I used a metal testing kit (about $125) as a resource, you might find it to be helpful. I purchased the kit from Amazon or you can check online for other purchase options. My nickel response was very intense. No response to the other tested metals including titanium. I am aware there are trace amounts of nickel in titanium, that may over time be an issue.

Look for : Allergy Test Kit Medical Metal Implants - Tests 7 Common Metals Including Nickel - Wearable Easy Use - Visit the Sensiband Store

Of course, diagnosis must come from a physician.
Best wishes in your journey with your knee implants.