From river blindness
to
interventional cardiology

Black flies (Simulium damnosum) are known not only for their nuisance causing economic losses in different areas of human activities, but also for transmission of pathogens and parasites to man and animals. In some areas, black flies are vectors of a filarial worm (Onchocerca Volvulus) which causes a serious endemic disease whose final stage is known as river blindness. "Onchocerciasis, or river blindness, is one of the major endemic, parasitic diseases which in addition to causing untold human suffering is a major obstacle to socioeconomic development. It is found in the Americas, in the south-western part of the Arabian peninsula and in East, Central and West Africa. It is estimated that between 20 and 30 million people are infected by onchocerciasis throughout the world" ^[1].  In 1975 the World Health Organization (WHO) launched the Onchocercosis Control Program (OCP) in West Africa.  The strategy chosen was to break the chain of transmission by destroying the vector at its most vulnerable state, that is, the larval state. To control black fly larvae in running waters special products have been developped with targeted toxic effects. Helicopters are used to periodically spray the rivers at prescribed sites over very large geographical areas. To reduce the costs of operations, it is important to determine the amounts of product and locations of the injection sites to minimize the total quantity of sprayed larvicide while maintaining a given level of mortality along the river to be treated.

An electrical engineer, Dominik M. Wiktor (Bellcore, Morristown) underwent open heart surgery to correct an aortic dissection in 1984. Following the procedure, he wondered why such a vascular repair couldn't be done with less surgical trauma, and began to read about angioplasty. He came up with a variety of stent designs and signed a consulting agreement with Medtronic in 1988.  The``Wiktor Stent," an intravascular stent (U.S. patent No. 4,886,062) provides an important solution to coronary artery reconstruction and recanalization. The stent keeps a diseased vessel open and prevents reclosure...  In the case of the Wiktor Stent, the delivering catheter is inflated to expand and deploy the stent to maintain the opening. The balloon is then deflated and the catheter removed. Within a month, the stent becomes incorporated into the artery wall. Today, Medtronics Wiktor stent has a 20 percent market share in Europe. ^[2]

More than forty percent of patients treated for atherosclerosis present re-stenosis within six months of the operation.  Indeed, implanting a stent generally leads to complications, thrombosis and proliferation of smooth-muscle cells being the root causes of restenosis. A great deal of research has been conducted on medicinal agents designed to have an anti-thrombotic effect or to limit the proliferation of smooth-muscle cells. This type of medication can be delivered in two ways: systematically or locally.  The primary disadvantage of systemic delivery is generally  its greater toxicity in the body, since, to be effective, a much higher dose of the medication is a priori required than for local delivery. For this reason, the local solution is the preferred choice. For instance a typical system consists of a stent coated with a thin layer of polymer which has been impregnated with a molecule that has an effect on the proliferation of smooth-muscle cells. There are many aspects to the design of a stent.  Mechanically the stent has to be strong enough to exert a sufficiently strong pressure on the wall of the vessel to keep it open and to restore a normal flow of blood. Once the purely mechanical parameters are set, there are a number of parameters left to control the delivery of the molecule to the wall. It is generally accepted that it is the effect of the concentration of the product over time that effectively cures the damaged wall of the vessel. This naturally leads to the notion of dose in each point of the artery. In this work we study the effect of the density of the struts and the pattern of the stent on the distribution of the dose.