Artificial Disc Surgery in the Cervical Spine
Part 1: Current Treatment Options without Artificial Discs
What is a Cervical Disc?
The spine is a column made of up bones, discs and ligaments. The blocks of bone (or vertebrae) provide the anterior support and structure of the spine. Discs are located between the bones (vertebrae) and function as "shock absorbers". The discs also contribute to the flexibility and mobility of the spinal column. The discs are made up of two parts:
1) The inner portion of the disc is a jelly-like material and is called the nucleus pulposus.
2) The outer part is called the anulus fibrosus of the disc. It is stronger and more fibrous than the nucleus pulposus. The anulus fibrosus surrounds and supports the inner jelly material.
Disc material is mainly composed of water and other proteins. As a normal part of aging, the water content gradually reduces. This can cause the disc to flatten out and even develop tears or cracks throughout the anulus fibrosus. These discs are often referred to as "degenerative" discs and may or may not cause pain.
In the case of a degenerative disc, the inner jelly material (nucleus pulposus) can bulge out and press up against the anulus fibrosus. This can stimulate the pain receptors causing pain to occur. The cracks or tears that develop in the anulus fibrosus can also become a source of pain. Finally, the inner nucleus can also come out through the cracks in the anulus and compress nerves or spinal cord, a condition that may cause weakness, pain, pins and needles or numbness, and may require surgery.
Current Treatment Options for Prolapsed Discs
Non-surgical options for people with disc protrusions in the neck include rest, heat, pain medications and physiotherapy. When non-surgical treatment options fail, surgery is often the next step. This usually means spinal fusion surgery.
Neck pain with compression of the nerves in the neck or spinal cord is a common condition that affects the spine and may require surgery. If only nerves are compressed, with symptoms in one arm, a period of non-surgical management is instituted. If this fails surgery is contemplated.
Early surgery is performed if there is severe weakness or pain that cannot be effectively controlled with available analgesia (pain relieving medication). If the spinal cord is being compressed, surgery to decompress the spinal cord is usually recommended. Compression can be caused by a bulging disc or bony spurs (bony overgrowths).
Surgery on the spinal cord is performed either from the back of the neck (laminectomy) or through the front of the neck (cervical discectomy or vertebrectomy). If compression of the spinal cord is from the front, then the decompression must be done from the front (anterior decompression).
Typically, if the entire disc is removed, a wedge of bone is taken from the hip and put into the empty disc space. A plate and screws may be used to hold the bone wedge in place (see below). This is commonly referred to as an anterior cervical decompression and fusion.
Figure 1: MRI scan of the cervical spine showing a typical disc protrusion between the 5th and 6th cervical vertebra compressing the spinal cord
Figure 2: A Schematic diagram of a typical anterior discectomy and fusion procedure. A block of bone graft is placed into the space left when the disc is removed
Figure 3: A post-operative x-ray on the patient shown to the left. The disc has been removed, a block of bone has fused the 5th and 6th vertebrae and a plate with screws holds it into place
This is a common operation and whilst it takes pressure off the spinal cord, it necessitates that at least two of seven bones in the neck are fused. This does reduce some of the movement in the neck, but patients typically do not notice it unless several levels are fused. Typically after this surgery, the patients wear a neck brace for six weeks.
Fusing Adjacent Levels of the Spine
The problem fusing bones in the neck are that adjacent levels in the neck are placed under more strain. This increases "wear and tear" at the surrounding disc space levels and has been termed "adjacent segment disease". We now know that if 10 people undergo single-level fusion, at 10 years three patients have had to undergo another operation to treat narrowing at the next level either above or below the original fusion.
Secondly, the bone does not always heal or "fuse" correctly. In fact, the overall success rates for these procedures range from 48% to 89%. Finally, spinal fusion at one or more levels increases stress to the rest of the spine. This transferred stress may cause new problems to develop at other levels, which may lead to the need for additional surgery.