NEW YORK (CBS 2) — There are nearly half a million knee and hip replacements done every year in this country. They’re very effective for eliminating pain from arthritis and restoring mobility to the patient.

As CBS 2’s Dr. Max reports, that joint replacement may also be a weapon in the fight against obesity.

It stands to reason that if your hips or knees are so painful and arthritic that even walking is difficult, then you’re not going to be burning many calories and weight gain is almost inevitable. But the correlation between joint replacement and weight loss has been notoriously hard to show, until now.

Garson Goldstein is a pretty trim and fit man these days. The 62-year-old had always been active, playing hockey well into his fifties. But then his hips started to go.

“It was just terrible. The pain was terrible. I couldnt walk,” the hip replacement patient said. “Sitting was impossible. Sitting in a car was impossible. I was miserable.”

The pain made Garson so sedentary that his weight ballooned up to 200 pounds. The combination made him pretty unhappy.

“I look at pictures of myself during the period before my surgery and I never smiled,” he said.

The surgery Garson had was a double hip replacement. Like most such operations, it stopped the pain and enabled him to become active again and lose weight.

Makes sense.

Trouble is most studies that have looked at weight loss after joint replacement surgery have not found that these patients lose weight. That’s because previous studies failed to take into account that most people gain weight as they grow older.

When that correction was factored in, as Dr.  Michael Bronson of Mt. Sinai Medical Center explains, “20-percent of the patients had an absolute loss of weight after hip and knee replacement and then when you factored in what they would be if you factored in their age differential and what they would have been, 60-percent of the patients had a loss of weight.”

And the fact that patients can now move, even if just a little, brings other benefits. “You’re making them heart healthier, you’re making their pulmonary function healthier, you’re making their circulatory function healthier, so it’s a win-win situation for these patients,” Bronson said.

After his hip replacements, Garson lifts weights, bikes, and swims, all pain free.

And just as important, “my weight is down. This morning I weighed 184 and I’m very proud of that,” Garson said.

And it’s not just weight. Physical activity is good for the heart, lungs, prevents diabetes and even cancer. So joint replacement can have a beneficial effect across the board.

But Garson is a pretty young man. Won’t he have to have those hips done again in ten years or so, especially if he stays active like that?

Actually no. That’s a common misconception.

Modern hip and knee replacements with better materials and surgical techniques now routinely last 20, 25, 30 years. So chances are that Garson’s good for life.

And these days, doctors say you can do almost anything with your new joints: play tennis, run, bike, and hike.

You might want to avoid tackle football or pro wrestling, but everything else? Go for it.

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