The human spine is made up of 33 vertebrae, which provide physical strength while allowing the spine to be flexible. A number of conditions can damage or change the structure of the spine and surrounding tissue, including:

  • Spinal deformity, the most common form of which is scoliosis. In the UK, spinal scoliosis affects 3–4 of every 1000 children and young people, and 7 out of 10 adults aged 65 years or older (NHS Choices 2013a).

  • Spinal fractures, which happen most frequently in people with osteoporosis and as a result of trauma. Approximately 120,000 cases of vertebral fractures happen each year in the UK (van Staa et al. 2001).

  • Age‑related degenerative diseases of the spine, including osteoarthritis (spondylosis), spinal stenosis and degenerative spondylolisthesis. In the UK approximately 8.5 million people have radiologic evidence of osteoarthritis of the spine (Arthritis Research UK 2004).

  • Primary and secondary (metastatic) tumours of the spine. Primary spinal tumours are rare and make up less than 5% of bone neoplasms, but spinal metastases are more common and affect about 40% of people with terminal cancer (Tidy 2011). They can also lead to the development of metastatic spinal cord compression. According to NICE's guideline on metastatic spinal cord compression there are about 4000 cases in England and Wales each year.

All of the conditions above can cause pain and restrict movement. The treatment recommendations will depend on the condition but in severe cases, spinal surgery may be offered (NHS Choices 2013b, 2013c). There are 3 broad types of spinal surgery:

  • decompression of the neural elements without spinal fusion

  • decompression of the neural elements with spinal fusion

  • correction of deformity with spinal fusion.

The last 2 types of surgery involve spinal fusion. Spinal fusion is a procedure in which 2 or more vertebrae are joined together using a bone graft to stabilise and strengthen the spine (NHS Choices 2013d, 2013e). Spinal instrumentation, which involves the use of metal implants to hold the spine in place, may be used in spinal fusion (Awasthi and Thomas 2004).

The standard method for performing spinal instrumentation may involve drilling pilot holes using a sharp surgical tool such as a pedicle awl, and inserting metallic screws into the pilot holes in the vertebrae. This procedure can also be performed percutaneously, although this is a relatively new method and there is little evidence as to its clinical effectiveness. The metallic screws are often referred to as pedicle screws because they are inserted through a small canal of bone called the spinal pedicle; they can act as a foundation for spinal implants. A pedicle probe can be used to measure the depth and trajectory of the pilot hole (Awasthi and Thomas 2004).

The main risk associated with placing pedicle screws is pedicle perforation, which occurs when the screw exits the vertebrae. This can result in dural tears, vascular injury, nerve injury or, rarely, spinal cord injury. The rate of pedicle perforations reported in the literature varies greatly. A systematic review and meta‑analysis calculated a perforation risk of 6–15%, depending on the insertion method used (Shin et al. 2012). Evidence suggests that the risk of severe complications resulting from pedicle screw perforations ranges from 0.8% to 1.4% (Amato et al. 2010; Oh et al. 2014).

A reliable method to ensure safe drilling through the vertebral pedicle and into the vertebral body may reduce the rate of pedicle perforations and therefore improve patient outcomes and reduce costs.