Arthroplasty increases the risk of postoperative urinary retention. Treatment of postoperative urinary retention is indwelling or intermittent catheterization. The need for catheterization is most commonly determined with an ultrasound scanner. Catheterization increases the risk of urinary tract infection, which may further lead to haematogenic infection of the artificial joint. There is no evidence of an optimal way to monitor the onset of spontaneous urination. The aim of the study is to determine whether monitoring the onset of spontaneous urination after fast track knee arthroplasty is safe based on symptoms alone without ultrasound monitoring.