title: Development of an index to characterize the "invasiveness" of spine surgery: validation by comparison to blood loss and operative time.
abstract: METHODS
  Prospective cohort study.
  
  
  OBJECTIVE
  To create and validate an index describing the extent of spine surgical intervention to allow fair comparisons of complication rates among patients treated by different surgeons, devices, or hospitals.
  
  
  BACKGROUND
  Safety comparisons in spine surgery are limited by lack of methods that adjust for important variations in the surgical "case-mix." Among other factors, the magnitude of an operation is likely to have a substantial influence on the likelihood of complications.
  
  
  METHODS
  We created a spine surgery invasiveness index defined as the sum, across all vertebral levels, of 6 possible interventions on each operated vertebra: anterior decompression, anterior fusion, anterior instrumentation, posterior decompression, posterior fusion, and posterior instrumentation. We assessed the validity of this index by examining its association with blood loss and surgery duration in 1723 spine surgeries, adjusting for important factors including age, gender, body mass index, diagnosis, neurologic deficit, revision surgery, and vertebral level of surgery.
  
  
  RESULTS
  Blood loss increased by 11.5% and surgery duration increased by 12.8 minutes for each unit increase in the invasiveness index. The invasiveness index explained 44% of the variation in blood loss and 52% of the variation in surgery duration. For specific surgical components, blood loss increased by 9.4% and surgery duration by 11.4 minutes for each vertebral level of anterior decompression, 19.4% and 33.8 minutes for each segment of anterior instrumentation, 12.9% and 22.7 minutes for each level of posterior decompression, and 25.1% and 18.8 minutes for each segment of posterior instrumentation.
  
  
  CONCLUSIONS
  An "invasiveness" index based on the number of vertebrae decompressed, fused, or instrumented showed the expected associations with both blood loss and surgery duration. This quantitative description of surgery invasiveness may be useful to adjust for surgical variations when making safety comparisons in spine surgery.
authors.affiliation: Department of Orthopedics and Sports Medicine, Center for Cost and Outcomes Research, and University of Washington, Seattle, WA, USA. mirza@u.washington.edu
authors.forename: Sohail K
authors.initials: SK
authors.lastname: Mirza
authors.forename: Richard A
authors.initials: RA
authors.lastname: Deyo
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Heagerty
authors.forename: Mark A
authors.initials: MA
authors.lastname: Konodi
authors.forename: Lorri A
authors.initials: LA
authors.lastname: Lee
authors.forename: Judith A
authors.initials: JA
authors.lastname: Turner
authors.forename: Robert
authors.initials: R
authors.lastname: Goodkin
mesh_terms: D000328:Adult; D000368:Aged; D016063:Blood Loss, Surgical; D019299:Decompression, Surgical / Q000009:adverse effects*; D005260:Female; D006305:Health Status Indicators; D006801:Humans; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D011446:Prospective Studies; D019984:Quality Indicators, Health Care; D012042:Registries; D015203:Reproducibility of Results; D018570:Risk Assessment; D012307:Risk Factors; D012720:Severity of Illness Index; D013123:Spinal Fusion / Q000009:adverse effects* / Q000295:instrumentation; D013131:Spine / Q000601:surgery*; D013997:Time Factors