Compartment pressures will be recorded using a slit catheter technique. Insertion of the slit catheter will be performed by either the chief investigator or Orthopaedic Registrar who will have been trained in the technique. The slit catheter is inserted using a sterile technique - the skin over the front outer part of the leg is cleaned and a catheter inserted into the anterior muscle compartment at this point. The catheter should be sited at the approximate midpoint of the leg, although this can be adjusted up or down if this would lead to insertion into a fracture site. The catheter is secured with a sterile dressing and connected to a monitor from which recordings can be taken, the catheter can be left in place for the duration of the study with no need to re-site it unless it stops recording accurately. At the same time as compartment pressure measurements are taken the patient's blood pressure will be monitored and the 'Perfusion Pressure' for the leg calculated, where: