An Overview of Artificial Disc Replacement (ADR) for Cervical Spine
The artificial disc replacement (ADR) for cervical spine procedure could theoretically be used in any case where an anterior cervical decompression and fusion (ACDF) is used. There are mechanical advantages, disadvantages and limitations associated with this procedure.
The reason the artificial disc was invented was because degenerative disc disease can occasionally progress above and below the level of a cervical fusion. If some motion could be preserved, the levels adjacent might be spared further degeneration. Movement sparing devices can take some of the stress off of the levels above and below by preventing some increased forces that could cause degeneration.
Disc arthroplasty is designed to replace the motion of the damaged segment. It does so by restoring the height of the disc space and implanting bearing surfaces in place of the disc itself. Most disc replacements use chrome cobalt alloy. The metal is designed to press fit into the disc space and the bone will either grow into the metal or attach to it by scar formation. The problem with this metal is it is not MRI compatible. This does not mean that an MRI cannot be performed but the metal obscures the signal of the MRI around the area of the artificial disc replacement.
The replacement disc shares some of the characteristics of the biological disc but not all of them. The articulation or joint itself is a type of elongated ball and cup design. This design allows bending forward and backwards, side to side bending and some rotation. The design will not allow shock absorption.
The one disc replacement that does have shock absorption is the Bryan disc. This disc uses a membrane contained fluid filled core of saline (the same fluid as in the body) to act similarly to a normal human disc. The membrane allows only a specific amount of fluid in to create a cushion for impact activities. This membrane also provides stability similarly to the annulus of a normal disc.
It turns out that genetics plays a big role in the determination of the health of a disc. There have been reports of degenerative changes above and below implanted artificial discs so the additional motion may not solve the dilemma of continuing degenerative changes.
Limitations of Artificial Disc Replacement
The artificial disc replacement (ADR) for cervical spine will not work in certain situations. The artificial disc replacement depends upon the vertebral segment other than the native disc to be intact to function properly. That is, the facets and the ligamentous connections have to be “normal”.
If the facets exhibit degenerative changes such as wear of the cartilage, this could create pain with motion (what the surgery is trying to avoid). If the facets have worn down enough to create a condition called a degenerative spondylolysthesis (where the vertebra above has shifted forward on the vertebra below) this instability would preclude using an artificial disc arthroplasty.
The long term viability of the disc is not yet determined as these have not been around for more than 10 years. For artificial joints like a hip or knee, we know that they will last 10 -15 years before the need to be replaced. Because of that, artificial hip and knee joints tend to be implanted in individuals older than 60 years of age.
Disc replacements are being surgically implanted in younger adults and we do not know the life expectancy of the ADR at this point. Replacement procedures will have to be planned for. The artificial disc in the cervical spine is generally easy to revise to a fusion if necessary.
Finally, some individuals, especially women, will develop osteoporosis as they age. The interface between the metal of the disc replacement and the bone of the vertebra may fail with advancing osteoporosis and the metal disc may subside into the bone.
Types of Artificial Disc Replacements
There are 3 types of cervical disc implants- Medtronics Bryan disc, Medtronics Prestige and Synthes Prodisc C.
The Bryan disc consists of two titanium saucers that have ingrowth surfaces for bone and a membrane outer lining that holds water inside to recreate the normal disc space. This disc allows shock absorption and translation by stressing the membrane through compression of the fluid inside the disc. The titanium is also much more MRI compatable if a future MRI of the cervical spine is necessary.
The ProDisc-C consists of two chrome cobalt alloy endplates (similar to stainless steel) with a plastic bearing surface fixed to the lower plate. The edges of the implant are attached to the vertebra by a central keel with an in-growth surface. The keel stabilizes the implant until bone growth occurs into the endplate.
The prestige total disk replacement is also made of chrome cobalt alloy and has a ball-and-socket design. The entire disc is made of metal with the bearing surfaces entirely metal on metal. It literally screws into the vertebral bodies so it is stable immediately after surgery.
The differences in the three discs are not significant for a one level surgery but do become significant with more than one level disease. There is no approval currently for 2 ADRs in a row in a cervical spine. If a fusion (an ACDF) is needed in conjunction with an ADR for more than one level of disease, the ProDisc-C will allow fixation of the ACDF level with a plate. This would allow a better chance of healing for the fusion level. Using a Prestige disc will not allow a plate to be used in an adjacent vertebra.The Bryan disc is compatable with multiple level fusions and should be the best one for this purpose.
If two ProDisc-C ADRs are used next to each other (such as at C5-6 and C6-7), the keels that attach the disc to the vertebra can act as a wedge and possibly split the vertebral body in-between with significant impact. If two Prestige discs are used in adjacent segments, there may not be enough room to place both sets of screws into one vertebral body.
Dr. Donald Corenman, spine surgeon and neck specialist, welcomes new patients at his practice. To learn more about the artificial disc replacement (ADR) for cervical spine procedure as a treatment option for spine related conditions, please contact Dr. Corenman at his Vail, Aspen, Denver and Grand Junction, Colorado area office.