Anterior cervical diskectomy and fusion (ACDF) is a common treatment for cervical spondylosis, with clinical success in relieving the pain and improving the neurologic symptoms associated with the disease. However, adjacent segment disease (ASD) has been associated with cervical spine fusion.

AAOS Now: What is ASD?

Dr. Cho: The exact definition of ASD varies in the literature. The term “adjacent segment degeneration” describes radiographic changes observed at levels of the spine next to the previously fused segment or segments; this degeneration may not correlate with clinical symptoms. In contrast, ASD refers to the development of new radiculopathy or myelopathy referable to a segment adjacent to the site of a previous anterior arthrodesis in the cervical spine.

Is ASD “fusion disease” caused by undue stress placed on segments neighboring the fused spine? Or is it simply a consequence of an aging spine? Without a prospective randomized trial, we won’t be able to truly answer this question, and ASD is most likely multifactorial.

AAOS Now: How prevalent is ASD in ACDF patients?

Dr. Cho: In 1999, Alan S. Hilibrand, MD, and colleagues published an often-referenced study in which they documented the incidence and prevalence of ASD in 374 consecutive patients who underwent a total of 409 ACDF procedures to manage cervical spondylosis with radiculopathy or myelopathy. The annual incidence of symptomatic ASD during the first 10 years after surgery was 2.9 percent; overall prevalence was 13.6 percent at 5 years and 25.6 percent at 10 years.

AAOS Now: Why does ASD develop in some patients and not in others?

Dr. Cho: We know from biomechanical studies that fusing the spine places additional stress on neighboring segments. However, patients who undergo cervical spine surgery already possess predisposing risk factors—either genetic or environmental—for disk degeneration. These patients are more likely to have disk problems—in adjacent segments or in the lumbar spine, for example—in the future, compared to age-matched controls. We also know that the intervertebral disks degenerate over time as part of the natural aging process.

According to the Hilibrand study, risk factors for developing ASD among patients who had anterior cervical fusion included preexisting spondylotic changes on radiographs and index surgery performed next to the C5-C6 and/or C6-C7 levels. In contrast to their initial hypothesis, these authors found that multilevel cervical fusions led to a statistically significantly lower rate of symptomatic ASD compared with a single-level ACDF.

AAOS Now: What role does surgical technique play in the development of ASD?

Dr. Cho: Some surgeons argue that, in addition to fusion, surgical damage to the anterior longitudinal ligament or to the longus colli muscles can contribute to adjacent segment degeneration. In a retrospective radiographic study of 87 consecutive patients who had undergone a single- or a two-level ACDF, Ahmad Nassr, MD, and colleagues reported a threefold increase in the risk of developing adjacent segment degeneration when the disks were incorrectly marked with a needle for localization during surgery.

In 2005, Jong-Beom Park, MD, PhD, and colleagues retrospectively reviewed radiographs of 118 patients who had undergone anterior cervical arthrodesis with plate fixation. They reported that the likelihood of developing moderate to severe adjacent level ossification was high when the anterior cervical plates were less than 5 mm from adjacent disk spaces. Although a direct correlation between symptomatic ASD and ossification was not made, we may reasonably assume that ossification likely contributes to early degeneration.

AAOS Now: What can surgeons do to reduce the risk of ASD?

Dr. Cho: Based on our surgical experience, to minimize violating adjacent intervertebral disks during surgery and thus lessen the potential development of adjacent segment degeneration or disease, we suggest the following:

• Radiographic and anatomic landmarks should be used to best identify the intended level for surgery before the needle is introduced into the disk for localization. For example, prominent anterior osteophytes may be more pronounced in the degenerated segment that requires surgical intervention than in other better preserved segments and can often be seen on preoperative radiographs. These osteophytes can be visualized during the exposure and even palpated with appreciable difference. The carotid tubercle can also be palpated to locate the C6-C7 level, enabling the surgeon to count up or down to arrive at the intended level.
• When it is time to place the needle for radiographic verification, it should be inserted into the vertebral body rather than into the disk to prevent unintended damage to the wrong disk.
• A short anterior cervical plate should be used to ensure that the plate is at least 5 mm from both the cranial and caudal end plates to decrease the incidence of adjacent level ossification.

AAOS Now: Would an alternative treatment method reduce the risk of ASD?

Dr. Cho: Because of its motion-sparing benefits, artificial disk replacement (ADR) has been proposed as a viable alternative method of managing cervical spondylosis in response to clinical concerns and complications related to fusion. A number of biomechanical studies have suggested that cervical arthroplasty mimics natural cervical motion at the operated level while ACDF does not. However, short-term studies comparing ACDF and ADR have failed to show any significant difference in the rate of adjacent level pathology following surgery.

Furthermore, although several biomechanical and finite element studies indicate that increased stress and strain can be expected in segments next to a fusion, no significant difference has been demonstrated in revision surgery based on available evidence. Existing studies comparing ACDF and ADR have been limited by small sample sizes and by the lack of long-term follow-up. Longer follow-up studies are needed to assess the clinical effects of ADR on ASD.

Dr. Cho’s coauthor is K. Daniel Riew, MD.

Disclosure information: Dr. Cho—no conflicts; Dr. Riew—Biomet; Medtronic Sofamor Danek; Osprey; Amedica; Benvenue; Expanding Orthopedics; Nexgen; Paradigm Spine; PSD; Spinal Kinetics; Spineology; Vertiflex; Cerapedics; Journal of Bone and Joint Surgery–American; Spine; Korean American Spine Society; Cervical Spine Research Society; AOSpine; North American Spine Society; Scoliosis Research Society.

Maureen Leahy is assistant managing editor of AAOS Now. She can be reached at leahy@aaos.org

References

1. Hilibrand AS, Carlson GD, Palumbo MA, Jones PK, Bohlman HH: Radiculopathy and myelopathy at segments adjacent to the site of a previous anterior cervical arthrodesis. J Bone Joint Surg Am 1999 Apr;81(4):519-528.
2. Nassr A, Lee JY, Bashir RS, Rihn JA, Eck JC, Kang JD, Lim MR: Does incorrect level needle localization during anterior cervical discectomy and fusion lead to accelerated disc degeneration? Spine (Phila Pa 1976). 2009 Jan 15;34(2):189-192. doi: 10.1097/BRS.0b013e3181913872.
3. Park JB, Cho YS, Riew KD: Development of adjacent-level ossification in patients with an anterior cervical plate. J Bone Joint Surg Am 2005 Mar;87(3):558-563.