Hi BJB
Just be aware they will only be able to consider the evidence at the time of the claim - so the fact that your husband may need a second knee replacement sometime in the future probably won't be of interest to them, unless you made it clear in your original claim that he had problems with both knees.
Make sure that your husband is absolutely clear about the restrictions to his walking, distances, how often he has to stop, any aids, the pain he experiences, balance issues any other symptoms and how often he experiences them and whether his movements are such that there are issues with reliability and safety. Also if he drives at all, be ready to answer questions on that. Many people who drive feel they are penalised by the DWP.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
