Lunges like DD said are pretty good. Some stretches and excercises that work the legs and thus the knees are always a good thing as well. Some herbal supplements, most the ones that are geared towards joints usually have Gluecosamine in them.I take 1 tablet 3 times a day for my joints. I gots myriad of things to stress them out so yeah. And my one teammate also got some of these supplements as well and his slide knee was really buggin him, and he loves it. Also a knee brace is a good preventive measure as well. Now this I know is hard for athletes to swallow, but, go see a doctor, get a diagnosis and sit it out and strengthen it up before you return. The last part very gets competitors...until they get hit in a game or try to bowl and their knee explodes again or they ribs end up broken instead of fractured or bruised. The old adage goes something like, "Time heals all wounds." Man if I had a dollar for every time I had to beat that into someone's skull I'd make Bill Gates look poor.
Anyways, something else stood out to me as well and this something else that tends to happen with injuries...the development of bad habits. I'm not saying that planting your foot is a bad habit, but in your case it isn't helping at all. It's probably putting more strain on your knee than a slide would. For example, we've all seen bowlers fall or been the one that fell, usually because we go to slide and something is on our slide shoe. If you look at what happens to the knee, it bends forward and the weight and momentum is forced onto the leg and since the knee is the major joint in the leg as well as support it takes a majority of the strain as well. By planting your leg you're effectively forcing more weight and force onto your leg and thus your knee, while you are throwing the ball instead of transferring it to the ball. And it's not like I'm saying you need a tremendously long deep low slide, it helps. You can do things to help out your game, but if you can't no big deal
. Do what fits for you and your game If a little bend is what you can do, work from there. When I first ventured into the realm of working on my knee bend and getting lower, I watched the female pro bowlers and thought D@mn! My knee definately can't do that. So I worked at, figured some stuff out that is right for me, and I can get nice and low. There isn't a right or wrong way to bowl. Each person is unique and different and thus their swings and approaches are and will be different.
Anywho, I hope this helps,
The Demolition Man