Table 2. Baseline characteristics of the included studies.
| Study, year | Country | Study period | Type of database | Type of surgery based on ICD-9-CM | No. of patients | Age | The definition of morbidity | Surgeon volume grouping and category, cases [year] | Multivariate OR (95% CI) for morbidity related to surgeon volume | Covariates in fully adjusted mode |
|---|---|---|---|---|---|---|---|---|---|---|
| Farjoodi et al., 2011 | America | 1992–2005 | National Inpatient Sample | Posterior lumbar decompression with fusion (81.08) or exploration/decompression of the spinal canal (03.09) | 1,148,963 | More than 18 years old | Surgical complications | Quartiles [1992]: low volume ≤1; high volume ≥9 Quartiles [2005]: low volume ≤1; high volume ≥22 |
1 (reference); 0.97 (0.83–1.13); 0.95 (0.82–1.01); 0.94 (0.81–0.99) | Age, gender, race/ethnicity, insurance type, and comorbidity index |
| Dasenbrock et al., 2012 | America | 2005–2008 | Nationwide Inpatient Sample | Lumbar spine decompression (03.09) or fusion (81.06, 81.07, 81.08) | 48,971 | More than 18 years old | Any postoperative complications | Quintiles: <15; 15–28; 29–47; 48–81; >81 | 1 (reference); 0.85 (0.75–0.96); 0.77 (0.68–0.87); 0.73 (0.64–0.83); 0.73 (0.62–0.84) | Age, sex, comorbidities, primary payer, hospital bed size, teaching status, and location |
| Paul et al., 2015 | America | 2001–2010 | Nationwide Inpatient Sample | Revision adult spinal deformity arthrodesis (81.00, 81.03–81.09) | 68,132 | More than 21 years old | Major medical complications | Quartiles: <1; 1–3; 3–10; 10–112 | 1 (reference); 0.60 (0.21–1.707); 0.53 (0.223–1.275); 0.44 (0.196–0.966) | Surgical index, revision, osteotomy, and comorbidities |
| Paul et al., 2015 | America | 2001–2010 | Nationwide Inpatient Sample | Adolescent spinal deformity arthrodesis (81.00, and 81.03 through 81.09) | 6,100 | 10–18 years old | Major medical complications | Quartiles: <1; 2–7; 8–19; 20–97 | 1 (reference); 1.03 (0.916–1.145); 1.02 (0.909–1.14); 0.75 (0.662–0.846) | Age, race, sex, neuromuscular scoliosis, operative complexity index, and comorbidities |
| Basques et al., 2016 | America | 2003–2009 | Nationwide Inpatient Sample | Anterior cervical fusion (81.02) | 419,212 | More than 18 years old | Surgical complications | Tertiles: <5; 5–67; >67 | 3.8 (3.1–4.9); 1 (reference); 0.7 (0.6–0.8) | Age, sex, Elixhauser Comorbidity Index, and insurance status |
| Rafael et al., 2016 | America | 2003–2009 | Nationwide Inpatient Sample | Metastatic spinal tumors decompression (03.01, 03.09, 03.4, 03.54) or fusion (81.00–81.08 and 81.61) | 3,069 | More than 18 years old | Any postoperative complication | Two groups (volume was not defined) | 1 (reference); 0.70 (0.58–0.85) | Age, sex, race, comorbidities, expected primary payer, hospital bed size, hospital teaching status, hospital location, presence of visceral metastases, and provider volume |
| Cole et al., 2017 | America | 2006–2010 | MarketScan Longitudinal Database | Anterior cervical discectomy and fusion procedures | 24,461 | More than 18 years old | Any postoperative complication | Two groups: ≤29; ≥30 | 1 (reference); 0.72 (0.64–0.82) | Age, sex, tobacco use, procedure characteristics, decompression, and comorbidities |
| Rafael et al., 2017 | America | 2009 | Nationwide Inpatient Sample | 1- and 2-level anterior cervical discectomy and fusion (81.62) | 11,249 | More than 18 years old | Surgical complications | Quintiles: <12; 12–23; 24–35; 36–47; ≥48 | 1 (reference); 0.81 (0.62–1.05); 0.72 (0.53–1.01); 0.78 (0.54–1.11); 0.58 (0.41–0.84) | Age, sex, race, Elixhauser score, insurance status, hospital teaching status, hospital location, hospital volume, and use of iliac crest autograft |
| Blais et al., 2017 | America | 2011–2014 | Florida Statewide Inpatient Database | Cervical spine fusion | 66,068 | More than 18 years old | Any postoperative complications | Two groups: Anterior cervical fusion: low volume <40; high volume ≥40 Posterior cervical fusion: low volume <30; high volume ≥30 |
Anterior cervical fusion: 1 (reference); 0.55 (0.50–0.61); Posterior cervical fusion: 1 (reference); 0.69 (0.59–0.81) |
Age, race, biologic sex, insurance status, income quartile by zip code, medical comorbidities, and hospital length of stay |
| Schoenfeld et al., 2017 | America | 2011–2014 | Florida Statewide Inpatient Database | Lumbar spine surgery | 187,185 | More than 18 years old | Any postoperative complications | Two groups: Decompression: low volume <25; high volume ≥25 Discectomy: low volume <40; high volume ≥40 Lumbar interbody fusion: low volume <43; high volume ≥43 Lumbar posterolateral fusion: low volume <35; high volume ≥35 |
Decompression: 1 (reference); 0.61 (0.58–0.65) Discectomy: 1 (reference); 0.64 (0.60–0.68) Lumbar interbody fusion: 1 (reference); 0.87 (0.82–0.93 Lumbar posterolateral fusion: 0.68 (0.63–0.72) |
Age, biological sex, hospital volume, race, insurance status, and age-adjusted Charlson score |
| Schoenfeld et al., 2017 | America | 2011–2014 | Florida Statewide Inpatient Database | Spinal metastases | 3,135 | More than 18 years old | Any postoperative complications | Two groups: low volume <49; high volume ≥49 | 1 (reference); 0.71 (0.59–0.86) | Age, biologic sex, Charlson comorbidities, race/ethnicity, and insurance status |