Sean762 wrote:So my limited understanding is that an implant will inflate the shaft but not the glans.
Would pumping up the implant, followed by placing the penis in a VED at say 5 inches of Hg for 5-10 minutes cause the head to fill normally and stay somewhat filled (with or without an erection ring) for more 'normal' sex with a partner?
I have often used the concept that the implant inflates the corpus cavernosum inside the tunica albuginea. This is not unlike those fabric tubes being blown full of air and dancing around outside of car lots and furniture stores. But to be hydraulically correct that concept is not totally accurate. The implant gets inflated and supports the tunica (the tube of fibrous tissue inside the penis that is normally filled/pressurized/supported by the corpus cavernosum - the porous erectile tissue).
The implant operation often removes the corpus cavernosum tissue, replacing it with the implant. (Some surgeons use a "tissue-sparing" technique, but even this leaves the cavernosum pretty much ineffective for erections.)
So, the implant itself stiffens by the pressure inside it. This stiff implant causes the tunica and the penis to be straight and resistant to bending (erect), but not necessarily "inflated". As I understand it, the AMS implants resist expanding width-wise because their structure is of a woven polyester cloth covered inside and out by medical grade silicone. Thus cannot expand much in girth. The Coloplast devices are made of a proprietary material called "Bioflex" which can stretch a bit, this may be able to press against the sides of the tunica, simulating inflation of the tunica (and thus, the shaft).
The implant stiffens the shaft and expands it somewhat, but the glans is completely unaffected. However, with a stiff shaft underneath the glans, stimulation often will cause the glans to engorge (note that the blood supply that engorges the glans is separate from the blood supply that used to engorge the corpus cavernosum and the glans blood supply is usually unaffected by ED). Also note that the shaft outside the tunica is engorged in the same way as the glans. The tissues in the glans and shaft that engorge is called the "spongiosum".
Except for that nuanced correction, your understanding is essentially correct.
Spongiosum engorges by a different mechanism than the cavernosum in an erection of a normally operating penis. Spongiosum engorgement is independent of the erecion, whether by engorgment of the cavernosum or inflation of an implant.