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 – described in detail below.
This refers to the situation when no new bone (or not enough new bone) forms between the two vertebral bodies being fused together in the months following surgery. With the advent of modern biological bone graft substitutes this risk is now very low but in smokers it is still considerable.
It usually takes 6-9 months for a fusion to become mature and solid and it is only when this is confirmed (usually on a CT scan) that physical restrictions are lifted and an assessment of how good the patient is can be made. During this period damage can occur to the reconstruction if the patient does too much, and subsidence can occur (collapse of the bone around the fusion cage). The more segments in the spine that are fused, the greater the risk of non-union in one of them.
Total disc replacement surgery (TDR, or Total Disc Arthroplasty, TDA) on the other hand simply starts working when the patient starts walking – usually the next day. The prosthesis still has to stabilise, bond to the bone, and become surrounded by a nice tight capsule of scar tissue, but this is a much quicker process than for a fusion to complete.
Adjacent Segment Degeneration (ASD)
This is the situation where, as a result of many fusions, adjacent discs degenerate prematurely. Fusions load-transfer (in other words they transfer their workload) to other discs, whereas TDR's load-share. With modern surgical techniques, it is actually unknown whether ASD even exists.
ASD is however a very real entity in the situation where there are 3 or 4 degenerate discs in the spine but only the worst 2 (usually the lowest) need to be reconstructed for pain relief. Leaving 2 slightly degenerate discs in the presence of 2 fusions, in this example, almost certainly commits the patient to a significant risk of ASD. If a moving prosthesis can be used as part of the reconstruction to 'load-share' then the risk is reduced.
A very good scientific study performed by Huang and colleagues proved that the risk of ASD when a TDR is used in the reconstruction is very low when that total disc replacement moved physiologically with the disc above i.e. moved enough but not too much, which is usually between 5 and 15 degrees. More than 15 degrees might be harmful for the facet joints behind, less than 5 means it probably isn't working as a TDR.
Types of Prostheses
Figure 1 (below) shows a typical prosthesis called the Prodisc. Figure 2 (below) 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.
Figure 3 (below) 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.
Figure 4 (below) is a prosthesis called the Extreme Lateral Total Disc Replacement (XL-TDR). 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.
- The Anterior Longitudinal Ligament (one of the biggest stabilisers in the spine – it resists rotation, and is the only restraint to extension in the spine) is preserved which produces a more physiological range of motion than other disc replacements.
- The very large front-on dimension gives it great stability in soft bone.
- Hospital stay is reduced dramatically – often just overnight.
- It can be inserted at all levels in the spine from T12/L1 to L4/5. All disc replacements that are inserted via the anterior approach can only really be inserted at L3/4 to L5/S1.
- It is true minimally invasive surgery and negates the need to access the spine from the front of the abdomen and move the abdominal contents around.
Figure 5 (below) is an x-ray showing the XL-TDR 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.
Figure 6 (below) is an x-ray showing 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.
Figure 7 (below) shows what 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 (illustrated in Figure 8 below), 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.
Figure 9 is an x-ray showing the prosthesis 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.