND: Age-related macular degeneration (AMD) is a frequent cause of an irreversible loss of the ability to read. The non-exudative form of AMD has not been therapeutically approached in the past in contrast to the exudative form with choroidal neovascularizations (CNVs). Parafoveal laser coagulation can be applied, and in cases of subfoveal location a pars plana vitrectomy with subretinal resection of the CNV is possible. MATERIAL AND METHODS: Since 1995, we have operated 46 eyes of 45 patients with CNV developing from AMD. Patient ages ranged from 63 to 85 years (mean 71.8 years). Pre- and postoperatively we performed vision tests, fluorescence angiographies with sodium fluorescein and indocyanine green. Follow-up times ranged from 3 to 28 months (mean 12.3 months). RESULTS: Pre-operative vision was 0.10 (range: hand movements to 0.4). Postoperative vision at the end of the follow-up period was 0.12 (range: hand movements to 0.4). Vision at the end of the follow-up was lower in 41%, unchanged in 20% and improved in 39%. In 43 eyes, a non-exudative form of AMD developed. Two eyes had a recurrent CNV, which was removed successfully with a second pars plana vitrectomy. Three patients developed a retinal detachment, which was successfully treated by pars plana vitrectomy, encircling buckle and gas tamponade. CONCLUSIONS: We still have to wait for the results of the photodynamic study trials and a randomized study of macular dislocation. Subretinal removal of the CNV by pars plana vitrectomy allows a stabilization of the visual function in most of our cases of AMD. This method inhibits the development of large pseudotumour-like scars. Postoperatively remaining pigment epithelial defects with choroidal atrophies however limit a visual rehabilitation so that reading vision can only be achieved in cases with good pre-operative vision. Long-term results of photodynamic therapy are still lacking and have to show its effectiveness over greater time spans.

Aged↗
PubMedDate not supplied

A modified technique to stain the internal limiting membrane with indocyanine green.

PURPOSE: Peeling of the internal limiting membrane (ILM) has improved the outcomes in vitreoretinal surgery of macular diseases. Indocyanine green (ICG) is used to stain and improve the visualization of the ILM. This paper aims to describe a modified approach to stain and peel the ILM avoiding potential adverse side effects. METHODS: After a core vitrectomy, 0.05 ml of ICG in a concentration of 5 mg/ml is quickly injected under water around the macular region. The tip of the syringe is positioned about 5 mm from the macular tissue, in a way that about a 3-mm diameter area around the fovea is stained by the ICG. Additional ICG is immediately removed by aspiration. Peeling of the ILM is accomplished with a forceps. RESULTS: Neither residual ICG in the vitreous cavity nor any clinical signs of phototoxicity like retinal edema were detected in any patients operated by this technique. CONCLUSIONS: This modified technique to stain the ILM limits the amount and concentration of ICG. The locally limited contact of ICG with the retinal surface seems to be a safe procedure to stain the ILM.

Coloring Agents↗
PubMedDate not supplied

Intravitreal staining of the internal limiting membrane using indocyanine green in the treatment of macular holes.

Surgical management of macular holes consists of pars plana vitrectomy, removal of the posterior hyaloid facia, and peeling of the epiretinal membranes (ERM). Additionally, removal of the internal limiting membrane (ILM) may enable an increase in the anatomic and functional success rates. However, recognition of fine ILM is difficult thus increasing the time that the macula is exposed to intraoperative light. Staining the ILM with indocyanine green (ICG) dye during vitrectomy facilitates recognition of the ILM and assures that all adjacent ERM are removed. Therefore, ICG-assisted ILM peeling has gained worldwide popularity among vitreoretinal surgeons. However, there are some concerns about the intravitreal ICG application. Reports in the literature described a variety of application techniques using different concentrations. The postoperative outcomes were controversial reporting heterogeneous anatomical and functional outcomes after ICG application, as well as descriptions of adverse effects related to the dye. We discuss the indications, techniques, surgical results, and complications after intravitreal ICG injection for the treatment of macular holes.

Basement Membrane↗
PubMedDate not supplied

Effect of topical diclofenac and ketorolac on patient discomfort and corneal sensitivity.

PURPOSE: To determine if at clinical dosages, the topical nonsteroidal anti-inflammatory drugs diclofenac and ketorolac decrease corneal sensitivity, and if there is a difference in discomfort on instillation. METHODS: In a double-masked study, 30 patients were randomized to receive two of three topical drops, Endosol, diclofenac sodium, and ketorolac tromethamine. A microaesthesiometer was used to evaluate corneal sensation. Baseline corneal sensitivity was obtained on each patient. Corneal sensory thresholds were remeasured at 7 and 15 minutes following eyedrop application. Patients also completed a questionnaire that graded the burning caused by the medications. RESULTS: No significant decrease in corneal sensory thresholds was found with diclofenac or ketorolac compared with control at baseline (P = .50), 7 minutes (P = .41), or 15 minutes (P = .82). There was significantly more burning with diclofenac and ketorolac compared to control. There was a small but not statistically significant (P = .28) trend of more burning with ketorolac than diclofenac. CONCLUSION: Neither diclofenac nor ketorolac were found to decrease corneal sensation compared to control. There was no significant difference in burning upon instillation reported by patients.

Administration, Topical↗