The rise of the artificial disc
| September 25, 2013
A degenerative intervertebral disc is the most common diagnosis in cases of chronic lumbar pain. Constant pressure from above, coupled with shearing forces and absorbing repeated impacts causes weakening of the annulus, which may eventually lead to a herniation of the central nucleus. Pain then becomes more acute and more difficult to treat conservatively.
The mainstay of treatment at this point — until recently — has been a lumbar discectomy to remove the offending disc, often accompanied by a fusion of the vertebrae above and below. In this case, a small section of bone is often taken from the patients hip and inserted between the two vertebrae — where the disc was removed. The two vertebrae and the additional bone are then screwed together to fuse the segment.
There are however several problems with this procedure. The main concern is that the reduced segmental motion at the fused vertebrae, increases the stress on the discs and vertebrae above and below. Anecdotal evidence has shown that within 10 years of discectomy and fusion surgery, around 25 percent of patients require additional surgery due to wearing of the adjacent discs.
Lumbar disc replacement surgery was first performed in Germany 20 years ago, and the first artificial disc was approved for use in the U.S. back in 2004. For the right candidate, success rates are high, complications less frequent, return to work time is shorter and there is less risk of requiring further surgery down the line.
Who is the right candidate? Lumbar disc replacement surgery tends to be most successful in younger patients (aged 25-50) with degenerative disc disease. It is less effective in older patients due to the presence of more advanced degenerative changes in the spine which may mean that disc replacement does not cure the problem. Similarly, those with a history of traumatic spinal injury (such as victims of road traffic collisions and falls) are not suitable due to the possibility of spinal instability.
The recovery time of disc replacement is quicker than with traditional surgery, allowing patients to get back to work in record time. After spinal fusion a brace is worn for around two months. After disc replacement the expected return to work timescale is 3-4 weeks, with Navy troops being back to full combat within this time frame.
The reason for this is no immobilization time is required as there is no bone healing time — unlike with fusion surgery where the additional bone must knit together with the vertebrae above and below. Patients are instead encouraged to stand and move from day one.
However, there are some unknowns. As a relatively new procedure, there is no evidence available for what happens to patients more than 20 years down the line. Currently the effects of aging and bone density deficits are not known, but research continues to monitor those who have undergone such surgery for the answers.
Moving on up
Following the success of artificial lumbar discs, the first cervical discs (ProDisc-C and Prestige) were approved in 2007. These were followed by Medtronic's Bryan in 2009; Globus Medical's Secure-C in 2012 and most recently LDR Medical's Mobi-C. This most recently approved implant design is the first to potentially be used to treat degeneration at two intevertebral levels.
Cervical disc degeneration and herniation is far less common than lumbar disc injuries but can be just as debilitating when pressure on nerve roots causes pain, motor and sensory deficits throughout the upper limb. Whilst two lumbar disc implants now have full FDA approval for use on the general U.S. population (the Charite disc and the ProDisc-L), all cervical implants are currently still at the clinical testing level.
It is now six years since the first cervical implant was approved for testing, and research is starting to appear regarding the medium-term effectiveness of these devices. Back in 2010, the first of these studies used the Prestige cervical disc versus traditional cervical fusion. Results showed that "the Prestige disc maintains improved clinical outcomes and segmental motion after implantation at five-year follow-up."
A new study was published in August that observed the impact of disc implantation on the biomechanical function of the adjacent segments. The results showed increased stress on adjacent cervical discs of only 10 percent following replacement compared to an increase of 70 percent after fusion surgery.
So, while the early results look promising, as always patients are advised to proceed with caution and to arm themselves with the full information before opting for a disc replacement. Generally the risks are much the same as with any major cervical surgery (such as nerve or spinal cord damage, infection, blood loss), which are possible but uncommon. There are, however, a couple of risk factors which are increased in comparison to fusion surgery.
Another study published in August looked at the use and short-term effects of cervical disc arthroplasty (CDA) versus anterior cervical discectomy and fusion (ACDF) during the FDA trials from 2004-2007. The number of CDA procedures carried out increased significantly every annum, and CDA patients were more likely to be discharged home with shorter hospital stays. But CDA patients also had a higher rate of deep venous thrombosis than ACDF patients. In addition, as with all arthroscopic procedures, there is a risk — albeit small — of subsidence and implant failure.
The future looks bright
All in all the results of trials for cervical disc replacement look promising and hopefully these implants will gain full FDA approval as their lumbar counterparts have done. While disc replacement may not be suitable for everyone (especially those with increased risk of clotting and older patients with spinal degeneration of instability), it may well be the future of disc surgery for years to come.
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