Relative risk of OSF by follow up time
| Risk factors | Relative risk | 95% CI | p Value |
| Follow up until 31 December 1989, 37 OSF | |||
| Age (in years) | 0.96 | 0.94 to 0.99 | 0.004 |
| Valve size | 0.005 | ||
| ⩽ 21 and 25 | 1.00 | – | |
| 23 | 4.6 | 0.5 to 43.9 | |
| 27 | 4.2 | 0.4 to 40.3 | |
| 29 | 4.7 | 0.5 to 40.0 | |
| 31 | 12.2 | 1.6 to 95.1 | |
| 33 | 18.0 | 2.0 to 166.7 | |
| Manufacturing period | 0.02 | ||
| Before 1981 (baseline) | 1.0 | – | |
| 1981–1984 | 9.7 | 1.5 to 63.8 | |
| Shop order fracture rate | 1.4 | 1.2 to 1.60 | < 0.001 |
| Follow up 1 January 1990 to 31 July 1997, 19 OSF | |||
| Age (in years) | 0.95 | 0.92 to 0.98 | 0.004 |
| Valve size | |||
| ⩽ 21 and 25 | 1.00 | – | 0.55 |
| 23 | 6.2 | 0.7 to 56.0 | |
| 27 | 4.1 | 0.4 to 39.8 | |
| 29 | 3.9 | 0.4 to 36.7 | |
| 31 | 7.0 | 0.4 to 35.4 | |
| 33 | 6.7 | 0.4 to 111.8 | |
| Manufacturing period | |||
| Before 1981 (baseline) | 1.0 | – | 0.21 |
| 1981–1984 | 2.6 | 0.6 to 11.2 | |
| Shop order fracture rate | 1.3 | 1.1 to 1.7 | 0.011 |