(Philippe Desmazes/AFP/Getty Images)

Downhill skiing is one of the most exhilarating outdoor sports. Ask skiers why they spend huge amounts of time and money to feed their habit and they might describe the feeling of speeding down a groomed trail early in the morning, carving turns across icy mountain pitches or slicing through powder with a bunch of friends.

With the fun comes risk. While your next ski trip probably will not leave you with a broken leg or a head injury, the risk of knee injury persists, despite decades of improvements in skis, bindings and lessons.

Each year, an estimated 17,500 skiers tear an anterior cruciate ligament (ACL), accounting for about a third of all skiing injuries, according to researchers at the University of Vermont School of Medicine. This figure has remained steady for the past decade, while the number of skier and snowboarder visits to slopes has stayed at about 55 million per year, according to industry estimates. (As for fatalities, running into other skiers, trees or ski lifts accounts for about 38 deaths a year, according to the National Ski Areas Association.)

“Skiing has never been safer,” said David Byrd, director of safety and regulatory affairs at the association, a trade group whose members own and operate ski slopes. He notes that nearly 7 of 10 skiers and snowboarders now strap on a helmet before hitting the slopes, compared with fewer than 1 in 4 just a decade ago. The death rate (fatalities per overall skier visits) has declined 25 percent from 1992-1993 to 2013-2014, according to the University of Vermont studies, a trend the researchers associate with greater use of helmets.

Massanutten Ski Resort junior instructor Lucie Rutherford rides a lift with Casey Sneath, left, Ava Lewinsohn and Lauralee Maas. (Daniel Lin/Daily News-Record via AP)

Helmets are the low-hanging fruit of ski safety, Byrd said. As instructors pushed for kids to use them during ski lessons, more parents and young adults began wearing them as well.

Still, orthopedic surgeons get a mini-boom every winter in sprained and torn ligaments.

“We see a lot of knee injuries,” said Robert LaPrade, chief medical officer at the Steadman Philippon Research Institute in Vail, Colo., a nonprofit that studies sports injuries. He is also an orthopedic surgeon, treating hundreds of ski-related injuries each winter.

In the past 15 to 20 years, the ski industry has developed hourglass-shaped skis that make it easier for beginner and intermediate skiers to make a carved turn on the snow. Of course, there have also been unintended consequences, LaPrade said.

“With the improved ski design, skiers can go faster,” he said. “The problem is, when people go faster, they fall harder.”

As a result, ski manufacturers are working on bindings that sense a fall before it happens — just as crash-avoidance sensors do on automobiles. These sensors would pick up forces that are leading to a bad fall, then trigger the ski to release from the boot, preventing the twisting forces on the knee that result in an ACL tear.

To address the problem of ACL injuries, Irv Scher, a Seattle-based ski industry consultant, says he’s betting on a technological fix.

Scher, a biomechanical research engineer and scientific chairman of the International Society for Ski Safety, said “electro-mechanical” bindings are being tested by ski manufacturers but are still several years away from widespread use.

“It may be that these are bindings we are not used to and something that takes a while to permeate through the industry, but it will happen eventually,” he said.

ACL injuries strike weekend warriors, longtime instructors and elite athletes. During one week in January, America’s top male Alpine skier, Ted Ligety, tore his right ACL during a training run in Germany, while Norwegian Aksel Lund Svindal, the leader of skiing’s World Cup competition, crashed during a race in Austria, tearing the same ligament. Both skiers are out for the season.

American downhiller Lindsey Vonn tore her right knee ACL twice, in 2013. She returned this season after surgery and has won several World Cup races.

Still, not everyone is able to make such a comeback. Many people just quit skiing, pick up another sport or find they can’t quite walk, run or stand up the same afterward.

ACL injuries cost skiers and resorts money and time, according to Robert Johnson, a retired orthopedic surgeon at the University of Vermont School of Medicine who has compiled data on ski injuries for more than 40 years.

“Yes, it’s a big problem, a big expense, and not all those knees get back,” Johnson said.

One company that has taken up the quest to reduce the risk of those injuries, Vermont-based KneeBinding, says its binding releases the ski boot’s heel before too much force has built up from the side during a fall.

