Hello, There can be a lot of causes for poor vision following congenital cataract surgery and unless each cause is looked for by a thorough eye examination it would be difficult to pin point on the exact cause. However the likely causes for poor vision are uncorrected aphakia- absence of lens-natural or artificial ( has been off contact lenses/ not wearing corrective glasses), amblyopia ( due to uncorrected aphakia), dense posterior capsular opacification(PCO- very common after pediatric cataract surgery), glaucomatous optic nerve damage. So unless the above are clear, visual prognosis after seconday IOL implantation cannot be commented upon. For good results, the optic nerve should be fine, no PCO, no amblyopia. Best wishes for you son s surgery and a happy festive season too! Regards