When Daniel Riew, MD, began implanting artificial cervical discs in 2002, surgeons were waiting for answers to three questions. Would the operation work as well as fusion? Would the implant last? And could preserving motion prevent the levels above and below it from deteriorating?
More than two decades later, Dr. Riew believes the first two questions have largely been settled. The third has not delivered the answer many surgeons expected.
Cervical disc replacement has proved durable. In appropriately selected patients, long-term outcomes are comparable to, and in some studies slightly better than, anterior cervical discectomy and fusion. Many of the earliest implants Dr. Riew placed are still functioning. But the procedure did not eliminate adjacent-level disease.
“We thought, ‘If we do this, there will be no more adjacent-level disease,’” Dr. Riew, chief of cervical spine surgery, co-director of the spine fellowship and professor of orthopedic surgery at Columbia University in New York City, told Becker’s. “That was kind of naive.”
That disappointment did not make cervical disc replacement a failed experiment. It made the operation more honest. The procedure’s greatest value may not be protecting the rest of the spine from aging. It may be allowing the right patient to recover faster, retain motion and avoid fusing multiple segments.
After more than 20 years of experience, however, Dr. Riew believes the field’s next advance will depend less on expanding disc replacement to everyone than on recognizing who should never receive one.
The promise that did not hold
Artificial disc replacement entered cervical spine surgery with a compelling theory. A fusion stops motion at one level. The levels above and below must compensate. Over time, the added stress was thought to accelerate degeneration and lead to additional surgery.
Preserve motion, the theory went, and the rest of the spine would be protected. Dr. Riew said long-term evidence has challenged that explanation.
“The answer is because people get old,” he said. “When you operate at one level, you have to understand that all of the discs are exactly the same age.” A 50-year-old patient does not have one aging disc and several young ones. Every segment has experienced the same five decades, although some may deteriorate more quickly than others.
That natural history helps explain why patients can develop disease at several cervical levels without ever undergoing surgery. Dr. Riew pointed to several lines of evidence.
In one study, researchers compared patients born with a congenitally fused cervical segment with patients whose spines developed normally. If fusion itself caused adjacent-level disease, those born with a fusion should have needed surgery far earlier. They did not. Both groups generally underwent their first operations in their 50s, Dr. Riew said.
Another prospective study compared fusion with posterior foraminotomy, an operation that preserves motion. The motion-sparing group did not experience less degeneration at neighboring levels.
Then came the artificial disc trials. Dr. Riew and his colleagues found a statistically significant difference in adjacent-level reoperation between disc replacement and fusion when they examined longer-term data, but the benefit was far smaller than originally anticipated, roughly 4 percentage points at 10 years in their analysis. Artificial discs may reduce the risk. They do not stop the clock.
What the operation actually offers
If cervical disc replacement does not dramatically protect neighboring levels, why has it continued gaining momentum? Because it still solves other problems.
“It has withstood the test of time,” Dr. Riew said. “The long-term data is as good as a fusion.”
Patients in the early FDA trials often recovered quickly, avoided cervical collars and returned to normal activities sooner. Those advantages persisted even as the first generation of implants was replaced by newer designs. Dr. Riew said he allows most artificial disc patients to return to work and recreational activities such as golf, tennis and pickleball almost immediately. He is more cautious with professional athletes involved in contact sports.
Fusion patients face a different recovery. Although some surgeons do not use a postoperative collar after ACDF, Dr. Riew routinely does. Fusion requires two bones to knit together, and he believes limiting motion improves the speed and reliability of healing.
“You can go to a surgeon who is going to make you happy for the first six weeks,” he tells patients. “But if they tell you in one year that you’re not healed, you’re not going to like them.”
That difference makes artificial disc replacement attractive to patients seeking a faster return. The amount of motion preserved at one level, however, may be modest. Dr. Riew said most patients would struggle to distinguish the range of motion produced by a one-level disc replacement from that of a one-level fusion.
The value becomes more noticeable as the number of treated levels increases. Losing one motion segment may be imperceptible. Losing three can change how the neck feels and functions. That is where motion preservation begins to matter more.
The field’s most important skill may be saying no
The growing popularity of cervical disc replacement has created a new risk: treating the operation as inherently preferable to fusion. Dr. Riew does not.
“A nail doesn’t fit every job,” he said. The operation can be an excellent choice for a younger patient with a soft disc herniation, healthy bone and enough room in the spinal canal. In that setting, it can relieve radiculopathy or myelopathy while preserving motion and accelerating recovery.
But an artificial disc cannot correct every cause of cervical compression. Patients with extensive bone spurs, ossification of the posterior longitudinal ligament or diffuse idiopathic skeletal hyperostosis may continue forming bone around a motion-preserving implant. If the surgeon cannot fully remove the compressive pathology through the disc space, symptoms can recur.
Bone spurs, Dr. Riew said, are the body’s attempt to stabilize an unstable or collapsed segment. Restoring motion without removing the full spur may allow the same process to continue. He has treated patients whose artificial discs had to be removed because the initial decompression was incomplete. Many ultimately required the fusion they might have been better served by receiving at the first operation.
