| Grade | Description |
| Grade 0 | Segments without any new HO formation after prosthesis implantation. |
| Grade I | Segments with new HO formation not reaching the intervertebral space. |
| Grade II | HO reaches the intervertebral space but segmental movement is not limited. |
| Grade III | Important bridging ossifications with limited, but possible movement. |
| Grade IV | Segmental fusion. |