News from AAOS: BMI Associated with Higher Complication Rate in THA, TKA

A study of 13,250 patients undergoing total hip or knee arthroplasty (THA/TKA) showed that patients with a higher body mass index (BMI) had higher overall complication rates compared with patients of normal weight (P = 0.0006).

The study, presented last week at the American Academy of Orthopaedic Surgeons (AAOS)annual meeting by Hasham M. Alvi, M.D., of Northwestern University, also found a correlation between elevated BMI and the surgical complication rate, including superficial wound infection and deep infection, return to operating room, and total length of hospital stay.

In subgroup analysis, elevated BMI was associated with increased risk of superficial wound infection but not with deep wound infection following TKA. The converse was true following THA. Pairwise analysis demonstrated that the risk for surgical complications following THA, including superficial and deep infection, was most profound in patients with a BMI higher than 40. The correlations all were statistically significant.

There was no association between increasing BMI and medical complication rate, readmission rate, or 30-day mortality rate in the overall analysis, which was derived from the American College of Surgeons National Surgical Quality Improvement Program. The program collects preoperative, intraoperative, and 30-day postoperative outcomes for patients who undergo major surgical procedures.

BMI was divided into the following five categories, per the World Health Organization’s guidelines:

• healthy weight—18.5 to < 25
• overweight—25 to < 30
• obese class I—30 to < 35
• obese class II—35 to < 40
• obese class III—≥ 40

Since BMI is highly associated with multiple comorbidities that may also affect outcomes, investigators created matched quintuplets (quints) for age (younger than age 75 or 75 years and older), gender, procedure type (THA or TKA), and American Society of Anesthesiologists Physical Status (ASA) classification (1 or 2, 3, and 4 or 5).

“We thought that this classification would adequately control for overall health status of the patient and would be more effective than matching for specific comorbidities. For example, diabetes has an innate variability (mild or severe; controlled or not), which the ASA classification takes into account,” Alvi noted.

Alvi said the findings have important clinical implications. “Specifically, via adjusted pair-wise comparison, it is clear that BMI of 40 or greater is an independent risk factor for medical and surgical complications following THA and TKA compared to normal-weight individuals,” he said.

Impact greater with TKA
Based on sub-group analysis, the effect of BMI on risk of postoperative complication is greater in the TKA population compared to the THA population. With respect to the specific complication of infection, a BMI of 40 or higher was significantly linked to superficial infection following TKA and deep infection following hip arthroplasty.

Alvi noted that the incidence of postoperative complication following THA and TKA is relatively low, and so investigation of the effect of a single risk factor benefits from the use of a large data set. Previous reviews of similar but significantly smaller datasets showed either no relationship between BMI and complication rates or very limited association between BMI and superficial surgical site infection only. In this investigation, a longer period, larger sample side, and more data “improved our ability to substantiate the significant relationships between BMI class and postoperative complications.”

In addition to using a large sample from a previously validated high-quality data set, the study methodology of creating and comparing matched BMI class quints allowed for greater statistical control of covariables that have previously been shown to be linked to complication risk, Alvi said. Results generated by comparing matched BMI quints were further controlled by adjusting for ASA class, age, sex, race, procedure, smoking, preoperative steroid use, hypertension medication, history of chronic obstructive pulmonary disease, preoperative test results, and anesthesia type.

“Our adjusted results represent refined observations of the effect of BMI on risk for postoperative complication following THA and TKA,” Alvi said.

Study limitations
Although the use of the large ACS-NSQIP data set confers advantages, Alvi noted, it is not a randomized sampling. However, the data include a broad spectrum of institutions. He also pointed to the short follow-up period as another limitation because periprosthetic joint infections may develop after 30 days occur within that period.

“Although THA and TKA patients who are obese have been shown to gain functional benefits similar in magnitude to those seen in normal-weight patients, it is important to note that the risk of postoperative complications increases with elevated BMI, most specifically, for patients with a BMI of 40 or greater. Obese patients should be appropriately counseled regarding the risk of complication following joint replacement surgery, and obesity potentially considered a modifiable risk factor that should be optimized prior to any surgical intervention,” Alvi concluded.