Now scientists are testing ways to strengthen the entire leg so it stays straighter, in hopes of slowing the knee's deterioration.
"People with knee arthritis have kind of a bow-legged appearance," notes Laura Thorp of Rush University Medical Centre. An assistant anatomy professor, Thorp thinks that misalignment is a big key to the painful condition. She is not alone.
Specialists now agree the more out of alignment a leg is, the faster arthritis in the knee worsens. "You start with a little pain, then everything north and south of it has to compensate," explains Dr John J Callaghan of the University of Iowa, a knee and hip specialist with the American Academy of Orthopaedic Surgeons. "It's not enough to concentrate on the knee."
What to do about it?
At issue is how your knee handles its load. Being even a little bow-legged puts extra pressure on the knee, especially the inner side. It has to carry a higher-than-normal load each time your foot strikes the ground.
Weight adds to the knee's load, too. Being 7kg or more overweight is a key risk factor for developing knee arthritis. So is having had a knee injury earlier in life, and the one unavoidable risk factor - getting older. Millions of people have osteoarthritis, the wear-and-tear that breaks down the cartilage that cushions joints, causing pain and stiffness as the ends of bones rub on each other. Hundreds of thousands eventually turn to knee replacement for relief.
Even when researchers account for weight, Thorp says a misaligned leg compounds the extra load. Exercise has long been prescribed for knee arthritis, mostly exercises that strengthen the hamstring and quadriceps – muscles that support the knee. Stronger muscles do in fact alleviate some knee pain, but they do not slow the inevitable worsening of arthritis. An Australian study last year showed quad strengthening had no effect on knee load. Interestingly, people with more misaligned legs also got less pain benefit from those exercises.
Exercising hip muscles
Rush's Thorp is trying a different, top-down approach - exercising not muscles that support the knee but those that support the hip.
Knee arthritis sufferers tend to have weak muscles on the outside of the hip, muscles that are key for gait, she found. Every time you take a step, the hip muscle on the standing side has to keep your pelvis level. If it's weak, it changes your pelvis alignment in a way that magnifies the already misaligned leg and adds even more twisting pressure to the knee.
The exercises themselves are similar to what patients are prescribed after a hip replacement. In a handful of knee patients who underwent a month of physical therapy to learn those exercises, Thorp measured a drop in knee load. Now she's enrolling 30 people with mild to moderate arthritis in a more formal study. They get not only the physical therapy, but are told to do the exercises at home six days a week for a second month, too.
If the exercises do reduce knee load, it would take far larger - and years longer - studies to see if that in turn slowed the arthritis, both Thorp and Callaghan caution. Thorp's is among a handful of studies getting under way to look at hip exercises. Swedish researchers saw some drops in knee pressure when people who did exercises affecting both the knee and hip climbed stairs.
Foot and ankle treatment
Other doctors are focusing on the south end, the foot and ankle. Repositioning the leg's alignment with custom-measured shoe inserts, called orthotics, sometimes helps. The challenge: Finding people who agree to stick with exercise. People with painful joints tend to become sedentary if they weren't already.
Iowa's Callaghan already prescribes whole-leg exercises, including the hip, for knee patients - but he praises the Rush study for hunting proof of which specific activities truly help, information long missing when doctors give the exercise pep talk.
He advises the newly diagnosed to try to keep in shape and suggests consulting a good physical therapist about how to work the whole leg, saying it can't hurt.
"It's hard for people to buy off on that, especially people who are out of shape," Callaghan says. "There's just not that much data out there." – (Sapa, August 2009)