Shortening of the penis is a very real side-effect/complication of radical prostatectomy.
There's no relevant, significant "movement" of either the bladder (held in place by ligaments connected to the pelvis) or the urethra (also contstrained "below" where the prostate was); there's enough "give" to pull these two sections back together.
Rather, shortening is caused by one or both of two mechanisms (each individual case has more or less of each of these mutually independent causes).
1. Permanent nerve damage experienced during surgery: When nerve damage occurs (anywhere in the body), the tissues to which the damaged nerves supply impulses usually experience atrophy or breakdown. The lack of impulses to smooth muscle of the penis leads to an atrophy or breakdown of the tissues responsible for erections and causes the penis to shrink both in length and girth.
2. Decreased transport of blood and oxygen through the erectile tissue of the penis: Loss of erections is a common side effect of prostatectomy, particularly in the first few months after surgery. Erections lead to the circulation of oxygen-rich blood through the penis which nurtures it and keeps it healthy. If no erections occur for extended periods of time, the lack of circulation of this oxygen-rich blood leads to fibrosis or scarring of the erectile tissue of the penis, also leading it to shrink in terms of length and girth.
The most effective method of minimizing the chance of shortening the penis depends on avoiding the permanent damage to the penis that I described above. This requires some work by both surgeon and patient.
In the first case, damage to the nerves around the prostate needs to be avoided by the surgeon. The way to accomplish this task is to perform a meticulous nerve sparing prostatectomy. Of course, every prostatectomy is always a fine balance between cancer control and nerve sparing. That is why a skilled, experienced surgeon is vital to performing a prostatectomy that removes the prostate with a minimal sacrifice of the nerves. No nerve-sparing, more nerve damage.
In the second case, we are all (mostly) aware of the job which falls to the hands of the patient...we like to call it "rehab." The purpose of rehab is to ensure the continued circulation of oxygen-rich blood through the penis and prevent the scarring of the penis that I mentioned above.
How much shortening any one man has depends uniquely on the fine details of his individual surgical procedure, and how vigorously (or not) he rehabed. One is not
in the patient's control (except choosing an excellent, experienced surgeon, although that by itself does not assure no nerve damage), and the other is
in his control.
The other element not in his control is luck..which seems to randomly bless some with more or worse side effects than others with nearly identical case characteristics. Luck, I'm finding, is huge.