Lumbar Total Disk Replacement
John H. Peloza, M.D.,
Director of the Center for Spine Care, Dallas, Texas
 Chronic low back pain is a significant problem for thousands of people, accounting for large health care expenditures, loss of productivity and decreased quality of life. The traditional treatment for degenerative low back conditions has been lumbar spinal fusion surgery with mixed results. But modern spinal fusion techniques and technologies have dramatically improved fusion outcomes and predictability. But patients still fear this treatment and seek new alternatives, such as disc replacement surgery.
Lumbar Disc Replacement
Lumbar disc replacements or lumbar artificial discs will soon become available and there is a great deal of interest in the general public and medical community. There is a lot of hype about disc replacements being the answer to low back pain - but it is not a miracle cure.
The outcome of any treatment, especially surgery, relies on the accurate or precise diagnosis of the source of pain. There are also psychological, physical, and social factors involved in the successful treatment of chronic pain. Collectively, these are known as surgical indications. Diagnostic procedures, spine imaging, surgical indications, and surgical techniques have been refined over the years so that experienced spinal practitioners can reliably and predictably achieve excellent clinical outcomes with modern spinal fusion surgery. The indications, as well as the type of disc replacement implants, are not nearly so refined.
Disc replacement surgery is an immature technology and not fully developed. I believe that the preservation of motion at a diseased segment is a very useful concept, but there are facts that everyone needs to understand.
First, the average age of patients undergoing surgery for chronic low back pain related to degenerative disc disease is 40-45 years. This is much younger than patients undergoing total hip or knee replacements. The demands on a lumbar artificial disc will be very great and may not outlast the lifetime of the patient - meaning patients may need revision surgery. And because of the nearby vascular structures and scar tissue from the original surgery, revision procedures for a lumbar artificial disc can be very dangerous.
Second, not all disc replacements are the same. They have drastically different designs and are made of different materials. This has a dramatic impact on how long the implant will last, the forces transmitted at the operated level and adjacent levels, and ultimately the clinical outcome for the patient.
Third, the long-term data for disc replacement is unknown. The European studies have some significant shortcomings and the latest long-term data for the JNJ DePuy Charite disc is very worrisome. I will now discuss each disc replacement currently in U.S. FDA clinical trials.
In conclusion, lumbar disc replacement surgery is in its infancy and is presently indicated in single, possibly two level, painful degenerative disc disease that has failed at least six months of intensive spine focused rehabilitation in a patient without major psychological, physical or social/emotional contraindications.
Types of Lumbar Artificial Discs:
Name: SB Charite III/ LINK
 Manufacturer: Johnson and Johnson/ DePuy
Web site: www.depuyaccromed.com
FDA Status: Approved 10/2004
Design:
The Johnson and Johnson/ DePuy Charite disc consists of two metal plates and a polyethylene spacer. The polyethylene spacer can move or slide with modest constraint between the metal plates. The metal plates attach to the bone by three small pegs in front and three small pegs in back. This lumbar disc has been used in Europe since the mid 1980s and has undergone three design revisions because of significant complications. The European disc has a coating, which allows bone to grow into the metal. The disc being introduces to the U.S. has no coating and, therefore, no ability for bone to grow into the metal.
Results:
There are multiple European studies that show success rates for the JNJ DePuy Charite between 63% and 79%. However, revisions were necessary in close to 25% of patients in one study due to continued pain. Dr. Andre Van Ooij, from the Netherlands, has revised 50 patients (10% of the patients at one study site) at an average of 4 1/2 years after surgery. The patients had incapacitating low back and/or leg pain. The diagnoses were subsidence, polyethylene failure or dislocation, metal loosening, degeneration of the facet joints at the same level, degeneration of adjacent level and bone formation around the implant. No patient benefited from the usual posterior instrumented fusion as a salvage situation and all did poorly in terms of their pain. This implies that the implant will need to be removed from an anterior approach, which as a revision will be potentially life threatening in every case because the blood vessels in front of the spine will be difficult to expose. The Charite Hospital in Berlin also reported on their 17-year follow-up of the JNJ DePuy Charite implant. More than half of their patients had poor results with incapacitating pain for many of the reasons cited above. The Charite Hospital no longer uses the implant and recommends other newer disc replacements.
In the U.S. clinical trial, the JNJ DePuy Charite implant was compared to a spinal fusion technique that is not widely used today because of inconsistent results. And the clinical trial results showed the JNJ DePuy Charite was only marginally better than this outdated fusion procedure. The U.S. clinical trial reported an overall success rate of 63% with 26% of patients either no different or worse in regard to their preoperative low back pain. Because the manufacturer only had to demonstrate that the implant was at least equal to a fusion technique, the JNJ DePuy Charite was recommended for approval with stringent conditions by the FDA panel on June 2, 2004.
Potential Problems:
There are many problems with the JNJ DePuy Charite implant design. The metal plates do not match the shape of the vertebral bone. It is difficult to get a good fit between the bone and the implant. The fixation between the metal and bone is completely inadequate. The small pegs on the metal base are known to fail almost immediately on implantation and there is no ability of the bone to grow into the metal in order to secure fixation. The polyethylene core is made of non-cross linked polyethylene and is only 3mm thick at the point of maximal stress concentration. This guarantees that the implant polyethylene will fail in a short time of cyclical loading.
