Demographics of UTKA and BTKA discharges
We identified a total of 670,305 admissions between 1998 and 2006 during which a TKA procedure was performed. This represented a weighted national estimate of 3,270,836 hospitalizations. Of those 6.52% had bilaterally performed procedures. The average age was 67.46 (CI=[67.43, 67.49]) years for admissions undergoing UTKA and 66.14 (CI=[66.05, 66.23]) years for BTKA procedures (P<0.0001).
contains information on patient and health care system related demographic variables. Length of hospital stay was significantly longer for BTKA compared to UTKA recipients (4.71 days (CI=[4.68, 4.74]) vs. 3.99 (CI=[3.98, 4.00]) (P<0.0001). Comorbidities under study tended to be more prevalent among UTKA than BTKA recipients, except for obesity, cardiac valvular and pulmonary circulatory disease (). The average comorbidity index among admissions for BTKA recipients was significantly lower compared to those for UTKA (0.48(CI=[0.47, 0.49]) versus 0.55(CI=[0.54, 0.56]) (P<0.0001)).
Depicted is the prevalence of selected comorbidities among uni- and bilateral total knee arthroplasty discharges. BTKA=Bilateral Total Knee Arthroplasty; CNS= Central Nervous System; UTKA=Unilateral Total Knee Arthroplasty
Outcomes after UTKA and BTKAs
Complications considered procedure related were more frequent among BTKA versus UTKA recipients (9.45% vs. 7.07% (P<0.0001)) (). The incidence of pulmonary embolism (0.82% vs. 0.39%), venous thrombosis (1.21% vs. 0.72%), and Adult Respiratory Distress Syndrome (0.48% vs. 0.25%) was also increased among BTKA patients compared to UTKA recipients (P<0.0001). Acute posthemorrhagic anemia was coded at about double the rate among BTKA versus UTKA procedures (27.02% vs. 14.52% (P<0.0001)). In-hospital mortality was higher among BTKA compared to UTKA recipients (0.30% vs. 0.14% (P<0.0001). The average age of fatalities after UTKA and BTKA was similar (74.37 (CI=[73.74, 75.01]) vs. 73.07 (CI=[71.54, 74.60]) years, (P=0.1144). Mortalities after BTKA occurred sooner after admission to the hospital than after UTKA procedures (6.86 (CI=[5.20, 8.53]) days versus 8.41 (CI=[7.55, 9.27]) days, but this difference was not significant (P=0.1184).
Procedure Related Complications Among Uni- and Bilateral Total Knee Arthroplasty Discharges
Staged versus simultaneous BTKAs during the same hospitalization
22.33% (9688) of all entries for BTKA did not allow for determination of timing of one or both procedures and were therefore excluded from the sub-group analysis. To make sure that the observed data could sufficiently represent the target patient population, we conducted a sensitivity analysis and found that our results are reliable and robust in so far as 1) the observed BTKA and the missing BTKA followed similar distributions in patient demographics, and 2) in the extreme case when all missing BTKAs were considered as simultaneous BTKAs or staged BTKAs, the risk factors for mortality and any procedure related complications found by logistic regression retained their significance.
Of the BTKA discharges included, 74.8% were performed simultaneously, while the remainder was performed on separate days of the hospital admission. The average time between staged procedures was 3.59 (CI=[3.39, 3.79]) days. The average age of discharges associated with staged procedures was 66.18 (CI=[65.96, 66.39]) years and 66.00 (CI=[65.88, 66.12]) with simultaneous BTKA (P=0.1567). There was no difference in the overall comorbidity severity between the simultaneous versus staged group (comorbidity index 0.48 (CI=[0.47, 0.49]) for simultaneous and 0.49 (CI=[0.48, 0.51]) for staged BTKA, respectively (P=0.3332)). Length of stay was longer after staged BTKA compared to simultaneous procedures (5.37 (CI=[5.30, 5.43]) days vs. 4.52 (CI=[4.48, 4.56]) days, respectively (P<0.0001)).
Generally, procedure related complications were more frequently encountered in discharges associated with staged compared to simultaneous BTKA (10.30% vs. 9.15%, respectively (P<0.0001)). details the incidence of specific procedure related complications. For all categories, staged procedures had either a higher or similar incidence of complications.
Adverse events such as Adult Respiratory Distress Syndrome, and posthemorrhagic anemia occurred at higher rates after staged procedures compared to simultaneous BTKA (0.62% vs. 0.40% (P<0.0001) and 29.61% vs. 25.17% (P<0.0001), respectively). Venous thrombosis and pulmonary embolism occurred more frequently among simultaneous procedure recipients (1.48% vs. 1.22% (P=0.0002) and 0.89% vs. 0.77% (P=0.0218), respectively).
No statistical difference in the rates of in-hospital mortality was seen between either BTKA approach (0.29% for simultaneous and 0.26% for staged BTKA, respectively (P=0.2875)).
