TKA is an abbreviation of 'total knee arthroplasty'.

In a total knee arthroplasty the joint surfaces of tibia, femur, and often the patella too, are replaced with metal and plastic.
Synonyms:
total knee arthroplasty
total knee replacement
TKR
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CLINICAL PUBLICATIONS
MR Imaging of Knee Arthroplasty Implants. Fritz J, Lurie B and Potter HG. Radiographics. September-October 2015; 35(5): 1483–1501.
Retention versus sacrifice of the posterior cruciate ligament in total knee arthroplasty for treating osteoarthritis. Verra WC, van den Boom LGH, Jacobs W, Clement, DJ, Wymenga AAB and Nelissen RGHH. Cochrane Database Syst Rev. 2013 Oct; 2013(10): CD004803.