Chien is a vitreoretinal specialist and surgeon at OCL Vision, Great Ormond Street Hospital for Children (GOSH) and Moorfields Eye Hospital. He is ranked top five globally by Expertscape, based on publications in vitreoretinal surgery over 10 years (2013-2023). At OCL Vision, the current holder of the Best in Healthcare Outcomes award (Laing Buisson) and leading private ophthalmology clinic group in England, he is a co-director, managing partner and clinical lead for adult retina.

At GOSH for Children, he is the head of the de facto UK national NHS centre for retinal detachment in retinopathy of prematurity where he introduced a novel surgical technique in 2014. This supports neonatal care throughout the British Isles, reducing the risk of lifelong blindness in premature babies. In 2020, he delivered the first-in-man retinal gene therapy treatment in a child with an ultra-rare inherited retinal disease (AIPL1) at GOSH, as part of a University College London (UCL) research team.

He is passionate about training surgical teams around the world, supporting eyecare charities. He has worked closely with the Armenian Eye Care Project (AECP) since 2014, a not-for-profit California-based organisation supporting eye care in Armenia, with over 13 trips to the country, training the national surgical team. Additionally, he recently donated time in Rwanda, training the national team at Kabgayi Eye Unit whilst his two children helped out in the hospital kitchen.