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Adjacent Segment Disease Following CDR and Cervical Fusion
Over the past decade, total disc arthroplasty via cervical disc replacement (CDR) has continued to gain popularity in cervical spine surgery as a motion-preserving alternative to fusion. One potential contributor to this growth is the concern for the development of adjacent segment degeneration (ASDeg) and adjacent segment disease (ASDi). Consequently, recent literature has investigated whether TDR offers better protection than fusion. Hilibrand and Robbins distinguished between ASDeg, which refers to radiological evidence of changes at levels adjacent to a previous spinal fusion procedure without necessarily correlating to any patient symptoms, and ASDi, where new clinical signs arise that correlate with the radiographic observations of ASDeg adjacent to the level of a previous spinal fusion or other spine procedure (Figure 1).1
ASDeg is a common radiographic finding characterized by signs of degenerative changes and instability, such as loss of disc space height, disc impingement, osteophytosis, listhesis, central or foraminal stenosis, and endplate irregularity.2 ASDi generally involves radiographic signs of ASDeg and may present clinically as an isolated symptom or a combination of symptoms, including myelopathy, axial pain, and radicular pain.1,3 As a result, patients who develop ASDi have poor longterm outcomes and commonly require reoperation procedures to relieve symptoms that their initial surgery was intended to solve. Therefore, understanding the etiology of ASDi and its incidence following CDR and fusion is vital to prevent further operative intervention in this patient population.
Pathophysiology of ASDi
Controversy remains over whether adjacent segment pathology originates from surgical intervention or if it follows the natural course of the degenerative process. However, current literature suggests that no single factor alone explains its development. 4-6 Instead, several etiologic factors have been proposed as contributors to the pathology. Cervical spine degeneration often begins with age-related changes in the intervertebral discs and facet joints that are ultimately treated with primary CDR or cervical fusion at the most severe level(s) and continue at adjacent levels. Following the procedure, existing ASDeg progresses, influenced by several interrelated postoperative mechanical factors, including intradiscal pressure changes, anatomical disruption, and biomechanical alterations that result in ASDi when symptomatic.4,7
At the microscopic level, ASDeg is characterized by decreased hydration, reduced proteoglycan content, and loss of elasticity, resulting in microstructural alterations, including annular tears and fissures, which can then lead to a gradual decline in disc height and integrity.8 Macroscopically, following cervical fusion, there is a loss of motion at the operated level resulting in an altered mechanical load distribution of the adjacent segments.4,7,9,10 Fused vertebrae act as a single larger vertebra, which engages in compensatory hypermobility with adjacent segments and affects the natural curvature of the spine, leading to sagittal malalignment.7,9,10 In addition, malalignment can also be caused by improper implant size or cervical positioning intraoperatively, which may result in subsidence.11
Although CDR aims to preserve motion at the operative level, it can lead to compensatory hypermobility at adjacent levels.10,12 These alterations in the biomechanical environment increase intradiscal pressure and stress in adjacent discs, promoting further disc degeneration via herniation or collapse. 4,7,9,10 Reduced or altered mobility at operated levels may impair microvascular circulation and nutrient diffusion in adjacent segments, accelerating degenerative processes.13 Adjacent facet joints are also affected by altered kinematics and increased loads, leading to hypertrophy, inflammation, and osteophyte formation, which can narrow neural foramina and compress nerve roots.14 Together, existing adjacent disc degeneration at nonoperated levels, biomechanical changes, and facet joint degeneration can result in ASDi at levels that were previously asymptomatic.
