David A. Share, MD

The use of inferior vena cava (IVC) filters has increased substantially in recent years. These medical devices, which are used to prevent pulmonary embolism in patients considered to be at high risk of venous thromboembolism, were placed in 167,000 patients in 2007.¹ In 2012, it is estimated that 259,000 patients will undergo placement of an IVC filter, an increase of 55%.[1] Increasing use of IVC filters is attributable to the development of retrievable versions of the devices, which have expanded indications for use such as in bariatric surgery.

Unfortunately, the increase in the use of IVC filters has been accompanied by an increase in reports of adverse events in patients receiving them. The United States Food and Drug Administration (FDA) has received more than 900 adverse event reports involving IVC filters, prompting the agency to issue a warning about their use.[2] A prior study by our group demonstrated a lack of benefit of IVC filter insertion for the prevention of pulmonary embolism among bariatric surgery patients but lacked statistical power to prove harms associated with this practice.[3]

In the current study, we analyzed data from the prospective, statewide, clinical registry of the Michigan Bariatric Surgery Collaborative. Our study population now includes 35,477 bariatric surgery patients from 32 hospitals whose procedures were performed between 2006 and 2012. Since the publication of our prior study, the use of IVC filters in bariatric surgery has decreased significantly in Michigan. For this reason, our study population now includes many more high‐risk patients who did not undergo IVC filter placement, allowing us to match IVC filter patients to similarly high‐risk patients who did not receive IVC filters. We used these data to compare outcomes within 30 days of surgery, including rates of venous thromboembolism, overall serious complications, and death among patients who did and not receive IVC filters.

METHODS

RESULTS

Matching resulted in cohorts of IVC filter and control patients who were well balanced on all baseline characteristics (Table 1). In contrast, there were large and significant differences between IVC filter patients and unmatched control patients. For example, mean body mass index was 58 and 57 in the matched cohorts and 47 in the unmatched control patients. Prior history of venous thromboembolism was present in 39%, 39%, and 2% of the IVC filter, matched control, and unmatched control patients, respectively. With regard to procedure mix, unmatched control patients were less likely to have open gastric bypass and more likely to have adjustable gastric band procedures than IVC filter or matched control patients.

With regard to outcomes (Table 2, Figures 1 and 2), IVC filter patients had significantly higher rates of venous thromboembolism (1.9% vs 0.74%; OR, 2.7; 95% CI, 1.1‐6.3; P=0.027) and deep vein thrombosis (1.2% vs 0.37%, OR, 3.3; 95% CI, 1.1‐10.1; P=0.039) than matched control patients. Rates of pulmonary embolism were higher among IVC filter patients, but the difference was not statistically significant (0.84% vs 0.46%; OR, 2.0; 95% CI, 0.6‐6.5; P=0.232). Rates of pulmonary embolism were similar for patients with a low baseline risk of venous thromboembolism (0.27% vs 0.27%; OR, 1.0; 95% CI, 0.1‐7.7; P=0.965) but were higher for medium‐risk patients (2.1% vs 0.87%; OR, 2.5; 95% CI, 0.5‐12.7; P=0.288) and high‐risk patients (2.1% vs 0.97%; OR, 2.2; 95% CI, 0.2‐24.3; P=0.530).

Figure 1

Figure 2

Rates of other complications were also higher among IVC filter than matched control patients (Table 2 and Figure 3). There were significantly higher rates of complications (15.2% vs 11.6%; OR, 1.3; 95% CI, 1.0‐1.7; P=0.048), serious complications (5.8% vs 3.8%; OR, 1.6; 95% CI, 1.0‐2.4; P=0.031), and permanently disabling complications (1.2% vs 0.4%; OR, 4.3; 95% CI, 1.2‐15.6; P=0.028) among IVC filter patients. Rates of death (0.7% vs 0.1%; OR, 7.0; 95% CI, 0.9‐57.3; P=0.068) were also higher among IVC filter patients than matched control patients, but this difference was not statistically significant.

Figure 3

Among the 7 IVC filter patients who died, 4 had fatal pulmonary embolism, and 2 had IVC filter thrombosis/occlusion. Other IVC filter‐specific complications included IVC filter migration requiring heart valve replacement surgery in 1 patient, contrast‐induced nephropathy in 1 patient, IVC filter incision site infection in 1 patient, and technical difficulties removing a temporary IVC filter requiring that the device stay in place in 1 patient.

