The Lumbar Artificial Disc
The quest for an artificial disc, as a more physiologic answer to stabilizing the spine, began about half a century ago and has involved many novel approaches. At the time of FDA approval of the Charité device for total disc replacement these devices had been implanted for over 15 years in Europe.
Perhaps the most innovative approach to an artificial disc was that initiated by Fernströ>m in the 1950s. He replaced intervertebral discs with steel ball bearings in approximately 250 patients. General enthusiasm for this approach never developed.
In addition to metals disc nucleus replacements, as well as total disc arthroplasties, have been constructed of plastics, ceramics, polymers, injectable fluids, hydrogels, inflatable devices, and elastic coils.
Clearly the most intriguing narrative in the history of artificial disc development belongs to Canadian surgeon David Kuntz, who first began to personally fabricate cervical artificial discs for his patients in the late 1970s. Kuntz’s surgery was intended “to restore the spine to the manufacturer’s original specifications.” Kuntz reported a 98% success rate in a series of 300 patients.
Kuntz (shown above) established satellite helicopter clinics for isolated Northwest British Columbia Canadian clinics and survived mishaps including crashes in order to bring medical care to his outlying patients. He has been honored by the “Giraffe Project“, a non-profit Canadian organization, in recognition for “sticking his neck out”.
In other venues, however, the development of artificial discs followed the more mundane orthopedic concepts of joint replacement as practiced elsewhere in the body. In 1955 Hamby and Cleveland injected discs with methylmethacrylate and Nachemson injected self-curing silicone in 1962. Gardner reported on the use of plastic pegs in “hundreds of cases”. It was in 1964 that Fernström published his results of implanting ball bearings under local anesthesia. Urbaniak designed a number of elastomeric artificial discs, Froning developed a liquid filled bladder and Lee created a composite viscoelastic replacement. The first mechanical hinge-spring and rubber disc devices were those designed by Kostuick and Steffee.
The most extensive application of a mechanical disc replacement device, however, was the Charité initially designed by Büttner-Janz and Schellnack in the German Democratic Republic in 1982. This was modified by Zippele as the Mark II model in 1984. The SB Charité III model was introduced by Link Design Group in 1987. In addition to these technologies it is important to point out that neurosurgeon Charles Ray first implanted his prosthetic disc nucleus (see below) in 1996 and this continues in investigational trials with continuing modifications and improvements. Other investigational prosthetic disc devices being evaluated are the: ProDisc (Spine Solutions/ Synthes) which was developed by Dr. Thierry Marnay in the late 1980s, and Flexicore (Spinesore) devices. Other disc replacements in development include the Newcleus, the Aquarelle Hydrogel Nucleus, the Interpore Cross ceramic, the Disc Dynamics polyurethane implant, the Cryolife Biodisc, the Disc Augmentation Technologies thermopolymer, and Replication Medical’s Aquacryl.
Shown here is an early example of the Raymedica PDN (Prosthetic Disc Nucleus) consisting of a hydrogel core in a flexible, inelastic, woven polyethylene jacket designed to replace a damaged or herniated nucleus pulposus invented by Charles D. Ray.
The ProDisc, TDR, Charité, and the Bryan Cervical Disc System are shown above. Unlike the Raymedica device these are total disc protheses (artificial discs) intended to replace the entire disc while also maintaining some degree of motion.
The most recent addition to the list of lumbar arthroplasty devices is the NuBac (shown to the left).
The Cervical Artificial Disc
It is clear that the initial tsunami wave of enthusiasm for the lumbar artificial disc also was carried into the cervical area. In 1989 the Department of Medical Engineering at the Frenchay Hospital in Bristol, U.K took on this challenge and by 1996 22 of their Cummins-Bristol devices had been implanted in patients.
Second generation improved designs and a new Frenchay artificial disc were introduced in in 2002. In 200 clinical trials were initiated for the Bryan artificial disc which is shown to the left.
This is a metal on plastic device and by 2008 approximately 2,00 of these artificial devices were implanted throughout the world with good short-term results being reported. A major advantage of these devices was the hope of avoiding stress related “adjacent level” disease.
The Synthes Pro-Disc C disc is shown to the right. This design has been of particular interest because the studies have included the placement of stereometric markers (dots seen on the x-ray)which have allowed more sophisticated monitoring of the maintenance of post-surgical segmental movement.
