BMI has been increasing almost linearly for decades in the United States. In 1960, only 14% of men, 6% of women nationwide were obese (BMI > 30). In 2013, the rate of obesity in every state is over 20%, the vast majority of states are over 30%, and West Virginia and Alabama have rates over 35% . There is no sign of a plateau.
The trend toward obesity in TJA is similarly impressive. Obesity (BMI >30) and Morbid Obesity (BMI>40) in the TJA population climbing more steeply than in the general population  . This is particularly notable in TKA, which shows closer correlation to weight.
Recognizing that obesity is becoming more prevalent, it is important to understand its impact on TJA outcomes. While obesity appears to be an independent risk factor for adverse events, the correlation is complicated by its close association with many comorbidities.
Obesity has been studied widely and yet there is no consensus about what BMI cut off confers increased risk for adverse events. Its exceedingly rare in medicine for complications to neatly abide by a single numerical cut off (“You will have increased wound complications only at BMI >40”), while more commonly, complication risk occurs within a spectrum, where increasing risk follows increasing deviation from “normal”.
The summation of studies were used to formulate guidelines for managing obese patients by AAHKS . It stated that BMI > 40 showed a high correlation with increased complications, particularly surgical site infections, while all obese patients (BMI> 30) likely have some increased risk and should be counseled preoperatively about this increased risk. A study has proposed thinking of BMI as a continuous variable for complication risk (ie dose response) rather than a threshold model (risk only occurs over 30 or 35 or 40) which appears more in line with the general consensus that BMI is correlated to increased complication risk .
▪ Wound Complications. Significant increase from <1% in non-obese to over 4% in morbid obese.
-THA: [9, 10] 2x increased risk at BMI of 35 .
▪ Length of stay. Increased in both TKA and THA in obese. 
▪ Cardiopulmonary, GI, and dermatologic, and renal complications may only be statistically significantly higher in BMI > 40.  BMI > 35 confers risk for unplanned ICU admission.  
-THA: Some found increased loosening.   but appears less significant than TKA, and recent studies have found no strong correlation at any BMI .
▪ Patient satisfaction.
-THA: Despite increased risk for complications, obesity does not affect satisfaction and obese patients show similar improvements in WOMAC, HHS, SF-36 and this must be remembered. [18, 19]. Others however, report worse outcomes at 1 and 5 years .
What can be done?
The biggest challenge, once identifying the independent risk of obesity, lies in modifying this risk. It remains a question whether obesity is truly a modifiable risk factor. Obesity peaks in the 7th decade, when people have become ingrained in a certain lifestyle, activity has slowed down, and the ability to make dramatic changes is incredibly difficult. There isn’t great evidence in TJA to suggest that patients can lose significant weight through dieting. One study suggests that only 12% of patients lost at least 5% weight (that would be 10 lbs in a 200 lbs person) within a year before TJA surgery (while 80% remained unchanged, and 8% gained weight) . The risk for surgical site infections unfortunately did not decrease in the patients that were able to lose weight, which reflects that modest weight loss (5%) is insufficient to modify surgical risk. A study of weight loss not specific to TJA patients showed that exercise produced 2.4% weight loss, diet produced 8.5% weight loss, and a combined routine produced 10.8% .
Patients may argue that inability to lose weight is related to knee or hip pain preventing exercise. However, Kahn et al showed that improved pain and quality of life after TKA did not translate into postop weight loss, nor substantial increased activity .
Bariatric Surgery. Because weight loss programs appear to have marginal benefit at best before TJA procedures, surgeons have examined more dramatic techniques, such as bariatric surgery, to achieve lower BMI at the date of surgery. Bariatric surgery is shown to effectively promote weight loss and improve glucose control .
However, the clinical effect on complication risk after TKA is unclear. While early studies  showed that bariatric surgery helped improve outcome measures, and reduce complications, other studies suggest less promising results, with increased complications despite weight loss . The time interval between bariatric surgery and TKA is important because its been suggested that patients enter a catabolic state for 2 years following bariatric surgery, which possibly introduces the risk of poor nutrition status, poor tissue quality and thus increased wound healing complications.
Concerning still is a recent study that showed patients with BMI >40 experienced fewer complications than patients that were previously morbidly obese but were corrected to a BMI < 40 after bariatric surgery .
Many orthopedic surgeons continue to recommend bariatric surgery because many studies have shown that patients become healthier overall after the procedure, with tighter glucose control, improved cholesterol and other health factors. Furthermore, weight loss alone addresses the reports of increased risk for aseptic loosening and malalignment secondary to the obesity. Surgeons believe that as long as bariatric surgery does not show convincing inferior outcomes, there is little harm and notable benefit to the surgery.
However, the effect of intensive weight loss therapy with nutritionist consultation is something that requires further study, and remains a great hope. This provides both the requisite weight loss (a specific numerical target is again not available) without the concerns of malnutrition or poor soft tissue.
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