Demographics and characteristics of the studies.
| Author | Journal | Year | Country | Level of Evidence | Subjects | Mean Age, y | Type of Procedure | No. of Procedures | % Women | Follow-Up |
| Guzman et al23 | Spine | 2014 | USA | III: Retrospective database analysis | 423,050 | 61.8 | Lumbar spine surgery | 2,568,994 | 51.8% | NR |
| Peng et al19 | Medicine | 2019 | China | III: Retrospective single-center analysis | 523 | NRa | Lumbar spine surgery | 523 | 43.6% | NR |
| Tanishima et al18 | Asian Spine Journal | 2019 | Japan | II: Multicenter prospective study | 61 | 69.5 | Cervical spine surgery | 61 | 43% | 12 mo |
| Hwang et al24 | Korean Journal of Neurotrauma | 2019 | Korea | III: Retrospective review | 92 | 68.4 | Lumbar spine surgery | 92 | 58.7% | 23.4 mo |
| Worley et al25 | International Journal of Spine Surgery | 2017 | USA | III: Retrospective review | 5904 | 61 | Cervical spine surgery | 5904 | 42.6% | NR |
| Langlois et al26 | European Spine Journal | 2014 | France | II: Prospective cohort study | 75 | 56.1 | Cervical, lumbar, and thoracic spine surgery | 75 | 62.7% | 3 mo |
| Khan et al20 | Global Spine Journal | 2019 | USA | III: Retrospective cohort study | 850 | 58.4 | Lumbar spine surgery | 850 | 53.3% | 22.99 mo |
| Moazzeni et al22 | International Journal of Spine Surgery | 2018 | Iran | II: Prospective study | 96 | 57.8 | Lumbar spine surgery | 96 | 65.2% | 12 mo |
| Machino et al21 | Spine | 2017 | Japan | II: Prospective cohort study | 505 | 66.6 | Cervical spine surgery | 505 | 39.2% | 12 mo |
Abbreviation: NR, not reported.
↵a Three hundred thirty patients were older than 60 y.