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Definition of preoperative ACS and progression of ACS after surgery. (A) Preoperative ACS was defined as anterior slippage of ≥2 mm on performing a cervical plain radiograph in the neutral position. (B) Progression of ACS was defined as follows: postoperative worsening of ACS ≥2 mm on the level with preoperative ACS (B.1) or newly developed ACS ≥2 mm at the final follow-up (B.2). ACS, anterior cervical spondylolisthesis.
