Figure 4. Plug valve. One early Surgitek implant, known as the Dahl implant, was inflated with silicone at time of placement (115). Shown here is the "plug valve" of such an implant placed in about April 1975, stoppered with a white plug. The inner edge of the hole in the shell is just discernible. Free gel is present on the implant surface. This type of valve should have a characteristic and identifiable cylindric appearance on MR images.