Artificial Disc
Motion-Preserving Surgery
Cervical Arthroplasty (Disc Replacement)
Dr Ales Aliashkevich is a strong advocate for motion-preserving spinal surgery. He has been using cervical disc replacement in hundreds of patients with chronic neck pain, radiculopathy and myelopathy since 2006, achieving excellent results. Over the years, he has gained extensive experience in single and multilevel arthroplasty and hybrid procedures.
History of Cervical Disc Replacement
Surgery of the cervical discs began evolving in the 1950s. Dr. Robert Robinson, an orthopedic surgeon, and Dr George Smith, a neurosurgeon, from John Hopkins Hospital in Baltimore, United States published an article “Anterolateral cervical disc removal and interbody fusion for cervical disc syndrome” in 1955.
Cervical Disc Implants
Decades of biomechanical research, engineering, and meticulous clinical studies were required to achieve advances in medical device technology and to produce a current line of artificial intervertebral discs. Modern artificial cervical disc implants are available in variable shapes, sizes, heights and articulation types.
Surgical Technique
The surgical approach can be performed from the left or right side, depending on the dominant symptoms. The incision is usually made slightly off midline after injection of local anesthetic.
Postoperative Care
For the first days at home, patients are recommended to avoid lifting weights over 2-3 kg and to engage in any strenuous or repetitive activities that may affect the neck. Most patients are capable of returning to their normal activities and light work within a few weeks of surgery.
Comparison with ACDF
Both artificial cervical disc replacement and anterior cervical discectomy and fusion (ACDF) offer very favorable clinical outcomes reaching satisfaction rates higher than 80 – 90% in appropriately selected patients.
SPINAL DEGENERATION
Cervical Disc Disease
Natural aging of the disc (degeneration) or trauma can affect its mobility and cause a reduction of its cushioning function. Structurally, it may manifest in a disc protrusion or formation of bone spurs (osteophytes) causing pressure on the neural structures and resulting in neck and arm/shoulder pain, numbness, or weakness.
Cervical arthroplasty is aimed at restoring the shock-absorbing function of the damaged or degenerated intervertebral disc with simultaneous decompression of the spinal cord and nerve roots. Similar to the well-established hip and knee joint Cervical Disc replacement procedures, the evolution of artificial disc prostheses may also result in a dramatic improvement in patients’ quality of life and achieve high levels of patient satisfaction.
The initial treatment of cervical disc disease involves rest, gentle physical therapy, painkillers or anti-inflammatory medications. Sometimes, spinal nerve root injections with local anesthetic or steroids can be applied. Surgical treatment is taken into consideration only as the very last resort to manage intractable pain or neurological deficit. Two main types of surgery can address cervical disc pathology – a fusion or.
Although they are similar in approach, technique, and initial results, the differences are becoming more obvious over time, in particular after more than 5 years. The cervical Disc replacement device preserves motion and reduces the load on the discs above and below protecting them against accelerated degeneration.
The long-term outcome study of Dr Jetan Badhiwala from the University of Toronto, Canada showed that cervical disc replacement reduces the chances of symptomatic adjacent segment degeneration by more than 50% when compared to fusion (Cervical disc arthroplasty versus anterior cervical discectomy and fusion: a meta-analysis of rates of adjacent-level surgery to 7-year follow-up; J Spine Surg 2020;6(1):217-232). Multiple other investigations have shown superior patient outcomes, higher patient satisfaction rates, reduced rate of re-operations, fewer complications, and reduced costs after cervical arthroplasty when compared with spinal fusion in properly selected candidates.
SURGERY OF THE CERVICAL DISCS
History of Cervical Disc Replacement
Surgery of the cervical discs began evolving in the 1950s. Dr Robert Robinson, an orthopedic surgeon, and Dr. George Smith, a neurosurgeon, from John Hopkins Hospital in Baltimore, United States published an article “Anterolateral cervical disc removal and interbody fusion for cervical disc syndrome” in 1955.
