People who earn less than $35,000 a year report less pain and better function at their two year check up following a total knee replacement than those with higher salaries, according to new research presented recently at the 2012 annual meeting of the American College of Rheumatology.

Researchers say the results were unexpected, as lower incomes typically translate to reduced access to care and a higher number of comorbidities (other diseases and conditions) that can complicate treatment.

“[Lower] income and socioeconomic status are associated with worse health outcomes in other conditions, so that made us think they might not have done as well, or worse, after joint replacement but paradoxically they did better. So that was surprising,” explains lead author Jasvinder Singh, MD, associate professor of medicine and epidemiology at the University of Alabama at Birmingham.

Researchers analyzed data from the Mayo Clinic Total Joint Registry, including 7,139 patients seen at a two-year check up following a total knee replacement (TKR) and 4,234 knee replacement patients seen five years after their surgery.

Patients in the two lowest income categories – those making annual salaries of less than $35,000 and those making $35,000 to $45,000 – were up to 40 percent less likely to have moderate to severe pain in the replaced knee after surgery, compared with those making more than $45,000 a year. Similarly, patients making less than $35,000 were also twice as likely to rate their overall improvement in knee function as “better” than those making more than $45,000. 

The study’s researchers say this could be because low-income patients tend to wait longer to have the surgery than those who can easily afford it – possibly due to financial challenges or a lack of education about the surgery and its benefits. Waiting longer often results in worsening knee pain and more limited function, so when those patients finally get the surgery, they may notice a bigger improvement than those who get the surgery earlier, Dr. Singh suggests.

This study only shows an association between income and reported satisfaction with TKR, so Dr. Singh says more research is needed to verify and explain this finding and to determine if other factors are at play. “Do they have more social support? Do they have better expectations or are there other factors having to do with their perception with healthcare or relationship with friends and family?” Dr. Singh asks. “Those are things that need to be investigated.”

David Pisetsky, MD, PhD, a professor of medicine and immunology at Duke University School of Medicine in Durham, N.C., says this study is interesting because it offers doctors another piece of information to better understand the outcomes of this common surgery.

“It’s probably difficult to know exactly what the origin of this difference was. There are probably multiple determinants of outcome after joint replacements that we should think about more. If socioeconomic status is a determinant, we should figure out why,” says Dr. Pisetsky, noting that recommendations can then be better tailored to patients.

He adds that timing of the surgery is always critical and this [study] helps doctors on that front, too. “For some, waiting can be a real problem in terms of life quality. But if you can get by and you have a reason to wait, your outcome may not be much worse,” Dr. Pisetsky says.