DISCUSSION

In this propensity matched, observational cohort study, we assessed the safety and effectiveness of prophylactic IVC filters among bariatric surgery patients. We found that patients with IVC filters had significantly worse outcomes than comparably high‐risk patients without IVC filters. Rates of venous thromboembolism were higher in the IVC filter patients, and a large proportion of the other complications among IVC filter patients were device related.

Our current study of IVC filters was prompted by an FDA advisory report regarding complications in patients receiving IVC filters.[2] The FDA's report was in turn prompted by a study indicating a high prevalence of strut fracture and embolization among 80 patients who received a certain type of retrievable IVC filter.[4] The FDA conveyed receiving 921 adverse events reports involving IVC filters between 2005 and 2009. Thirty‐six percent of these reports involved migration of the device, 16% were related to breakage and embolization of parts of the device, and 8% involved perforation of the IVC.

Research on the safety and efficacy of IVC filters in bariatric surgery patients has largely been limited to small, single‐center, case series or cohort studies.[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] A systematic review of this literature concluded that the evidence was insufficient to recommend IVC filters for patients undergoing bariatric surgery.[16] A 2010 study by our group was the largest and only multicenter study of IVC filters in the bariatric surgery population. We found no benefit of IVC filters in a comparison of 542 gastric bypass patients with prophylactic IVC filters to 5,834 gastric bypass patients without prophylactic IVC filters.[3]

When interpreting the results of this study, a number of limitations should be considered. Our study was observational, so there is the potential for unmeasured confounding variables to have influenced our results. To minimize the risk of confounding, we used propensity scores to match IVC filter patients to comparably high‐risk control patients, resulting in study cohorts that were well balanced on all baseline variables. Although this method accounts for confounding on the variables for which there are data, there is still the possibility that an unknown confounder could affect our findings. For example, our clinical registry lacks data on hypercoagulable states, so it is possible that a higher proportion of IVC filter patients could have had this risk factor and therefore a higher baseline risk of venous thromboembolism. However, most patients with a hypercoagulable state would have had a prior history of venous thromboembolism, which is a variable included in our database that patients were matched on.

The effects of changes in clinical care occurring during the time frame of this study should be considered in interpreting our findings. For example, bariatric surgery has been getting safer in general over time. Rates of death have fallen both in Michigan and in the rest of the country as bariatric surgeons have gained experience with this procedure. In Michigan during this time period, our group has developed and implemented a risk‐stratified, standardized approach to venous thromboembolism prophylaxis for patients undergoing bariatric surgery. For these reasons, we included the year of the procedure and the type of medical venous thromboembolism prophylaxis (unfractionated or low molecular weight heparin) used perioperatively as a matching variable in our analysis.

Another limitation that should be considered in interpreting our findings is statistical power. Although our study is the largest in this study population to date, many of the outcomes of interest are relatively rare. Considering the entire bariatric surgery population, rates of venous thromboembolism and death within 30 days are each less than 1%. Even in the high‐risk patients included in this analysis, there were a total of just 28 (1.3%) venous thromboembolism events and 8 (0.37) deaths. Nonetheless, our study did find significantly greater risks of multiple types of complications among patients receiving IVC filters.

Finally, our study captures events occurring within 30 days of bariatric surgery. Complications, including venous thromboembolism and other complications directly related to IVC filters, frequently occur after 30 days of bariatric surgery. Therefore, our study may be a conservative estimate of the risks associated with the use of IVC filters in bariatric surgery patients. Furthermore, certain brands of filters have been shown to be associated with higher risks of complications. Our study lacks data on the brand of IVC filter used and so cannot assess the extent to which this would affect our results.

CONCLUSIONS

In conclusion, our study indicates that IVC filters do not reduce the risk of pulmonary embolism in high‐risk bariatric surgery patients. They are also associated with other complications attributable to malfunctions of the device itself. We believe that the use of IVC filters among bariatric surgery patients should be discouraged.

Disclosure

This study was supported by a grant from the Agency for Healthcare Research and Quality (HS018050) and was presented at the Annual Meeting of the American Society for Metabolic and Bariatric Surgery (ASMBS), San Diego, California, June 20, 2012.