They initially described the technique and outcome of cervical disc fusion performed in 8 patients, and in 1962 reported their results of treating 55 patients. Dr. Ralph Cloward, a neurosurgeon from Honolulu, United States, reported about his technique of cervical fusion on 47 patients in the article “The anterior approach for removal of ruptured cervical disks” in 1958.
Dr Ulf Fernström, a surgeon from Uddevalla, Sweden, invented a solid stainless-steel sphere disc prosthesis now known as the Fernström Ball. In 1966 he published an article “Arthroplasty with intracorporal endoprosthesis in the herniated disc and in the painful disc”. He described the implantation of various metal ball bearings into the disc space at the cervical and lumbar levels. Because of complications related to ball migration, hypermobility, and subsidence, the use of Fernström ball implants eventually had to be abandoned.
The development of lumbar disc prostheses in the 1980s also rekindled the evolution of cervical devices. A two-piece stainless-steel ball-in-socket implant with anchoring screws (Cummins-Bristol artificial cervical joint) was designed in the Department of Medical Engineering at Frenchay Hospital in Bristol, United Kingdom and used in clinical studies in 1991. The results of this trial were reported in 1998 by Drs Brian Cummins, James Robertson, and Steven Gill.
Years of technological advancement and biomechanical studies lead to the development of further generations of robust and safe cervical implants. The modern cervical disc replacement device is typically composed of metal plates separated by an elastic material. The device can be secured in place with screws, anchors, or spikes that prevent migration.
Cervical total disc replacement surgery began in Australia in the 1990s, but Medicare stopped funding it mainly because of poor results with the first-generation prostheses. In 2004, a Consortium including Medtronic, Taylor Bryant, and Johnson & Johnson Medical (DePuy Spine) applied to the Medical Services Advisory Committee (MSAC) to list artificial disc replacement, specifically using the Bryan, Prestige, ProDisc C, Charitè, and Prodisc, for the treatment of degenerative disc disease. Based on the absence of adequate evidence of clinical effectiveness at the time of the evaluation, MSAC recommended that public funding for cervical disc replacement in the cervical spine should not be supported.
After new randomized comparative evidence of cervical disc replacement in the cervical spine has been published, MSAC agreed that cervical arthroplasty is more effective than ACDF in terms of the composite measure of “overall success” at 24 months and supported its public funding for symptomatic single-level cervical degenerative disc disease in 2011. Medicare still does not permit multi-level surgeries in Australia.
MATERIALS AND DESIGNS
Disc Implants
Decades of biomechanical research, engineering and meticulous clinical studies were required to achieve advances in medical device technology and to produce a current line of artificial intervertebral discs. Modern artificial cervical disc Replacement implants are available in variable shapes, sizes, heights and articulation types.
It means that there is still no ideal implant available to mimic the biomechanical properties of a healthy natural disc and perfectly cushion and transfer loads in the neck. An ideal artificial disc should achieve the following goals:
– distribute loads evenly across the front, back, and sides of the vertebrae
– provide stability and natural neck movements in different postures;
– bear prolonged loads of the head’s weight and have
– unload facet joints;
– restore healthy anatomy and space between the vertebrae;
– restore for aminal height decompressing the nerve roots;
– be biologically compatible, corrosion-resistant, and able to.
The crucial biomechanical factor in a cervical disc implant is the position of its center of rotation. As the natural human disc can move in all 3 planes and does not have a fixed rotational center, a surgeon needs to choose an implant design with similar kinematics. Old-fashioned ball-and-socket articulation provides only one fixed rotation center and can no longer be accepted as suitable to achieve the goal of replicating complex natural movements.
Many of the disc implant designs are uni- or bi-articular. Some old models are metal-on-metal ball-and-socket designs such as the Prestige ST and Prestige LP (Medtronic) and Kineflex-C (Spinal Motion). Others are metal-on-plastic with one or two ball-in-socket or saddle-type articulations such as ProDisc-C (DePuySynthes), PCM (Nuvasive), Discover (DePuy), Mobi-C (Zimmer Biomet), Secure-C (Globus Medical), ActivC (B Braun) and Synergy (Synergy).
