What Can Cause Pus From Knee Replacement Surgery Cut?

my husband had a total knee replacement 5wks ago, the bottom of the scar srarted weeping a nurse looked at it and said there was a pin hole size exit in the wound but it is fine, two days ago it started pouring with blood & pus, you could now get an ear in to the wound we are awaiting a swab test & hes been given antibiotic, but its not clearing up the the wound is getting bigger