Comparing Outcomes of ACDF vs CDR
CDR and anterior cervical discectomy and fusion (ACDF) are established surgical treatments for degenerative cervical disc disease. In single-level procedures, both techniques demonstrated comparable success in alleviating neck and arm pain, enhancing function, and achieving patient satisfaction. The prevalence of ASDeg and ASDi following cervical procedures has been the subject of ongoing research, with studies reporting varying rates depending on the length of follow-up and specific criteria used for diagnosis.10,12,15-18 Prior studies have reported the incidence of ASDi following anterior cervical discectomy and fusion (ACDF) to be as high as 25.6% within 10 years, with an annual rate of 2.9%.10 More recently, a study of 219 ACDF patients reported a 21% ASDi incidence over a minimum 5-year follow-up, identifying high BMI, severe osteoporosis, and a large C2-C7 cervical sagittal vertical axis as risk factors.15 Some studies have suggested that the incidence of ASDi may increase over time, potentially affecting long-term outcomes for patients who undergo cervical fusion. 4,7
The high prevalence of ASDi in ACDF patients has led to increased interest in motion-preserving techniques such as CDR, which aim to maintain more natural biomechanics of the cervical spine and potentially reduce the risk of adjacent segment complications and ASDi.19 Although initial studies showed similar rates of ASDi in ACDF and CDR patients,20 current meta-analyses of randomized controlled trials with longer follow-up showed that ACDF patients had a higher prevalence beginning around 5 years after surgery based on operation rates.10,21 In addition, there are several studies with at least 2 to 5 years of follow-up that support a lower rate of ASDi in CDR patients, including a meta-analysis by Toci et al, who reported lower rates of ASDeg (14.4% vs. 19.7%), ASDi (3.8% vs. 6.1%), and reoperation (3.1% vs 6.1%) in CDR operations compared to ACDF.12,16-18 In addition to the lower rates of ASDi, CDR has also been shown to have better neck disability index (NDI) improvement, faster cervical range of motion (ROM) recovery, and a lower incidence of radiographic ASDeg changes. Also, a small cohort study comparing CDR and ACDF for symptomatic ASDi suggested better NDI and ROM recovery in CDR over ACDF.23 Therefore, despite comparable success in improving pain, the lower rates of ASDi in CDR patients may suggest its superiority over ACDF.
In multilevel procedures, the benefits of CDR become more pronounced. A meta-analysis comparing multilevel CDR to ACDF found that patients undergoing CDR exhibited greater overall cervical spine motion and reduced rates of ASDi. 9 Additionally, the incidence of adverse events was significantly lower in the CDR group, suggesting that CDR may be a safe and effective surgical strategy for multilevel cervical degenerative disc disease.9 Furthermore, long-term studies have reinforced the advantages of CDR in multilevel cases. Research with 10-year follow-up data has demonstrated that CDR is superior to ACDF in terms of overall success rates, subsequent surgeries, and neurological success. The cumulative risk of adjacent-level surgery was notably lower in the CDR group, indicating a sustained benefit in preserving segmental motion and reducing ASDeg.24
Although both CDR and ACDF can be effective treatments for multilevel cervical disc disease in single-level operations, multilevel CDR offers significant advantages over ACDF, including better preservation of cervical motion, lower rates of ASDi, and improved long-term outcomes. These findings support the use of CDR, particularly in multilevel procedures, to optimize patient outcomes.25,26
Conclusion
It is important to note that this review focuses primarily on the difference between CDR and cervical fusion in developing ASDi and ASDeg. Despite better outcomes for these pathologies in CDR patients, both procedures have their place in cervical spine surgery, and the choice between CDR and ACDF should be tailored to individual patient factors, including age, number of levels involved, and specific pathology.
ASDi and ASDeg are common complications of cervical fusion and CDR procedures that can result in additional operations and continued pain. The etiology of ASDi is multifactorial, involving age-related changes, biomechanical alterations, and surgical factors. While both CDR and ACDF have demonstrated efficacy in alleviating pain and improving function, long-term outcomes, particularly regarding ASDeg and ASDi, favor CDR as a superior option. CDR, as a motion-preserving technique, aims to maintain more natural cervical spine biomechanics, producing additional advantages over ACDF, particularly in multilevel procedures.
References
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Contributors:
Adin Ehrlich, BA1
Andrea Pezzi, MD1
Kasra Araghi, BS1
Tomoyuki Asada, MD1,2
Sheeraz A. Qureshi, MD, MBA1,2
From the 1Hospital for Special Surgery and 2Weill Cornell Medical College, both in New York, New York.
