is it true that MRI is contra-indicated in if a person has a prosthetic knee?
yup I think so , same is true for pacemaker you can not do MRI when a patient is having heart pacemaker.
You cannot use an MRI if the body part being imaged has metallic prostheses/foreign bodies. So you cannot do an MRI-head in someone with a metallic foreign body in the eye but you CAN in someone with a prosthetic knee.
Besides pacemakers, cochlear implants, insulin pumps etc are also contraindications.
Hope this helps.