Today, surgery in an ambulatory setting is preferred for many reasons. Total hip and knee arthroplasty (THA and TKA) are now procedures where patients can be sent home on the same day of the surgical procedure. However, this requires substantial knowledge of factors affecting the length of hospital stay. In this prospective, observational cohort study, we will investigate factors that affect the duration of spinal anaesthesia which is often used in THA and TKA. The aim is to gather enough data to be able to determine how local anaesthetic volume and dose affect the duration of spinal anaesthesia. This will enable us to decide whether spinal anaesthesia is a good option when performing TKA and THA in an ambulatory setting.