To control pain after total knee replacement surgery a catheter (tubing) is sometimes inserted into an anatomic space containing nerves that provide sensation to parts of the knee. This space is called the adductor canal.

The catheters often stop working before we remove them for unclear reasons. The investigators think this is because the catheters become dislodged from where it was meant to be. This could be due to repeated movements of the catheter tip brought on by patients contracting their leg muscles when they ambulate or perform physio.

The investigators want to confirm this by measuring the distance from a fixed spot on the patient's thigh to the adductor canal using an ultrasound machine. The leg will be measured in various positions to simulate muscle movements. A significant change in the distance could possibly contribute to catheter dislodgement and result in catheter failure.

Inclusion Criteria:

• Patients over or equal to the age of 18 years old who can understand the study protocol and are able to give consent
• Patients must be undergoing a primary total knee arthroplasty with neuraxial anaesthesia

Exclusion Criteria:

• Patients with an allergy to ultrasound transducer gel or measuring tape

We are recruiting patients undergoing total knee replacement. Per hospital protocol, patients will receive a spinal anesthetic and adductor canal block as part of their anesthetic regimen. After surgery, previously consented patients will have their skin to adductor canal distance measured by ultrasound in 5 different positions.

Adverse event date was not collected since this was an observational study

There was no opportunity for adverse event since the object of the study was ultrasound measurements of anatomy.