Short Review/Perspective on Adjacent Segment Disease (ASD) Following Cervical Fusion Versus Arthroplasty.

Background: Although the incidence of radiographic Adjacent Segment Disease (ASD) following anterior cervical diskectomy/fusion (ACDF) or cervical disc arthroplasty (CDA) typically ranges from 2-4%/year, reportedly fewer patients are symptomatic, and even fewer require secondary surgery.
Methods: Multiple studies have documented a 2-4% incidence of radiographic ASD following either ACDF or CDA per year. However, fewer are symptomatic from ASD, and even fewer require additional surgery/reoperations.
Results: In a meta-analysis (2016) involving 83 papers, the incidence of radiographic ASD per year was 2.79%, but symptomatic disease was present in just 1.43% of patients with only 0.24% requiring secondary surgery. In another study (2019) involving 38,149 patients undergoing ACDF, 2.9% (1092 patients; 0.62% per year) had radiographic ASD within an average of 4.66 postoperative years; the younger the patient at the index surgery, the higher the reoperation rate (i.e. < 40 years of age 4.56 X reoperations vs. <70 at 2.1 X reoperations). In a meta-analysis of 32 articles focusing on ASD 12-24 months following CDA, adjacent segment degeneration (ASDeg) occurred in 5.15% of patients, but adjacent segment disease (AS Dis) was noted in just 0.2%/ year. Further, AS degeneration occurred in 7.4% of patients after 1-level vs. 15.6% following 2 level fusions, confirming that CDA's "motion-sparing" design did not produce the "anticipated" beneficial results.
Conclusion: The incidence of radiographic ASD ranges from 2-4% per year for ACDF and CDA. Additionally, both demonstrate lesser frequencies of symptomatic ASD, and the need for secondary surgery. Further, doubling the frequency of ASD following 2 vs. 1-level CDA, should prompt surgeons to limit surgery to only essential levels. Copyright:

INTRODUCTION

The incidence of radiographic Adjacent Segment Disease (ASD) following cervical spine surgery, whether following anterior diskectomy/fusion (ACDF) or cervical disc arthroplasty (CDA), typically ranges from 2-4%/year [Table 1].[1-12] However, the frequency of symptomatic ASD and the requirement for secondary surgery in these two populations is less well-defined [Table 1].[1-12] In this short review/ perspective, studies were carefully selected to include those that focused on the frequencies of ASD following predominantly ACDF and CDA, while also looking at different rates of postoperative symptomatic ASD (0.62-1.43%), and the need for reoperations (0.2–0.24%).

Table 1:

Summarizing Adjacent Segment Disease following Cervical Spine Surgery.

DEFINITION OF ASD

Virk et al. (2014) defined the various degenerative, radiographic, and/or biomechanical factors predisposing patients to the development of ASD [Table 1].[12] These factors included; disc disease, stenosis, spondylosis, spondylolisthesis, arthrotic changes of facet/uncovertebral joints, or fractures. Although ASD was typically initially described as due to greater stress at adjacent levels following a fusion, subsequent literature showed it also occurs with comparable frequencies following CDA.[6,11]

INCIDENCE OF ASD AFTER CERVICAL SURGERY

Several studies cited 2–4% frequencies of ASD following ACDF, with some also citing comparable rates with non-fusion techniques [Table 1].[2-5,10,12] Kepler and Hilibrand (2012) observed a 2–3% per year incidence of ASD following cervical fusion [Table 1].[4] They recommended that ASD frequencies could be mitigated utilizing improved surgical fusion techniques, while also questioning the future potential efficacy of CDA through “motion preserving surgery.” Cho an Riew (2013) found a 3% overall incidence of ASD/year typically following ACDF, noting that this frequency would rise to 25% over a 10 year period.[3] They also showed that the frequency of ASD remained relatively unchanged for non-fusion procedures (anterior diskectomy without fusion or laminoforaminotomy) including CDA. Using the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis 2009) guidelines, Kong et al. (2016) evaluated 83 cervical surgical studies utilizing multiple databases, and found that ASD occurred in 2.79% of patients/year; however, only 1.43% were symptomatic/year, while just 0.24% required secondary surgery/year.[5]

