Two courses of Hylan G-F 20 ( Synvisc) administered by intra-articular injection at 6 monthly intervals. MRI examinations of the knee for estimation of cartilage volume prior to treatment, before second course and at 12 months in 40 subjects with Grade 2/3 osteoarthritis of knee . A further 40 subjects with similar degree of osteoarthritis of knee who do not receive Hylan G-F therapy will be assessed by MRI in identical manner.