Junctional Degeneration of the Cervical Spine

CHAPTER PREVIEW

CHAPTER SYNOPSIS:

Junctional degeneration of the cervical spine, or adjacent segment disease, is a well-established complication after anterior cervical fusion. Although its cause is still highly debated, many believe it is either the natural progression of disc disease at the adjacent segment or an accelerated progression caused by increased forces and intradiscal pressures seen at the adjacent segment secondary to decreased motion at the fused segment. One of the goals of cervical disc arthroplasty is to maintain segment motion, thus decreasing the transferred forces to adjacent segments and decreasing the risk for development of adjacent segment disease. This chapter reviews the current scientific knowledge focusing on adjacent segment disease of the cervical spine, emphasizing the more recent and relevant literature.

IMPORTANT POINTS:

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The cause of adjacent segment disease is a highly debated topic among spine surgeons.
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Adjacent segment disease after anterior cervical disc fusion occurs at a rate of 2.9% per year for the first 10 years for a total occurrence rate of 25.6% at 10 years.
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Cervical disc arthroplasty was designed to maintain disc height and motion, thus potentially decreasing the rate of adjacent segment disease.
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Currently, no good intermediate- to long-term follow-up studies exist that document a decreased 10-year rate of adjacent segment disease after cervical arthroplasty.

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Junctional degeneration of the cervical spine, also known as adjacent segment disease, has become a popular topic in recent years with the advent of cervical disc arthroplasty. The phenomenon of junctional degeneration was first reported in the literature by Gore and Sepic in 1984, and described as degeneration of the intervertebral disc at the level immediately cephalad or caudad to the fused segments. Although classically shown in patients after anterior cervical fusion (ACF), it has also been seen in patients with Klippel–Feil syndrome, with multiple autofused cervical segments, as Guille et al. reported. This complication of cervical fusion has inspired the development of motion-sparing devices, such as the cervical disc replacement, whose purpose is to maintain the innate disc kinematics and height to reduce the transfer of stresses to the adjacent levels, and possibly reduce the rate of subsequent degeneration at adjacent cervical levels.

The concept of adjacent segment disease after ACF was described more extensively in the late 1990s by Hilibrand et al. Reviewing long-term outcomes of 374 patients undergoing 409 ACFs for degenerative disease, the authors constructed a survivorship analysis that predicts that 25.6% of these patients would be expected to experience development of symptomatic junctional degeneration at 10 years. They observed a relatively steady 2.9% rate of new disease referable to an adjacent level. When looking at the radiographic rate of junctional breakdown, some authors have reported changes as great as 92% within the first 10 years after ACF.

To date, much controversy exists as to the origin of this disease. Some authors suspect that this is simply the natural history of spondylosis in these individuals at the intervertebral discs adjacent to fusions. Still others believe that there are increased stresses at these adjacent levels directly because of the fusion, leading to new or increased degeneration. The purpose of this chapter is to review the existing scientific knowledge with regard to adjacent segment disease of the cervical spine, with an emphasis on the more recent literature involving this disease. We will also discuss potential treatment options for patients who go on to experience development of junctional breakdown.

OUTCOMES/RESULTS OF TECHNIQUES

Anterior Cervical Fusion

ACF achieved by performing discectomy and interbody grafting was described in the 1950s by Cloward and Smith and Robinson. This procedure has become the gold standard for treatment of cervical degenerative disease. However, a risk exists for degeneration on the adjacent intervertebral disc, both the cephalad segment and the caudal segment.

One of the most quoted reports of this adjacent segment disease is by Hilibrand et al., published in 1997. In that article, the authors retrospectively review the outcomes of surgery performed for new disease adjacent to a prior anterior cervical decompression and fusion. They report a much greater rate of nonunion among these patients when compared with rates of union among patients undergoing primary anterior cervical disc fusion (ACDF). The authors hypothesize that increased stresses from the long lever arm caused by the fused segments might make this a more difficult environment for obtaining a solid ACF. Previous long-term follow-up studies of ACF report that approximately 10% of patients who underwent ACF required additional surgery for spondylosis or disc herniation at an adjacent level. By contrast, Hilibrand et al. report the rate of new symptoms caused by new disease at an adjacent level after ACF, defined as adjacent segment disease, regardless of whether the patient underwent additional surgery for these symptoms. For this reason, they report greater rates of adjacent segment disease than prior authors, who were reporting only patients who underwent additional surgery. Hilibrand et al. report an average annual incidence rate of adjacent segment disease of 2.9% per year over a 10-year horizon. In addition, they applied Kaplan–Meier survivorship statistics, taking loss of patients to follow-up into account, to predict an overall 10-year prevalence rate of 25.6%.

In addition, Hilibrand et al. identified that the most mobile segments (C5-6 and C6-7) were also the ones at greatest risk for development of adjacent segment disease. Based on this correlation, as well as the lack of any correlation between the length of the prior fusion and the development of adjacent segment disease, the authors conclude that this new disease “may be part of the biologic process of progressive cervical spondylosis and unrelated to an adjacent arthrodesis.” A recent cadaveric study by Rao et al. supports this theory. In that article, the authors show neither increase in sagittal motion nor increase in intradiscal pressures at the adjacent segments in patients after one-level ACDF.

These findings have also been confirmed in vivo. Using sensitive computerized motion analysis, Reitman et al. report no difference between preoperative and postoperative motion at the levels adjacent to ACDF in 21 patients at a mean follow-up of 13 months. However, the study was limited because the authors could consistently measure motion only at the cephalad adjacent level.

Other in vitro studies examining changes at the levels adjacent to ACDF have suggested that the fusion increases forces at the adjacent levels. In the first of three cadaveric studies, Eck et al. demonstrate that the increased rigidity at the fused segment leads to increased shear stresses and increased intradiscal pressure at the adjacent motion segments. This is supported by two additional studies by Chang et al., where the authors show increased segmental motion, increased intradiscal pressures, and increased forces imposed on the facet joint complex at the levels adjacent to a single-level cervical fusion. However, these studies are flawed in that the authors consistently potted and measured motion below C2, which does not take into account the accommodation of motion by the upper cervical spine, which normally provides about half of all cervical motion.

Arthroplasty

The goal of cervical disc arthroplasty is to maintain normal disc height and motion after discectomy, theoretically preventing the abnormal loading and motion at the adjacent segments ( Fig. 33-1 ). A recent biomechanical study by Chang et al. reports a decrease in forces at the adjacent segment facet joint complexes after disc replacement. Clinically, Duggal et al. in 2004 showed no increased adjacent segment motion in 26 patients followed up to 27 months after arthroplasty. This led to no incidences of junctional degeneration in this short-term study. Although these authors report an overall increase in cervical motion, they assume that this increase is absorbed over all remaining cervical motion segments.