This is a common concern to patients who I believe they have metal allergies. Although no definitive research has been done that shows that patients who have sensitivity to metal ever experienced adverse symptoms that can be proven to be directly related to the materials that their knee arthroplasty components are made of. If the patient does have a metal allergy, almost specifically a nickel allergy then special types of knee replacement do exist that do not contain any trace elements of this metal.
Unicompartmental knee replacements, however, do not have a wide range of metal alloys that they are made from. Consequently if you are concerned that your metal allergy will be a factor when deciding whether or not to undergo unicompartmental knee arthroplasty, this is something that you should discuss with your surgeon as you may better served with a total knee arthroplasty that does not contain trace elements of nickel in it.