Don’t Be Scared by (or of) Statistics

A study came out recently showing that having a joint replaced increases the risk of having a heart attack by at least a factor of three in the month after surgery. Gah! What’s a person to do, then, if you’re barely able to walk because your knee or your hip is causing you so much pain? The first thing to do is look at the actual numbers behind those statistics.

How Large Is Large?

Researchers in Great Britain reviewed records of patients through the Health Improvement Network, which captures information about more than 10 million people. They found that during the period 2000–2012, nearly 14,000 people in that database had a total knee replacement, and more than 6,000 had a total hip replacement. When compared to similar patients who had not had a joint replaced, those who had a new knee were more than 8 times as likely to have a heart attack in the following month, and those with a new hip were more than 4 times as likely to experience a heart attack.

Those figures sound dreadful, but the raw numbers were so low that the change in absolute risk is negligible. And over the longer term, the risk of heart attack evened out between the two groups. In the 4 years following surgery, there were 306 heart attacks in the replacement knee group and 286 in the born-with knee group. For the hip folks the numbers were 128 and 138 (yes, fewer in the replacement group). For the knee group those 20 extra heart attacks sent the total risk zooming from 2.0% without replacement all the way up to 2.2% with joint replacement. Hardly a cause for excitement. And in a group that was older (average age 71) and heavier (BMI of 28—not obese but still overweight) to begin with, it’s not surprising that there were heart attacks.

What, Me Worry?

Other recent studies had shown a reduced risk of heart attack long-term in those who have had a joint replaced. Research in Toronto showed that over a followup period averaging 7 years, joint replacement reduced a person’s risk of heart attack, stroke, or other cardiovascular events by about 12%. Other research, conducted in Taiwan, showed a 7% reduction in CV risk over 5 years.

There could be many reasons for this health improvement. It could come from a change in diet after surgery, or less for NSAID pain medication, or increased physical activity. Regardless, the benefit is there.

The press release from the publisher of this current study was titled, “Knee and Hip Replacements May Be Bad for the Heart.” This seems like a title aimed at getting noticed rather than one that tells the truth. Now that you’ve seen how statistics get cherry-picked to make a point, you can be a smarter consumer of information.