Thiamine is an essential vitamin in the body, particularly for brain and nerve function. Many people in the UK are prescribed thiamine due to poor diet or excess alcohol use. Several preparations of thiamine are available, but it is not clear what dose or method is best absorbed by the body. This study aims to compare several forms of thiamine by testing the absorption into the blood and cerebrospinal fluid (CSF) at different time points after taking a dose. A total of 113 adults having an orthopaedic operation are invited to participate as gaining access to their CSF and blood is part of standard care.