What Causes Numbness In The Left Leg?

I have Psoriatic Arthritis, I have Degenerative Disc Disease with the L4 L5 mri said obiterated, and S1 and problems with other joints, my Neurosurgeon wants to do a Transforaminal lumber fusion L4 to S1 my health insurance has denied it said it was not considered medically necessary, It is effecting my Soriactic nerve, left leg is going numb alot. have had some bowel problem, constant pain. left leg keeps giving out.