Advances in Artificial Disc Technology 

Introduction

Disc arthroplasty with an artificial disc is a promising new technique being used in patients with chronic low back pain due to degenerative disc disease. These patients have mechanical low back pain, decreased disc space height on plain films, and decreased T2 signal on MRI. Currently, the mainstay of operative treatment for these patients is a lumbar fusion. There is a great deal of controversy over the use of lumbar fusion in the treatment of degenerative disc disease, including the need for instrumentation, the use of interbody cages, anterior vs posterior approach, and whether lumbar fusion is useful at all.

A major pitfall of lumbar fusion is that the operative segment is immobilized, shifting the mechanical stresses to the adjacent segments.^[1] Subsequent degeneration at the adjacent levels can lead to facet joint arthritis, disc degeneration, spinal stenosis, or even instability. This, in turn, can lead to repeat surgeries until the entire lumbar spine is fused.

Disc arthroplasty is promising because it is intended to preserve motion, thereby preventing the adjacent segment degeneration associated with fusion. The pathologic -- and presumably painful -- disc is removed, which should reduce the patient's pain. The implant restores or maintains the normal lumbar lordosis. Therefore, after the procedure, the pathologic tissue has been removed and the biomechanics of the intact spine have been restored.

There are several designs of total disc replacement as well as prosthetic disc nucleus replacement. Disc replacement systems are typically composed of 2 metal plates that have a keel or teeth to gain purchase in the endplates of the vertebral bodies above and below the disc being replaced. The metal plates have a porous ingrowth surface that allows the bone to interdigitate with the prosthesis, providing very solid fixation. A polyethylene spacer is then placed between the 2 metal plates and allows motion to occur.

Prosthetic disc nucleus devices are placed into the center of the disc space through an annulotomy. One example is a device that has a hydrogel core with a polyethylene jacket. The core is flexible and expands once it is implanted by absorbing water. The polyethylene jacket preserves the hydrogel in its optimal form.

The potential disadvantages of disc arthroplasty are significant. The anterior approach to the lumbar spine has intrinsic risks. A surgeon familiar with this approach is necessary to perform the approach since the aorta and common iliac vessels lie anterior to the lower lumbar spine. In men, the superior hypogastric plexus can be injured, resulting in retrograde ejaculation. Furthermore, a repeat retroperitoneal approach is difficult, if not impossible, and dissection and mobilization of the vessels places the patient at significant risk of hemorrhage, particularly if performed by a surgeon unaccustomed to this approach.

It is also easy to understand why dislodgment of the polyethylene or the metal plates could have disastrous consequences and may be extremely difficult to remove. Although the anterior spine has an excellent blood supply, infection after anterior lumbar surgery is not uncommon. Late infection is also a potential problem, as hematogenous seeding of the lumbar spine with bacteria commonly occurs. Obviously this would present a formidable challenge because removal would be difficult and the prosthesis would act as a retained foreign body, providing refuge from antibiotics.

Another potential drawback is polyethylene wear. Small particles of polyethylene can cause significant histologic reaction in other joints.^[2] The long-term consequences of polyethylene debris in the retroperitoneal space are unknown. Some researchers believe that since the disc is not a synovial joint, there is not as much of a concern about reaction to polyethylene debris. Further answers will likely come only after a very large number of devices have been implanted.

Several designs are under investigation by the US Food and Drug Administration (FDA). Disc arthroplasty has been performed in Europe since at least the early 1990s. It is expected that some of these devices may be FDA-approved in the next several years. For the appropriately selected patients treated by a skilled surgeon, this procedure holds great promise.

Long-term Outcomes for Disc Replacement

Russel Huang^[3] presented flexion-extension range-of-motion (ROM) data on 34 patients who underwent disc replacement with 50 prostheses. Follow-up ranged from 7 to 10 years with a mean of 8.6 years. Cobb angles were measured at each disc replacement level to determine the ROM at latest follow-up. The mean ROM for the instrumented levels was as follows:

Spinal Level	Flexion/Extension
L2-3	3.5°
L3-4	4.0°
L4-5	4.5°
L5-S1	3.2°

Thirty-four junction levels were examined radiographically and 9 (26%) were found to have evidence of degeneration. Of interest, the mean ROM for a prosthesis below a degenerated level was 1.4°, while the mean ROM for a prosthesis below a normal level was 5.0° (P = .015). Four of the 50 levels fused and there was no subsidence or other mechanical problem. Foraminal height increased by 18% at final follow-up compared with preoperative measurements.

This study is significant because it shows that ROM is maintained years after the device is implanted. However, the Cobb angle that is used to measure spinal curvature has a measurement error of approximately 3°.^[4,5] Thus, ROM shown in this study may actually be much smaller than it appears. Furthermore, ROM levels were well below normal measurements.^[6]

It was interesting that the mean ROM level below a degenerated disc was significantly less than the mean level below a normal disc. Having a degenerated disc at the level above, and perhaps throughout the lumbar spine, may contribute to the decreased ROM of the arthroplasty level. Additionally, the arthroplasty may have been placed such that it led to degeneration at the level above. It is also likely that in some cases degeneration at the level above would have occurred regardless of whether an arthroplasty was performed.

