The inhibition of calcium oxalate monohydrate crystal growth by maleic acid copolymers.

The crystallization of calcium oxalate monohydrate (COM) was investigated at conditions of constant supersaturation both in the absence and in the presence of synthetic maleic acid copolymers at 37C, 0.15 M NaCl. The dependence of the rates of COM crystallization in the absence of inhibitors was found to be second order at low and first order at higher supersaturations suggesting a surface diffusion controlled mechanism. The presence of all copolymers tested at concentration levels up to 5 ppm retarded the rates of COM crystal growth up to 90%. The decrease of the COM crystal growth rates by the polymers depended on the nature of the comonomer polymerized with maleic acid and the order of inhibition was found to be vinyl acetate > N-vinylpyrrolidone > styrene. Taking into consideration kinetics data published in the literature concerning the inhibition of COM crystal growth, it is suggested that molecular weight also plays a role, with more inhibition at higher molecular weights. The morphology of the COM crystals grown was unaffected yet the crystals growing at lower rates in the presence of the copolymers were larger and their size more uniform. It is concluded that maleic acid copolymers are strong inhibitors of the crystallization of COM, the inhibitory activity being more pronounced in the case of the linear copolymers.

Metal stents: a new treatment of malignant ureteral obstruction.

PURPOSE: We report our experience with the use of metallic self-expandable and balloon expandable stents for the treatment of malignant ureteral obstruction. MATERIALS AND METHODS: We treated 12 consecutive patients with malignant ureteral obstruction, for a total of 14 ureters with stents placed. We placed metallic balloon expandable stents in 6 patients and self-expandable metallic stents in the remaining 6. Mean patient age was 65 years and mean followup was 9 months (range 8 to 16). RESULTS: Of the ureters 11 were patent without any additional manipulations during followup of 8 to 16 months. Secondary interventions were needed in 3 cases because of obstructive urothelial hyperplastic reaction, tumor ingrowth and local recurrence of the primary cancer invading the upper end of the stent. Two patients died 2 and 10 months after placement of the stent. CONCLUSIONS: Both types of metal stents have advantages and disadvantages that must be balanced against each other when choosing the ideal device for the treatment of obstruction. Implantation of a metal self-expanding or balloon expanding stent is safe and effective for the palliative treatment of malignant ureteral obstruction in late stage cancer patients.