High BMIs linked to worse total knee replacement outcomes

Obesity has been identified as a contributing factor for dozens of other serious health conditions. According to the U.S. Centers for Disease Control and Prevention, obesity increases the risk of heart disease, Type 2 diabetes and stroke. Obesity is also a major factor in the development of joint conditions such as arthritis – with more weight placed on the hips and knees, cartilage is more likely to degrade faster than in people of healthy weights.

According to a recent study from researchers at the Mayo Clinic's College of Medicine and published in the Journal of Bone and Joint Surgery, high body mass indices are directly linked to longer hospital stays and worse patient outcomes after total knee replacement surgeries. While obese patients had more damage to their natural knee joints that had to be addressed, the presence of co-occurring disorders and their requisite treatment accounted for the majority of the longer stays and worse outcomes.

Examining the data
According to the CDC, 719,000 patients undergo surgery for a total knee replacement every year. While some of them need the procedure to address disorders like arthritis, many have degraded joints due to obesity. 

The Mayo Clinic study found that high BMIs may be a reliable indicator of patient outcomes in TKR surgeries.To reach their findings, the researchers reviewed the medical records of 8,129 patients who had undergone some form of TKR. The majority – 6,475 patients – had primary TKRs, while 1,654 procedures were revisions and follow-ups. All procedures occurred between Jan. 1, 2000, and Sept. 30, 2008.

Of the patients whose medical records were reviewed as part of the study, the median age was 68, and 57 percent were female. The average BMI of patients was 31.6, which slightly surpasses 30, the lower limit for obesity.

The researchers found that every 5-unit increase in a patient's BMI beyond the overweight range resulted in higher hospitalization costs and admission periods compared to patients of healthy BMIs. With each incremental step in a patient's BMI, costs rose $250 to $300 and hospitals stays increased by 0.11 days for primary TKRs, and $600 to $650 and 0.6 days for revisions.

While these numbers may not seem significant, one-third of Americans are categorized as obese. Though that classification normally starts when a person's BMI rises above 30, there is no upper limit to the category. This means that patients may have substantially higher costs and longer hospital stays than people in the healthy or overweight ranges.

Trimming down
The study also found that the increased costs of TKRs did not disappear after adjusting for comorbidities. In fact, the presence of conditions like diabetes and hypertension directly caused by obesity was the major factor in these higher costs.

The study did not find any correlation between longer hospital stays and co-occurring disorders.

David Lewallen, M.D., an orthopedic surgeon at Mayo Clinic and one of the study's authors, warned patients that even though TKR is a relatively low-impact surgery, obesity can only serve to introduce more complications to the process.

"The bottom line is that obesity is increasingly common among patients undergoing joint replacement, which creates a myriad of technical and medical challenges, and likely contributes to the financial burden of the surgery," Lewallen said.

As obesity rates continue to rise across the U.S., the number of TKRs is likely to surge as well. Though orthopedic surgeons have made great strides in their surgical techniques, they may be powerless in the face of higher obesity rates and co-occurring disorders during surgeries.