It would have been useful to have controls for comparison data. This would allow evaluation and comparison of the ROM of patients who did not have disc replacement and were treated with either fusion or nonoperative measures vs the ROM of those who underwent disc replacement. This may have helped to determine the cause-and-effect relationship of the level above degeneration, since the study author believed that this was about the same as after spinal fusion.

Experiences in Disc Arthroplasty From France

Thierry Marnay^[7]presented his results of using disc arthroplasty to treat patients with chronic low back pain who had failed conservative treatment. He evaluated them with the visual analog scale for back pain and leg pain, neurologic exam, and the use of pain medications. He compared preoperative and postoperative Beaujon scores to evaluate low back pain, leg pain, neurologic status, walking capacity, medication, and everyday life function. He also evaluated them with plain films and computed tomography (CT) examining the height of the implant, resorption around the implant, motion (flexion/extension/lateral bending), and subsidence.

A total of 64 patients received 93 prostheses implanted at various levels from L2-3 to L5-S1. Thirty-nine cases had 1-level implants, 21 had 2-level, and 4 had 3-level. Mean follow-up was 8.6 years. No implants were removed, but 5 patients underwent spinal fusion for persistent pain. There were no cases of periprosthetic lucency, subsidence, or implant migration. Some complications were associated with the anterior approach; there was 1 vascular injury which was recognized intraoperatively and repaired, and 2 cases of temporary sexual dysfunction. There was also 1 deep venous thrombosis, 2 ventral hernias, and 4 cases of heterotopic ossification.

Visual analogue scale scores improved from a preoperative score of 8.5 to a score of 3.0 at final follow-up. Leg pain improved from 7.1 preoperatively to 1.9 at final follow-up. Sixty-five percent of patients were entirely satisfied with their result, 28% were satisfied, and 7% were not satisfied.

Dr. Marnay's presentation was impressive for several reasons. He was very candid about his results and was fair in that although he was pleased with his results, he felt that the only way to be certain about the utility of disc replacement was to perform a study directly comparing the results of disc replacement with the results of fusion. It was also obvious that physicians in the United States are at a significant disadvantage in developing new treatments and devices and providing them to patients because of our current legal system. Dr. Marnay was offering disc arthroplasty to French patients at a time when American physicians were battling teams of lawyers over the use of pedicle screws that have been the standard of care for years.

Dr. Marnay showed CT scans of patients pre- and postoperatively. It was impressive that the facet joints looked very similar at the levels of arthroplasty as well as at adjacent levels even 10 years after disc arthroplasty. He also believed that spinal canal stenosis may be decreased by increasing the disc space height, although this was not evaluated directly in the study. Dr. Marnay reported that he had no problems with polyethylene debris, although there was no histology since no devices were removed.

At the end of the session, the panelists were asked about technical tips that they could offer to ensure the best result. Paul McAfee, MD, recommended that the prosthesis be placed in the center of the disc in the frontal plane, and approximately 2 mm posterior to the center of the disc in the sagittal plane. Placing the disc anterior to this point would result in decreased ROM since the center of rotation is posterior to the middle of the disc. Dr. Marnay added that it is important not to distract the disc when placing the prosthesis, as this may result in decreased ROM. Furthermore, placing a segment into too much lordosis may load the facet joints, causing pain.

A New Animal Spine Model

Ledet and colleagues^[8] evaluated the baboon lumbar spine as a model for future testing. Disc nucleus replacements were placed into 3 baboon lumbar spines through a lateral approach. In 1 animal, 2 customized implants were placed into the disc place. In the remaining animals, a single implant was placed at the levels replaced. Plain films and CT scans were obtained and assessed at 2, 4, 12, and 26 weeks after the procedure. Implant movement, disc height, and endplate morphology were evaluated. At 26 weeks, the specimens were harvested and evaluated with microradiology and histologic studies.

The 2 animals with the single device showed displacement within the first 2 weeks. They were taken back to surgery and the disc implants were revised. The revision technique involved endplate disruption, and smaller devices were used. These levels showed progressive decrease in disc space height, endplate degeneration, prosthetic subsidence, circumferential lucency, and increasing sclerosis of the adjacent vertebrae on radiographs. Osteophytes were noted to bridge the disc space by 26 weeks. Histology showed mild inflammation surrounding the implants. There was no wear debris, no evidence of neoplasia, and no granulation or histologic change adjacent to or involving the thecal sac.

In the animal with the paired prostheses, there was mild inflammation at 26 weeks, no evidence of implant movement, no abnormal endplate morphology, no sclerosis, and no change in osteophytes. The study authors believed that the responses of the baboon spine to the implants in all 3 animals were similar to those in the human lumbar spine.

