Cervical spine fusions have been performed since the 1950s and have achieved good, reproducible clinical results. Anterior cervical fusion is a current standard of care for cervical degenerative disc disease (DDD), and 1-level fusions have predictable results, and are a successful procedure for surgeons and patients. However, fusion by definition eliminates the motion that normally occurs in a spinal segment and, as more levels of the spine are fused, the lack of motion created in the neck may cause additional problems for the patient. With the down classification of cervical cages by the FDA in 2007, patients and surgeons were offered more options for on label treatment of 1-level cervical disc disease. There are not as many options for on label multi-level fusion, and this has led to the need for an alternative on label treatment for patients requiring surgery at one or two levels.
The natural mobility of the neck is what allows us to easily perform many common daily tasks, such as driving, participation in sports (such as swimming), and recreational enjoyment (such as golf, photography, etc.). When a fusion is performed, this natural mobility of the whole neck is altered by removing motion at the surgical level(s). Interestingly, the total motion in the cervical spine can return to pre-operative conditions and, it is theorized that, this compensatory mechanism results in an increase in motion and increased loads on the levels of the spine just above and below the fusion. When multiple levels of the cervical spine are fused and rendered immobile, the functional implications can be noticeable to patients as this motion compensatory mechanism is amplified, which can cause greater impact to their daily life activities. Patients with multi-level fusions are believed to have an accelerated rate of adjacent level degeneration, as the altered motion and stresses at the levels next to the fusion facilitate and amplify the progression of their disease. If true, there is an increased likelihood they will be back for more treatment, potentially additional surgery, at a later time.
The body of evidence for non-fusion alternatives, mainly disc replacement, has grown quite large over the past 10 years. Disc replacement studies have shown a lower rate of reoperation and also a trend towards reduced adjacent level degeneration versus fusion. This is promising for patients. Long term data and improvements in non-fusion alternatives will continue to be a need in cervical surgery, as will the need for providing FDA approved, on-label options for patients and surgeons.
Artificial Cervical Disc Replacement
Artificial cervical disc replacement is a surgical procedure of the neck whereby a degenerated native cervical disc is removed and replaced with a prosthetic device made of biologically compatible metals, plastic and/or ceramics, depending upon the device. Cervical disc replacements are designed to maintain vertebral alignment and allow for normal physiological cervical motion. Therefore, cervical disc replacements are often referred to as motion preservation devices. Motion preservation is achieved by surfaces that articulate with other pivoting or sliding surfaces. There are more than 15 cervical disc replacement devices in the global market today, though only six are approved for commercialization in the United States.
Disc Replacement vs. Spinal Fusion Surgery
Fusion surgery and disc replacement begin the same as the disc is removed and the spinal nerves are decompressed in order to alleviate pain and/or neurologic symptoms. Fusion surgery places a mechanical device made of metal or plastic, along with bone graft material, or other material that promotes bone growth, in the disc space in order to support and grow new bone in place of the removed disc. This eliminates motion of the segment while maintaining normal space between the vertebrae to relieve compression of the neural elements. Benefits are predictable bone growth patterns yielding high fusion rates in the cervical spine, contributing to long-term pain and/or neurological relief. The theorized drawbacks include the loss of segmental motion and the transfer of loads to the adjacent levels above and below the fusion. Healthy discs next to a successful fusion site may degenerate at an accelerated rate. Disc replacement utilizes a motion preservation device that allows the segment to move more similar to a native disc. This re-creation of physiological motion provides the same neurological pain relief of fusion without the healing time required for biological fusion or the potential for increased adjacent level problems.
While fusion is suitable for many indications, including patients with cervical instability, cervical trauma, or longer term degenerative changes, cervical disc replacement, or arthroplasty, may be an alternative for many patients. Disc replacement is generally indicated for patients with arm pain and/or a neurological deficit, sometimes accompanied by neck pain, resulting from an abnormality in the disc space (i.e., herniated disc). It is, of course, very important for patients to have a detailed discussion with their surgeon when considering the best treatment option for them.
Anterior cervical discectomy and fusion (ACDF) has been the standard treatment for cervical degenerative disc disease with a long history of good outcomes. Beginning in the 1950s with the Cloward technique, cylindrical bone dowels were easily placed in the cervical disc space to fill the void, restore, and maintain disc height, and provide biological fusion for a solution to pain relief. This procedure has been modified over time to optimize bone graft options (materials and shapes) coupled with many device options for predictable outcomes.
The initial use of anterior cervical plates came about in cervical trauma surgery in the late 1960s by Dr. Lorenz Böhler in Vienna to provide rigid bone fixation that would allow bone growth and graft healing at a fracture site. This was more popularized in 1980 by Dr. Wolfhard Caspar in the Department of Neurosurgery at the University of Saarland, Germany, with the first-generation trapezoidal cervical plate, described for tumor, trauma, and revision applications. The Caspar plate was known to be very strong and stiff with generous open holes for screw fixation. Screw placement in these early plates required bi-cortical purchase to provide stability, as there was no locking feature for the screw/plate interface.
