This study has provided a large-scale and health-economic analysis of postoperative delirium after TKA. From the year 2005 to 2008, the incidence of postoperative delirium was increasing nearly from 1.12–1.21%. Then, the incidence of postoperative delirium decreased annually to 0.71% in 2014 (Fig. 1). Interestingly, this trend had never been reported in previous studies. Although there was no change of the definition of delirium over this decade, according to ICD-9-CM, the diagnosis of delirium may vary among institutions . One possible explanation for this trend may be that the number of TKA performed was increased with aging of population, however, the lack of recognition and medical interventions led to a higher incidence of postoperative delirium. Then postoperative delirium received more and more attention and this trend was reversed after 2008. We identified an overall incidence of 1.00% after TKA procedures, which is much lower compared with most of the previous studies ranging from 3.1–25% [1, 8–13], with the exception of an Asian study by Huang et al. reporting the incidence of 0.59% . There are two possible reasons accounting for the former obvious difference. First, most of previous literatures observed small-scale and selected senior patients, resulting in an over-reported incidence. Second, the diagnostic accuracy varied between institutions may also contributed to the difference [16, 29]. Huang et al. observed the Asian population with a lower incidence while our study observed a diverse race mainly in the Whites indicating a racial difference in the occurrence of postoperative delirium. It was also found that patients with delirium were less likely in Asian or Pacific Islander compared with those without delirium (P < 0.0001) (Table 2). Interestingly, another significant difference of race distribution between the two groups was found, the Whites occupying a larger proportion in the postoperative delirium group. This is consistent with the previous report that the Whites undergoing general or orthopedic surgery were more likely to develop postoperative delirium . Additionally, in logistic regression analysis, the Hispanics was a protective factor for postoperative delirium in comparison with the Whites, which was never been found before. However, very few studies have focused on racial difference in postoperative delirium.
Regarding to another demographic characteristic, patients with postoperative delirium were significantly 9 years older than those without. Besides, in terms of age distribution, as observed in clinical practice, elderly patients take a greater proportion in the postoperative delirium group. Further, in logistic regression analysis, older than 71 yrs. was identified as an independent risk factor of postoperative delirium (Table 4). This was highly consistent with previous studies which had identified advanced age as a common independent predictor of postoperative delirium [1, 4, 9, 11–13, 21–25].
The CCI score of patients with postoperative delirium was significantly higher. This is reasonable as higher CCI score means relatively worse healthy condition before surgery, and may increase postoperative complications including delirium. Postoperative delirium has been reported to increase hospitalization duration, medical cost, and mortality [1, 4, 9, 11–14]. Similar findings were observed in our study (Table 2). With the presence of postoperative delirium, the average LOS was 1 day longer and the total hospital charges was $5150 more per admission. This may be due to that patients with postoperative delirium can not follow instructions of nursing and rehabilitation [16, 31]. Another explanation is that, postoperative delirium may be associated with medical perioperative complications, including acute renal failure, acute myocardial infarction, pneumonia, pulmonary embolism, stroke, and urinary tract infection (Table 3), which commonly delay discharge and prolong hospital stay [18, 26]. Consistently, patients with delirium were more commonly pay the charges through the medicare than those without delirium. Further, the medicaid, the private insurance, and the other type of insurance were protective factors for postoperative delirium in logistic regression analysis, illustrating that the medicare played a major role among the type of insurance. Thus, the in-hospital mortality of patients affected by delirium was more than three times to those unaffected.
Several studies on postoperative delirium after total joint arthroplasty suggested that pre-screening, risk stratification and appropriate management is essential to improve outcomes [1, 4, 11–13, 21]. Consequently, in order to prevent postoperative delirium, it is critical to understand the risk factors before surgery. Logistic regression was applied and the results were consistent with previous publications [1, 4, 9, 11–13, 21–25]. As expected, neurological disorders before surgery introduced the highest odds ratio (OR, 8.35) for delirium (Table 4). Age greater than 71 years had a considerably high OR (3.07). Patients with a history of the other neuropsychiatric disorders such as drug abuse (OR = 1.90), psychosis (OR = 2.18), alcohol abuse (OR = 2.79) and depression (OR = 1.43) were at increased risk of postoperative delirium [1, 4]. A diagnoses of either chronic deficiency (e.g., iron, Vitamin B12) anemia (OR = 1.36) or chronic blood loss anemia (OR = 1.54) were independent risk factors, confirming previous findings that patients with perioperative anemia were more likely to experience delirium [4, 16, 23, 32–34]. Other comorbidities such as fluid and electrolyte disorders (OR = 2.41), congestive heart failure (OR = 1.59), coagulopathy (OR = 1.53), renal failure(OR = 1.36), peripheral vascular disorders (OR = 1.29), uncomplicated diabetes (OR = 1.20) and diabetes with chronic complications (OR = 1.87) had also been previously reported as risk factors of delirium [4, 14, 22, 23]. Furthermore, to the best of the author's knowledge, for the first time chronic pulmonary disease (OR = 1.14), weight loss (OR = 2.02), pulmonary circulation disorders (OR = 1.48), and teaching hospital (OR = 1.13), were identified as independent risk factors of postoperative delirium. Interestingly, female (OR = 0.79), obesity (OR = 0.85), and urban hospital (OR = 0.87) were found to be protective factors. Additionally, female patients with delirium occupied smaller proportion indicating that whether female sex hormone has an effect on postoperative delirium will need further study.
Several limitations exist in utilizing the NIS database. First, information of each patient is only recorded before discharge, meaning any complication that occurs after discharge will not be included in the NIS database. This limitation might contribute to the lower incidence of postoperative delirium as only early period medical records were analyzed. Second, only risk factors recorded in the NIS database could be analyzed. There are other known risk factors that were not available in the NIS database, such as a history of dementia, type of anaesthesia, commonly used perioperative medications (opioids, benzodiazepines, and ketamine), sedation during anesthesia recovery, vision impairment, functional impairment, and so on [4, 16, 23, 24, 35]. Further more, as a retrospective database analysis, the results obtained need clarification to confirm their etiology.