KneeBinding gear is more expensive than other bindings, but it does have its supporters. One ski area manager says the KneeBinding equipment saved his resort money by preventing injuries among those who work on his slopes.

“Our philosophy is, we want to send employees home in the same or better condition, and we want to watch our workers’-comp premiums,” said Chris Bates, general manager at Cataloochee Ski Area in Maggie Valley, N.C.

Bates said his staff hasn’t had any ACL injuries since he provided all 200 ski instructors and ski patrollers with KneeBinding equipment.

Resorts in Colorado and Vermont have put employees on the same device, but Scher, the ski industry consultant, says there isn’t firm evidence yet whether KneeBinding works. It could be that the skiers who use them ski less aggressively, reducing their risk of falling and tearing an ACL, for example.

“It’s an interesting idea,” Scher said of the new design. “But I haven’t seen any data that it will reduce likelihood of ACL tears. If there is, it would be great.”

The popularity of what are often called “fat skis” — they have a wider platform designed to help skiers stay above soft snow and deep powder — has also caused problems for skiers’ knees. While an extra inch in width may not seem like much, it creates significantly greater twisting force on a skier’s knee, according to John Seifert, a professor of sports physiology at Montana State University.

“What we are finding is not real good unless you have a substantial change in ski technique,” Seifert said. “We see increased torques and stress [on the knee].”

Seifert fastened tiny accelerometers onto volunteers from Montana State’s ski racing team in an experiment to measure the force generated on wider skis.

With fat skis, the volunteers used a lot more muscle effort to make a turn, Seifert said.

Seifert says he is working with colleagues in Europe on additional research, but he doesn’t have the funding to collect data on whether people are getting injured more.

A Czech study of downhill skiers published last year in the Journal of Sports Science Medicine found that the increased torque connected with fat skis could result in more degenerative knee injuries over time.

LaPrade, the Vail orthopedist, says he’s seeing more patients with soreness and even torn knee ligaments as a result of using fat skis on hard-packed snow.

“On powder days, we don’t see any injuries,” he said. “It’s usually four or five days without snow, when it gets icier. I don’t see people changing over [to skinnier skis]. Most people are skiing with powder-type skis. Once the snow gets packed down, then we see torn ACLs.”

Some experts say prevention is a better approach than switching skis. They are working on strengthening the muscles around the knee, reducing fatigue that occurs during skiing. Less fatigue means less risk of a fall, especially the kind of fall that can produce a torn ACL, according to Michael Decker, a biomechanical engineer at the University of Denver.

Decker said that asymmetry between the left and right leg may be a factor in ACL injuries.

“You are seeing big differences in right and left legs,” Decker said. “What we are finding is the non-dominant leg has been injured, especially the ACL, much more than the dominant. Females have two times the rate as males of the non-dominant leg.”

Decker has been placing probes and measurement devices on junior ski racers at Vail Resort to pinpoint which muscles are involved in turning the ski and to learn whether they can be trained to even out. Decker says training the gluteus muscles in the buttocks may be the key. The harder they work, the more the ACL is protected.

He is also testing Opedix, a new kind of stabilizing tights that fit around the hips and knees to bring skiers into a more balanced stance. Decker’s studies showed these compression tights help balance and improve turns without changing the dominance patterns of the legs.

Spending more time in the gym and less time looking for new gear could be the key for many.

“Most people are physically not as equipped as they probably should be,” said Roy Pumphrey, a certified strength and conditioning specialist in Baltimore who trains many skiers.

Pumphrey agrees that the key to protecting the knee from injury is keeping the gluteus muscles tight while skiing, strengthening the muscles around the knee beforehand and learning how to increase your body’s mobility, no matter how old you are.

“A lot of people look at the gym and weights, and say, ‘That’s too hard,’ ” he said. “They will have skeptical view of doing a squat or dead lift or a lunge. They are afraid of going down on one knee. But they more than willing to go 25 miles an hour down the mountain.”

Ski injuries:

17,500 skiers tear an anterior cruciate ligament each year, according to one estimate.

7 of 10 skiers and snowboarders wear a helmet.

38 deaths occur in skiing mishaps in an average year.