Gregory A. Hill

Soft contact lenses

sensory deficit of varying degree in the trigeminal territory. The denser the anaesthesia, the lower the recurrence rate, whichever the technique used. Hendersons results after alcohol injection are a perhaps unique exception in this respect. Microsurgical techniques are mostly aimed at reducing the morbidity from facial sensory loss either by fractional rhizotomy, dividing only the portio major, or by relieving presumed arterial compression. It will be surprising if in the long run these procedures find any more general acceptance than Taarnhoj s decompression.1 Surgical treatment is unlikely to reduce the recurrence rates unless it produces complete anaesthesia at least in the affected division of the trigeminal nerve and should, therefore, be confined to those patients willing to accept the sensory loss in return for a more or less permanent cure. We feel that on most occasions percutaneous radiofrequency thermocoagulation has more to offer than any other form of therapy because sensory deficit can be controlled and confined to the distribution of the pain. As the aim of this technique is to obtain selective loss of pain sensation only in the affected area, with minimal injury to the nerve, the chance of delayed recurrence is relatively high in comparison with any other method of surgical treatment. This recurrence, however, can be treated by repeating the technique more swiftly and simply than by intracranial surgery. The treatment is tolerated best and the incidence of facial dysaesthesiae lowest if as with other procedures under basal sedation thermocoagulation is carried out when the pain is at its height, and not during a remission.

Clear Nonionic Polyurethane Hydrogels for Biomedical Applications

Clear nonionic polyurethane hydrogels having a broad range of mechanical properties and degrees of swelling were prepared by both bulk (compression molding) and solution polymerization processes. Hydrogels con taining 70% water were also prepared which had an elongation of 1150% and a tensile strength of 280 kPa. The effects of the chemical structure, molecular weight, and functionality of polyether polyols and type of diisocyanate on hydrogel properties were studied. In addition, the type and concentration of crosslinker, and concentration of ethylene glycol, which was used as chain ex tender were investigated. In order to achieve transparency in the hydrogels, it was determined that poly(oxypropylene) glycols (PPGs) should be present in the system to disrupt the crystallinity of the poly(oxyethylene) glycol (PEG) soft segments. The PEG segments of the network which contain the hydrophilic moiety are responsible for the absorption of water. However, in addition to the concentration of oxyethylene, the degree of swelling of the hydrogels was also determined by measuring the elasticity of the polymer network. The elasticity of the polymer network is determined by the molecular weight between cross links (crosslink density) and the concentration of hard segments in the net work. The concentration of hard segments was controlled by the concentration of chain extender. The crosslink density was controlled by the diol/triol ratio and the respective molecular weight of each component.