Cervical disc replacement update (Research article)

Cervical artificial disc

The cervical intervertebral disc is an amazing mechanical structure from an engineering perspective. It has the ability to absorb a large compressive load while still providing adequate motion between two adjacent bony vertebrae. Duplicating the disc’s form and function with a synthetic disc is challenging. However, several cervical artificial discs are in various phases of development and testing in the US and may soon be available for use in the general patient population.

The first report of a cervical disc replacement in the medical literature came from the South African Medical Journal in 1964.¹ Since then there has been great progress in the research and the technology of the cervical disc arthroplasty field. In addition there has been great progress made in understanding the disease process that can result in the need for a disc replacement.

This review will discuss degenerative disc disease of the cervical spine and its resultant clinical consequences. Next the current “gold standard treatment” (fusion) will be discussed including the rationale for the use of a disc replacement. Finally the different types of cervical disc replacement technologies will be described.

Cervical degenerative disc disease

Cervical degenerative disc disease of the spine is a prevalent condition in the population. Studies have shown that after age 40, almost 60% of the population has radiographic evidence of cervical spine degeneration and by age 65, 95% of men and 70% of women will have some sort of degenerative change on x-ray.² Fortunately, many of these changes seen on x-ray can be considered a general aging phenomenon and not pathological as most people with degenerative changes on an imaging study do not have any pain or other symptoms. However, in some patients, as the disc degenerates it can result in a general arthritis of the bony elements of the cervical spine or a herniation of the disc elements. One of the clinical manifestations of disc degeneration can result in neck and radicular type of arm pain (pain that radiates from the neck down the arm and possibly into the hand and fingers). In addition to neck pain and/or arm pain, symptoms may include numbness, tingling or weakness, and again these symptoms may radiate into the arm.

There are many causes of disc degeneration:

• Dehydration of the disc

• Facet joint arthritis

• Breakdown of the collagen framework.

Most instances of pain or other symptoms from cervical degenerative disc disease (or a cervical disc herniation) will resolve spontaneously and not require any type of interventional treatment. Unfortunately there are some cases that are recalcitrant to conservative treatments and require some type of surgical intervention. Most surgical strategies require a formal anterior approach to the cervical spine (from the front of the neck) and removal of the cervical disc along with the offending herniation. The space where the disc was is then replaced with a block of bone that leads to a fusion of the two vertebrae in place of the disc space. Reports in the literature indicate that in the properly selected patient this form of treatment can result in over 90% success rate.

Cervical fusion vs. cervical disc replacement

While removing the disc and replacing it with a block of bone provides excellent clinical results, it unfortunately has some adverse long term effects. In reviewing the long-term results of patients after an anterior cervical discectomy and fusion, Hilibrand et al was able to show that a significant proportion of these patients required further operations due to adjacent level disease.³ This consisted mainly of a breakdown of the joints above and below. It has been theorized that this occurs due to the increased stiffness from the bony fusion.

The cervical spine has a fluid range of motion that occurs through seven vertebral bodies. Intuitively it makes sense that fusing one of these segments will result in adverse consequences to the remaining joints. This adjacent level disease has encouraged researchers and surgeons to look for a solution other than fusion. This has been the impetus for the use of a cervical disc replacement.

In examining the articulations of a cervical spine segment, it is clear that the joint consists of articulations between two uncovertebral joints located in the lateral margins anteriorly, and two facet articulations posteriorly. The goal in cervical disc replacement is to remove the offending disc as well as provide a normal range of motion. The hope is that providing a normal range of motion can prevent the risk of adjacent level disease that can develop after anterior cervical discectomy and fusion.

In addition the prosthesis should have a means to be fixed to the cervical spine so as not to dislocate. Finally the prosthesis should have a means to permanently bond to the adjacent bone to provide a stable means of secondary fixation.