Knee Replacement is a costly operation, so will be reviewed by the doctor panel of the insurance company. I believe that they are as good as your current doctor to find out, when accident happen and steel rod etc. (or may go for further tests to find out).

Why this new problem happen? Is it relate with the old problem? Only doctors can say.

Pre-existing disease are covered after 4-5 years of the policy. So I feel it's better to declare the true state.