The measure reflects the percent of patients receiving spine surgery who were readmitted to a hospital within 90 days of surgery. Planned "readmissions" for two-stage procedures or other reasons are not counted. Rates are reported for all-cause readmissions, although the registry allows for separate analysis of spine-related vs. unrelated readmissions. Rates are reported on a risk-adjusted basis, with the risk adjustment model based on a set of demographic and clinical characteristics, as well as case-mix variables (e.g., fusion vs. non-fusion) so that rates are truly comparable from surgeon to surgeon.