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.

Adult patients undergoing knee arthroplasty will be recruited to this study. Participants will be randomized to the ultrasound monitoring group or the symptoms alone group. In the symptoms alone group, urination is monitored by asking at regular intervals about the onset of spontaneous urination and the patient is catheterized only on a symptomatic basis or if spontaneous voiding has not started 10 hours after the spinal anesthesia. Symptoms that indicate catheterization are lower abdominal pain or need to void. In the ultrasound group, urinary retention is monitored according to current practice with an ultrasound scanner and the patient is catheterized if residual urine exceeds 800 ml or if the patient is symptomatic.

The primary outcome variable is the change in IPSS score from the baseline to 3 months after the procedure. The baseline score is evaluated preoperatively.

Inclusion Criteria:

• elective total knee arthroplasty

Exclusion Criteria:

• previous surgery for urologic cancer
• fracture as indication for surgery
• lack of co-operation
• untreated urinary retention
• nephrostoma
• general anesthesia

in 1:1 ratio. Participant eligibility is based on self-representation of gender identity.