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Retrospective analysis of adjacent disc degeneration (ADD) after anterior cervical decompression and fusion (ADF).To elucidate the influence of the number of levels fused in ADF on the incidence of ADD.ADD is known as a complication associated with ADF. However, how the number of levels fused affects the incidence of ADD is not well understood.One hundred and two patients with cervical degenerative disease, who underwent ADF and were followed for more than 24 months, were retrospectively analyzed. They were classified into 2 groups, a long group (L group) consisting of 50 cases with ADF of 4 or more disc levels, and a short group (S group) consisting of 52 cases with ADF of 3 or fewer disc levels. Furthermore, the patients were also divided into 2 groups according to inclusion or exclusion of C5-6 and C6-7 (C group: including both, NC group: not including both). The incidence of ADD, and that of symptomatic ADD (sADD), was compared between the 2 classifications.In the L group, there were 13 cases of ADD (26.0%), including 1 case of sADD (2.0%), whereas in the S group, there were 22 cases of ADD (42.3%), including 11 cases of sADD (21.2%). The incidence of sADD was significantly lesser in the L group (P=0.024). Three cases with sADD in the S group required revision surgery, whereas no additional surgery related to ADD was performed on patients in the L group. In addition, in the C group, ADD occurred in 20 of 71 cases (28.2%) and sADD occurred in 4 of 71 cases (5.6%), whereas in the NC group, ADD occurred in 15 of 31 cases (48.4%) and sADD occurred in 8 of 31 cases (25.8%). The incidence of ADD and sADD were significantly lesser in the C group (P=0.048).ADD occurs less frequently among patients in whom C5-6 and C6-7 are fused than among those in whom C5-6 or C6-7 is left at an adjacent level, irrespective of the length of the fusion.