It's difficult to define the absolute most common complication of total knee replacement because this can vary by hospital and geographic area and can also be influenced by various social and economic factors.
That said, a 2017 study examining a Medicare beneficiary population found that the overall complication rate for knee replacement was 2.09%. In this study, the most common complication type for knee replacements during the procedure was pneumonia at 0.50%, whereas the most common complication after discharge was infection at 0.46%.
Another 2022 study separated complication rates by in-patient vs outpatient procedures. In-patient means the person was hospitalized for the procedure, whereas out-patient means that person could go home that day. Overall, in-patient procedures had more complications than out-patient procedures. In this study, a superficial surgical site infection and urinary tract infection (UTI) were most common for both types of procedures.