The concept of developing a true artificial disc is but one step in the quest, as Dr. Kuntz most aptly observed, “to restore the spine to the manufacturer’s original specifications.”
Artificial discs are, however, only one in a spectrum of more rational stabilization technologies now existing and also emerging for the purpose of effective spine stabilization and restoration. Artificial discs are as attractive as artificial hips were when they were first introduced. Prior to the latter all hips were fused. Today this is obsolete thinking. Unfortunately multi-level instrumented fusions for the treatment of low back pain are not yet obsolete (as they should be) and because of this, due to adjacent level stress, there is a domino effect of adjacent level disease, following which patients are often advised to have more in the way of rigid fusion. Lumbar artificial discs have not yet demonstrated reasonable safety or efficacy in the Editor’s opinion.
Cervical artificial discs are a completely different ballgame as the present “gold standard” procedure of anterior interbody discectomy and fusion (ACDF) is also guilty of promoting adjacent level disease. Fortunately the stresses directed to the cervical spine are many orders of magnitude less than the lumbar spine. Even thought these devices are prone to having their mobility compromised by post-surgical bone ossification and scarring the initial studies have shown that after a year there still remains what has been termed “micromotion”. The jury, however, is still out on the longevity of these cervical devices.
Artificial disc prostheses are implanted through anterior neck and abdomen surgical approaches. In the abdomen this typically means that a general surgeon needs to be involved to provide exposure. The risks of this anterior approach are not insignificant, they are:
- Finding a surgeon who is experienced in providing adequate surgical exposure to the anterior lumbar spine.
- Increased cost for second surgeon (general surgeon).
- Risk of injury to the great vessels.
- Risk of impotence, sterility, or retrograde ejaculation in males.
- Risk of injury to ureters and lymphatic ducts.
- Revision surgery is potentially life-threatening
At this point in time the lumbar artificial disc has been used in Europe for over 15 years. A review article by de Kleuver (Eur Spine J, 12:108-116, 2003) covered nine case series in which 564 replacements were performed. de Kleuver found that here was a lack of controlled studies as well as a lack of adequate data in order to judge the efficacy of this approach to the treatment discogenic back pain. While initial complication rate were found to be low the mobility of the motion segment was not always retained, removal of the device was not always possible, spontaneous fusion occurred in as many as 26% of patients in one series, and device
subsistence was not infrequently seen.
van Ooij, a Dutch Orthopedic surgeon, noted in a letter to the Editors of the North American Spine Society Newsletter (SpineLine, Nov.-Dec. 2004) that he had now seen 55 surgical failures and noted that “these patients represent the most disabled group of patients that I personally have seen in 24 years of spine practice”.
Orthopedist Charles Rosen and associates, at the Orthopaedic Biomechanics Laboratory in Irvine, California believe that the high level of disc replacement failures reflect significant basic flaws in design, poor data, inappropriate influence of industry on researchers and the approval process, and recommend that these designs should be abandoned.
Shown above is a patient who has had a Prodisc 5 level implantation. This would not be consistent with United States FDA indications for use and may actually represent an example of surgical “irrational exuberance” (NASS SpineLine, Nov.-Dec., pp. 23, 2004) .
Shown to the left is an example of a two level ProDisc insertion performed in a patient with genomic spine disorder who was a smoker. The upper device has subsided into the vertebral body causing the device to dislocate. Removal of such dislocated artificial discs is life threatening.
Another important concern regarding the use of artificial discs is that placement of such a device does not necessarily mean that the patient has been adequately treated in regard to concomitant nerve compression and that adequate surgical decompression to address this has occurred prior to segmental stabilization. Given the fact that the most common reason for failure of back surgery is inadequate decompression of impaired nerves this must be recognized as a very meaningful concern. In fact, this issue must take precedence over any consideration of accompanying stabilization.
Assuming that the concerns expressed above are adequately addressed attention can then be focused on the disc prosthesis itself. The Charité device approved for use in the United States hardly represents the best example of the state-of-the art in avoiding the loosening of vertebral attachment or subsidence of the implant into the adjacent vertebrae.
Only time will tell if the marketing hype and high financial expectations regarding the sale of artificial discs will come to fruition. What seems clear is that the present devices, if applied according to the FDA criteria, will, and for the present time should have limited use associated with careful patient screening and scientifically accurate outcome monitoring. Once again, this technology is in its infancy and will require a great deal more in the way of development and careful study before it can be considered a success in regard to patient welfare.