In this study of nationally representative data collected for the NIS between the years of 1998 and 2006 we found an increased incidence of perioperative complications and adjusted risk of in-hospital mortality among hospital admissions undergoing APSF and ASF when compared to PSF procedures. The highest rates of fatal outcomes and complications were associated with procedures utilizing the anterior thoracic approach. Risk factors for in-hospital mortality included: male gender, advanced age, procedures indicated for metastatic disease and trauma as well as the presence of a number of comorbidities and perioperative complications.
In view of the increasing utilization of spine fusion procedures, these findings are of importance to the perioperative physician, in order to better assess the chance of morbidity and mortality, identify patients at risk and adequately inform patients of such risks before embarking on this procedure.
Procedures involving the anterior spine were associated with higher morbidity and mortality in our study, despite being performed in younger individuals with lower comorbidity burden. The risk of perioperative mortality was increased even when controlling for the indication for surgery. Complications associated with the anterior approach have long been recognized (16
), but a paucity of comparative analysis addressing outcome in this patient population exists. The reason for the increased rate of complications is likely associated with the entry of the abdominal and thoracic cavity and the proximity of vital organs (16
). This fact is supported by our finding of increased gastrointestinal and pulmonary complications among patients undergoing APSF and ASF compared to PSF procedures. The highest rate of morbidity and mortality was seen in APSF patients, which can be explained by longer surgical times, more blood loss and increased surgical complexity. Shen et al., without studying the specific approach, identified thoracic procedures to be associated with higher rates of complications and mortality compared to lumbar fusions (9
), which is in concordance with our findings.
While the approach is often dictated by the individual patient's condition, newer access methods utilizing the retroperitoneal space, thus avoiding intra abdominal structures, and thoracoscopic exploration may be considered to reduce morbidity and mortality whenever feasible (17
). Unfortunately, information on the utilization of these particular techniques is not discernable from the NIS data and their impact could therefore not be studied.
When studying patient demographics and their association with mortality, we found increased independent risk of a fatal event after spine fusion among men. Although this finding has been reported in the past in spine surgery patients (9
), little is known about the causality between gender and mortality risk.
We identified an increased incidence of morbidity and risk for mortality in patients with advanced age. Patients over the age of 75 made up almost one third of all mortalities, despite representing less than 9% of the spine surgical population in this study. Similarly, Li et al. was able to show that mortality increased in elderly patients after lumbar laminectomy compared to their younger counterparts. He further concluded that mortality was higher in the patient population over 85 years of age when comorbidities were present compared to otherwise healthy, elderly individuals (1.4% vs. 0.22%) (21
). In our study of spine fusion patients, advanced age remained an independent predictor for mortality even when controlling for comorbidity burden. However, it must be mentioned that advanced age has not consistently been associated with increased mortality in the past (22
Increasing comorbidity burden and the presence of a number of diseases were associated with an increase in mortality risk in this and other studies (9
). Pulmonary circulatory disease, congestive heart failure, renal disease and coagulopathies were associated with the highest increases of risk for perioperative mortality. While the latter comorbidities have been implicated as risk factors in the past (9
), pulmonary circulatory disease as a risk factor has been less well documented. Patients with pulmonary hypertension and decreased right ventricular reserve may be less likely to deal with the consequences of pulmonary embolization of bone and marrow material during instrumentation, resulting in increases in pulmonary vascular resistance. Urban et al. was able to demonstrate an adverse pulmonary effect of perioperative events in the form of an increase in pulmonary vascular resistance in 15% (8/55) of patients undergoing anterior/posterior spine fusion, usually during or after posterior instrumentation (24
). In a follow up study, the same author analyzed bronchoalveolar specimens and linked the presence of lipid laden macrophages to possible embolization of fat and debris entering the blood stream during the surgical procedure (25
). This mechanism of lung injury is supported by echocardiographic studies, in which 80% of spine surgery patients experienced moderate to severe embolic events during instrumentation of the spine (26
Perioperative complications were also associated with increases in the odds of a fatal event. Pulmonary embolism, perioperative shock, ARDS and cardiac complications were associated with the highest risk of mortality. All of these events had the highest incidence among APSF patients. While the mentioned findings are not surprising, it is noteworthy that local surgical complications including infection, wound dehiscence and hematoma/seroma formation also significantly increased the risk for mortality. The impact of wound complications, especially infections has been studied in the past and has been associated with increased morbidity and mortality (27
). Our data confirm the importance of measures to reduce this complication and implement strategies to achieve this goal (28
This study is limited by a number of factors inherent to secondary data analysis of large administrative databases. As such, clinical information and that detailing the surgical procedure (i.e. type of anesthesia, amount of blood loss, length of surgery etc.) available in the NIS is limited and our analysis must be interpreted in this context. Although gathering data on the number of levels operated on may theoretically be possible through the use of the ICD-9-CM coding system, this information was missing in about two thirds of patient entries, thus making the inclusion of this variable in our analysis not feasible. Because of the nature of the NIS, only in-patient data are available and thus complications and events after discharge are not captured. Furthermore, readmissions cannot be discerned from this database. Thus, conclusions should be limited to the acute perioperative setting with the notion that mortality and complications are likely underestimated.
An additional limiting factor is the bias associated with the retrospective nature of our study. Nevertheless, because of the availability of data from a large, nationally representative sample, this type of analysis may provide a more accurate estimate of events surrounding primary non-cervical spine fusion procedures than various prospective studies that are limited in sample size and thus lack the ability to capture low-incidence outcomes.
In conclusion, using a nationally representative database we determined that APSF and ASF carried an increased adjusted risk of in-hospital mortality and greater incidence of in-hospital complications when compared to PSF procedures.
Until more data are available, careful selection of candidates for ASF and APSF and an in depth discussion about risks and alternatives with the patient cannot be overemphasized. Risk factors identified in this analysis may be used to gage the preoperative mortality risk for individual patients.