I had a distal femoral osteotomy (along with a Tibial tubericle transfer and medial/lateral ligament reconstruction) back on June 13, 2011. I think the reason you cannot do quad sets is because you have no muscle. Have you really been on crutches for 6 MONTHS? If so...that will cause a bunch of atrophy. I was on crutches for 6 weeks and I lost a significant amount of muscle. And I didn't come close the past few months of strength training to getting it back to where it was before...its harder to grow that muscle back. I just had the hardware taken out last week so I am currently recovering from that.
I am sorry that your DFO has yet to heal...that is really strange. You should not still be on crutches. I hope you are doing therapy?
Are you sure your doc is proficient in performing DFO's? If you are interested in a second opinion, I could highly recommend two doctors who both specialize in the operation. I have had a very good outcome from the one. I don't come on here often, so email me if I could be of any further help with this.
Hope you start doing better.