Artificial disc for cervical disc replacement (Research article)

An artificial cervical disc is a device inserted between two cervical vertebrae after an intervertebral disc has been surgically removed in the process of decompressing the spinal cord or a nerve root. The intent of the device is to preserve motion at the disc space. It is an alternative to the use of bone grafts, plates and screws in pursuit of a fusion following such a disc removal, which necessarily eliminates motion at the operated disc space in the neck.

Cervical disc replacement surgery would most typically be done for patients with cervical disc herniations that have not responded to non-surgical treatment options and are significantly affecting the individuals' quality of life and ability to function.

An artificial disc surgery may be done instead of an anterior cervical discectomy and fusion. The theoretical advantages of the artificial cervical disc over a fusion include:

• Maintaining normal neck motion

• Reducing degeneration of adjacent segments of the cervical spine

• Eliminating the need for a bone graft

• Early postoperative neck motion

• Faster return to normal activity

Postoperative neck braces are not required for disc replacement operations.

There are currently a number of artificial disc technologies undergoing clinical trials in the US to evaluate their safety and effectiveness. Various cervical artificial discs are in use outside the United States, with the oldest prototype device dating back to the early 1990s in England.

Artificial cervical disc background
Many people may experience neck, shoulder, and/or arm pain in their lifetime due to disc abnormalities in the neck. These complaints can be signs of disc herniations or disc degeneration, and/or arthritis of the neck.

The cervical spine (neck) is composed of vertebral bodies (the boney building blocks of the spine) and intervertebral discs, which act as combination universal joints and shock absorbers between the vertebrae. With time, the discs can become worn out and cause pain and/or other symptoms, a condition typically referred to as degenerative disc disease, a subgroup of which will include cervical disc herniations. This means the disc becomes compressed, frayed, and/or herniates into the adjacent spinal canal where it can press on nerves or the spinal cord.

Most patients with these types of symptoms do not need surgery. They typically can improve with conservative (non-surgical) treatment, which may include anti-inflammatory medications, physical therapy, or cold/heat therapy. Over 90% of patients will experience pain relief with these modalities within four to six weeks.

Anterior cervical discectomy and fusion
If a person continues to have significant neck pain and/or radicular arm pain (meaning that the pain is "shooting" down from the neck into the arm) after a trial of conservative treatment, then he or she may be a candidate for cervical spine surgery. An anterior discectomy and fusion is the most common operation for treating patients with symptoms related to a degenerative or herniated disc in the neck.

This procedure consists of removing the problem disc entirely and replacing it with a piece of bone taken either from the patient’s hip or a human cadaver. A metal plate with screws and/or a cervical collar may also be used to help hold the bone in place and to allow this segment of the neck to fuse together. The purpose of an anterior cervical discectomy and fusion surgery is twofold:

• To remove the offending agent—either the herniated disc or the osteophytes that are compressing the nerves and/or spinal cord.

• To eliminate motion by inducing a fusion at the disc space where the disc has been removed, aided by the use of bone grafts and possibly plates/screws, etc. and thereby creating stability and/or eliminating pain associated with the motion.

This type of surgery typically improves the pain in over 90% of people with one-level disease. However, there are potential complications in using bone grafts in pursuit of a fusion. Harvest of one's own bone may be associated with both acute and potentially long-term pain from the donor site. Any type of bone graft may fail to heal, resulting in a so-called 'non-union', which may require another fusion operation. Sterile bank bone (cadaver bone) is more convenient and not associated with donor site complications, but it tends to heal a lesser percentage of the time. Studies have also shown that by fusing a segment of the spine, the levels of the spine above and below the fused area are now forced to absorb more load since there is no longer any intervening motion shock absorption. These adjacent levels will then wear out and become symptomatic in more than 25% of these patients within ten years, meaning possibly more surgery. This is called adjacent-segment degeneration.

The development of artificial cervical discs is intended to accomplish the same objectives as the traditional decompression and fusion surgery in terms of providing pain relief and stability, but with fewer drawbacks. For example, the cervical artificial disc does not include the potential complications involved with using bone graft (e.g. 'non-unions') and theoretically should lessen the risk of developing adjacent-segment disc degeneration or disease.