The use of inferior vena cava (IVC) filters has increased substantially in recent years. These medical devices, which are used to prevent pulmonary embolism in patients considered to be at high risk of venous thromboembolism, were placed in 167,000 patients in 2007.¹ In 2012, it is estimated that 259,000 patients will undergo placement of an IVC filter, an increase of 55%.[1] Increasing use of IVC filters is attributable to the development of retrievable versions of the devices, which have expanded indications for use such as in bariatric surgery.

Unfortunately, the increase in the use of IVC filters has been accompanied by an increase in reports of adverse events in patients receiving them. The United States Food and Drug Administration (FDA) has received more than 900 adverse event reports involving IVC filters, prompting the agency to issue a warning about their use.[2] A prior study by our group demonstrated a lack of benefit of IVC filter insertion for the prevention of pulmonary embolism among bariatric surgery patients but lacked statistical power to prove harms associated with this practice.[3]

In the current study, we analyzed data from the prospective, statewide, clinical registry of the Michigan Bariatric Surgery Collaborative. Our study population now includes 35,477 bariatric surgery patients from 32 hospitals whose procedures were performed between 2006 and 2012. Since the publication of our prior study, the use of IVC filters in bariatric surgery has decreased significantly in Michigan. For this reason, our study population now includes many more high‐risk patients who did not undergo IVC filter placement, allowing us to match IVC filter patients to similarly high‐risk patients who did not receive IVC filters. We used these data to compare outcomes within 30 days of surgery, including rates of venous thromboembolism, overall serious complications, and death among patients who did and not receive IVC filters.

METHODS

RESULTS

Matching resulted in cohorts of IVC filter and control patients who were well balanced on all baseline characteristics (Table 1). In contrast, there were large and significant differences between IVC filter patients and unmatched control patients. For example, mean body mass index was 58 and 57 in the matched cohorts and 47 in the unmatched control patients. Prior history of venous thromboembolism was present in 39%, 39%, and 2% of the IVC filter, matched control, and unmatched control patients, respectively. With regard to procedure mix, unmatched control patients were less likely to have open gastric bypass and more likely to have adjustable gastric band procedures than IVC filter or matched control patients.

With regard to outcomes (Table 2, Figures 1 and 2), IVC filter patients had significantly higher rates of venous thromboembolism (1.9% vs 0.74%; OR, 2.7; 95% CI, 1.1‐6.3; P=0.027) and deep vein thrombosis (1.2% vs 0.37%, OR, 3.3; 95% CI, 1.1‐10.1; P=0.039) than matched control patients. Rates of pulmonary embolism were higher among IVC filter patients, but the difference was not statistically significant (0.84% vs 0.46%; OR, 2.0; 95% CI, 0.6‐6.5; P=0.232). Rates of pulmonary embolism were similar for patients with a low baseline risk of venous thromboembolism (0.27% vs 0.27%; OR, 1.0; 95% CI, 0.1‐7.7; P=0.965) but were higher for medium‐risk patients (2.1% vs 0.87%; OR, 2.5; 95% CI, 0.5‐12.7; P=0.288) and high‐risk patients (2.1% vs 0.97%; OR, 2.2; 95% CI, 0.2‐24.3; P=0.530).

Figure 1

Figure 2

Rates of other complications were also higher among IVC filter than matched control patients (Table 2 and Figure 3). There were significantly higher rates of complications (15.2% vs 11.6%; OR, 1.3; 95% CI, 1.0‐1.7; P=0.048), serious complications (5.8% vs 3.8%; OR, 1.6; 95% CI, 1.0‐2.4; P=0.031), and permanently disabling complications (1.2% vs 0.4%; OR, 4.3; 95% CI, 1.2‐15.6; P=0.028) among IVC filter patients. Rates of death (0.7% vs 0.1%; OR, 7.0; 95% CI, 0.9‐57.3; P=0.068) were also higher among IVC filter patients than matched control patients, but this difference was not statistically significant.

Figure 3

Among the 7 IVC filter patients who died, 4 had fatal pulmonary embolism, and 2 had IVC filter thrombosis/occlusion. Other IVC filter‐specific complications included IVC filter migration requiring heart valve replacement surgery in 1 patient, contrast‐induced nephropathy in 1 patient, IVC filter incision site infection in 1 patient, and technical difficulties removing a temporary IVC filter requiring that the device stay in place in 1 patient.

