STUDY DESIGN:
Systematic review.

OBJECTIVE:
We performed a systematic review to determine the risk and to define potential identifiable risk factors for the development of adjacent segment pathology (ASP) after cervical fusion surgery.

SUMMARY OF BACKGROUND DATA:
During the past several decades, the indications for spinal arthrodesis have expanded, with a dramatic increase in the rate of cervical spine fusion in the United States during the past decade. However, as more of these procedures are performed over time, there have been concerns regarding the potential for these patients to develop changes at levels adjacent to the index procedure. Questions remain whether the development of clinical ASP (CASP) are iatrogenic in nature or part of natural history.

METHODS:
A systematic review of the literature was undertaken for articles published in English language between 1990 and March 15, 2012. Electronic databases and reference lists of key articles were searched to identify articles reporting risk factors for CASP after cervical fusion. Two independent reviewers assessed the level of evidence and the overall quality of the literature using the Grades of Recommendation Assessment, Development, and Evaluation criteria. Disagreements were resolved by consensus.

RESULTS:
We identified 5 studies (4 retrospective cohorts, 1 database study) from our search strategy that met the inclusion criteria from a total of 176 possible studies for review. The prevalence of CASP ranged from 11% to 12% at 5 years, 16% to 38% at 10 years, and 33% at 17 years. Factors that may contribute to the development of CASP include age less than 60 years, fusing adjacent to the C5-C6 and/or C6-C7 levels, a pre-existing disc herniation, and/or dural compression secondary to spinal stenosis with a mean anteroposterior diameter spinal canal of 13 mm or smaller.

CONCLUSION:
CASP remains a controversial topic despite multiple attempts of elucidating an iatrogenic effect of spinal fusion versus the natural history of spinal degeneration. The mean rate of the development of symptomatic degeneration in the cervical spine after arthrodesis is estimated between 1.6% and 4.2% per year. The mean rate of reoperation for CASP is estimated at 0.8% per year. In addition, fusing adjacent to but not including the C5-C6 and/or C6-C7 disc spaces seems to consistently increase the risk of developing CASP.

CONSENSUS STATEMENT:
1. The risk of developing new symptoms secondary to adjacent segment pathology causing radiculopathy and/or myelopathy after cervical fusion surgery ranges from a cumulative rate of 1.6% to 4.2% per year. Strength of Statement: Strong. 2. The risk of developing adjacent-level symptoms may be increased if disc protrusion, disc degeneration, or cord effacement is present at C5–C6 and/or C6–C7 and if those levels are adjacent to the planned surgical level. Strength of Statement: Strong.





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