The Surgical treatment of Lumbar Spinal Stenosis
Following your recent MRI scan and consultation with Mr Paremain you have been diagnosed with a narrowing of your lumbar spinal canal (stenosis). This is usually related to the wear and tear of the spine.
The normal spinal column has a central canal (or passage) through which the spinal cord passes down. To each side of the canal, spinal nerve roots branch out at every level. The spinal cord stops at the top of the lumbar spine (low back) and below that tiny nerve rootlets splay out like a horse’s tail (cauda equina). The spinal cord, nerve roots and cauda equina are surrounded by cerebrospinal fluid (CSF) and are all contained within a membrane, or covering, called the dura mater, rather like the thin layer that covers a boiled egg with the shell removed.
The ligamentum flavum is a tough band of elastic tissue (ligament) that connects the vertebrae (bones of the spine) and provides stability for posture and protection for the dura mater.
The intervertebral disc is a structure between the vertebrae, which acts as both a spacer and a shock absorber. Over time, as disc degeneration (wear and tear) occurs, the disc will lose water and height and, as such, close down the bony passage (foramen) where the nerve root passes through on leaving the spine.
In spinal stenosis, the spinal nerve roots and/or cauda equina become trapped or compressed by the narrowing of these nerve passages, by arthritic joint swelling and bony overgrowths (osteophytes) which grow into the spinal canal and the bunching up (buckling) of the ligamentum flavum, like an elastic band losing tension, or bulging of the intervertebral discs. Rarer cases include cysts or fatty collections or tumours in the spinal canal.
Diagram 1 below is an impression of the MRI scan on the right which demonstrates spinal stenosis of the central spinal canal.
IThe image below illustrates the progressive degenerative changes that lead eventually to narrowing of the nerve root canals through which pass the branches of the sciatic nerve.
When nerves are compressed they can produce symptoms of pain, numbness or tingling in the area of the leg that the particular spinal nerve supplies. In rare cases they can produce severe pain and even weakness in the legs, such that the ‘legs don’t work’.
In most cases, the symptoms are produced when standing or walking and are relieved by sitting or bending forward, as this can temporarily open up the nerve passages. In rare cases the nerves which control your bladder, bowel and sexual function can be compressed. This is known as cauda equina syndrome(CES) and often requires urgent surgical intervention. Fortunately, immediate spinal surgery is only necessary in a few cases.
Unfortunately, most conservative treatments (manipulation, physiotherapy, medication or injections) are unlikely to be of much long-term benefit, and the symptoms rarely improve permanently without surgery to take the pressure off the nerves (decompression).
The objective of surgery is to remove the material (for example the excess bone and ‘thickened’ ligament) from the back of the spinal canal to give the nerve roots and/or cauda equina more room.
The nature of spinal surgery is not to ‘cure’ and it cannot prevent further degeneration of the spine but is aimed to provide benefit with a good percentage improvement and relief of leg symptoms. Good relief from leg symptoms following decompression surgery usually occurs in approximately 70–80% of cases (up to 8 out of 10 people). This is not necessarily felt immediately, but over a period of time, often several months. Sometimes however, numbness or weakness can persist, even with a technically successful operation. This can occur when people have more extensive stenosis before they have surgery. Rarely, the surgery may make your symptoms worse than they were before. Many patients will also experience some relief from their back pain as well as from their leg pain. However, the results of the operation are not nearly as reliable for the relief of lower back pain. Much of the back pain experienced comes from the arthritis and associated muscular spasms, therefore, decompression surgery cannot eliminate this and it should not be regarded as the main aim of the surgery.
There are different techniques when performing an operation for spinal stenosis. Expected outcomes from all methods are very similar and the best choice of operation will be decided by Mr Paremain.
Laminectomy and/or laminotomy
This is performed through an incision in the midline of the lower back. The position and length of incision is determined by which levels of the spine and how many nerves are involved. The muscles are then held apart to gain access to the bony arch and roof of the spine (lamina). Next, the surgeon needs to gain entry into the spinal canal by removing some bone, either by cutting away the whole area of lamina (laminectomy) or making a small window in the lamina (laminotomy) with a high-speed burr (like a dentist’s drill) or an osteotome. Further bone and ligament are then removed and often the facet joints, which are directly over the nerve roots, are undercut (trimmed) to relieve the pressure on the nerves and give them a wider passage as they pass out of the spine.
The illustration below shows the ligamentum flavum has been removed to expose the contents of the spinal canal. The flavum forms part of the roof of the spinal canal and removing it allows the canal contents to expand. This in turn relieves the pressure on the nerves running within the canal.