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.

The purpose of this chapter is to discuss the indications and diagnostic work-up for patients who may be candidates for total disc replacements, describe the basics of the surgical procedure for implanting these devices, and review the clinical results and complications reported to date.

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).

Fig. 131.1 Model and radiographic images of the Charité Artificial Disc. Motion at the implanted level can be seen when comparing the flexion (B) and extension (C) views.

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.

Fig. 131.2 Model and radiographic images of the ProDisc lumbar device. Motion at the implanted level can be seen when comparing the flexion (B) and extension (C) views.

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.

As with any arthroplastic procedure, active infection is a contraindication for disc replacement. Patients should also be asked if they have any known allergy to metal or any other material in the artificial disc implant.

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

The role of nerve root compression in the patient’s symptoms must be carefully evaluated. If the compression is related only to a bulging disc, this can be addressed by the prosthesis, which generally increases disc space height. However, if symptoms are related to nerve root compression caused by a migrated, extruded disc fragment, this cannot be addressed by disc replacement and such patients are not considered good candidates for disc replacement.

Spondylolisthesis of greater than a 3 mm slip is a contraindication for total disc replacement. A severe slip will likely make it difficult, or impossible, to correctly place the device in alignment with the vertebral bodies. This may also lead to an increased chance of device failure due to altered loading or displacement of the device.

Disc degeneration is often associated with degenerative changes of the facet joints. One must carefully assess the condition of the facets prior to undertaking total disc replacement in the lumbar spine. If the facets are compromised, the motion allowed by the disc prosthesis may eventually lead to facet joint pain. This is particularly true if the implant is not ideally positioned or not of the ideal size, altering the natural loading pattern on the facets. Also, severe facet joint degeneration can cause stenosis. This condition may not be addressed by total disc replacement.

Previous surgery at the painful level is not an absolute contraindication for total disc replacement, but does require special consideration. The primary concern is the condition of the posterior elements. Unlike fusion, the disc is designed to allow motion of the segment. If the posterior elements have been compromised by previous surgery, the patient should not receive a total disc replacement. If the patient has undergone a percutaneous discectomy or a minimally invasive microdiscectomy, he or she may be a viable candidate for disc replacement.

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.

Obesity

As with anterior lumbar interbody fusion, accessing the spine is more difficult in an obese patient. There is also the possibility that additional weight may alter the biomechanical properties of the implant, leading to subsidence or displacement.

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.

The skin is prepped and draped in the usual fashion. A small incision of approximately 4–6 cm is made either transversely or vertically. Imaging is used to identify the lumbar levels. This imaging also verifies that the spine is positioned with the pedicles on the same horizontal plane and perpendicular to the spinous processes. The anterior rectus sheath is incised over the disc space to be operated. The left rectus muscle is released and mobilized laterally, taking care not to injure the inferior epigastric vessels. The posterior sheath is incised to reveal the peritoneum. Using one’s fingers, a plane is defined between the peritoneum and the internal oblique and transversus muscles. This allows entry into the retroperitoneal space. The psoas muscle is identified. As the peritoneum is moved away from this muscle, the ureter is revealed. Although rare, damage to the ureters can occur during anterior spinal surgery. The peritoneum is mobilized further and a retractor is used to hold the peritoneum while further dissection is performed. The ascending iliolumbar vein is often ligated during exposure of the L4–5 disc space. During exposure to the disc space, injury to the left sympathetic trunk may occur, resulting in a warm leg. Fortunately, this condition is usually temporary. One must be cautious not to injure the superior hypogastric plexus at the L5–S1 level, which can result in retrograde ejaculation. Fortunately, this is a relatively rare complication and usually resolves by 6 months after surgery. However, the problem does not resolve in some cases. When operating above L5–S1, the vessels are gently mobilized. At the L5–S1 level, the disc space may be accessed under the bifurcation of the vessels.

Once access to the spine has been safely achieved, the midline of the disc space is marked with a small screw into the vertebral body or an osteotome is used to mark the vertebral endplate, depending on the type of prosthesis being implanted. The disc tissue is removed. The spine may be placed into a slightly extended position to widen the access to the disc space. Preoperative templating can be used to estimate the size of the endplates and polyethylene core needed. However, the final determination of device size is made intraoperatively. The prosthesis size should be selected to fit within the disc space but cover as much of the vertebral body endplates as possible. This will help prevent subsidence. Obliquely-shaped endplates are selected to match the lordosis of the spine at the level to be implanted. If the vertebral endplates have a relatively posterior lip, it may be beneficial to remove them with a Kerrison ronguer or burr to increase the amount of contact area between the vertebrae and the metallic endplates of the device, allowing for a firmer fit. The disc space is temporarily distracted to make space for the device. Prior to finalizing the device placement, the spine is returned to a neutral position, rather than the slightly extended position.

Once the device is implanted, images are taken to make sure that the prosthesis is properly positioned. In the anteroposterior (AP) view, the prosthesis should be centered in the disc space. On the lateral view, the prosthesis should be centered in the disc space or placed approximately 2 mm posteriorly for the Charité. Cinotti et al. have reported that optimal prosthetic sizing and positioning are related to outcome.¹⁶ Coverage of at least 80% of the vertebral body endplates by the device endplates was related to the amount of motion at the operated segment at follow-up. These authors also found that prostheses positioned anteriorly to midline were related to decreased motion at follow-up. Lemaire reported that implantation of the prosthesis of more than 4 mm anterior to the center was related to posterior facet pain.¹⁷ After verification of proper device positioning, the retractors are removed and the layers of fascia are closed with absorbable sutures. The patient is usually hospitalized for 1–3 days. A light brace or corset is worn for 2–6 weeks.

Although the indications and surgical approach for total disc replacement are similar to those for anterior lumbar interbody fusion, the postoperative rehabilitation is quite different. There is no need to have a period of significantly reduced activities to allow the formation of bone to create a solid fusion. With respect to activities, one must keep in mind that many of these patients have become deconditioned during the months of symptoms leading to surgical intervention. After surgery, rehabilitation can be helpful in increasing strength, through core stabilization exercises, and general fitness. Also, these programs will hopefully help modulate symptoms and prevent future episodes of back pain. A description of postoperative rehabilitation following total disc replacement has been provided by Keller.¹⁸ During the first 3 weeks after total disc replacement, activities are oriented toward symptom management and not stressing the area of the surgical incision. The patient is encouraged to walk frequently. Other activities are oriented toward gentle range of motion exercises, excluding extension. After 3 weeks, the patient is progressed to rotation and side-bending, as well as stabilization exercises. Controlled resistance training is also introduced at this time. After 6 weeks, the patient is progressed to more strenuous exercises, and activities involving extension of the spine are introduced.

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