Biomechanical Evidence

Nearly 50 years ago, cervical spine fusion was introduced by Cloward, Bailey, and Badgley, and Robinson and Smith.⁵ Today, anterior cervical discectomy and fusion (ACDF) is a common surgical treatment for symptomatic cervical spondylosis. It is widely accepted, but it has been postulated that levels adjacent to a fused cervical segment(s) are prone to accelerated degenerative changes possibly owing to the significant forces transmitted to the adjacent levels after interbody fusion.

Certain biomechanical studies have generated data that lend credibility to the notion that fusion could cause degeneration of adjacent levels. Eck et al.⁶ evaluated motion and intradiscal pressures at adjacent levels in a cadaveric C5-6 ventral fusion model. Compared with intact specimens, flexion at C4-5 and C6-7 both increased, with greater increases in motion at the rostral level. Extension also increased at both levels adjacent to the fused level; however, motion was greater at the caudal level. Intradiscal pressures increased by 73% at C4-5 and by 45% at C6-7 compared with control specimens, indicating that fusion causes a transfer of load to adjacent levels. Park et al.⁷ reported similar findings in a cadaveric C5-7 ventral fusion model. The authors demonstrated that intradiscal pressures at C4-5 increased during physiologic range of motion. Cheng et al.⁸ compared fluoroscopic imaging in patients with normal, degenerative, and fused (C5-6) cervical spines. Range of motion and estimated in vivo forces at levels adjacent (C4-5 and C6-7) to a C5-6 ventral fusion were both higher compared with normal and degenerative cervical spines. In contrast to these studies, Rao et al.⁹ reported that intradiscal pressures and motion are not statistically different at adjacent levels after ventral fusion. Overall, most literature suggests that there is heightened loading and motion at levels adjacent to a fused segment(s).

Adjacent-Level Degeneration

Several authors have investigated adjacent-level degeneration radiographically. Baba et al.¹⁰ retrospectively reviewed 133 patients who underwent ventral cervical fusion with an average follow-up of 8.5 years. The authors reported a 25% incidence of spinal stenosis and increased motion at the adjacent rostral level on dynamic radiographs. In another retrospective evaluation with an average 5-year follow-up of 121 patients who underwent ventral cervical fusion, Gore and Sepic¹¹ reported a 25% incidence of progression of spondylosis. In a radiographic review of 44 patients with a 4.5-year follow-up, Herkowitz et al.¹² reported rates of 41% and 50% of adjacent-level degeneration in patients who underwent ACDF and posterior foraminotomy, respectively. This would suggest that ACDF did not directly cause the adjacent-level degeneration, because foraminotomy is a not a fusion procedure. There was no correlation found in any of these studies between adjacent-level degeneration and clinical deterioration.

Adjacent-Level Disease

Attempts to define the incidence of adjacent-level disease, or clinically apparent adjacent-level degeneration, have been made by several authors. In 1968, Williams et al.¹³ retrospectively studied 60 patients who underwent ACDF with an average follow-up of 4.5 years. Seventeen percent of these patients required additional surgery for progressive spondylosis after fusion. In a larger series with an average follow-up of 5 years, Gore and Sepic² reported that 14% of patients required further surgery after ACDF. In 1993, Bohlman et al.¹⁴ reported a 9% incidence of the adjacent-level degeneration in 122 patients after ACDF, with an average clinical and radiographic follow-up of 6 years; 81% (9 of 11) of patients with adjacent-level degeneration required further surgery at the adjacent level. Hilibrand et al.¹⁵ performed a 10-year survivorship analysis of 409 ventral decompression and stabilization procedures. They reported a 2.9% annual incidence of adjacent-level disease, and an overall prevalence of 14% (follow-up, 2–21 years). Kaplan-Meier survivorship analysis predicted a prevalence of adjacent-level disease of 13.6% at 5 years and 25.6% at 10 years. The authors also concluded that the greatest risk of adjacent-level deterioration involved single-level fusions, particularly at C5-6 and C6-7. The authors could not determine from these results if degeneration was a result of the prior fusion or simply a manifestation of the natural progression of the spondylosis. These four studies indicate that there is 9% to 17% prevalence and an annual incidence of 1.5% to 4% of adjacent-level disease after ACDF.¹⁶

In addition, the control arms for the Investigational Device Exemptions trials for the various cervical artificial discs brought to market provide level I evidence and shed more light on the incidence of adjacent-level disease after one-level ACDF with allograft and plate. Mummaneni et al.¹⁷ and Murrey et al.¹⁸ reported a 3.2% and 1% incidence, respectively, of secondary surgeries for symptomatic adjacent-level degeneration at 2-year follow-up. Clearly, this refers to surgeries performed for adjacent-level disease and does not provide the absolute incidence. Presumably, there was a cohort of patients treated nonoperatively for adjacent-level symptoms.

Several authors have compared fusion and nonfusion procedures to further study adjacent-level disease. Lundsford et al.¹⁹ reviewed 334 patients who underwent anterior discectomy, with and without fusion. They reported an annual prevalence of adjacent-level disease of 7% and an annual incidence of 2.5%. There was no difference in the development of adjacent-level disease between fusion and nonfusion procedures. In a review of 846 patients who underwent posterior foraminotomy without fusion, Henderson et al.²⁰ reported an overall prevalence of adjacent-level disease of 9% and an annual incidence of about 3%. Thus, these two reports provide evidence that suggests adjacent-level disease is not necessarily related to prior fusion procedures.

Prevention of Adjacent-Level Degeneration

As outlined previously, whether fusion causes adjacent-level degeneration remains a topic of debate. Regardless, there are steps the surgeon can attend to in order to minimize iatrogenic injury to adjacent levels. Needle level localization during ventral cervical surgery has been studied recently by Nassr et al.²¹ They retrospectively followed 87 patients who underwent one- or two-level ACDF, 15 of whom had incorrect needle placement during level localization. In these patients, there was a threefold increase in adjacent-level degeneration compared with patients who had correct needle localization. Given these compelling data, it is very reasonable instead to place the needle into a vertebral body to avoid a potential disc injury. Surgeons should also avoid excessive subperiosteal dissection above or below the surgical levels. Typically, only 3 to 4 mm of the adjacent body needs to be exposed for plate placement. This minimizes the risk of injuring the adjacent level or compromising the integrity of the anterior longitudinal ligament and longus colli muscles.

Ventral plate placement has also been implicated in adjacent-level degeneration. Park et al.²² retrospectively reviewed 118 patients with solid ventral plated fusion. The authors demonstrated a statistically significant higher incidence of adjacent-level ossification when the end of the cervical plate is less than 5 mm from the adjacent disc space. Sixty-seven percent of adjacent rostral levels and 45% of adjacent caudal levels developed ossification disease when the plate was less than 5 mm from the adjacent disc. To maintain an appropriate distance from the adjacent disc, the graft-vertebral body interface should be visible through the terminal screw holes of the plate when placing the screws. The rostral screws are placed in a superior trajectory and the caudal screws are placed in an inferior trajectory.