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Letter to the editor concerning ''The Rate of Heterotic Ossification Following Cervical Disc Arthroplasty: A Systematic Review and Comparison of Data'' by Dowell et al W ith great interest, we have read the article by Dowell et al 1 published in Spine.The author pooled heterotic ossification rates and annum rates after cervical total disc replacement. We highly appreciate their contributions to this important topic. However, there is a point that may mislead readers, and some supplementary explanations would be of benefit.In this article, severe heterotic ossification may not be able to protect adjacent segments and may lead to degeneration. However, natural process plays an important role in the occurrence of adjacent segment degeneration (ASD). ASD is indeed a common phenomenon, but it may reflect the natural history of cervical spondylosis 2 . Matsumoto et al 3 found more than 60% of 201 asymptomatic volunteers had cervical degeneration within 10 years, and degeneration rate was comparable between surgical group and control group at C5-6. Park et al 4 found 85% of patients with ASD had degenerative changes in the adjacent segments before surgery, which indicate that preoperative degeneration contributes to the increasing risk of ASD in patients receiving anterior cervical spine surgery. Therefore, natural degeneration should not be ignored when analyzing the causes of ASD. If possible, a subgroup analysis of ASD in this article can give us more clues to reach more comprehensive and rigorous conclusions.Once again, we thank the authors for their contribution in this field and hope readers can benefit from it.