I have removed a JNJ DePuy Charite implant two years after implantation in an actual patient. The polyethylene already had pits, abrasions and cracks in the surface. There was a visible gap between the metal endplates and the bone and the metal endplates popped off the bone with minimal force. The polyethylene core can also dislocate from the implant. The center of rotation of the implant is in front of the center of rotation of the spinal segment. This causes increased loading of the facet joints (behind the disc), which are a known cause of low back pain. In addition the implant causes about 150% rotational loading of the facet joints increasing the degeneration of the joint and therefore potential pain. The metal endplates also tend to sink into the vertebral bone (subsidence), which completely alters the forces acting on the operated lumbar segment. Although motion is usually preserved to some extent with the implant, the chance that it reproduces normal or near normal biomechanics to the operated segment is highly unlikely. The likelihood that it will prevent degeneration at the adjacent levels is also highly unlikely.
My view:
The JNJ DePuy Charite is a poor alternative to modern spinal fusion surgery. It has only marginally better results at one-year follow-up and equivocal results at two-year follow-up than an out-of-date, traditional fusion procedure. It has worse results than an anterior fusion alone with an LT-CAGE Device and BMP, and far worse results than minimally invasive fusion techniques. It does not reproduce normal forces at the operated level and its materials and fixation are guaranteed to fail during the patients' lifetime. Traditional, low risk posterior revision techniques do not work and the implants will probably need to be removed anteriorly with an anterior and posterior instrumented fusion. This is very risky surgery and is potentially life threatening.
Name: ProDisc II
 Manufacturer: Spine Solutions/Synthes
Web site: www.synthes-stratec.com
FDA status: A FDA decision regarding release will likely occur in 2005.
Design:
The ProDisc II is a semi constrained three-part device. It has two metal endplates with a midline keel and a surface coating for bone in-growth. The spacer is a non-cross linked polyethylene core with a dome shape that articulates with the metal cup shape superior endplate. The center of rotation of the implant is anterior to the center of rotation of the spinal segment, which means the loading of the facet joints is still significant. The bone fixation to the metal endplates is excellent but the polyethylene wear issues are still significant. Even though the loading environment of the lumbar spine is less severe than the hip, no polyethylene has been able to stand the degree of wear that lumbar disc replacements will require in order to avoid revision surgery.
The convex bearing surface, snap-fit into the inferior end plate, is made of ultra-high molecular weight polyethylene (UHMWPE). The artificial disc is attached through a large central keel and two spikes on each endplate
Results:
The European experience with the ProDisc is very good. Three studies report good/excellent results in 90% of patients. However, the U.S. clinical trial results published at one follow-up show equal outcomes between the ProDisc and a standard open 360 fusion. This is very disappointing. The outcome with the ProDisc should be much better than a fusion technique that is unpredictable with a high morbidity rate.
My View:
The Pro Disc is a much better implant than the Charite. However, the polyethylene issues are worrisome. Also, this disc replacement will predictably cause excessive loads to the facet joints. It is surprising that the results so far in comparison to open, instrumented 360° fusion (not minimally invasive fusion) are not dramatically better.
Name: Maverick
 Manufacturer: Medtronic Sofamor Danek
Web site: www.back.com
FDA status: A FDA decision regarding release will likely occur in 2006.
Design:
The Maverick disc is a semi constrained metal-on-metal device. The metal endplates come in multiple sizes that precisely fit the shape of the vertebral bone. The endplates have a midline keel and surface coating for bone in-growth. It comes in multiple heights and combinations to reproduce the normal disc height and lordosis. It has a posterior center of rotation that matches the center of rotation of the disc segment. This unloads the facet joints and reproduces near normal force transmission at the operated segment. Metal fixation into bone is excellent and there is no polyethylene wear issue. Metal-on-metal wear for a ball and socket implant that is semi constrained has been tested extensively and there is essentially no metal wear at 31.5 years of simulated loading.
Results:
The European experience with the Maverick disc is very good. Unpublished, but presented studies report greater than 82% good/excellent results at two-year follow-up. I am an investigator for the Maverick disc FDA trial. All patients have been entered and had surgery at this time. We are monitoring the results at this time.
My view:
My personal experience is that the Maverick disc results are exceptionally good, much better than traditional spinal fusion surgery and equivalent to modern spinal fusion surgery. This implant has the best design features and material to last the patient's lifetime. It also replicates the normal biomechanical forces at the operative and adjacent spinal levels. Of all the disc replacements at this time, the Maverick will most likely have the best results and longevity.
Name: Flexicore
 Manufacturer: Spinecore
Web site: www.spinecore.com
FDA status: A FDA decision regarding release will likely occur in 2007.
Design:
The Flexicore disc is a constrained metal-on-metal device. The endplates have small pegs for fixation into the vertebral bone and surface coating for bone in-growth into the metal. The small pegs are again inadequate for bone fixation but the surface coating may allow for bone fixation in time. The constrained ball and socket design is similar to a constrained ball and socket total knee replacement design that predictably failed at the ball and socket junction. This implant will most likely break at the ball-and-socket joint or will pull off the bone prior to bone in-growth fixation.
Results:
Presently, there is no published mechanical testing or clinical data to report.
My view:
This disc has significant design flaws. The metal pegs are inadequate for bone fixation, but at least it has a surface coating. The totally constrained metal on metal construct concentrates forces lending to metal breakdown and implant failure. Predictably, this implant will fail within the patient's lifetime, necessitating a revision procedure.
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