The DiscoCerv device (Alphatec) has titanium plates with ceramic-on-ceramic articulation. Although these articulating devices can maintain the mobility of the affected segment, they don’t produce natural motion. They also don’t deliver the shock-absorbing property of a healthy disc and may increase loading on the facet joints.
The healthy disc is elastic, and apart from flexion/extension, lateral bending and rotation can also be compressed to absorb axial shock. There are several prostheses which feature elastic properties: older Bryan (Medtronic) and Advent (Orthofix) and more recent M6 (Spinal Kinetics), Cadisc (Rainier), CP-ESP (FH Orthopedics) and Freedom (AxioMed). The latter two models come very close to duplicating the natural disc’s shape and function.
Less important is the implant classification into constrained, semi-constrained, and unconstrained. A constrained disc has a mechanical stop that limits its range of motion and provides physical stability. Semi-constrained devices can move slightly outside of the physiological range of motion, and unconstrained prostheses rely only on natural limiters of segmental mobility like spinal ligaments and facet joints.
Titanium is the most common disc implant material used, followed by cobalt alloys. Special surface treatment increases its integration with the vertebral bones:
– large footprint to cover the maximum area of the vertebral endplate;
– porosity;
– keels, spikes and screws;
– specialized coatings (calcium phosphate, plasma-sprayed titanium, aluminum oxide, hydroxyapatite).
The metallic components may produce distortions (artifacts) on magnetic resonance imaging (MRI) but because they are fixed to the bone, should not be considered as a contraindication for a scan.
ADVANCED SPINAL TECHNOLOGY
Implant Examples
The market of cervical disc prostheses is rapidly expanding and constantly changing, bringing new models into consideration and making some older versions, e.g. Charité or Bryan obsolete. Several implants, e.g. CerviCore, Kineflex C, or NeoDisc, had only very limited use.
– Bryan disc (Medtronic) is a semi-constrained device made of titanium alloy plates and a polyurethane center surrounded by a saline-filled sheath. The plate surfaces are porous. An anterior stop is present to prevent posterior dislocation.
– Discover prosthesis (Depuy Synthes, Centinel Spine) is an unconstrained ball-and-socket device made up of two titanium plates with a stiff polyethylene center. The plates are coated with porous titanium and hydroxyapatite and have small teeth.
– Mobi-C disc (Zimmer Biomet) is made from cobalt-chromium-molybdenum alloy coated with a titanium and hydroxyapatite spray. It has a polyethylene center which is supposed to move between two stops providing translational freedom.
– Prestige implant (Medtronic) is an unconstrained metal-on-metal device coming in two variations: ST made of stainless steel, and LP made of titanium. ST model fixates to the vertebra by screws, and the LP has rails coated with titanium plasma spray.
– ProDisc-C disc with Nova and Vivo variants (Depuy Synthes, Centinel Spine) is a ball-and-socket semi-constrained device made of a cobalt chrome alloy with a convex firm plastic insert. It has keels or teeth for fixation.
– PCM (Porous Coated Motion) disc prosthesis (NuVasive) has cobalt-chromium-molybdenum alloy endplates with a polyethylene spacer. The endplates have three rows of teeth to strengthen the bone fixation.
– CerviCore disc (Stryker) is a semi-constrained device made of cobalt-chromium-molybdenum coated with titanium spray. It has a saddle-bearing design, which allows for two separate centers of rotation. It is supposed to stay in place supported by two fins, each containing three spikes.
– SECURE-C implant (Globus Medical) is a semi-constrained device with two cobalt-chrome alloy plates and a polyethylene sliding center. The superior and inferior surfaces of the centerpiece have different shapes, spherical and cylindrical, allowing for the front-to-back sliding and moving axis of rotation.
– Baguera-C prosthesis (Spineart) consists of a high-density polyethylene nucleus that articulates between two titanium endplates. Its nucleus allows about 0.15 mm elastic deformation to absorb shocks.
– M6 disc (Spinal Kinetics) is a single-piece device with titanium alloy plates and a complex polymeric centerpiece surrounded by polyethylene woven fiber. There are three keels on each endplate. This implant allows for motion in all six degrees of freedom.
