Annual incidence of surgery for ASD.
| Years After Initial Fusion | Number of New ASD Surgery | Number Censored | Effective Sample Size | Annual Incidence of ASD |
| 0 | 0 | 0 | 49.0 | 0 |
| 1 | 0 | 0 | 49.0 | 0 |
| 2 | 4 | 15 | 41.5 | 0.0964 |
| 3 | 1 | 11 | 24.5 | 0.0408 |
| 4 | 0 | 13 | 11.5 | 0 |
| 5 | 0 | 5 | 2.5 | 0 |
Abbreviation: ASD, adjacent segment disease.