Anterior cervical plating applications grew over time to cross over from trauma to cervical DDD surgeries. The idea of placing a cervical plate over the interbody graft to protect it during the healing process and provide stability of the operated segment was an attractive proposition. Anterior cervical plating was also seen as a solution for anti-expulsion of the bone graft, as well as antikyphosis of the cervical spine.
By 1990, the cervical DDD market was saturated with many styles and choices of anterior cervical plates made by numerous spine device manufactures. Third generation anterior cervical plates featured enhanced locking mechanisms with anti-screw backout capabilities, variable screw angles, load sharing designs (thinner and less stiff), and customized drill guides and instrumentation that streamlined the surgical technique. Anterior cervical plates have enjoyed success, though complications have been noted over time for screw backout or breakage, esophageal irritation (dysphasia), and potentially stress shielding of the graph preventing bone healing.
By the early 2000s, companies began to explore options to place polyether ether ketone (PEEK) cages embedded with bone graft material in the cervical interbody space, and integrated fixation through the cage directly into the vertebral bodies. This alleviated the need for anterior cervical plates for some patients and simplified the overall procedure.
Today there is a mix of anterior cervical plating and stand alone PEEK cervical cages with integrated fixation on the market. While anterior plating is still more widely utilized in the market, we believe that application of stand-alone devices represents a rapidly growing segment.
Anterior cervical procedures account for more than 240,000 surgeries per year in the US. It is anticipated that motion preservation procedures will grow rapidly as more devices become available and new and long term evidence is published. The FDA approval of such devices is the obvious first step, and as more options are provided for on-label use, the increased usage of motion preservation devices is likely.
There are six cervical discs currently approved in the US—three of which were approved between 2007 and 2009, and another two approved by the FDA in 2012 (Globus’ Secure-C, and NuVasive’s PCM). Five of the available cervical discs in the US are approved for 1 level cervical disease only. In late August, LDR, Austin, TX, received approval from the FDA for the Mobi-C cervical disc replacement device, the first and only cervical disc replacement device to receive FDA approval to treat both onelevel and two-level cervical disc disease. (See Figure 1)Differentiating Mobi-C from other artificial cervical discs is its unique mobile bearing core which allows for controlled angulation and translational motion. In addition, it is implanted without a keel that is cut through the adjacent vertebral bodies like many existing discs on the market. The device’s patented mobile bearing core allows for flexion/extension and lateral bending coupled with translation similar to natural cervical motion.
Mechanical and Intrinsic Benefits
Artificial discs provide motion at the operated level, which allows the whole neck to move in a way that is more physiologic. This, compared to fusion, prevents the potential load transfer to adjacent healthy disc levels and represents the largest mechanical benefit of total disc replacement vs. fusion. The fundamental benefit is that artificial disc technology is intended to create an environment that is more natural.
As with hip or knee replacement, motion preservation for the spine is believed by many to provide a more natural surgical solution. And by maintaining motion, it is hoped that there will be a decreased likelihood, or a decreased rate of, degeneration at levels adjacent to the surgery.
Published clinical results for cervical disc replacement have shown:
- Lower reoperation rates for cervical disc replacement as compared to fusion
- Sustained motion at operated levels
- Trends toward lower rates of adjacent level degeneration
Many of the surgical steps to implant a cervical artificial disc are the same as performing an anterior cervical fusion. So the good news is that most of the surgical procedure is familiar to surgeons. However, there are specific considerations for an artificial disc regarding placement and sizing, and patient indications must be respected for good outcomes. We believe that many surgeons will be open to learning about artificial discs as second generation discs become available, and the quantity of consistent data to support good clinical outcomes is compelling.
The Challenge of Two-Level Cervical Discs
Why aren’t there more two-level discs on the market? This is a combination of factors. To begin, you must start with the design of the devices themselves. Some of the currently approved devices incorporate features that may make them less attractive for two-level applications. So in order to pursue two-level approval, you must start with a device specifically designed for both 1 and 2-level use. For example, the keels associated with many devices make them difficult to implant on two adjacent levels without compromising the vertebra in between the two keel cuts. If a device with anterior screw/flanges is used, the anterior vertebral body real estate may not accommodate so much hardware.
LDR is one of only two companies to have completed enrollment of a two-level cervical disc replacement study, and was the first to receive approval. In order to market a cervical disc device for two-level applications in the US, it is necessary to complete an investigational device exemption study and gain approval through the premarket approval process. This is a very arduous process and few device manufactures to date have made the necessary investments to study this large patient population.
The Mobi-C cervical artificial disc was first implanted in 2004 in Europe. To date, it has been implanted in more than 17,000 cases outside the US, in countries around the world. The device entered an IDE study in 2006, with approximately 600 patients enrolled in the US to study 1- and 2-level disc replacement and was recently approved by the FDA.
Information in this article was supplied by Joe Ross, Executive Vice President of Global Marketing, LDR Spine, Austin, TX. For more information, Click Here .