Disc replacement surgery in the lumbar spine with moving prostheses is not a new philosophy. The concept has been around for 30 plus years. The drive for motion preservation when reconstructing the lumbar spine came from the observation that fusing the spine has two undesirable complications. One is non-union, and the other is adjacent segment degeneration.
Types of Prostheses
To the left are photographs of a typical prosthesis called the Prodisc. And above it is a sagittal x-ray showing what one looks like in situ. Keels hold the implant firmly in place. The metal parts have a coating on the outside, which almost instantly bonds to bone because of its rough, porous nature.
Over a time period of about 3 months after the disc replacement surgery, bone grows onto these pores and the prosthesis is solidly bonded. Sandwiched between the two metal components is a core made out of ultra-high molecular weight polyethylene. This core forms the articulation, which is a very low friction movement, which means that the prosthesis is very unlikely to wear out in a patient's lifetime.
Polyethylene in total hip and knee replacements has been linked with a syndrome called osteolysis. Osteolysis, where tiny particles of wear debris are taken up by the synovial cells of the joint and set up an inflammatory reaction, which can eat away at the bond between the bone and the prosthesis and loosen the prosthesis, is a big problem in hip and knee replacement surgery. It led to the development of alternate bearing surfaces such as ceramics.
This is not seen in the spine because the arc of motion is very small resulting in much fewer wear particles and the complete absence of synovial cells means that any wear particles produced simply remain stuck in scar tissue - inert and reactionless. Nonetheless, because of these fears, and for some other metallurgic reasons, prostheses have been designed with metal bearings and no polyethylene present.
For example, to the right is a total disc replacement call a Maverick. This is a metal-on-metal type prosthesis and has a slightly higher level of constraint than many others on the market, which I rarely use, but has a place in patients with the mildest of facet joint arthropathy.
Beware of surgeons who only know, understand and use one prosthesis alone for disc replacement surgery. Different patient's anatomy, pathology and degree of degeneration are best suited by technology-matched prostheses, and your surgeon should be skilled and experienced in using a range of prostheses.
A new prosthesis called the Extreme Lateral Total Disc Replacement (XL-TDR) is shown below. The exciting thing about this prosthesis is that it can be inserted via the extreme lateral approach to the spine, which means no anterior approach is needed. It has other theoretical advantages over other total disc replacements, and for this reason is my preferred disc replacement for L4/5 and higher.
On the left is what the XL-TDR looks like in situ. As you can see it sits right across the apophyseal rim of the bone - the hardest part of the spine. This device cannot be used at L5/S1 because the crest of the pelvis is in the way of the approach. In the USA where this has been implanted the most, patients stay overnight in hospital only. I have performed about 50 XL-TDR in Australia and am very impressed with the clinical results.
On the right is an example of an XL-TDR in a hybrid situation where one level is fused (L4/5) and one level is replaced (L3/4) Often hybrid surgeries like this are accompanied by percutaneously placed pedicle screws (key-hole surgery) to prevent subsidence.
Not everyone is suitable for a total disc replacement surgery. If the facet joints are arthritic, there is a spondylolisthesis or scoliosis, auto-immune disease such as rheumatoid, osteoporosis (soft bone), known metal allergies then, at least with currently available technology, a fusion is preferred.
On the right is probably the most advanced anteriorly placed TDR on the market and this is my preferred option for most young patients at L5/S1 because it has a unique bearing mechanism, which allows motion in 6 directions, unlike most other total disc replacements that only allow for 3 or 4. The extra motion this prosthesis allows is compression, which means it mimics the natural disc's ability to absorb load and shock. In other words, it has shock absorbing characteristics. This means that this total disc replacement has no moving bearing surfaces, meaning no debris, which may be safer, especially in young women of child-bearing age.
The photo on the left is what the prosthesis looks like on an X-ray at L4/5. Again this is a hybrid setting performed in conjunction with an anterior fusion at L5/S1. It is a one-piece design with teeth to grip the end-plates of the vertebral body above and below and the matrix of the shock-absorbing mechanism is clearly visible in between the endplates. This prosthesis has been implanted over the last decade in France and Germany with an excellent track record and is now routinely used in Australia.
Total disc replacement in the lumbar spine is now an accepted means of treating lower back pain and radiculopathy from degenerate discs. The scientific literature is replete with studies that prove the efficacy of this technique and technology. It is a continually evolving field and I think we as a speciality are getting close to the situation where we will rarely fuse the spine at all for simple degenerative conditions.