Hello, i am dr. Selmani and I read carefully you question. It seems to me that due to sports activities and previous ACL and meniscus tears you may have developed also an early arthritis sighs even though you are young in age. it may have starteg as what we call chondromalatia or chondropathy which in the first stages is softening of the snoth layer of the joint cartlidge abd later erods more until the bone rubs agains bone. i would like to have more information from your previous arthroscopy. Anyway, since your doctor has recomended to you the gel injection which i think you mean Acid Hyaluronic, I suppose that during the surgery he has observed some sort of cartlidge erosion or wear so I would recommend you to go ahead and do the gel injection. Prior to that , you should discuss with your doctor about risk for any alergic reaction from the injection especialy if you alergic from eggs or chicken meet. The injection is in 2 forms. You can either do the single shot or 3 separated doses with 2 weeks interval each. The later one is more time consuming as you have to go 3 to 5 times to the doctor office but has less chance for complicarion since the dose is smaller. The effect of the gel lasts 6 months at least and you may need to repeat it if pain continues. Also i would recommend you to take from month some pills with Condroitine and Glucosamine for at least 3 to 6 month to help regenerate the smoth layer of cartlidge. i wish you all the best.