REDUCING THE RISKS AND/OR PREVENTION OF ASD FOLLOWING CERVICAL SURGERY

Several cervical spine studies, mostly involving ACDF, identified risk factors predisposing patients to developing radiographic ASD, symptomatic ASD, and the need to reoperate on ASD [Table 1].[1,2,6,10,12] Tobert et al. (2017) observed a 2-4% frequency of ASD following both cervical and lumbar surgery, largely attributed to; pre-existing spondylotic disease at the adjacent level, degenerative changes at those levels, and/or altered adjacent level “biomechanics” following the index surgical procedures.[10] Looking at the incidence of ASD after 70 ACDF (2005–2012) followed between 3-10 years, Alhashash et al. (2018) showed reoperations for ASD (i.e., based upon X-rays/MR studies) were warranted an average of 32 months after the index ACDF surgery (i.e. 54% single level procedures).[1] Two major factors predisposing to secondary surgery included preoperative “degenerative changes” (74%), or postoperative sagittal malalignment (i.e. lack of adequate lordosis). Butler et al. (2019), similarly noted a 3% incidence of ASD following ACDF per years; ways to avoid ASD included “consider non fusion alternatives,” improve patient selection, pay more attention to preserving lordosis during ACDF (i.e. sagittal alignment), and considering CDA’s for their “motion sparing” design.[2] Most notably, in Wu et al. (2019) series of 38,149 patients undergoing ACDF, 2.9% had radiographic evidence of postoperative ASD observed an average of 4.66 years postoperatively (1092 patients: 0.62%/per year) [Table 1].[11] Of interest, younger patients were more susceptible to ASD and required more frequent reoperations vs. older patients (i.e. < 40 years of age 4.56 fold incidence of reoperations vs. < 70 at 2.17 fold incidence). Further, patients exhibiting depression or psychoses also exhibited higher frequencies (1.42 and 1.45 fold respectively) of warranting additional surgery.

ACDF Data Largely Excluded ASD Results with Polyetheretherketone (PEEK) Cages

Notably, we did not specifically focus on the results for ACDF performed with stand-alone polyetheretherketone (PEEK) cages. In Shiban et al. (2016), one year following 265 PEEK cage fusions, 16 (6%) warranted reoperation to address radiographic ASD [Table 1].[8]

CDA’S “BIOMECHANICAL MOTION SPARING” DESIGN FAILED TO REDUCE ASD

CDA’s, by maintaining “motion” at operated levels, were devised to “theoretically” reduce stress transmitted to adjacent segments [Table 1].[4,6,7] In 2008, Seo and Choi evaluated the frequency of ASD after cervical fusion vs. CDA, focusing on the potential pros and cons of each technique.[7] They questioned whether ASD in part reflected the “natural history” of progressive degeneration rather than simply representing the response to a “fusion.” They further emphasized that CDA were recommended for younger patients without significant spondylosis who clearly demonstrated adequately preserved range of motion. Although Kepler and Hilibrand (2012) also emphasized CDA’s potential “motion-sparing” capabilities that might limit the risk of ASD, they nevertheless found that so far, the literature did not; “...distinguish a difference in the rate of ASD between fusion and disc replacement.” [4]

CDA’S COMPARABLE ADJACENT SEGMENT DEGENERATION (AS-DEG) VS. ADJACENT SEGMENT DISEASE (AS-DIS) VS. ANTERIOR CERVICAL FUSIONS

Two studies emphasized that CDA’s failed to limit or eliminate ASD when compared with largely anterior cervical fusions [Table 1].[6,9] In their meta-analysis of 32 studies, Shriver et al. (2016) found that at 1-2 postoperative years, there was a 5.1% frequency of AS-Deg, and a 0.2% incidence of AS-DIS.[9] Long-term follow-up data over 24 postoperative months showed that the incidence of both AS-DEG and AS-DIS further increased to 16.6% and 2.6% respectively. Additionally, higher frequencies of ASD occurred after 1-level (7.4%) vs. 2-level CDA procedures (15.6%). Parrish et al. (2021) also noted CDA’s comparable 1-2% frequencies of radiographic ASD vs. anterior cervical fusions.[6]

CONCLUSION

This short review/perspective of select studies emphasizes how to limit the risk of adjacent segment degeneration (i.e. radiographic, symptomatic, and/or requiring reoperations). This requires; carefully selecting “symptomatic” patients for either ACDF or CDA (i.e., for CDA, younger patients with preserved range of motion without spondylosis), stringently limiting the number of operated levels, and optimizing the surgical “technique” to preserve lordosis.