Ethical approval was obtained from the National Research Ethics Service (Edgbaston), reference 13/WM/0139. Upon admission to hospital for THA, eligible patients were invited to participate in the study. Inclusion criteria were: patients undergoing primary THA, in good general health and fitness, age 18 years or over and the ability to provide written informed consent. Patients were taken to the ultrasound suite within the first three days following surgery.