Nuclear spin conversion in polyatomic molecules.

Except for ortho- and para-H2, little is known about nuclear spin isomers (or spin modifications) of molecules. The main reason is the lack of practical enrichment techniques. Recently, a few enrichment methods were developed, which opened up new possibilities in the field. These methods are briefly reviewed. Substantial progress in the field has been made by the introduction of light-induced drift as a gas-phase separation tool. This is illustrated by extensive data on CH3F, which reveal that the gas-phase ortho-para conversion is governed by intramolecular mixing of the nuclear spin states. The role of direct ortho-para transitions is small. Various aspects of the conversion were investigated in detail: pressure and collision partner dependence, isotope effect, and temperature dependence. The most decisive information on the spin conversion mechanism is derived from the observation of level-crossing resonances in an electric field and the quantum Zeno effect induced by collisions.

Visual results after corneal transplantation.

The results of corneal transplantation can be measured by the percentage of clear grafts and by the subsequent visual acuity. In this study we summarized the visual results in five corneal diseases with a low rate of graft failure. Keratoconus patients achieved an average visual acuity of 0.5 one year after transplantation, while patients with corneal decompensation after previous cataract operation only reached an average of 0.2. Sometimes visual results were hampered by clouding of the graft and the reason for clouding was analysed in such cases. Reversible and irreversible rejections occurred with the highest frequency immediately after transplantation, and while in primary grafts 60% of rejections was reversible, only two out of ten repeat transplants could be saved. This indicates that more measures are necessary to prevent failure of secondary transplants in order to obtain the goal of useful vision.

Follow-up of corneal transplantations at the Academic Medical Center of Amsterdam.

In order to determine risk factors in corneal transplantations, corneal transplants performed at the Amsterdam Academic Medical Centre (AMC) were registered and the results evaluated by descriptive statistics and actuarial survival curves. The results showed that the diagnosis prior to transplantation, the number of previous transplants, and the vascularization were important prognostic factors for graft survival. The best results were obtained in the absence of vascularization, in primary transplants and in keratoconus, while vascularization in one or more quadrants and a diagnosis such as herpes resulted in poor graft survival. When, for instance, the negative influences of vascularization can be overcome, better results will be obtained.