Across the past 50 years, anterior cervical discectomy and fusion (ADCF) has been the gold standard surgical intervention for cervical radiculopathy and myelopathy that is unresponsive to conservative treatment. For the past 15 years in Europe and more recently in the United States, orthopedic and neurosurgeons have sought to determine if disc arthroplasty is a viable alternative to fusion. By mimicking the natural anatomy of the cervical spine, surgeons seek to avoid some of the risks and complications inherent to cervical fusion.
The development of prosthetic intervertebral discs evolved out of the success of hip and knee arthroplasty, now standard treatment for severe degenerative changes of the knee and hip joints. The goals in designing prosthetic discs include establishing an axis of rotation that promotes normal kinematics, maintaining or restoring the normal range of motion and disc height, and transmiting loading forces through the vertebral bodies in a natural fashion.
The vertebral segment consists of three joints: two facet or apophyseal joints, which guide segmental motion, and one interbody joint that connects the intervertebral disc with two vertebral bodies, which distribute force loads across the vertebral column.1 The intervertebral disc is comprised of a thick, gel-like core, called the nucleus pulposus, surrounded by a multi-layered fibrous encasement called the annulus fibrosis.1 The nucleus provides compressive forces that keep the annulus taut and allows for transmission of forces across the annulus. The disc lies between two vertebral bodies and is firmly attached to cartilaginous endplates, preventing excessive torsional and translational movements.2 Normal motion occurs between two vertebrae around an axis of rotation that varies in location, but is generally in the posterior half of the upper portion of the inferior vertebral body.3
Normal changes occur in aging, causing the intervertebral disc to become dehydrated and brittle. This reduces the efficient transmission of forces across the annulus causing tears to develop in the annulus fibrosis. Disc degeneration can result in disc herniation or spinal stenosis, depending upon whether the spinal nerves or cord itself are compressed.2 After age 40, 60% of the population shows evidence of degeneration on radiograph.3 After age 65, 95% of men and 70% of women have degenerative changes.3
Arthritic changes can lead to the degeneration of facet joints, and to the development of bony spurs and osteophytes, which can hamper movement as well as place pressure on the spinal cord or the spinal nerves leading to radicular or myelopathic symptoms. Herniation is more common in the initial stages of degeneration and stenosis in the later stage.2
Indications for Surgery
While the incidence of degenerative disc disease and spondylosis are high, the vast majority of people require no surgery. They may be asymptomatic or their symptoms may be mild. Additionally, few patients meet the strict criteria for eligibility for disc arthroplasty. Fusion remains far more common.
Indications for cervical disc arthroplasty are the same as for discectomy and fusion. Patients referred for surgery have a symptomatic cervical disc at one level that is not responding after at least six weeks of conservative treatment consisting of NSAIDs and physical therapy. The symptoms include radiculopathy and/or myelopathy with neck pain, resulting from disc herniation or spondylosis. While in the United States, arthroscopy is indicated for one segmental level, multi-level arthroplasty surgery is being performed in Europe.4
Contraindications to total disc arthroscopy include osteoporosis, spondylolisthesis, bone disease, facet joint abnormalities, instability, prior laminectomy, RA, ankylosing spondylitis, ossification of the posterior longitudinal ligament and diffuse idiopathic skeletal hypertosis.3,5,6 Patients with these conditions are better candidates for ACDF or another intervention.
Surgery and Postoperative Treatment
Anterior cervical discectomy and fusion (ACDF) has been a successful surgery for a majority of patients suffering radiculopathy and/or myelopathy due to disc herniation or spondylosis. A drawback to the surgery is the loss of segmental motion at the fusion. Loss of mobility increases with the number of segments involved. Hypomobility at the fused segment leads to increased mobility in adjacent segments.3 The increased intradiscal pressures accelerate degeneration at adjacent vertebral levels, with 2.9% of post-fusion patients developing symptomatic adjacent segment degeneration (ASD) each year, and 25.6% of patient with ACDF developing ASD within 10 years.7 Adjacent segment degeneration occurs more commonly at C5-6 and C6-7 levels.3
Success in knee and hip arthroplasty motivated development of artificial vertebral discs. The theory was that preservation of the normal anatomy and kinematics of the spine would place less pressure on adjacent discs thereby reducing the risk of adjacent disc disease. One study showed that 24 months after prosthesis implantation, 17.5% of the arthroplasty patients showed degeneration, while 34.6% of fusion patients showed degenerative changes in adjacent discs.8 However, other studies have found no difference in the incidence and risk of adjacent level degeneration.9
In designing an artificial disc, the goal has been to recreate the natural function of the cervical disc as closely as possible. Artificial discs approved for use in the United States have some variation in design and classification. Materials are metal alloys designed to last a lifetime, thereby preventing the need for second replacement surgeries.
