More than 800,000 joint replacement procedures are done in the United States every year.

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SAN FRANCISCO — Lynn Rhynes was in his early 50s when his body started falling apart, seemingly all at once. His knees ached and his shoulders were stiff and sore — he had problems lifting his arms over his head to brush his hair.

He had arthritis and he was desperate for relief. He tried supplements, and steroid shots into his deteriorating joints. When he started crying from the pain while trying to remodel the kitchen of his San Jose home, he decided he’d had enough.

Over the course of four years, Rhynes, now 68, had both shoulder joints replaced, along with his left knee. He thinks the right knee will need to be replaced eventually.

“You reach a point where you’re willing to do anything,” said Rhynes, who said he’s never had misgivings about the joint replacements. “I can lift and hold my grandkids now. I can reach up and change a light bulb and scratch my head. It’s incredible.”

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As baby boomers age and older people want to retain the active lifestyles they’ve grown accustomed to, cases like Rhynes’ are becoming more common. It’s been years since joint replacements were only for the elderly with broken hips and walkers. Today, people in their 40s and 50s are getting new knees, hips and shoulders.

More than 800,000 joint replacement procedures are done in the United States every year, according to the American Academy of Orthopaedic Surgeons. Knee replacements are the most common, followed by hips.

Over the past two decades, research into joint replacement technology has exploded. The basic mechanisms are similar to the first artificial joints in the 1960s, but doctors are using new materials, surgical equipment and procedures that help artificial joints last longer and work more like organic joints.

Joint replacement surgery is almost always performed when arthritis has eaten away the cartilage that ordinarily cushions and protects the bones that make up joints. Surgeons will cut away what’s left of the cartilage, along with any diseased sections of bone; for shoulder and hip surgery, they’ll remove the “ball”and “socket”of the joint.

They’ll replace the removed sections with artificial pieces made of metal, plastic or ceramic materials, depending on a variety of factors, including the patient’s age, how active he or she intends to be after the surgery, and the nature of the damage in the original joint.

Doctors who specialize in joint replacement procedures say artificial joints are highly effective for a vast majority of patients. Most replacement parts are designed to last 15 to 20 years, and give patients almost the same amount of mobility as if their joints had remained healthy.

For the most part, they don’t worry that there is anything dangerous about the technology — it all has to go through a lengthy safety review process by the U.S. Food and Drug Administration.

“It’s hard to improve on something that’s 95 percent effective. You really just need to fine-tune it at this point,” said Dr. Jeffrey Margolis, an orthopedic surgeon at Kaiser Permanente San Jose. “A lot of the new technologies have not stood the test of time yet.”

But they point to instances in which manufacturers have pitched technologies like custom-made joints or gender-specific knees directly to consumers, hoping patients will seek out doctors who promise the most cutting-edge treatments.

They also question whether the so-called improvements are just a way to bump up the cost of an already expensive surgery. Joint replacements can cost tens of thousands of dollars, and are already among the largest expenses for Medicare.

“There has to be balance between innovation for purposes of patient improvement, and innovation or purposes of marketing and selling new technologies,” said Dr. Kevin Bozic, vice chairman of orthopedic surgery at UCSF.

Dr. John Costouros, an orthopedic surgeon at Kaiser San Jose who did Rhynes’ shoulder replacements, said he mostly welcomes research into new technology. Shoulder surgery in particular is still a relatively new field, and the technology has improved greatly over the past decade, he said.

Costouros went to Switzerland in the 1990s to learn a technique known as “reverse shoulder replacement,”which has been widely used in Europe for decades but won FDA approval only in 2004. The procedure reverses the position of the ball and socket of the shoulder joint, allowing patients with injuries like torn rotator cuffs to get artificial joints that wouldn’t have worked on them before.

Still, Costouros tells patients to be skeptical of new innovations and to make a thoughtful decision on the treatment that’s best for them.

“There are some people that stretch the envelope and promise things which are not realistic and possible,” Costouros said. “It’s important for patients to do their own research and get multiple opinions.”

E-mail Erin Allday at eallday@sfchronicle.com.