Enhanced Mechanistic Understanding Through the Detection of Radical Intermediates in Organic Reactions.

Two applications of a radical trap based on a homolytic substitution reaction (SH2') are presented for the trapping of short-lived radical intermediates in organic reactions. The first example is a photochemical cyanomethylation catalyzed by a Ru complex. Two intermediate radicals in the radical chain propagation have been trapped and detected using mass spectrometry (MS), along with the starting materials, products and catalyst degradation fragments. Although qualitative, these results helped to elucidate the reaction mechanism. In the second example, the trapping method was applied to study the radical initiation catalyzed by a triethylboronoxygen mixture. In this case, the concentration of trapped radicals was sufficiently high to enable their detection by nuclear magnetic resonance (NMR). Quantitative measurements made it possible to characterize the radical flux in the system under different reaction conditions (including variations of solvent, temperature and concentration) where modelling was complicated by chain reactions and heterogeneous mass transfer.

Development of a procedure for determining the mixing ratios in ecotoxicological experiments and its application in binary salt mixture experiments.

Although a plethora of models exist to describe the characteristics and risk assessment of chemical mixtures in ecotoxicology, there is no specific procedure to decide on the mixing ratios (i.e. proportions of the individual chemical substances that form the mixture) at any desired level of concentration in an ecotoxicological mixture experiment. In this study, an attempt was made to develop a procedure for determining the mixing ratios in ecotoxicological experiments. In brief, from a single salt exposure test, the relative toxic fractions, which represent the toxic effect exerted by the individual salts, are determined. Thereafter, the proportions of each individual salt at any level of concentration in the mixture are estimated by multiplying the desired concentration with the relative toxic fraction of that particular salt. The procedure was applied to ecotoxicological experiments involving four binary salt mixtures (MgCl2 + MgSO4, NaCl + Na2SO4, MgCl2 + Na2SO4 and NaCl + MgSO4) and Caridina nilotica, an indigenous South African freshwater shrimp. It is hoped that the application of this developed procedure will ensure administering the correct proportions of individual chemical substances in chemical mixtures in order to obtain the desired levels of concentration in aquatic ecotoxicological mixture experiments.

Dislocation of the donor graft to the posterior segment in descemet stripping automated endothelial keratoplasty.

PURPOSE: To report a series of dislocations of the donor graft into the posterior segment associated with Descemet stripping endothelial keratoplasty (DSAEK) and to identify possible risk factors for dislocation and clinical outcomes. DESIGN: Retrospective case series. METHODS: Cases of donor graft dislocation into the posterior segment associated with endothelial keratoplasty were identified from the clinical experience of 7 surgeons. Observations included the preoperative surgical history of each eye, preoperative and postoperative visual acuity, management of the complication, and the postoperative clinical course. No identified cases were excluded from this series. RESULTS: Eight posterior graft dislocations were associated with DSAEK surgery. Each eye had a history of vitrectomy. Five eyes had sutured posterior chamber intraocular lenses, 1 eye had a sulcus intraocular lens, and 2 eyes were aphakic. Each eye required repeat grafting, and in 6 of 8 eyes, pars plana vitrectomy was used to remove the dislocated graft. Final visual acuities ranged from 20/30 to no light perception. CONCLUSIONS: Graft dislocation into the posterior segment is a rare complication of DSAEK surgery that can lead to permanent vision loss. It has occurred in eyes that have undergone previous vitrectomy and complicated intraocular lens placement or were aphakic. As is the case with a dropped lens nucleus during cataract extraction, visual acuities after a dropped DSAEK graft range from very good to no light perception. Better postoperative results seem to be associated with prompt removal of the posteriorly dislocated graft.

Corneal transplant rejection rate and severity after endothelial keratoplasty.