Artificial discs have ball-and-socket or ball-and-trough articulations, which may be articulating, uniarticulating or biarticulating.10, 3 Prosthesis stability is classified as constrained in certain planes if the disc restricts motion to less than normal physiological range of motion.3 It is semi-constrained if the disc permits a range of motion similar to physiological range, and non-constrained if there are no mechanical stops except for soft tissue, like muscles and ligaments, and compression across the disc space to prevent excessive movement.3
In both cervical arthroplasty and fusion, the surgeon approaches the vertebrae from an anterior approach to perform a discectomy, ensuring that the affected disc and all material compressing spinal nerves or the spinal cord is removed. Additionally, the surgeon removes any osteophytes to maximize the potential for normal range of motion.3 Some surgeons remove the posterior longitudinal ligament to ensure complete decompression of the disc space and to restore intervertebral height.3 Other surgeons leave the posterior longitudinal ligament as long as it is undamaged.11
For cervical arthroplasty, the endplate surfaces are prepared by removing the cartilaginous endplate and burring the surfaces until they are parallel to ensure even insertion of the prosthesis. Then the disc space is distracted to a normal height and the prosthesis is implanted. Surgeons are exacting in identifying the midline for prosthetic implantation as it is critical for normal range of motion, the proper axis of rotation and transmission of forces across the cervical spine.3 Implants more than 1mm to 2mm from the midline risk uneven facet loading, which can lead to postoperative pain.4
Patients remain in the hospital for 24 to 48 hours after surgery. They are sent home with few movement restrictions. After two weeks, patients are permitted to perform mild flexion and rotation activities.5 Extension is initiated after the incision has healed.5 Rehabilitation is similar to fusion, but recovery and healing occurs in shorter time.
The primary goals of physical therapy are to promote good healing of scar tissue and to prevent adhesions, institute good posture and body mechanics, maximize range of motion and to address any strength deficits in the neck. Total disc arthroscopy is relatively new and there is a lack of research regarding postsurgical physical therapy.
In addition to a lack of research regarding rehabilitation following cervical disc arthroplasty, there is no evidence to support the use of modalities as adjuncts to physical therapy. Searches of PubMed, Proquest, Cinahl and Google Scholar using search terms such as, “artificial cervical disc”, “cervical disc prosthesis”, and “cervical disc arthroplasty” failed to uncover any published research. Since the advent of cervical disc arthroplasty is relatively recent, research continues to focus on determining the benefits of cervical disc arthroplasty in comparison to cervical disc fusion. Other research concerns individual prostheses or surgical methods. For related information, please refer to the Morphopedics pages concerning Mid-cervical Disc Fusion and Cervical Disc Herniation. While potential treatment methods may overlap, there may be contraindications to the use of ultrasound or electrical modalities due to desired location of application, the materials in the prosthesis or impaired sensation surrounding the site of surgery.
Outcomes of Surgery
While cervical disc arthroplasty has been performed in Europe for the past 15 years, it was approved in the United States only since 2007. At this point, studies are largely short-term, with the longest following patients for 4-6 years. Studies generally compare outcomes for patients receiving artificial discs with patients receiving fusion because fusion has been the gold standard.
Studies show the cervical disc arthroplasty is as effective as ACDF, with the possibility of a slightly better outcome. However, there is no solid evidence that it is superior to fusion. Complication rates are similar in the two surgeries with 1.5% of patients experiencing complications in arthroplasty and 2.9 percent in fusion.12 At 12 months 90% of the TDA and 80% of the fusion patients had patient-rated “good” outcomes, and at 24 months 93% of the TDA, and 82% of the fusion patients responded favorably.12
Arthroplasty is advantageous in maintaining range of motion12, possibly reducing stress on discs in adjacent segments. Preoperative range of motion and disc insertion angle has been found to influence postoperative range of motion. Disc angle and depth of insertion place the axis of rotation close to the natural axis, thus allowing restoration of natural kinematics and column support.11 Patients with preoperative disc space collapse combined with contracture of spinal ligaments and facet capsules may continue to experience deficits in range of motion postoperatively.11
While the 10% difference in satisfaction between the TDA and ACDF patients must be taken into consideration, functional outcome measures do not reflect this difference.12 The time of greatest difference is just after surgery. Mummaneni et al. demonstrated that patients returned to work 16 days earlier after TDA as compared to fusion.13,14 Patient satisfaction outcome measures are statistically superior or comparable to ACDF.12 However, differences in scores narrow over time, and are similar by 4 years of follow up.9
Adverse outcomes include delayed fusion in the area around the prosthesis, postoperative kyphosis due to asymmetric endplate preparation, and reduction in vertebral height.15 Another common complication is heterotropic ossificans (HO), an abnormal formation of bone within soft tissue. Some patients have initially demonstrated good range of motion, which later decreased and is possible associated with HO and newly developed osteophytes.6
HO is a frequent complication of the surgery, with figures ranging from as low as 17.8% to as high as 76.2% across several studies.15 Several studies show high rates of HO and a correlation between HO and restricted range of motion.6 Rates of HO occurrence differed by prosthesis type, possibly due to design, metal debris from wear or bone dust from endplate drilling.15 HO can be triggered by a trauma, such as surgery, and originates from osteoprogenitor stem cells lying dormant within affected tissues.
Patient's Research Guide to Cervical Artificial Disc Replacement
Artificial Disc Replacement
Patient's Guide to Cervical Artificial Disc Replacement
American Chronic Pain Association
Prestige Cervical Disc
Prodisc-C Cervical Disc