A chronic weak stifle is, in great part, due to a general lack of muscle tone in the hind limbs. That is aggravated by poor confirmation, deep footing, poor shoeing (too much toe or extended heels) or pre-existing hock or fetlock arthritis.
Assuming that your vet has ruled out a stifle joint problem (OCD, arthritis, cruciate lig.), the cause for the instability/injury will probably always be present, but there are several things you can do to minimize it. The most important aspect to treatment is a well thought out training schedule. The horse must achieve a high level of fitness (muscle tone) to compensate for the joint's instability. The schedule that I recommend is a build up to two 25 minute trot sets with a 5 minute walk in between. This should be done at least five days a week (if not 6 or 7) for thirty days. Then you can start to mix in some dressage or jumping on an every other day basis. Once you have achieved an adequate level of fitness, you then have to maintain it! A horse with chronic weak stifle cannot be ridden three days a week and, if you're away on holiday, you must have someone ride or lounge in your place.
You have to get the horse sound first - stall rest for two to three days is usually enough and a little anti-inflammatory will help. Once sound you can proceed one of two ways: commence training as described above or, have your vet inject the stifles (stifle ligs & muscle groups) with a 2% iodine in almond oil solution (internal blister). This treatment is an old one but, in my opinion, very effective. After the injections, the horse should be ridden - one 25 minute trot will do! Each day before riding, application of some kind of hot liniment around both stifles helps "pre-heat" the joints and makes re-injury less likely.
If, after due diligence, the stifle continues to fall out then surgery may be your only other option. A Medial patellar Desmotomy is a procedure that achieves joint stability by resecting (cutting) the medial patellar ligament. As long as there is no primary joint pathology, the procedure is quite effective. It does, however, have a down side - and that is an increased risk of patellar arthritis. I would simply say that if you have a horse that is unable to be an effective athlete due to a chronic stifle and nothing else has worked, then you can do the surgery and extend the horse's useful life (realizing some long-term risk) or lower your expectations in terms of what you and your horse can/will do.
Taken From Wiwfarm