Finally got around to ACL reconstruction. After the surgery the nurse got me up on crutches. My knee collapsed and I fell hard on the floor. I was carried back to bed. The next morning the surgeon saw me and said that it was most likely the nerve block that stopped my thigh from holding me up. The physio put a full leg brace on and sent me home on crutches. It's 5 days since surgery and my knee hurts to be straight which is necessary. Knee still collapses without the full leg brace but I can weight bear. It's a painful and frustrating recovery. I see my own physio in 2 days. Does anyone have experience of ACL hamstring reconstruction?
I had my ACL replaced several years ago. My experience was very similar to what you are describing. My physical therapist told me that sometimes a nerve or two will be severed during surgery and the brain will not recognize the correct signals to keep the leg bearing weight. That's why it collapses at random moments. My knee gave out for a couple years (at increasingly large intervals). My pain was stupid high right after surgery. It got better the harder I continued to work.