I believe it is vital to ask patients before surgery regarding their expectation of their surgical knee - as a surgical knee is not a natural knee!

This review article highlights that while Total Knee Replacement is a very reliable procedure, up to 20% of patients can be dissatisfied after their operation. This dissatisfaction may be because of unrealistic expectations about what a knee replacement can achieve for patients.

Pain relief after surgery is often unpredictable, gradual, variable and INCOMPLETE! Up to 1/3rd of patients will still report some knee pain 1 year after a knee replacement, however it should be less severe. Almost all patients report a "knocking noise" in their knee - which is normal and to be expected. Difficulty kneeling, squatting and climbing stairs can persist for 2 years.  

Here is the link to the paper - well worth a read.