PURPOSE: To examine the corneal transplant rejection rate and the rate of graft failure subsequent to rejection in the first 2 years after endothelial keratoplasty (EK) and to compare this to background data for similar cases of penetrating keratoplasty (PK). METHODS: Anonymized data from consecutive cases of EK [deep lamellar (DLEK) or Descemet stripping (DSEK)] for Fuchs corneal endothelial dystrophy or pseudophakic bullous keratopathy operated on before August 2004 were collected in 4 participating centers. The number and timing of rejection episodes and cases of subsequent graft failure were recorded, together with the time to cessation of topical steroid medication. Two-year postoperative findings were compared with background data for similar patients undergoing PK recorded in the Swedish Corneal Transplant Registry. RESULTS: Rejection episodes were less frequent after EK than PK (P = 0.035). Fifteen (7.5%) of 199 EK cases had a rejection episode in the first 2 years after surgery versus 92 (13%) of 708 PK cases. Graft failure after rejection in EK may have been less frequent than in PK (P = 0.063), with only 1 (6.7%) case of rejection after EK versus 26 (28.3%) cases of rejection after PK proceeding to graft failure. A strong trend toward continued use of low-dose topical steroid medication was observed in the EK group. Eighty percent of EK patients were still taking topical steroid medication 2 years after surgery, whereas topical steroid medication had been ceased within a year of surgery in almost all PK patients in the comparator group. CONCLUSIONS: Corneal transplant rejection is less frequent and may be less severe after EK than after PK. It is not yet clear whether these apparent differences are simply a product of relatively prolonged postoperative topical steroid cover in EK patients.

Microkeratome versus femtosecond laser predissection of corneal grafts for anterior and posterior lamellar keratoplasty.

PURPOSE: To compare 2 different techniques for predissection of human anterior and posterior lamellar corneal grafts for eye bank storage. METHODS: A mechanical microkeratome (group 1, N = 5) and a femtosecond laser (group 2, N = 5) were used to dissect intended 350-microm-deep lamellar planes in deepithelialized donor corneas mounted on an artificial anterior chamber. These corneas were replaced in Optisol GS at 4 degrees C postoperatively and examined 2 days later to simulate a clinical scenario. Ultrasonic pachymetry of corneal lamellar sections was measured before and after separation of the lamellar grafts. Group 1 sections were separated by the mechanical microkeratome, whereas group 2 sections were manually separated 2 days after laser dissection. Endothelial cell viability was evaluated in posterior grafts. RESULTS: Total corneal thicknesses immediately before dissection were 559 +/- 61 (group 1) and 578 +/- 79 microm (group 2; P = 0.46). Immediate postdissection anterior and posterior graft thicknesses were 361 +/- 68 and 203 +/- 74 microm (group 1), respectively. Achieved anterior and posterior graft thicknesses 2 days later were 282 +/- 44 and 413 +/- 35 microm (group 1) and 324 +/- 112 and 397 +/- 51 microm (group 2), respectively. Percentage of devitalized endothelial cells were 3.4% +/- 1.6% (group 1) and 1.6% +/- 1.2% (group 2; P = 0.35). CONCLUSIONS: Centralized predissection by both techniques, cold storage, and shipping by airmail results in viable grafts without significant endothelial cell loss 2 days later.

Microkeratome preparation of lamellar corneal grafts.

PURPOSE: To evaluate the thickness and viability of microkeratome-prepared lamellar corneal grafts in cold storage. METHODS: Ten human corneas were sectioned with a mechanical microkeratome with a 350-microm depth head and stored in Optisol GS at 4 degrees C for 2 days to simulate an eye bank scenario. Central corneal thickness before and after mechanical microkeratome sectioning was measured by ultrasonic pachymetry. Endothelial cell viability was evaluated by trypan blue and alizarin red staining. RESULTS: Total corneal thickness immediately before microkeratome dissection was 562 +/- 51 microm. Anterior and posterior graft thicknesses were 296 +/- 111 microm and 270 +/- 74 microm, respectively, immediately after dissection, and 282 +/- 38 microm and 429 +/- 31 microm, respectively, 2 days after storage. There was significant swelling in the posterior (P=0.005) but not the anterior grafts (P=0.386). The percentage of devitalized endothelial cells was 3.0% +/- 1.2%. CONCLUSIONS: Corneal lamellar grafts may possibly be precut in a centralized facility and stored cold before further distribution.