If artefill is permanent non surgical nose surgery than how come it only lasts 18 months?

It's not permanent: Artefill is an injectable filler substance which is used as an alternative to surgery to correct deformities in the nose. Any filler used for a non-surgical nose job is temporary and will eventually be broken down over a period of time. The only way to permanently fix deformities of the nose is through traditional surgery.

"Duration" varies: Artefill has two components - pmma (polymethlymethacrylate) microspheres (permanent) and bovine collagen (temporary). The bovine collagen component will gradually dissipate but the tissue also responds to the presence of the pmma microspheres by building collagen. Long-term improvement (duration of effect) depends on the effectiveness of the collagen building process but also other factors...

Should last longer: The PMMA component of Artefill is not broken down by the body and stays in the skin. However, Artefill Non Surgical Rhinoplasty is performed in 3 to 5 sessions, spaced 6 weeks apart. One session is not enough. After doing hundreds of these procedures, I think that some people's collagen building reaction around the PMMA can fade partially over time. These people just need a touch up treatment.