DISCUSSION

In this propensity matched, observational cohort study, we assessed the safety and effectiveness of prophylactic IVC filters among bariatric surgery patients. We found that patients with IVC filters had significantly worse outcomes than comparably high‐risk patients without IVC filters. Rates of venous thromboembolism were higher in the IVC filter patients, and a large proportion of the other complications among IVC filter patients were device related.

Our current study of IVC filters was prompted by an FDA advisory report regarding complications in patients receiving IVC filters.[2] The FDA's report was in turn prompted by a study indicating a high prevalence of strut fracture and embolization among 80 patients who received a certain type of retrievable IVC filter.[4] The FDA conveyed receiving 921 adverse events reports involving IVC filters between 2005 and 2009. Thirty‐six percent of these reports involved migration of the device, 16% were related to breakage and embolization of parts of the device, and 8% involved perforation of the IVC.

Research on the safety and efficacy of IVC filters in bariatric surgery patients has largely been limited to small, single‐center, case series or cohort studies.[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] A systematic review of this literature concluded that the evidence was insufficient to recommend IVC filters for patients undergoing bariatric surgery.[16] A 2010 study by our group was the largest and only multicenter study of IVC filters in the bariatric surgery population. We found no benefit of IVC filters in a comparison of 542 gastric bypass patients with prophylactic IVC filters to 5,834 gastric bypass patients without prophylactic IVC filters.[3]

When interpreting the results of this study, a number of limitations should be considered. Our study was observational, so there is the potential for unmeasured confounding variables to have influenced our results. To minimize the risk of confounding, we used propensity scores to match IVC filter patients to comparably high‐risk control patients, resulting in study cohorts that were well balanced on all baseline variables. Although this method accounts for confounding on the variables for which there are data, there is still the possibility that an unknown confounder could affect our findings. For example, our clinical registry lacks data on hypercoagulable states, so it is possible that a higher proportion of IVC filter patients could have had this risk factor and therefore a higher baseline risk of venous thromboembolism. However, most patients with a hypercoagulable state would have had a prior history of venous thromboembolism, which is a variable included in our database that patients were matched on.

The effects of changes in clinical care occurring during the time frame of this study should be considered in interpreting our findings. For example, bariatric surgery has been getting safer in general over time. Rates of death have fallen both in Michigan and in the rest of the country as bariatric surgeons have gained experience with this procedure. In Michigan during this time period, our group has developed and implemented a risk‐stratified, standardized approach to venous thromboembolism prophylaxis for patients undergoing bariatric surgery. For these reasons, we included the year of the procedure and the type of medical venous thromboembolism prophylaxis (unfractionated or low molecular weight heparin) used perioperatively as a matching variable in our analysis.

Another limitation that should be considered in interpreting our findings is statistical power. Although our study is the largest in this study population to date, many of the outcomes of interest are relatively rare. Considering the entire bariatric surgery population, rates of venous thromboembolism and death within 30 days are each less than 1%. Even in the high‐risk patients included in this analysis, there were a total of just 28 (1.3%) venous thromboembolism events and 8 (0.37) deaths. Nonetheless, our study did find significantly greater risks of multiple types of complications among patients receiving IVC filters.

Finally, our study captures events occurring within 30 days of bariatric surgery. Complications, including venous thromboembolism and other complications directly related to IVC filters, frequently occur after 30 days of bariatric surgery. Therefore, our study may be a conservative estimate of the risks associated with the use of IVC filters in bariatric surgery patients. Furthermore, certain brands of filters have been shown to be associated with higher risks of complications. Our study lacks data on the brand of IVC filter used and so cannot assess the extent to which this would affect our results.

CONCLUSIONS

In conclusion, our study indicates that IVC filters do not reduce the risk of pulmonary embolism in high‐risk bariatric surgery patients. They are also associated with other complications attributable to malfunctions of the device itself. We believe that the use of IVC filters among bariatric surgery patients should be discouraged.

Disclosure

This study was supported by a grant from the Agency for Healthcare Research and Quality (HS018050) and was presented at the Annual Meeting of the American Society for Metabolic and Bariatric Surgery (ASMBS), San Diego, California, June 20, 2012.

