After surgically extending a penis with fat injections, would it still get as hard? Would it work properly?

Thinking the penis is a muscle. Wouldn't only the muscle get hard and the fat stay soft? And could the man stay erect for as long? Are there any bad side effects? And also would just surgically stretching the penis be a better option? I'm comfortable with my size. Im lucky to not have one that is too small. But adding an inch or 2 one day would be great! I'm only 23 but I do know that it's not growing anymore. It's a little over 6 inches long as of right now. I wouldn't want much more.

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