Fusion vs Disc Replacement for Discogenic Pain: Part 2
Functional disc replacement is not a new idea. The initial steps of implantation dates back to the late 1950's performed by Fernstrom using a SKF ball bearing to produce a "ball joint" mechanism of the disc [Figures 3a, 3b]. The unique demands on spine arthroplasty implants necessitate that the intervertebral disc is not a true joint (with a center of rotation that is mobile) and serves a double function of mobility and damping with load repartition properties. The acceptance of arthroplasty to replace techniques of lumbar arthrodesis in disc disease will require a thorough analysis of the cost-benefit and risk assessment. Patient safety, efficacy, and value compared to current fusion techniques will need to be similar with this new and exciting technology.
Figures 3a and 3b. Complex nature of the intervertebral disc, as part of the 3-joint segment (disc and facet joints).
Compared to the medical advances in knee and hip arthroplasty, the progress in the last 30 years has been slow for the development of spinal arthroplasty. Over a decade ago, three mechanical disc prostheses were presented at the North American Spine Society (NASS) with optimistic insight to new methods for the treatment of degenerative disc disease. Although significant research has been conducted, knowledge, and expertise has been accumulated, there has yet to be a specific disc implant available in the United States for routine usage. Additional experience and research to evaluate the mechanical design, stability, and subsequent FDA regulatory pathways may result in an additional 3-5 years before this technology is universally available to clinicians globally.
The biomechanics of the lumbar motion segment have been well documented and studied in the past 2 decades. It may not be currently possible to mimic and reproduce all the mechanical properties and longevity of a natural disc without multi-components and materials. Contact stresses on an intervertebral arthroplasty will have to be minimized by having design characteristics of a significant cross-sectional surface area to distribute the load over the vertebral endplate. Unlike hip & knee arthroplasty, such large biomechanically stable implants will create specific surgical insertion problems. Disc arthroplasty implants need to have secure fixation methods to prevent catastrophic migration complications. Because of the complex structural and functional properties of the ankle, elbow, and wrist, arthroplasties in these joints have not been as successful as reconstructions of the hip and knee.
The treatment of symptomatic spinal diseases is fundamentally different than peripheral joints. The function of the peripheral joint is to allow a wide range of movements with cartilaginous surfaces. On the hand, intervertebral motion segments do not involve simple cartilaginous joints, but rather a highly complex structure consisting of peripheral collagenous bands, mucopolysacharide gels, and proteoglycans. The average spine motion segment undergoes approximately 100,000,000 cycles in a lifetime, and about 6 million each year. This highly complex structure of the disc allows small, precise movements around all three axise, and the center of rotation is mobile and not static. These unique structural, functional, and pathogenic factors create obstacles in the development of an efficient, predictable, and reliable artificial disc for the human spine. The average implant survivorship is estimated to be 30 million cycles (5 years of clinical usage), and therefore the demands on spine arthroplasty implants will be challenged.
In symptomatic degenerative disc disease, the origin of nociception is multi-factorial, and pain can originate at any of the components of the three-joint complex. When a patient undergoes a spinal arthrodesis, all the structures capable of nociceptions are rigid in contrary to motion preserving techniques. The literature is suggestive that the clinical outcome is not directly related to the occurrence of a biological osseous incorporation, and that a non-union of the spine does not preclude to a good outcome. The current published data on spine arthroplasty success is comparable to that of fusions.