Prevalence of adjacent segment disc degeneration in patients undergoing anterior cervical discectomy and fusion based on pre-operative MRI findings

Abstract

Anterior cervical discectomy and fusion (ACDF) is a widely accepted surgical treatment for symptomatic cervical spondylosis. Some patients develop symptomatic adjacent segment degeneration, occasionally requiring further treatment. The cause and prevalence of adjacent segment degeneration and disease is unclear at present. Proponents for motion preserving surgery such as disc arthroplasty argue that this technique may decrease the “strain” on adjacent discs and thus decrease the incidence of symptomatic adjacent segment degeneration. The purpose of this study was to assess the pre-operative prevalence of adjacent segment degeneration in patients undergoing ACDF. A database review of three surgeons’ practice was carried out to identify patients who had undergone a one- or two-level ACDF for degenerative disc disease. Patients were excluded if they were operated on for recent trauma, had an inflammatory arthropathy (for example, rheumatoid arthritis), or had previous spine surgery. The pre-operative MRI of each patient was reviewed and graded using a standardised methodology. One hundred and six patient MRI studies were reviewed. All patients showed some evidence of intervertebral disc degeneration adjacent to the planned operative segment(s). Increased severity of disc degeneration was associated with increased age and operative level, but was not associated with sagittal alignment. Disc degeneration was more common at levels adjacent to the surgical level than at non-adjacent segments, and was more severe at the superior adjacent level compared with the inferior adjacent level. These findings support the theory that adjacent segment degeneration following ACDF is due in part to the natural history of cervical spondylosis.

Introduction

Degenerative cervical disc disease is a common problem presenting to a spine surgeon. Typical indications for surgery include cervical radiculopathy and/or myelopathy. Anterior cervical discectomy and fusion (ACDF) is a proven effective surgical intervention for cervical radiculopathy or myelopathy secondary to cervical disc disease, and is considered by some to be the gold standard for surgical treatment following failure of non-operative measures.1, 2 The development of adjacent segment disc degeneration (ASDD) following ACDF is well reported in the literature.3, 4, 5, 6, 7, 8, 9, 10, 11, 12 In a frequently cited paper, Hillibrand reported a rate of symptomatic adjacent segment disease requiring further surgical intervention at 2.9% per level per year following single level ACDF, with about one quarter of patients requiring additional surgery within 10 years of their index procedure.⁵

Previous studies have demonstrated that the development of ASDD may be influenced by several factors. Advancing age and smoking both correlate with more advanced radiologic evidence of disc degeneration.¹³ Okada et al. suggested that a non-lordotic sagittal alignment of the cervical spine may predispose to disc degeneration,¹⁴ although they were unable to show a correlation with clinical symptoms. Furthermore, disc degeneration seen on MRI scan has been demonstrated to progress more rapidly in patients who have undergone ACDF compared with asymptomatic volunteers.⁹ Degeneration has also been shown to progress more rapidly at segments adjacent to the operatively fused segment compared with more remote disc segments.⁸

The theoretical advantages of cervical disc arthroplasty over ACDF have received a great deal of attention in the literature. Proponents of cervical arthroplasty suggest that preservation of motion at the operated level may decrease biomechanical strain at adjacent levels and thus reduce the incidence of ASDD.4, 10, 15, 16 Currently the lack of long-term outcome data following cervical arthroplasty neither supports nor refutes this theory.12, 17 Robertson and colleagues demonstrated a statistically significant decrease in the rate of of symptomatic adjacent segment disease requiring further surgery following cervical disc arthroplasty compared with ACDF.¹⁰ Most studies, however, have been unable to demonstrate a similar decrease in clinically symptomatic adjacent level disease.4, 8, 12, 18 Current best evidence in the form of a meta-analysis concluded that disc arthroplasty has not demonstrated improved clinical outcomes over ACDF.¹⁸ Up to 25% of cervical arthroplasties have been shown to auto-fuse in long-term follow-up studies without detriment to patient outcomes¹⁷ calling into question the veracity of the claim that motion preserving surgery improves patient outcomes. It remains controversial as to whether ASDD following ACDF surgery represents a true iatrogenic post-operative complication or a progression of the natural history of cervical spondylosis.

Significant work has been done to investigate the progression in asymptomatic individuals, but in the absence of clinical symptoms it is difficult to define these changes as pathologic.12, 13, 14, 19 The primary purpose of this study was to assess the prevalence of ASDD based on MRI findings in patients with symptomatic cervical disc disease prior to undergoing ACDF. A secondary goal of this study was to further assess the reliability and validity of the MRI grading scheme for cervical disc degeneration developed by Matsumoto and colleagues.