thanks for info. dont know your physio so wil llist al options and you can check if been considered. firstly agree that MRI doesnt always hold the whole truth. also where was the initial 'tearing'sensation u describe?

things to check,
ligament atttachements around back and buttock
piriformis and gluts in general - muscles in buttock
QL and paravertebrals - muscles in back
sacro-iliac joints, for dysfunction or actual injury some therapists do not believe it causes problems - but I do
slump test for neural scarring or tethering, reflexes, strength in leg, to differentiate actual sciatica from neural irritation
function of lower back, I am aware talk of disc but has lumbar manipulation been tried (with caution) if some degreee of facet involvement etc
also moving up the chain and checking thoracic and cervical spine, shoulders etc for good function

hope this helps, take it to your physio and discuss but it sounds as though he is confident of your problem and has probably considered all these things anyway

again diff over the net, but some things off top of my head