This would be a good opportunity to work on your shoulders (thrower's 10 and other exercises), abs, lower back, hips and other parts of your upper body. Some physical therapists and fitness clubs have a cardio "cycling" machine that uses your arms/shoulders rather than your legs. If you don't have access to something like this you might try various types of arms swings and cycling motions with moderate or lightweight dumbbells. You can execute some of these while executing a twisting motion of the upper body (at the waist).
Would you be allowed to perform some leg exercises that do not require much, if any, knee bending or impact the knee joints? Perhaps you could work on the muscles of your calves, shins and feet with exercises like toe-raisers. Probably be a good idea to work on your hip flexors as well.
You might want to buy (or see if you can gain access to) a neuromuscular
). These types of devices apply small electrical impulses to the muscles via electrodes. This category of devices includes EMS
. An EMS
device is probably the most applicable of these for your needs. This can possibly be used on your quads (and maybe for your hamstrings) to prevent atrophy and might even be useful for strength training in those muscle groups. I believe that Bruce Lee may have used something like back in the day (circa 1970) to promote muscle strength and definition (and/or he may have used it for rehab for a serious injury).