– Activ-C implant (B Braun) is made of cobalt-chrome-molybdenum plates and polyethylene inlay anchored into the inferior prosthesis plate forming a ball and socket joint with the superior plate. It has a titanium coating to facilitate bone growth. The inferior plate has a keel, and the superior plate has 3 spikes.
– ARAMIS disc (Osimplant) has two cobalt-chrome plates with a carbon fiber/PEEK interposing bearing.
– Cadisc-C disc implant (Ranier) is made of polymer with a graduated modulus design. The surface has a texture with a calcium phosphate coating. It has a mobile center of rotation.
– Procoral one-piece implant (RD Medical) is made of titanium and PEEK shaped like a tube at the inner mechanism.
– Freedom disc (Axiomed) is a one-piece viscoelastic implant bonded to two bead-coated titanium endplates with rails. The FLD and FCD polymer cores are designed to provide the kind of stiffness similar to that found in healthy human discs. Like human discs, the cores are viscoelastic, which means they respond to different loads and loading rates the way a human spine does. The material characteristics of the polymer used in all Freedom® Discs, in combination with their design, provide both stability and three-dimensional motion that biomechanical testing has shown functions within the natural biomechanics of the spine. The Freedom® Lumbar and Cervical Discs are designed to withstand the forces and wear characterized by decades of use as an implant in relatively young patients.
– CP-ESP (Elastic Spine Pad) disc (FH Orthopedics) is a one-piece implant that has titanium alloy endplates covered by hydroxyapatite with spikes to improve bone fixation. The core consists of concentric inner and outer parts made from elastomeric polycarbonate urethane. It has a variable center of rotation adaptive to natural movements and no surface bearing for an increased lifetime. The implant has been tested with load under different protocols and demonstrated the loss of height only to 0.12 mm after 10 million cycles. It compared favorably in aging tests with other devices (Bryan, Prestige, ProDisc-C, Active-C, PCM, Secure-C, Mobi-C and Prestige LP). This prosthesis has been implanted in France since around 2012.
Please refer to the following website for more information and examples of available cervical disc prostheses:
https://www.medicalexpo.com/medical-manufacturer/cervical-disc-prosthesis-4101.html
INDICATIONS AND CONTRAINDICATIONS FOR ARTHROPLASTY
Who can be a candidate for cervical disc replacement?
Patients with chronic neck pain, cervical radiculopathy, or myelopathy should be carefully selected before consideration of cervical disc replacement. Apart from a thorough neurological examination, they require MRI with 45° oblique views through the foramina to visualize the anatomy of the intervertebral discs, nerve roots, and the spinal cord.
Flexion/extension X-rays are routinely required to estimate the mobility of the affected segment. If imaging shows problems with several discs, clinical correlation, neurophysiological testing (EMG and nerve conduction tests) or diagnostic nerve blocks are required to identify which level should be considered as the dominant source of the problem.
All conservative treatment options (rest, medications, physical therapy, steroid injections) should be exhausted before the discussion of surgery. Intractable radicular shoulder/arm/hand pain and neurological deficits (tingling, numbness, weakness, loss of dexterity and balance, trouble with coordination or walking) caused by a damaged disc are the most common indications for cervical disc replacement. The final decision with regard to surgery is always the prerogative of the patient.
In Australia, the United States and many other countries, cervical disc replacement is only approved for use at one level. It can be used in combination with fusion at another level or for the treatment of symptomatic adjacent segment disease above or below the previous fusion. Based on MBS protocols, (http://www9.health.gov.au/mbs/search.cfm?q=51131) it can be used for a patient who:
(a) has not had prior spinal surgery at the same cervical level; and
(b) is skeletally mature; and
(c) has symptomatic degenerative disc disease with radiculopathy; and
(d) does not have vertebral osteoporosis; and
(e) has failed conservative therapy
Contraindications for Cervical disc replacement surgery
The procedure should not be performed in patients younger than 16 years (skeletally immature), in the presence of significant osteoporosis (reduced bone density), an active infection, or advanced malignancy. It is contraindicated if abnormal motion (>3.5 mm translation on flexion/extension lateral views) or instability is present at the affected level.
