The gear should not weigh anything in the water, its neutral.
While it may be ok to put your gear on in the water while in a pool, when you get into open water you must have all your gear on ahead of time. I know a few people with knee and hip replacement. It seems to be a very personal case by case thing. Some have no issues and are good as new, others have lots of issues and are limited. Diving is one activity that is hard on the back and knees. If you have a 40 lbs weight limit I would talk more with your doctor. Just an AL80 weighs 36 lbs. Then think about climbing back up onto a boat in 3 foot waves, or making a shore entree and having to walk in with your gear.