The Greenfield Vena Cava Filter: Mobin-Uddin Filter
In order to appreciate the development of the Greenfield filter, it is necessary to understand what has previously been available. The Mobin-Uddin filter was the first widely used transvenous device, having been introduced in the 1960s. It resembles an umbrella and consists of six stainless steel spokes radiating from a central hub. A thin fenestrated Silastic sheet covers the metal on each side but allows protrusion of the spokes by 2 mm. In its original design, the filter expanded to 23 mm in cross-sectional diameter, having been developed in such fashion based on autopsy studies of human vena cava morphology. However, due to filter migration in man, the filter was increased in size to its current 28 mm. This has helped reduce the number of proximally migrating filters but has not eliminated the problem. In one report 20 of 1,981 (1 percent) of 23 mm filters proximally migrated vs two of234 (0.85 percent) 28 mm filters; and in another report, 0.4 percent of the 28 mm filters migrated. This filter is efficacious in preventing pulmonary embolism; however, complications have been recognized, including caval thrombosis in 33 to 85 percent of patients, proximal migration of the filter resulting in death, venous stasis sequelae related to filter placement, and a higher than expected rate of recurrent pulmonary embolism perhaps related to the high incidence of caval thrombosis. natural breast enhancement
In an effort to resolve the problems of filter migration and caval thrombosis, Greenfield developed a conical stainless steel wire filter. It has been designed so that 80 percent of the depth of the filter can be occluded before reducing the cross-sectional area by 64 percent. This allows central trapping and capture of emboli with continued flow through the cava. The filter is 4.6 cm in length and 3.0 cm in diameter at its base when fully expanded (Fig 1). The ability of the filter to trap emboli 2.0 mm and larger has been demonstrated experimentally. The thrombus-en-trapment efficiency of the filter has been shown to be a function of the size of the emboli, and depth of filling. A significant gradient across the filter does not occur during repeated embolization until the filter is filled to approximately 70 percent of its depth. Because of the continued blood flow around trapped clot and the conical geometry of the device (funneling clot toward its apex in the middle of the vascular channel), clot lysis can take place, thus cleaning the filter. This has been shown to occur in dog studies and in clinical practice as well. Although not explicitly studied, it has been suggested that the filter will be less efficient if lodged obliquely within the cava.
Figure 1. The Greenfield filter (metric measure).