Removable partial denture design: a photoelastic study.

Quantifiable frozen-stress photoelastic techniques were used to analyze stresses induced in mandibular models by a conventional free-end saddle removable partial denture. Four quasi-anatomical mandibular models were constructed for processing, together with their respective calibration specimens, through identical time/temperature stress-freezing cycles. After processing and slicing, an unloaded control model demonstrated some low-order fringes adjacent to the coronal third of the abutment tooth roots, but was otherwise stress free. A lower bilateral free-end saddle partial denture was constructed and fitted in turn to each of the remaining three models. Each denture/model combination was then loaded and processed through a stress-freezing cycle. After processing, 6-mm slices were cut from selected regions for analysis for the presence of stresses. Using a polariscope with circular polarized, monochromatic light, values for maximum shear stress were calculated at selected points in the slices taken from the three loaded models. Variations up to 28% of the mean were obtained for the three experimental models as compared with the consistent results for the material fringe values obtained from the calibration specimens. The study pointed out the problems involved in using photoelastic stress analysis on complicated anatomical models. The interpretation of the results from such studies should be approached with caution.

Evaluation of acrylic bone cements and their performance standards.

Eight acrylic bone cement products were examined according to the methods set out in three published performance standards, viz., Australian Standard 2265 Part 1, 1979, ASTM specification F451-76, anda the ISO specification 5833/1, 1979. In addition, chemical analyses were carried out by GC, NMR, and IR spectroscopy as well as refractive index measurement, all to establish the main constituent of the products. All the products compiled to the physical, compositional, and sterility requirements of AS 2265. However, all but one sample failed to comply with the solubility requirements of ASTM F451-76. Also, none of the samples compiled with the doughing time requirements of ISO 5833/1. On further analysis, the results suggest that the water solubility and sorption requirements of ASTM F451-76 are restrictive and that the ISO doughing time test methodology requires modification. Various degrees of noncompliance to the requirement of all three specifications were noted on the labeling and in the information content of product inserts. Two additional clauses on mixing tools and radiopacity have been suggested for incorporation into the above standards.

Effects of roughness on the thrombogenicity of a plastic.

Tubings extruded from one batch of polyvinylchloride resin at five temperatures ranging from 110 to 205 degrees C were tested for thrombogenicity in the saphenous vein and aorta of sheep. There was more thrombus in both the vein and aorta on tubings extruded at lower temperatures. The surface areas of renal infarction were smaller when lower temperature tubings were inserted into the aorta. Scanning electron microscopy showed that lower extrusion temperatures caused greater roughness on the tubings. This greater roughness was confirmed by roughness measurements. It is suggested that the association which has been demonstrated between roughness and thrombus formation might be due to the degree of adhesion of the thrombus to the surface rather than to differences in thrombogenicity.

The role of heparin on platelet retention by acrylonitrile co-polymer dialysis membranes.

The role of heparin on platelet--foreign surface interactions was examined by platelet retention studies on acrylonitrile--dimethylaminoethyl methacrylate (AN-DMAEMA) dialysis membranes both with and without the bonding of heparin onto their surfaces. Heparin bonding significantly reduced platelet retention. Heparin in solution (4 units/ml.) increased platelet retention when the surface of the membranes was modified by ethylene oxide but had no significant effect on the platelet-retaining properties of unmodified membranes. Studies using heparin 99mTc demonstrated that unmodified membranes took up heparin from solution whereas ethylene oxide-modified membranes had little such affinity. The heparin bonding process greatly increased the heparin uptake achieved by simple soaking in heparin solution, and the leaching rate was less than 1% at 70 hours. The results indicate that heparin has two antagonistic effects in this platelet-foreign surface interaction: it acts directly on platelets to increase adhesiveness while acting on the foreign surface to reduce platelet retention.