Risk Factors for Perioperative Morbidity and Mortality after TKA
An over proportional number of deaths occurred among BTKA recipients (13.42% of total mortalities) compared to the prevalence (6.51%) of BTKA among the study sample. Multivariate regression revealed a number of independent risk factors for mortality after TKA. Patient related factors that significantly increased the risk for perioperative mortality were male gender (OR 2.02 CI=[1.75, 2.34], P<0.0001), and age (age above 75: OR 3.96 CI=[2.77, 5.66], P<0.0001; age between 65 and 75: OR=1.69 CI=[1.19, 2.40], P=0.0032 when compared to those aged 45–65 years). Entries for simultaneous (OR 2.23, CI=[1.69, 2.94], P<0.0001) and staged (OR 2.09 CI=[1.28; 3.41], P=0.0031) BTKAs had a significantly increased odds of perioperative mortality when compared to UTKAs. Health care system related factors associated with increased risk for mortality included only hospital size. Surgeries undertaken in large sized and medium sized hospitals were associated with higher odds of perioperative mortality (large: OR 1.48, CI=[1.16, 1.89], P=0.0015; medium: OR 1.44, CI=[1.10, 1.87], P=0.0075 when compared to small hospital size). No other patient demographic and health care system related factors (hospital location, hospital teaching status, type of insurance, and race) were significantly associated with altered risk of mortality.
The estimate of the impact of overall comorbidity burden on mortality was obtained by logistic regression Model 1 (). We found that for every unit increase comorbidity index, the odds of perioperative mortality increased by 13.6% (OR 1.136, CI=[1.055, 1.223], P=0.0007). A number of comorbidities detected by logistic regression Model 2 () increased the risk of a fatal outcome (), among which pulmonary circulatory disease was associated with the highest increase in the risk for perioperative mortality (OR 11.75, CI=[9.05, 15.25], P<0.0001). Interestingly, after controlling for covariates, the presence of obesity did not reveal to alter the odds of mortality after TKA.
When controlling for comorbidity severity and other patient and health care system related demographics, a number of procedure related complications and adverse events (logistic regression Model 3 ()) were associated with an increased risk for perioperative mortality (). Among admissions with the highest risk for mortality were those whose perioperative course was significant for complications affecting the central nervous (OR 22.77, CI=[14.28, 36.31], P<0.0001), cardiac (OR 14.19, CI=[11.25, 17.91], P<0.0001) and those suffering from shock (OR 15.10, CI=[3.88, 58.81], P<0.0001). Further, the occurrence of pulmonary embolism and Adult Respiratory Distress Syndrome increased the risk for mortality by 18- and 15-fold, respectively (OR 17.54 CI=[12.69, 24.22], P<0.0001 and OR 14.61 CI=[10.35, 20.63], P=<0.0001, respectively). The incidence of venous thrombotic events was not associated with a risk adjusted increase in perioperative mortality after TKA (OR 0.93, CI=[0.49, 1.79], P=0.8372).
Patient related factors that increased the risk for perioperative procedure related complications included: male gender (OR 1.41, CI=[1.38, 1.44], P<0.0001), older age (age>75 vs. age in 45–64: OR 1.40, CI=[1.34, 1.46], P<0.0001), and minority race (Black vs. White: OR 1.29, CI=[1.26, 1.39], P<0.0001; Other (excluding Black and Hispanic) vs. White: OR 1.23 CI=[1.15, 1.32], P<0.0001). Entries for simultaneous (OR 1.40, CI=[1.33, 1.47], P<0.0001) and staged (OR 1.66 CI=[1.52, 1.79], P<0.0001) BTKAs had a significantly increased odds of perioperative morbidity when compared to UTKAs; staged BTKAs had a significantly increased odds of perioperative morbidity compared to simultaneously performed BTKAs (OR 1.18, CI=[1.07, 1.30], P=0.0008).
Through the logistic regression Model 4 (), a set of comorbidities were determined that were associated with increased risk of a procedure related complications (), among which congestive heart failure (OR 2.01, CI=[1.91, 2.11], P<0.0001), pulmonary circulatory disease (OR 2.88, CI=[2.64, 3.14], P<0.0001), and electrolyte/fluid abnormalities (OR 2.43, CI=[2.35, 2.52], P<0.0001) were associated with the highest odds.
Multicollinearity was found absent for all variables (value inflation factor in the range of 1.01–1.76 and condition index in the range of 21.09). Lower Akaike’s information criterion scores were found for all full models (). The c-statistic values on both the training dataset and the validation dataset were estimated to be in the range 0.7– 0.8 indicating acceptable discrimination. No significant differences were found between the predicted and observed probabilities of death through Model 1 in both datasets and Models 2, 3 and 4 in the validation dataset for the H–L test. The low p-values for the H–L test for Models 2, 3 and 4 on the training dataset might have indicated that these two models are not well calibrated. However, the H–L test is known to not perform well with large sample sizes such as ours, and thus we are not deeming our model suspect of bad calibration.