A penile prosthesis is another treatment option for men with erectile dysfunction. These devices are either malleable or inflatable. The simplest type of prosthesis consists of a pair of malleable rods surgically implanted within the erection chambers of the penis. With this type of implant the penis is always semi rigid and merely needs to be lifted into the erect position to initiate sex. Today, many men choose a hydraulic, inflatable prosthesis, which allows a man to have an erection whenever he chooses and is much easier to conceal. A penile implant is usually used when there is a clear medical cause for erectile dysfunction and when the problem is unlikely to resolve or improve naturally or with other medical treatments. Sometimes a penile prosthesis is implanted during surgery to reconstruct the penis when scarring has caused erections to curve. The inflatable penile prosthesis consists of two cylinders which are placed surgically in the body. The tow cylinders are inserted in the penis and connected by tubing to a separate reservoir of fluid. The reservoir is implanted under the groin muscles. A pump is also connected to the system and sits under the loose skin of the scrotal sac, between the testicles. To inflate the prosthesis, the man presses on the pump. The pump transfers fluid to the reservoir, deflating the penis. While men who have had the prosthesis surgery can see the small surgical scar where the bottom of the penis meets the scrotal sac, other people probably will be unable to tell that a man has an inflatable penile prosthesis. When the penis is inflated, the prosthesis makes the penis stiff and thick, similar to a natural erection. Most men rate the erection as shorter than their normal erection; however, newer models have cylinders that may increase the length, thickness, and stiffness of the penis. A penile prosthesis does not change sensation on the skin of the penis or a man's ability to reach orgasm. Ejaculation is not affected. Once a penile prosthesis is put in, however it may destroy the natural erection reflex. Men usually cannot get an erection without inflating the implant. If the implant is removed, the man may never again have natural erections.