1 Department of Neurosurgery, Western General Hospital, Crewe Rd, Edinburgh EH4 2XU, United Kingdom; Edinburgh Spinal Surgery Outcome Studies Group, Edinburgh, UK. Electronic address: andreas.demetriades@gmail.com.
2 Department of Neurosurgery, Western General Hospital, Crewe Rd, Edinburgh EH4 2XU, United Kingdom; Edinburgh Spinal Surgery Outcome Studies Group, Edinburgh, UK.