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Table 1 Classification of HO following CDR

From: The impact of smoking on outcomes following anterior cervical fusion-nonfusion hybrid surgery: a retrospective single-center cohort study

Grade Definition
0 No HO was observed
1 HO does not occur within the disc space
2 HO is present between the planes formed by the vertebral endplates but does not block spinal motion
3 The range of motion of the vertebral endplates is blocked by the formation of HO or osteophytes
4 HO causes bony fusion
  1. HO heterotopic ossification, CDR cervical disc replacement