Disc Jockeying

Disc Jockeying

Back pain is as old as time, and the searchfor an effective cure is nearly as ancient.

Janet Cotton, Ph.D.
Contributing Writer

Picking up your kids.Throwing your computer bag over your shoulder. Bending over to tie your shoes. We often don’t give such everyday actions a second thought. However, throughout the course of a typical person’s life, the spine does a lot of work. In fact, in a lifetime of variable cyclical loading, the vertebral column experiences as many as 100 million flexion cycles, while a further 6 million cycles per year may be attributed to the minor motions induced by breathing.1 Given such frequency and variation in the loading of the spine, degeneration is likely and as such it is not surprising that lower back pain is a common ailment and is experienced by as much as 80 percent of people at some point in their lifetime.2

The Functional Spinal Unit

The spine functions to provide axial support and motion to the upper body, including flexion-extension, lateral bending and axial rotation. This support and motion is achieved by the three-joint complex that exists between each of the vertebrae in the spine, consisting of two adjacent vertebrae that attach to and sandwich an intervertebral disc forming one of the many joints in the vertebral column. Facets project superiorly and inferiorly from the posterior of each vertebra, interlocking with those of the adjacent vertebrae. The interactions of the bilateral facet joints of adjacent vertebrae complete the three-joint complex. The three-joint complex—called the functional spinal unit (FSU)—is the smallest component in the vertebral column that represents the biomechanics of the overall spine.

The loads experienced by the spine are distributed across the functional components, with the vertebral body and disc supporting 70-90 percent of static axial loads.3 The remaining axial load is taken up by the facets, supporting up to a maximum of 33 percent in full extension.4 An additional and equally important role of the facets is to provide restriction to the extent of motion, reducing the exposure of the intervertebral discs to torsional and shear loads, as well as limiting the extent of spinal flexion, extension and lateral bending.

Degeneration

Degenerative disc disease (DDD) is the most common disorder associated with the FSU.1 Progression of DDD presents as a degenerative cascade through three distinct stages, beginning with the dysfunctional phase occurring as a result of cyclic flexion-extension loads as well as torsional stresses producing radial tears in the outer ring of the intervertebral disc, reducing the disc’s integrity.2 Without the rigid outer ring, the disc can slip or herniate and impinge on nerves causing pain. Without the necessary stiffness of the disc, the degenerative cascade enters the instability phase. The ensuing cellular responses result in the formation of bony growths on vertebral endplates that further intrude on nerve canals.

Degeneration of the facets occurs in a manner that can be likened to the degenerative cascade of the intervertebral disc. Excessive loads result in the breakdown of cartilage, effecting a reduction in the contact surface and a loss of motion control. The increased motion imparts excessive loads to the facets and once again cellular responses result in the development of bony growths and the subsequent impinging of nerves.

A generally accepted hypothesis is that DDD precedes facet joint degeneration.5 The process progresses concurrently as DDD enters the instability phase, and increased joint laxity is experienced in the FSU. The excessive motion results in increased and more frequent loading of the facet joints, initiating their degeneration. Further development of DDD, resulting in severe disc space narrowing, yields excessive contact forces between facets during normal, extension and hyperextension postures.6 Such contact forces ultimatelyadvance the deterioration of facet surfaces.

In contrast, there exists an argument for degeneration of posterior elements preceding DDD. The aging process of the spine sees the facets become more sagittally aligned; decreasing their ability to resist and control shear forces.4 Ordinarily protected from shear loads, the intervertebral disc undergoes degenerative changes due to the imposed torsional loading.

Facet tropism is a condition of asymmetry in facet joint angles; one facet is orientated more coronally than the other. The incidence of tropism in patients who experience DDD is greater than in the normal population.3 It is for this reason that facet tropism also presents as a strong candidate for initiation of early DDD.7 Under an axial compressive load, the asymmetry of the facets results in axial rotation toward the more coronally orientated facet, imparting torsional stresses in the outer ring of the intervertebral disc, contributing to disc injury and deterioration.3

In review of the literature, it appears that DDD and facet degeneration are intrinsically linked. Sources note “[that] modifications of the facet joint influence disc degeneration and vice versa.”1

The link between pain and a degenerative joint often is a result of mobility, which in the past has resulted in motion-restriction measures for the relief from pain. Since its conception, fusion has been the favored treatment of degenerative spinal disorders, particularly DDD.8 The resulting loss of motion often is perceived to be of little functional consequence to the overall spine, whether fusion of one or several motion segments is induced.1 However there are disadvantages and fusion has been shown to result in morbidity, both short and long term. Of particular relevance are those incidents in the long term related to adjacent level degeneration (ALD) as various studies demonstrate a link between motion restriction and ALD.8 This knowledge more recently has resulted in the peeked interest in preserving natural spinal motion. Such enthusiasm has resulted in the development of a variety of arthroplasty devices, aimed at the reduction of pain as well as deterioration of adjacent levels.