The use of inferior vena cava (IVC) filters has increased substantially in recent years. These medical devices, which are used to prevent pulmonary embolism in patients considered to be at high risk of venous thromboembolism, were placed in 167,000 patients in 2007.¹ In 2012, it is estimated that 259,000 patients will undergo placement of an IVC filter, an increase of 55%.[1] Increasing use of IVC filters is attributable to the development of retrievable versions of the devices, which have expanded indications for use such as in bariatric surgery.

Unfortunately, the increase in the use of IVC filters has been accompanied by an increase in reports of adverse events in patients receiving them. The United States Food and Drug Administration (FDA) has received more than 900 adverse event reports involving IVC filters, prompting the agency to issue a warning about their use.[2] A prior study by our group demonstrated a lack of benefit of IVC filter insertion for the prevention of pulmonary embolism among bariatric surgery patients but lacked statistical power to prove harms associated with this practice.[3]

In the current study, we analyzed data from the prospective, statewide, clinical registry of the Michigan Bariatric Surgery Collaborative. Our study population now includes 35,477 bariatric surgery patients from 32 hospitals whose procedures were performed between 2006 and 2012. Since the publication of our prior study, the use of IVC filters in bariatric surgery has decreased significantly in Michigan. For this reason, our study population now includes many more high‐risk patients who did not undergo IVC filter placement, allowing us to match IVC filter patients to similarly high‐risk patients who did not receive IVC filters. We used these data to compare outcomes within 30 days of surgery, including rates of venous thromboembolism, overall serious complications, and death among patients who did and not receive IVC filters.

METHODS

RESULTS

Matching resulted in cohorts of IVC filter and control patients who were well balanced on all baseline characteristics (Table 1). In contrast, there were large and significant differences between IVC filter patients and unmatched control patients. For example, mean body mass index was 58 and 57 in the matched cohorts and 47 in the unmatched control patients. Prior history of venous thromboembolism was present in 39%, 39%, and 2% of the IVC filter, matched control, and unmatched control patients, respectively. With regard to procedure mix, unmatched control patients were less likely to have open gastric bypass and more likely to have adjustable gastric band procedures than IVC filter or matched control patients.

With regard to outcomes (Table 2, Figures 1 and 2), IVC filter patients had significantly higher rates of venous thromboembolism (1.9% vs 0.74%; OR, 2.7; 95% CI, 1.1‐6.3; P=0.027) and deep vein thrombosis (1.2% vs 0.37%, OR, 3.3; 95% CI, 1.1‐10.1; P=0.039) than matched control patients. Rates of pulmonary embolism were higher among IVC filter patients, but the difference was not statistically significant (0.84% vs 0.46%; OR, 2.0; 95% CI, 0.6‐6.5; P=0.232). Rates of pulmonary embolism were similar for patients with a low baseline risk of venous thromboembolism (0.27% vs 0.27%; OR, 1.0; 95% CI, 0.1‐7.7; P=0.965) but were higher for medium‐risk patients (2.1% vs 0.87%; OR, 2.5; 95% CI, 0.5‐12.7; P=0.288) and high‐risk patients (2.1% vs 0.97%; OR, 2.2; 95% CI, 0.2‐24.3; P=0.530).

Figure 1

Figure 2

Rates of other complications were also higher among IVC filter than matched control patients (Table 2 and Figure 3). There were significantly higher rates of complications (15.2% vs 11.6%; OR, 1.3; 95% CI, 1.0‐1.7; P=0.048), serious complications (5.8% vs 3.8%; OR, 1.6; 95% CI, 1.0‐2.4; P=0.031), and permanently disabling complications (1.2% vs 0.4%; OR, 4.3; 95% CI, 1.2‐15.6; P=0.028) among IVC filter patients. Rates of death (0.7% vs 0.1%; OR, 7.0; 95% CI, 0.9‐57.3; P=0.068) were also higher among IVC filter patients than matched control patients, but this difference was not statistically significant.

Figure 3

Among the 7 IVC filter patients who died, 4 had fatal pulmonary embolism, and 2 had IVC filter thrombosis/occlusion. Other IVC filter‐specific complications included IVC filter migration requiring heart valve replacement surgery in 1 patient, contrast‐induced nephropathy in 1 patient, IVC filter incision site infection in 1 patient, and technical difficulties removing a temporary IVC filter requiring that the device stay in place in 1 patient.

