Persistence and improvement of nasolabial fold correction with nonanimal-stabilized hyaluronic acid 100,000 gel particles/mL filler on two retreatment schedules: results up to 18 months on two retreatment schedules - Narins,R.S.; Dayan,S.H.; Brandt,F.S.; Baldwin,E.K. (Dermatol Surg, 2008)