Several minor flaws of this study were brought to light by members of the audience. First, the technique used was different from that used in humans (ie, the anterior approach). The implants are intended to be implanted in pairs, yet in 2 animals a single implant was placed. In the human system, preoperative templating is used, and this was not used in the current study. There was concern from some audience members that perhaps the appropriate-size implants had not been placed. Some members of the audience thought that the lumbar spine of the baboon has very different biomechanical characteristics from the human lumbar spine. The presenting panelists believed that the biomechanical properties were similar enough to provide useful data.

It may be premature to presume that because a similar response occurs in 2 experimental systems responding to 2 different stimuli, one can serve as a model for the other. However, no animal model is perfect and there were certainly similarities in radiographic and histologic response in the baboon lumbar spine compared with that seen with other devices in the human. The baboon seems to be a good model in the product-development phase of research, but ultimately the only way to know how the human body will respond to a given procedure is to perform it in people.

A Potential Tool to Study Heterotopic Bone Formation

A study by McAfee and coworkers^[9] was designed to develop a classification system for heterotopic bone formation after disc arthroplasty. A classification system is useful because the major advantage of disc arthroplasty is maintenance of spinal motion. If ankylosis occurs, motion is not preserved, eliminating the presumed benefit of arthroplasty. Heterotopic bone formation may lead to a decrease in motion, but in order to study this, a classification system is needed to assess the amount of heterotopic bone present. This study proposes such a classification scheme and presents data regarding the reliability and reproducibility of the system.

One hundred and one radiographs from 84 patients and histologic microradiographs from 17 baboons were evaluated by 7 reviewers. Three were full-time spine surgeons, 3 were basic scientists, and 2 were technicians. The following grading system was used:

• Class 0: no heterotopic ossification (HO)
• Class I: islands of bone in soft tissue, bone not present between planes formed by the endplates
• Class II: bone present between the planes of the endplates, but not blocking motion
• Class III: motion blocked by HO and/or postoperative osteophytes
• Class IV: inadvertent bony ankylosis

Intraobserver reliability was determined by having the same reviewer review the films at 2 different times. The r-value was found to be 0.8949. The interobserver reliability was found to be r = 0.9683. Both values are quite good and show that this system is useful in assessing HO.

A member of the audience raised the point that the presence of HO may not correlate with outcome. Furthermore, the statistics performed in the current study did not provide information about the effect of different classes of HO on lumbar spine motion. Dr. McAfee pointed out that HO after disc arthroplasty is very difficult to study, as the incidence of HO after disc arthroplasty is very small; of the 360 disc replacements that he has performed, he has encountered only 4 or 5 cases of HO.

It may turn out that the location of heterotopic bone is important with regard to its effect on motion. The location of the bone may have a significant effect on which motions are affected, as well as the degree to which they are affected. Furthermore, it is unknown what effect limitations of different motions will have on the ultimate outcome in patients with disc arthroplasty. Ultimately, this classification scheme may need to be modified, but for the time being it will certainly be valuable in assessing heterotopic bone formation in the many studies that will be done as these devices become commercially available.

References

1. Carman DL, Browne RH, Birch JG. Measurement of Scoliosis and Kyphosis Radiographs. Intraobserver and Interobserver reliability Variation. J Bone Joint Surg. 1990;72:328-333.
2. Lohmann CH, Dean DD, Bonewald LF, Schwartz Z, Boyan BD. Nitric oxide and prostaglandin E2 production in response to ultra-high molecular weight polyethylene particles depends on osteoblast maturation state. 2002. J Bone Joint Surg Am. 84:411-419.
3. Huang R, Girardi F, Cammisa F, Marnay T. Long-term flexion-extension range of motion of the Prodisc I disc prosthesis. Program and abstracts of the 17th Annual Meeting of the North American Spine Society; October 29-November 2, 2002; Montreal, Quebec, Canada. Abstract 171.
4. Cobb J. Instructional Course Lecture -- Outline for Study of Scoliosis. Am Acad Orthop Surg. 1948.
5. Lehmann TR, Spratt KF, Tozzi JE, Weinstein JN, el-Khoury GY, Colby H. Long-term follow-up of lower lumbar fusion patients. Spine. 1987;12:97-104.
6. Miyasaka K, Ohmori K, Suzuki K, Inoue H. Radiographic analysis of lumbar motion in relation to lumbosacral stability. Spine. 2000;25:732-737.
7. Marnay T. Lumbar disc replacement: 7 to 11-year results with Prodisc. Program and abstracts of the 17th Annual Meeting of the North American Spine Society; October 29-November 2, 2002; Montreal, Quebec, Canada. Abstract 172.
8. Ledet E, Dirisio D, Tymeson M, Andersen L, Kallakury B, Sheehan C, Sachs B. The Raymedica PDN prosthetic disc nucleus device in the baboon lumbar spine. Program and abstracts of the 17th Annual Meeting of the North American Spine Society; October 29-November 2, 2002; Montreal, Quebec, Canada. Abstract 173.
9. Mcafee P, Cunningham BW, Devine JG, Williams E, Yu-Yahiro J. Classification of heterotopic ossification in artificial disc replacement. Program and abstracts of the 17th Annual Meeting of the North American Spine Society; October 29-November 2, 2002; Montreal, Quebec, Canada. Abstract 174.