Background
Total hip arthroplasty (THA) has been called the operation of the century. Primary THA is the second most common joint replacement surgery performed in the U.S., associated with a significant improvement in patient outcomes. The utilization of primary THA is rapidly increasing and 478,000 primary THA procedures were performed in the U.S. in 2009. Recent studies have described the time-trends in THA utilization, outcomes and select patient characteristics (age, obesity). On the other hand, time-trends in important characteristics of patients undergoing THA, including body mass index (BMI), medical/psychological comorbidity etc. associated with THA outcomes, have not been examined. Most studies were done in non-U.S. settings, except three key studies using data from the Medicare, the National Inpatient sample (NIS) and the National Hospital Discharge Survey (NHDS). Several knowledge gaps exist in this area, as summarized in the following section.
In Medicare recipients 65 years and older, the number of comorbidities increased from 1.0 to 2.0 and obesity rates increased from 2.2% to 7.6% from 1991 to 2008. Representative studies of the U.S. NIS from 1998 to 2008 and U.S. NHDS data 1990 to 2004, that included all age groups, showed that among patients undergoing THA, age decreased by 2.1 years, Deyo comorbidity index increased from 0.48 to 0.65 and obesity increased from 2.2% to 3.8%. Data from these Medicare, NIS and NHDS studies are limited in several important aspects: (1) obesity, linked to THA outcomes, was captured with a diagnostic code, which is associated with significant under-coding compared to when using the weight and height data, as evident in low obesity rates <5% in these studies vs. U.S. obesity rates of 37% in 2009, proving that the use of ICD-9 code for obesity leads to high misclassification rate; and (2) anxiety, depression and underlying diagnosis, important predictors of THA outcomes, were not assessed. Data from Medicare study are additionally limited in that they are not representative, since at least 1/3rd of all THAs are performed in those younger than 65 years, the fastest growing age group for the receipt of THA over time. A recent study in TKA cohort found that rates of extreme obesity, Deyo-Charlson index score > =3, depression and anxiety increased 2–3 fold from 1993–2005. To our knowledge, there is lack of such data for time-trends in BMI, anxiety, depression and underlying diagnoses in patients undergoing THA in the U.S.
One of the most remarkable changes in the epidemiology of primary THA in the last two decades is the expanding indication of THA to both younger and older patients. Studies have not examined whether the time-trends in important clinical characteristics (BMI, underlying diagnosis and medical and psychiatric comorbidity) vary by age, the most rapidly changing characteristic of patients undergoing primary THA. Knowing how the patient complexity has changed over time is critically important, since these patient characteristics can have a significant impact on THA outcomes. Our objectives were to: (1) examine the time trends in BMI, underlying diagnosis and medical and psychiatric comorbidity in patients undergoing primary THA; and (2) assess whether these time-trends are different in various patient age groups.