“The patient would have been much better off having an ACDF to begin with,” he said.
The bone-quality blind spot
Bone quality presents another dividing line. Artificial discs rely on the vertebral endplates for support. In patients with osteoporosis or significant osteopenia, the implant can subside into weakened bone, collapse the disc space and create deformity.
Dr. Riew said he has revised several patients whose implants collapsed. In some, the damage was extensive enough to require a corpectomy, in which the surgeon removes part or all of the vertebral body. He believes surgeons should evaluate bone quality before cervical disc replacement using a DEXA scan, CT imaging or Hounsfield unit measurements derived from the CT.
A normal DEXA result may not tell the entire story. Bone density varies across the cervical spine, and the commonly operated C6-7 level may have lower density than more central cervical vertebrae. A patient can appear acceptable on a generalized screening test while having poor bone quality exactly where the implant will sit. Skipping that evaluation can turn a motion-preserving operation into a much larger revision.
When more motion causes more compression
The size of the spinal canal also matters. A patient with a large soft disc herniation causing myelopathy may do well with disc replacement if removing the disc adequately decompresses the spinal cord.
A patient born with an unusually narrow canal may not. Even after the disc is removed, the cord may remain crowded. Continued motion can cause the ligamentum flavum at the back of the canal to buckle inward, recreating compression over time. Dr. Riew recently evaluated a physician who received an artificial disc but remained symptomatic. The implant was well positioned, but the patient had congenital stenosis and residual bone spurs compressing a nerve root. The patient now requires revision surgery and fusion.
“He was not a good candidate for an artificial disc because his canal was too small,” Dr. Riew said.
Myelomalacia, damage or softening within the spinal cord, raises an additional concern. Implanting an artificial disc often requires tapping the device into place. Dr. Riew worries that force can further injure an already compromised cord. Even if the patient does not wake with a new deficit, continued motion at the damaged level may prevent the spinal cord from settling.
The lesson is not that disc replacement is unsafe. It is that motion is not always therapeutic. Sometimes stability is the treatment.
Insurance still wants a binary answer
The most difficult cervical cases do not always call for choosing entirely between fusion and disc replacement. A patient with disease at several levels may have one segment well suited to arthroplasty and another that requires fusion.
That is the logic behind hybrid surgery. A surgeon might fuse an unstable, severely arthritic level while placing an artificial disc at a healthier adjacent level. The approach preserves motion where it remains useful without forcing arthroplasty into anatomy that cannot support it.
Dr. Riew believes hybrid procedures are particularly valuable in three- and four-level disease. Multilevel ACDF carries a substantial risk that one or more levels will fail to fuse, a complication known as pseudarthrosis. A patient may require additional imaging, therapy and revision surgery. Replacing two levels and fusing two others can reduce the biological burden of fusion while preserving more movement.
“It saves the insurance company money,” Dr. Riew said. “It’s better for the patient because they don’t lose four motion segments, and in the end, everybody wins.”
Yet some insurers still do not cover a fusion and artificial disc during the same operation. That leaves surgeons with choices that may be worse for both the patient and the payer. They can place an artificial disc at a level that is not an ideal candidate, accepting the risk of later revision. Or they can fuse every diseased segment, increasing motion loss and the risk that a multilevel construct will not heal.
“It would be much better for patients if insurance would allow hybrid operations,” Dr. Riew said. The coverage restriction treats cervical surgery as a binary choice. The anatomy is rarely that simple.
Outpatient care has a boundary
Cervical disc replacement has also become part of the broader migration of spine surgery away from inpatient hospitals. Dr. Riew performs his cases in a hospital, but most patients go home the same day. He considers outpatient treatment reasonable for one- and two-level procedures in healthy patients who live close enough to return quickly if a complication develops.
The calculus changes as the operation grows. Three- and four-level anterior cervical procedures carry a higher risk profile and may warrant prolonged observation or a 23-hour stay, he said.
Payers have financial reasons to push procedures into outpatient settings. The lower-cost site can be appropriate. It should not become the only option.
“You always have to balance saving money with patient safety,” Dr. Riew said. A young, healthy patient undergoing a one-level operation near a hospital is different from a medically complex patient having several cervical levels treated far from emergency care.
The procedure code may be the same category. The risk is not.
A more mature form of motion preservation
Cervical disc replacement no longer needs the grandest version of its original promise. It does not have to prevent the rest of the spine from aging to be worthwhile. It has shown that it can last. It can relieve symptoms as effectively as fusion in the right patient. It can shorten recovery and preserve motion that becomes increasingly valuable when several levels are involved.
But the operation’s success has also revealed its limits. A poorly selected patient may face persistent compression, implant collapse, neurologic injury or a complex revision. An insurer that refuses hybrid coverage may push surgeons toward an all-disc or all-fusion solution that fits the policy better than the patient.
After more than two decades, the question is no longer whether cervical disc replacement belongs in spine surgery. It does. The harder question is whether surgeons, hospitals and insurers are prepared to use it with enough restraint.
The next breakthrough in motion preservation may not be another artificial disc. It may be knowing exactly where motion belongs, and where it does not.
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