There are two main penile prosthesis, the malleable and inflatable. The malleable or semi rigid prostheses consists of a silicone rod with a flexible steel core which allows it to be both stiff and bendable. The inflatable is a balloon that can be filled with fluid to create rigidity when an erection is needed.
The London team use the three part inflatable rather than the semi rigid as they were too rigid and prown too erosion. Also there is not much difference between the flaccid and erect states.
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