Innovations in Artificial Disc Replacement

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Innovations in Artificial Disc Replacement

Preston J. , MD, MS

On the final day of the NASS meeting, a symposium entitled " Artificial

Intervertebral Discs and Beyond " [1]presented information about what may be

the next great innovation in spine care.

The treatment of chronic back pain is multifaceted. Perhaps even more

challenging is the determination of the etiology of back pain, in the

absence of a fracture, infection, neoplasm, disc herniation, spinal

stenosis, or neurocompressive pathology. Spondylosis deformans and

degenerative disc disease are clearly defined clinical entities; however,

correlating radiographic findings with symptoms can at times be challenging.

History and Background

The history of artificial disc replacement was reviewed, beginning with the

use of methyl-acrylic injections and Lucite rings in the mid-1950s,

Vitallium spheres in the late 1950s, silicone injections and stainless steel

balls in the 1970s, and a silicone prosthesis in the late 1970s. Because of

complications associated with these technologies, artificial disc

replacements fell out of favor. However, a recent resurgence in this

approach to spinal pathology has prompted a re-examination of the concept

and newer prosthetic designs. In fact, more than 50 patents for artificial

disc replacements have been granted in the United States. The renewed

interest in artificial disc replacement has been in part stimulated by the

desire for an effective alternative to lumbar fusion -- the current gold

standard for the treatment of chronic discogenic low back pain unresponsive

to conservative treatment and confirmed by discography.

The objectives of disc replacement are to:

* Eliminate the need to perform a fusion, thus eliminating bone grafts and

donor-site morbidity

* Minimize stress on the adjacent level

* Preserve motion of the functional spinal unit

* Restore shock-absorption capabilities

* Restore disc-space height and neuroforaminal volume

* Maintain stability of the functional spinal unit

* Help protect neural elements

* Relieve pain associated with degenerative disc disease

Results of animal studies and preliminary clinical trials suggest that these

objectives are within reach.

Artificial discs are classified into 2 subtypes: total disc replacement and

disc nuclear replacement.

Total Disc Replacement

Several total disc replacement designs were presented. These similar designs

all derive from the common technological foundation of total joint

replacement (ie, hip and knee). The differences among designs are the nature

of the bone-implant interface, fixation techniques, degrees of freedom of

the articulating surface, and modularity. Most designs are porous-coated or

plasma-sprayed metal-backed implants with high-density polymer mobile

bearing surfaces.

The complications of total disc replacement are expected to be similar to

those encountered with total joint replacements. The special concerns in the

spine are due to the proximity of the implant to the neural elements. Wear

debris, the histochemical response to particulate matter, and loosening of

the prosthesis in the early or late postoperative period are the natural

concerns. Eventual failure of the prosthesis and the need for revision

brings up the question of the feasibility of revision total discs vs fusion

as a salvage procedure.

There have been reports of osteophyte formation and heterotopic ossification

around total disc prostheses. This periprosthetic bone formation has yet to

be classified. However, the concerns are progression to autofusion or, more

worrisome, the potential for the development of neurocompressive pathology

due to aberrant bone formation. The progression to autofusion would negate

the potential benefit of preserving motion of the functional spinal unit.

Heterotopic ossification or osteophyte/spurring resulting in

neurocompressive pathology would potentially require re-exploration and

decompression.

Four lumbar total disc prostheses were presented:

1. SB Charité III (LINK Spine Group Inc; Brantford, Connecticut)

2. ProDisc (Aesculap; Tuttlingen, Germany)

3. Acroflex (DePuy AcroMed; Raynham, Massachusetts)

4. New Jersey Disc (Stryker-Howmedica; dale, New Jersey)

Preliminary results from a multicenter US Food and Drug Administration

Investigational Device Exemption study of the SB Charité III, begun in March

of 2000, showed that 70% of cases demonstrated good-to-excellent results

(120 implants, 78 patients).

General contraindications to total disc replacement include:

1. Radicular leg pain greater that back pain

2. Osteoporosis

3. Spondylolisthesis

4. Spinal stenosis or neurocompressive pathology

5. Facet joint arthritis

6. Significant psychiatric problems

7. Noncontained herniated disc

8. Scoliosis

9. Spinal tumor

10. Infection

11. Morbid obesity

12. Arachnoiditis

Disc Nucleus Replacement

Prosthetic replacement of the nucleus pulposus was also discussed. The goals

are similar to total disc replacement. These devices are biocompatible

hydrogel polymers of varying shapes and sizes that have the capability to

absorb water and can potentially behave similarly to the native disc. The

data presented revealed that these devices have the ability to preserve --

but are limited in their ability to restore -- disc space height. Potential

complications include containment, displacement, or rupture of the implant.

The patient's body weight is also a limiting factor. Changes in design and

surgical technique have been made to facilitate prosthesis containment.

Two disc nucleus replacement prostheses were presented: PDN (Prosthetic Disc

Nucleus; Raymedica; Bloomington, Minnesota) and Aquarelle

(Stryker-Howmedica; dale, New Jersey) Nuclear Replacement. Clinical

trials with the Aquarelle implant are under way, and no clinical data are

yet available. Early clinical trials with the PDN in 65 patients yielded a

77% success rate, with 15 implants requiring removal. A subsequent study

with 251 patients yielded a 92% success rate, with 8 implants requiring

removal.

Cervical Disc Replacement

Cervical disc total replacement concepts and designs also were presented.

Again, the philosophy and prosthetic design are similar to the lumbar spine

approach, and the technological foundation derives from the total joint

experience. The implant is porous-coated for bony ingrowth. The mobile

bearing surface is contained within a sealed polymer capsular sleeve to

minimize wear debris. The indications are similar to those for anterior

cervical discectomy and fusion.

US clinical trials are at ground level, but clinical data from European

trials have been encouraging. There are several ongoing studies in the

United States, Canada, and Europe examining each type of implant. The early

European data again seem promising, but further study is required.

Reference

1. Blumenthal SL, R, Lee CK, et al. Artificial intervertebral discs

and beyond. Program and abstracts of the 16th Annual Meeting of the North

American Spine Society; October 31-November 3, 2001; Seattle, Washington.

Symposium.