MRI deffinitely. It is non-invasive, non-iradiating, very accurate in diagnosing soft tissue problems (muscle, ligaments, discs) which can be totally inapparent to X-rays, the single problem is that it is expensive (but most contries have insurance policies covering investigation for such a longstanding case, especially). It it excludes any sign of lesion, a pure functional syndrom benefits not only of homeopathy but also acupuncture and a good physiotherapist can work wonders.