ArgoSpine: Cervical Disc Replacement
BY EDITOR, DECEMBER 22, 2009
As part of our partnership with ArgoSpine, we're republishing select content from the ArgoSpine News and Journal for our audience. The article below, excerpted from a lecture by Robert P. Melcher, M.D. of the Klinikum Karlsbad-Langensteinbach Spine Centre in Germany, was first published in 2008 under the title "Total Disc Replacement: Cervical Disc Replacement."
We have seen interesting similarities in the development of the surgical treatment of spinal disorders and the treatment of disorders in the hip and knee: painful degenerative alterations of these joints were previously treated by gross cartilage resection followed by arthrodesis.
Achieving motion preservation was the goal of various implant techniques. The evolution of these joint replacement devices, especially for the knee, began with constrained or semiconstrained constructs. Fixation in the bone was achieved by large bone anchors, often with additional cement or screw fixation.
Loosening of the implant occurred after a short time and implant failures were frequently seen. The experience from the failures of the first and second generation prostheses led to multiple changes in the design but also the materials used. Today hip and knee total joint arthroplasties are considered to be routine procedures with minimal complication rates and construct survival time counted in decades rather than years.
It is interesting to see this evolutionary process being repeated for the cervical disc replacement with similarity in some developmental steps: constrained devices, solid screw fixation to the vertebrae and deep bony anchors.
Some models require wide resection of the endplates for their implantation. On the other hand, devices released to the market most recently are tiny unconstrained plates with a polyethylene core. Multiple sizes are available and the need for endplate trimming is minimal with good restoration of the sagittal alignment. Bony anchoring depends on rapid osseous bonding, supported by the sophisticated surface design. However, some constructs did not provide enough immediate fixation properties resulting in a higher incidence of device dislodgement.
The most demanding and challenging issue is the understanding of the physiological and the desired motion that should occur in the prosthesis. Whereas the ROM of any peripheral joint can be easily examined and standard values are well known and established, the knowledge and the understanding of the physiology and pathophysiology of intervertebral motion in the cervical spine is only rudimentary. In addition, the spinal motion segment consists of three articulations, two real joints (facet joints) and the intervertebral disc. It can be debated if replacement of the intervertebral disc should be called arthroplasty. Different investigational models are used to answer the questions of the motion modality of each cervical FSU. It becomes obvious, that the radius of the spherical facet joints may play a crucial role in the motion patterns of disc prosthesis in conjunction with the prosthesis' core radius.
Limitations for motion preservation in the cervical spine may be an already preexisting stiffening of the FSU. Once the body's self-healing process has reached an advanced stage towards spontaneous fusion, affording to reverse this process by implantation of a mobile device, it may be to a limited extent. The long term follow up will most likely reveal an outcome of fusion despite the prosthesis.
As it is the case in all new developments, in the early phase, predictions on the long term results are not possible. Very often the outcome of total disc replacement is compared to ACDF as the so called gold standard. However, a recently published study reported residual pain and disability in 70% of patients at 6-year follow up after ACDF.