DISCUSSION

In this propensity matched, observational cohort study, we assessed the safety and effectiveness of prophylactic IVC filters among bariatric surgery patients. We found that patients with IVC filters had significantly worse outcomes than comparably high‐risk patients without IVC filters. Rates of venous thromboembolism were higher in the IVC filter patients, and a large proportion of the other complications among IVC filter patients were device related.

Our current study of IVC filters was prompted by an FDA advisory report regarding complications in patients receiving IVC filters.[2] The FDA's report was in turn prompted by a study indicating a high prevalence of strut fracture and embolization among 80 patients who received a certain type of retrievable IVC filter.[4] The FDA conveyed receiving 921 adverse events reports involving IVC filters between 2005 and 2009. Thirty‐six percent of these reports involved migration of the device, 16% were related to breakage and embolization of parts of the device, and 8% involved perforation of the IVC.

Research on the safety and efficacy of IVC filters in bariatric surgery patients has largely been limited to small, single‐center, case series or cohort studies.[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] A systematic review of this literature concluded that the evidence was insufficient to recommend IVC filters for patients undergoing bariatric surgery.[16] A 2010 study by our group was the largest and only multicenter study of IVC filters in the bariatric surgery population. We found no benefit of IVC filters in a comparison of 542 gastric bypass patients with prophylactic IVC filters to 5,834 gastric bypass patients without prophylactic IVC filters.[3]

When interpreting the results of this study, a number of limitations should be considered. Our study was observational, so there is the potential for unmeasured confounding variables to have influenced our results. To minimize the risk of confounding, we used propensity scores to match IVC filter patients to comparably high‐risk control patients, resulting in study cohorts that were well balanced on all baseline variables. Although this method accounts for confounding on the variables for which there are data, there is still the possibility that an unknown confounder could affect our findings. For example, our clinical registry lacks data on hypercoagulable states, so it is possible that a higher proportion of IVC filter patients could have had this risk factor and therefore a higher baseline risk of venous thromboembolism. However, most patients with a hypercoagulable state would have had a prior history of venous thromboembolism, which is a variable included in our database that patients were matched on.

The effects of changes in clinical care occurring during the time frame of this study should be considered in interpreting our findings. For example, bariatric surgery has been getting safer in general over time. Rates of death have fallen both in Michigan and in the rest of the country as bariatric surgeons have gained experience with this procedure. In Michigan during this time period, our group has developed and implemented a risk‐stratified, standardized approach to venous thromboembolism prophylaxis for patients undergoing bariatric surgery. For these reasons, we included the year of the procedure and the type of medical venous thromboembolism prophylaxis (unfractionated or low molecular weight heparin) used perioperatively as a matching variable in our analysis.

Another limitation that should be considered in interpreting our findings is statistical power. Although our study is the largest in this study population to date, many of the outcomes of interest are relatively rare. Considering the entire bariatric surgery population, rates of venous thromboembolism and death within 30 days are each less than 1%. Even in the high‐risk patients included in this analysis, there were a total of just 28 (1.3%) venous thromboembolism events and 8 (0.37) deaths. Nonetheless, our study did find significantly greater risks of multiple types of complications among patients receiving IVC filters.

Finally, our study captures events occurring within 30 days of bariatric surgery. Complications, including venous thromboembolism and other complications directly related to IVC filters, frequently occur after 30 days of bariatric surgery. Therefore, our study may be a conservative estimate of the risks associated with the use of IVC filters in bariatric surgery patients. Furthermore, certain brands of filters have been shown to be associated with higher risks of complications. Our study lacks data on the brand of IVC filter used and so cannot assess the extent to which this would affect our results.

CONCLUSIONS

In conclusion, our study indicates that IVC filters do not reduce the risk of pulmonary embolism in high‐risk bariatric surgery patients. They are also associated with other complications attributable to malfunctions of the device itself. We believe that the use of IVC filters among bariatric surgery patients should be discouraged.

Disclosure

This study was supported by a grant from the Agency for Healthcare Research and Quality (HS018050) and was presented at the Annual Meeting of the American Society for Metabolic and Bariatric Surgery (ASMBS), San Diego, California, June 20, 2012.