This is a two-phase multicenter, double-blind, randomized, prospective evaluation of intra-articular injection(s) comparing APS to intra-articular HA injection(s). The maximum study duration for each subject will be 62 months; 60 months from treatment to last follow-up, and two additional months if the maximum visit window is realized. A total of 246 patients will be enrolled. These patients will meet specific inclusion and exclusion criteria, but can be generally characterized as patients with painful unilateral knee OA who have been unable to achieve satisfactory pain relief with previous conservative OA treatment.