Artificial Discs -- Surgical Fad or Real Breakthrough?
Doctor Lali Sekhon has quickly established himself as a leader in the field of spine surgery, particularly in the emerging field of artificial discs. SpineUniverse was fortunate to catch up with Dr. Sekhon at a recent meeting to chat with him about artificial discs and how they might help spine patients.
Dr. Sekhon, first of all, what is a "real" disc in the spine, and what is an artificial disc?
Dr. Sekhon: In technical terms, a real disc in the spine is a fibrocartilaginous structure that sits between adjacent vertebral bodies. The best way to picture a disc is to imagine a tough, spongy cushion. The cushion consists of a soft inner nucleus that gives it elasticity, bounce, and recoil. A firmer outer layer provides strength and support.
A key function that discs provide is they act as shock absorbers in the spine. For example, if you were to jump off a chair, about two-thirds of the load or pressure that passes through the spine would be dispersed by the discs.
An artificial disc attempts to replace this function if the real disc has been removed. The artificial disc sits between two adjacent bones; the vertebrae, and takes the place of the original disc. An artificial disc allows movement and acts as a shock absorber.
SpU: What is the idea behind artificial discs? Why might they be important?
Dr. Sekhon: First of all, you need to understand what happens when a disc deteriorates or ruptures. When a disc ruptures, the outer part bulges and sometimes the inner nucleus escapes and can press on nerves or the spinal cord. The pressure on the nerves can cause significant pain and neurological symptoms such as numbness or tingling in the extremities; the arms or legs.
If surgery is needed because of neurological symptoms, often the only way to take the pressure off the spinal cord or nerves is to entirely remove the disc. When this is done something must fill the empty space otherwise the bones keel forward and cause abnormal angling which, in themselves, may cause pain.
Most surgeons insert some form of bone into the space to fuse the vertebrae (bone) above and below the empty disc space. This often works very well in the short-term and can be done in many different ways that may include instrumentation such as cages, plates, and screws. Eventually the fusion becomes solid.
However, there is a price to pay for such a technique. The levels above and below the fused or solid area are now forced to absorb more load as there is no spongy disc between the vertebrae. We now know that up to 30% of discs above or below the level of the fusion wear out within 10 years and will require surgery. A stepladder effect can occur with multiple fusions over many years.
So what we needed to do was find a way to enable us to remove discs, but also to insert something into the space that would retain the spine's mobility and share the loads exerted on the spine. That is what we are trying to achieve with artificial discs.
We don't know yet if artificial discs indeed achieve this but, by keeping movement at the level that has been operated on, excessive loads are not placed on the level above or below.
SpU: What are artificial discs made from and are they available for all levels of the spine?
Dr. Sekhon: There are many different types of artificial discs. They share similar design principles that include keeping the empty disc space jacked open and allowing normal spinal movement at that specific level. Some artificial discs consist of elastic polymers headed by titanium shells, such as the BryanÂ® disc. Others use a metal-on-metal ball and socket joint such as the PrestigeÂ® disc. In the lumbar spine, ball and socket discs are the type commonly implanted. You can see the differences in construction between the BryanÂ® and PrestigeÂ® discs in these two pictures.
Figure 1: The BryanÂ® artificial disc prosthesis.
Figure 2. The PrestigeÂ® disc.
Getting back to your question, "are they available for all levels," in general, cervical artificial disc replacement is used at C4-5, C5-6, and C6-7 and in the lumbar spine, L3-4, L4-5, and L5-S1. These are the levels most likely to wear out and the levels that allow most movement.
SpU: How recently have artificial discs begun to be used? Are they used all around the world now? Do surgeons require special training to use them?
Dr. Sekhon: Lumbar artificial discs have been around for more than 10 years with good 10-15 year follow-up; such as the Charite and PRODISCÂ®. In the neck we have only had the BryanÂ® disc readily available for 2-3 years. Older versions have been around for over 10 years but a lot of these have fallen out of favor.
They are available throughout the world and especially widely used in Europe and increasingly used in the Pacific Rim. In the United States, both the BryanÂ® disc and PRODISCÂ® are part of a current FDA study. Surgeons do need specialist training in order to place these discs.
SpU: What types of spinal conditions are suitable for disc replacement? Are there particular circumstances that make one patient more suitable than another? What are these?
Dr. Sekhon: The ideal patient for a cervical disc protrusion arthroplasty is the same patient we would consider for an anterior cervical fusion; a patient with disc protrusion that causes arm symptoms or spinal cord compression.
Ideally, only one level of the spine is replaced, but up to three levels have been done. The same patients whom we would have fused in the past are receiving artificial discs. Of course, not all patients are suitable for this technology, and assessment by a skilled spinal surgeon is essential.
In the lumbar spine, the main indication for artificial disc technology is mechanical back pain with an MR showing one or two abnormal discs and a positive discogram.
SpU: How "proven" are artificial discs?
Dr. Sekhon: Artificial discs are proven in that they are safe to implant and patients are doing fine. The real question is what happens to these discs over time? Will they wear out? Will they need to be replaced? Lab tests up to 47 years on the BryanÂ® disc suggest not yet, although hip and knee replacements wear out after 10-15 years.
SpU: What are the developments that you expect to see in the next 2-5 years regarding artificial discs?
Dr. Sekhon: The next steps are (1) a few different types will appear, all with a little difference in how they do what they do, (2) the techniques to insert them will become easier and, (3) the indications for these operations will become clarified. The era of spinal arthroplasty is definitely here.
SpU: Thank you Dr. Sekhon.
Dr. Sekhon: You are welcome.