Arthroplasty

Arthroplasty devices are deemed advantageous due to the intended restoration of natural motion, and the reduction of ALD, however their effectiveness has not been proven.8 Such motion preserving arthroplasty devices often are cited as the optimum treatment for degenerative spinal disorders, yet the limited clinical evaluation does not support the ideology of superiority.8 Although promising, there are disadvantages to spinal arthroplasty devices that include mechanical failure, dislocation and migration, subsidence and same level of degeneration.8

It must be noted that many of the arthroplasty devices currently undergoing investigation are aimed at treating DDD and degenerative facets as separate conditions, often leading to further degenerative issues. Disc arthroplasty has a detrimental effect on same level facet joints resulting in degeneration five years post operatively.9 In addition the placement and fixated rotational center of artificial discs results in increased facet loading induced through flexion or extension of FSU undercompressive loads.10

Available Treatments

Of the current treatments available for DDD, fusion of adjacent vertebrae is the most common.2 Fusing of the vertebrae is achieved through the insertion of a cage containing bone fragments into the intervertebral space following resection of the disc. The bony fragments encourage further bone growth and the subsequent fusing of adjacent vertebrae.
The resulting cessation of motion within the FSU leads to further problems including ALD and the need for motion preservation has led to the development of alternative means of treating DDD.

An option for the treatment of DDD while maintaining FSU motion is total disc replacement (TDR) in which an artificial disc is inserted between the vertebrae, replacing the entire intervertebral disc.4 The artificial disc ideally then performs the functions of an intact intervertebral disc, maintaining disc height and permitting normal range of motion. Similar to TDR, disc nucleus replacement (DNR) is intended to replace only the disc cavity, leaving the outer ring intact.4 The device mimics the biomechanics of the intervertebral disc, restoring disc height and FSU range of motion.

While the aforementioned devices provide treatment options for DDD, further devices address issues related to degeneration of the facets.

Such devices replace the function of the facets and include, but are not limited to the Total Facet Arthroplasty System developed by Archus Orthopedics Inc. in Redmond, Wa., which has been classified as investigational and has not received approval from the U.S. Food and Drug Administration (FDA)11; the Acadia Facet Replacement System developed by Facet Solutions Inc., in Hopkinton, Mass., which also is investigational and not FDA-approved; the Total Posterior System developed by Impliant Inc.; and Dynesys developed by Wintethur, Switzerland-based Centerpulse Orthopaedics Ltd. (a division of Zimmer), which also does not yet have FDA approval. Unlike the total facet replacement devices, the Zyre Facet Arthroplasty System developed by Quantum Orthopedics in Carlsbad, Calif., replaces the contact surfaces between the facets. Zyre also hasn’t been approved by the FDA.

Set for Growth

The facet joint is critical to the stability of the spine, carrying more than 30 percent of the axial load in full extension.Further, the facet joint limits excessive stress on the intervertabral discs in both torsion and shear. It generally is accepted that degeneration of the facets follows after disc degeneration, because the laxity of the joint begins to overload the facets. If DDD is treated with a motion preservation device, the damage to the facet joints could go unnoticed.The patient still may experience pain following a motion preservation procedure.

Certainly it has been shown that facet degeneration is linked to degeneration of the spine. In the interests of motion preservation in the spine and associated treatment, in many cases, damage to the facet joints cannot be ignored if treatment is to be complete. The available treatments are limited to four options at this time, none of which have FDA approval. The four treatments available differ considerably from each other in design. This field of orthopedics is highly novel with essentially only a few experimental devices circulating the market.If motion preservation in the spine does become more popular and regulatory pathways are clear, this field of orthopedics is set to grow and develop rapidly.

Janet Cotton, director of 6° of Freedom, completed her undergraduate studies at the University of Cape Town in metallurgical engineering in 1995 and started her Ph.D. in 1996. Her Ph.D. focused on the development of a cast Cromanite alloy for Columbus Stainless, specifically on microstructure-property relationships of a Cromanite base composition with additions of precipitate (fine particles that harden the steel) forming elements. Cotton finished her Ph.D. in 2000 and worked for two more years at UCT as a post-doctoral fellow. Cotton is now the director of One Eighty (Pty) Ltd., a sister company to 6° of Freedom, and over the last few years has completed more than 200 projects for the manufacturing industry in South Africa, including research and development work, failure investigation or forensic engineering and general problem solving. The primary business of 6° of Freedom is to complete wear particle analysis for implant manufacturers in accordance with ISO and ASTM standards. 6° of Freedom has developed unique enzymatic digestion procedures for the processing of metallic and elastomeric debris in bovine serum. Additionally, 6° of Freedom has the capacity to analyze nano-sized particles by means of a high-resolution field emission gun scanning electron microscope. Some of the clients of 6° of Freedom include Medtronic Sofamor Danek, Abbott Laboratories, Synthes Spine, Blackstone Medical, DePuy Orthopaedics and Aesculap.

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