How have you managed for so long?
Can I suggest a couple of supplements to relax the back and restore some strength to the leg that gives way?
Magnesium is always a favourite recommendation as it is necessary for proper muscle function, specifically muscle relaxation and could very much help your back.
I would of course normally mention our Atrosan® Devils claw, which is licensed for bad backs, but I presume you've tried all sorts of strong anti-inflammatory medicines already.
The other supplement is Creatine. Creatine is needed for energy production within the muscle cells and has a reputation as a bodybuilders supplement but don't let that put you off.
It is not steroidal in any way and body builders simply use it to improve the force of muscular contractions.
For that same reason, I have heard and seen for myself excellent results in people with muscle wasting diseases.
One person in Northern Ireland I met would sit in the car while his wife shopped because he didn't have the strength to get out. After taking Creatine, his strength returned so that he could accompany her. If he manage that with his form of muscular dystrophy, which causes a great loss of muscle strength, I hope you might find it helpful too.
It's inexpensive (about £10 for 100g or 20 days worth at 5g a day) and if you find it helps, just halve the dose thereafter to half a teaspoon (2.5g) a day.
Make sure you drink plenty of water with it as it needs the water to enter your muscles.