I just had ACL replacement surgery as well as some cartilage in my knee repaired. As I understood from the orthopedic surgeon, I will not be able to ride for at least 6 months following surgery. I will also have to wear a brace while riding. I assume he will know what kind of brace I need to use in order to ride and not tear up my saddle, but I thought I would ask for any recommendations.
As a medical assistant for an orthopedic surgeon (who specializes in ACL repair), a Professional Choice brace (or any soft brace, for that matter) is not going to give you anywhere near the medial/lateral support you need after surgery. You will most likely be fitted for a custom ACL brace - here's a link to one brand: http://www.townsenddesign.com/Townse...ign/Rebel.html
Donjoy is another brand that's commonly used.
The brace will look similar to this although there also may be an additional strap going diagonally across the thigh. They are very rigid braces (I believe they're constructed with carbon fiber).
There are hinges on the outside and inside of the knee. You may be able to put padding on the outside of the medial hinges to protect your saddle but I don't think it will be comfortable to ride in.
PM me if you need more info. Your best bet is to talk to your doc at length during your next appt. and ask if you will need a custom ACL brace.
I had a custom brace similar to the one posted above. When I went in for the fitting I made sure that they measured me while I was wearing my breeches and boots. This way I knew everything would fit while I rode and figured that I could just make the velcro straps a little tighter when I was wearing it under my slacks or skirts. It worked out fine. It didn't cause any damage to my saddle and I hunted in that thing until I had the knee replaced.