The goal of cervical arthroplasty is to preserve but not to recreate motion, and severe degeneration of the facet joints with loss of mobility on flexion/extension x-rays or significantly reduced disc height (less than 3 – 4 mm) would make fusion more appropriate. Kyphotic or scoliotic spine deformity cannot be reliably corrected by Cervical disc replacement. Patients with neck pain due to facet arthropathy may not respond well to a Cervical disc replacement. Possible allergy to the device material should also be taken into consideration during preoperative planning.
ADVANCED SPINAL TECHNOLOGY
Surgical Technique
Preparing for cervical arthroplasty
To maximize the benefit of surgery, patients are usually advised:
– to take sufficient time to consider all available treatment options;
– to have an assessment of general health and fitness for general anesthesia;
– to discuss their medications with the surgeon and anesthetist;
– to quit smoking (if applicable);
-not to eat or drink at least 8 hours before the scheduled surgery.
Slight variations of the surgical technique are possible, depending on the patient’s anatomy and type of the implant. Before surgery, sequential compression stockings are fitted to maintain blood circulation in the legs and to minimize the risk of deep venous thrombosis. After induction of general anesthesia, the patient is positioned face-up on the operating table with the neck in the neutral position.
The level of surgery is visualized with an intraoperative X-ray (fluoroscopy). Sometimes, for the lower cervical levels, gentle traction of the shoulders may be needed to confirm the affected disc. The surgical approach can be performed from the left or right side, depending on the dominant symptoms. The incision is usually made slightly off midline after injection of local anesthetic.
After medialising the thyroid gland, trachea and esophagus and lateralizing the carotid artery and jugular vein, the ventral aspect of the cervical spine is approached bluntly without any muscle cutting. Careful dissection is carried out to identify the affected segment. Pins are inserted into the vertebral bodies, and an X-ray is taken to confirm the correct level. Self-retaining lateral retractor blades are placed allowing for a good view of the disc under the microscope.
The anterior longitudinal ligament is incised, and the disc is removed using Kerrison rongeurs and curettes. In certain situations, drilling of the osteophytes and degenerated uncovertebral joints is needed to decompress the spinal canal and nerve roots. In those cases, copious irrigation is used to prevent retention of the bone dust as a possible source for late ossification. The posterior longitudinal ligament and protruded disc material are removed. Possible venous bleeding can be controlled with bipolar coagulation or various hemostatic agents.
The disc implant trials are used to size the height, width, and depth of the implant. The artificial disc should have the largest possible footprint to cover the surface of the vertebral endplates. The height of the prostheses should match the discs above and below. The implant is positioned in the middle of the intervertebral disc space with some tension allowing its snug fit.
After the x-ray confirms would position of the artificial disc, the pin retractor is removed, and the holes are filled with bone wax. The wound is usually closed with absorbable future over the subcutaneous drain, and a sterile dressing is attached. The drain is removed on the day following the operation. No neck brace or collar is usually required. The average duration of uncomplicated cervical disc replacement surgery is one hour.
RECOVERY AFTER CERVICAL ARTHROPLASTY
Postoperative Care
After surgery, patients are taken to the recovery room to monitor their neurological function, to control postoperative pain, and to keep an eye on the heart rate, blood pressure, and blood oxygen saturation. About an hour later they are taken to the ward and allowed to mobilise. Physiotherapy and gentle exercises are usually commenced the next day after surgery. X-rays of the neck are performed within the first 24 hours after surgery to confirm implant position. The drain is removed the next day, and blood thinning medication is commenced to prevent deep venous thrombosis
Compression stockings are used until full mobility and discharge. Some patients may experience mild trouble with swallowing or speaking for a few days. Discharge can be planned after one or two nights, depending on postoperative progress and physiotherapist assessment. A cervical collar is usually not required. Postoperative pain is usually moderate and subsides markedly within a couple of days. Recovery times after cervical arthroplasty can vary. Neuropathic myelopathic symptoms usually improve with time. The longer the problems exist before surgery, the longer it may take to notice a benefit. In some cases, the final recovery can take a few months.
