Left low back pain = leg length discrepancy?

Hi all,

I'm a 36 year old male. About 10 years ago I started getting some pain on the inside of my right knee. After a while this was joined by some pain in my lower left back. Over those 10 years the pain seems to have transferred completely, such that my right knee is now absolutely fine (touch wood) but my lower left back pain is gradually getting worse (maybe 0.25 on a scale of 1 to 10 each year). I've been to two physios - one private, one NHS. Both seemed to think it was SI joint related. Had a few sessions of manipulation, was given some exercises which I did regularly for a few months, but none of it helped.

Any (fresh) ideas what it could be? It's not excruciating by any means, maybe 3 out of 10 at it's worst, but it's persistent and as I say, getting slightly worse each year. If I were to put my finger on the source of the pain it would be the bottom of the spine, where you have those two bony bumps - the left one. But it sometimes (e.g. when I'm walking) feels like it's coming from inside the ball joint of the hip/leg.

Leg length discrepancy keeps crossing my mind, as it would explain (in my layman's mind!) the asymmetrical/one-sided nature of the issue, but the consensus seems to be that most people have a small LLD and it's rarely structural so shouldn't be addressed directly (e.g. with a heel lift, which I haven't tried as I'm not sure which leg would be shorter!) I do appear to wear my right heel of my shoes down at a quicker rate if that factors in at all.

Really at my wits end with it. Do I try another physio? Or perhaps an osteo? I used to play tennis several days a week (and that is when the knee issue initially presented), I've taken a good 3-4 year break from this (which hasn't helped much). I really want to get back into tennis, but I played a couple of sessions this week and it's definitely flared up. I want to play more and could use the exercise but I don't want to do/accelerate any damage!?

Thanks in advance for any advice you can offer.

Rob