CHAPTER 131 Disc Replacement
INTRODUCTION
Maintaining motion has long been the quest in the treatment of painful spinal disorders. Movement of the neck and trunk is a large part of everyday life. Limitations due to pain or surgical intervention are frustrating for patients with back pain and can be related to significant loss of work and reduced quality of life. Beginning in the 1980s, clinically useful lumbar disc replacements were introduced. Today, there are several such devices available for use. Although these devices hold great promise for improving the long-term care of patients with back pain, they are certainly not a cure-all, and the basics of any spine surgery still apply with respect to the importance of proper patient selection.
HISTORY
The first patent for disc replacement was issued in 1956 to van Steenbrugghe in France.1 This was just one of many devices described in his patent application. The first clinically useful artificial disc was simply stainless steel spheres implanted into the disc space following discectomy.2,3 This procedure was first performed in 19624 and reported in 1966 by Fernström of Sweden, who implanted the spheres into 133 patients, 125 lumbar disc levels and eight cervical disc levels. Among the lumbar implants, in follow-up of 6–30 months, there were only two complications: one sphere displaced into the epidural space and one case of temporary paresis of the peroneus. Clinically, Fernström noted that patients receiving the sphere had a better result than patients undergoing discectomy alone. In 1971, Fernström reported the 4–8-year follow-up on 142 patients.4 He found that among the patients operated for disc prolapse, 65% had no pain and were able to work full duty. An additional 28% had reduced pain and were able to work in some capacity. These results were more favorable than those from a series of control patients undergoing discectomy without the placement of a sphere. In 1995, McKenzie reported follow-up of 10–20 years on 67 patients who had received the spheres described by Fernström.5 He noted a high success rate, 83% among patients with disc herniations and 75% among those with degenerative disc. Prosthesis removal was required in only one of 155 patients who received the implant. The author reported that although more than 90% of patients were not working prior to surgery, 95% were working after receiving the sphere. The author also reported that 95% of the patients felt that the procedure was worthwhile.
In 1982, Drs. Kurt Schellnack and Karin Büttner-Janz in East Germany at the Charité Hospital began the development of an artificial disc based on careful biomechanical analysis of the motion and properties of a normal lumbar disc. The design was a three-piece implant consisting of two metallic endplates and a sliding polyethylene core. The design concept was tested in the laboratory, and the first SB Charité artificial disc prosthesis was implanted in September 1984 in Berlin.6 There were problems with subsidence with this device. In 1985, a second design was introduced that had ‘wings’ on two sides of the circular endplates to increase the contact surface area between the metallic endplates and the vertebral body. This design had problems with fracture between the core and the wings. A third design was introduced in 1987. This design proved to be reliable and remains in use today (Fig. 131.1).
In the late 1980s, Thierry Marnay in France designed the ProDisc. It also allows motion through articulation between a concave and convex surface. The original design was anchored by two keels on each of the superior and inferior metallic endplates. The current design of this device has one keel in the center of each of the endplates (Fig. 131.2). In the last several years there have been numerous other designs, including metal-on-metal discs such as the Maverick (Medtronic-Sofamor-Danek) and the FlexiCore (Stryker). These four discs are currently being studied in IDE FDA studies with the Charité having achieved FDA clearance October 26, 2004. There are two additional devices that are being used outside of the US called the Mobidisc (LDR Medical), consisting of metal and polyethylene, and SpinalMotion’s Kineflex™, which is of similar design.
Indications
As with any surgical procedure, much of the success of total disc replacement is dependent upon the selection of patients being treated. There are indications that apply to any elective spine surgery procedure. These include the failure to achieve acceptable pain relief after an appropriate nonoperative course of treatment. In general, appropriate nonoperative management should include medication, active physical rehabilitation, education, activity modification, and often injections with at least 6 months of nonoperative care. As with any patient, careful and comprehensive history and physical examination are crucial to patient evaluation. These findings are then reviewed with respect to image findings. Most patients who have failed nonoperative management have had a magnetic resonance imaging (MRI) scan. This is reviewed to evaluate the disc, facets, bony structures, and to rule out pathologies such as spinal tumor. In most patients being evaluated for symptomatic disc degeneration, it is recommended that further evaluation be undertaken using discography. This evaluation is used as a confirmatory test to determine if the disc(s) appearing as abnormal on an MRI is the source of the patient’s symptoms. This is particularly important in view of the reports of the high rates of disc abnormalities seen on MRI scans made on individuals with no back pain.7 Also, the discogram can be used to better assess the condition of the discs adjacent to the suspect level. Discography should be undertaken with the patient awake and responsive. If the patient is too heavily sedated the pain response cannot be adequately evaluated. The patient should be asked the location of the pain provoked, if any, its location with respect to the location of their usual symptoms, and the intensity of the pain. If pain is provoked that is not concordant to the usual symptoms, the discogram is not considered to be a positive test.
Another important aspect in the evaluation of possible surgical candidates is psychological screening. While imaging studies are closely related to anatomical findings during surgery, psychological testing is more strongly related to surgical outcome.8 The presurgical psychosocial screening instrument has been found to have a high correlation to surgical outcome.9,10 Patients with a significant psychological component to their pain experience are likely to do poorly following surgery and should generally not undergo an elective procedure. Another important component to achieving a favorable surgical outcome is the establishment of realistic expectations. This may be addressed during the psychological screening. It can also be addressed during preoperative patient education. Patients must understand that results are not guaranteed and there is a very good chance that they will continue to have some level of pain or painful flare-ups following any spine surgery. The goal is to significantly reduce their pain and allow for improved function. Being totally and permanently pain free is not a realistic goal. Patients also need to understand that they play a major role in accomplishing these goals and must be willing to comply with a postoperative rehabilitation plan.
TOTAL DISC REPLACEMENT IN THE LUMBAR SPINE
The primary indication for total disc replacement is symptomatic disc degeneration or disruption at one or two disc levels unresponsive to nonoperative management. This condition is best diagnosed by the combined use of MRI and discography. Other diagnostic observations include disc space narrowing. The patient may have complaints of back pain with or without leg pain.
Contraindications
The contraindications for total disc replacement are very similar to those traditionally applied to anterior lumbar interbody fusion since the approach to the spine is the same. Details of inclusion and exclusion criteria have been discussed.11 One should screen patients for the number of types of previous abdominal surgery. If there have been several procedures, or surgery in the immediate vicinity of the painful disc, the patient may not be a good candidate. There is a risk of significant vascular injury related to scarring from previous surgery. One must evaluate the preoperative imaging studies to rule out patients with significant calcification of the vessels. This could result in significant vascular complications.
Diseases affecting bone quality, such as osteoporosis, Paget’s disease, and osteomalacia, are contraindications for total disc replacement. There is the risk of the device subsiding into osteoporotic bone, particularly if the positioning and size of the implant used are less than ideal. There may also be an increased risk of fracturing the vertebral body during, or after, surgery if the patient is osteoporotic. Poor bone quality may also negatively influence the anchoring of the device to the vertebral bodies. Vertebral body fracture following implantation of a total disc replacement into a patient with osteopenia has been reported.12
Bertagnoli and Kumar described what they defined as an ideal candidate for total disc replacement.13 Such patients have single-level disc degeneration, a disc space height of at least 4 mm, no facet joint changes, intact posterior elements, and no degeneration at the adjacent segments.
Procedure
The surgical approach for total disc replacement is the same as for anterior lumbar interbody fusion. The mini ALIF retroperitoneal approach has been described in detail elsewhere.14,15 In this chapter, we will provide an overview of the general approach to the anterior lumbar spine. The exact approach may vary by surgeon preference or design of the device to be implanted. In our practice, a general surgeon initiates the procedure to provide access to the spine and remains available in the event of a vascular injury or other difficulty. A radiolucent table is necessary, since imaging is needed during the surgery. It is helpful if a table is used that allows the spine to be put into a slightly extended position to aid during device implantation. Another alternative is to have an inflatable device under the level to be operated that can be inflated to create extension when desired during the implantation. During the rest of the procedure, this positioning is not needed.