For the first days at home, patients are recommended to avoid lifting weights over 2-3 kg and to engage in any strenuous or repetitive activities that may affect the neck. Regular short walks and a healthy diet are recommended. Prescription medications, such as opioids, may cause constipation and should be weaned off within the first week or two after surgery. Most patients are capable of returning to their normal activities and light work within a few weeks of surgery. Driving is allowed once strong pain medication is discontinued, and neck movements return to normal. Patients are advised to participate in a rehabilitation program and appropriate low-impact physical therapy.
Incision care is straightforward, and the dressing applied in the hospital should be kept dry and clean for 10 – 14 days. Dressing changes are usually not required. Taking showers is possible with an additional plastic occlusive dressing applied on top. Allowing water to soak the dressing or wound for a few minutes is not a problem but scrubbing or scratching should be avoided, and the incision must be kept dry. Local driving for up to 20 minutes can be resumed after discontinuing strong pain medications. A follow-up appointment with Dr. Aliashkevich is usually arranged six weeks following the procedure to plan ongoing care and individual prospects of recovery and return to work.
RISKS AND COMPLICATIONS
Risks of Cervical Arthroplasty
Cervical arthroplasty is a safe procedure, but not without risks. Candidates for surgery are provided with the relevant information materials and requested to sign a consent form before entering the operating theatre.
General surgery risks include:
– excessive blood loss,
– infection,
– an adverse reaction to medications,
– deep venous thrombosis,
– pulmonary embolism,
– heart attack,
– stroke, and
– other unpredictable complications/mortality.
Specific risks associated with the area of intervention may include:
– instability,
– implant migration or subsidence,
– spontaneous fusion/heterotopic ossification,
– adjacent segment disease,
– revision,
– recurrence,
– ongoing or worsening pain,
– material failure,
– adverse reaction to implant material,
– fracture,
– adjacent segment disease,
– cerebrospinal fluid leakage,
– injury of the spinal cord, nerves, vessels, trachea, esophagus, other vital structures,
– recurrent laryngeal nerve injury causing hoarseness of the voice,
– tetraplegia, hemiplegia,
– weakness/paralysis,
– numbness,
– bladder/bowel/sexual problems,
– difficulty with swallowing, speaking, breathing, and
– other unpredictable morbidities and mortality.
Some factors may increase the risk:
– poor general health,
– diabetes,
– chronic cardiac or pulmonary disease,
– chronic kidney problems,
– chronic pain condition,
– the prolonged and severe neurological deficit,
– mental health problems,
– smoking,
– multi-level surgery,
– advanced pre-existing degenerative disease,
– long-term intake of steroids and stronger painkillers.
Fortunately, the above complications are extremely rare, and the risk of permanent or significant disability is often less than 1%. The Cervical disc replacement surgery is well-established, and the quality of care and materials used are very robust. The likelihood of any implant-related complications is extremely low because the natural range of movements and stresses applied to the cervical intervertebral discs are significantly less than in other joints such as the hip and knee.
ADVANTAGES OF CERVICAL ARTHROPLASTY
Comparing cervical disc replacement with anterior cervical discectomy and fusion (ACDF)
Both artificial cervical disc replacement and anterior cervical discectomy and fusion (ACDF) offer very favorable clinical outcomes reaching satisfaction rates higher than 80 – 90% in appropriately selected patients. The advantages of motion-preserving surgery include:
– maintaining natural neck mobility;
– reducing the chances of adjacent segment degeneration above and below the operated disc;
– reduced need for re-operations;
– eliminating potential problems with bone graft harvesting;
– eliminating the risk of pseudoarthrosis or non-union;
– no need to wait for solid fusion;
Potential benefits of ACDF include:
– applicability to a wider range of patients, including cases where cervical disc replacement is contraindicated;
– ability to address multi-level pathology;
– wider insurance cover (some insurance plans don’t include joint replacements);
As a general rule, Dr. Aliashkevich would recommend considering cervical disc replacement as the preferred choice over fusion. Only if it is not feasible, e.g. in multi-level pathology consideration may be given to a hybrid procedure where disc replacement in one level is combined with fusion in another.