Chandler Crews sat on a bench outside of Sinai Hospital in Baltimore one recent day, put her feet on the ground and grinned.

Sitting this way was impossible for the 20-year-old from Arkansas just a few years ago because she was born with achondroplasia, a form of dwarfism in which limbs don't grow in proportion with the torso. With bowed legs and a 3-foot-10-inch stature, she said she was unable to do many such "normal" things.

She couldn't drive the family car or go for much of a walk without back and joint pain or even wash her hands in most sinks.

Now she stands just under 5 feet tall, thanks in part to a new procedure that was pioneered in part by Dr. John Herzenberg and Dr. Shawn Standard at Sinai, who had been searching for years for a better way to lengthen the limbs of those with trauma and genetic disorders such as dwarfism.

Limb lengthening itself is somewhat controversial among some with dwarfism who question the need to change. Some also wonder where surgeons should draw the line between medical necessity and cosmetic increases to height.

But Crews had no qualms, citing the pain she once experienced just trying to walk.

"Just a year ago, I was six inches shorter," Crews said. "I couldn't sit with my feet on the ground. Can you imagine what that was like?"

The new procedure is a radical departure from the past method, which involved forcing screws into bones through skin and muscles and building a sort of scaffolding, which Herzenberg likened to "a medieval torture machine."

The apparatus, called an external fixator, pulled the bone apart after it was surgically broken. New tissue would grow slowly in the gap, eventually hardening into new bone. It was effective but the system was cumbersome, painful and prone to infection. Doctors had to be careful not to turn the screws too much and risk a gap too big to heal.

The Sinai doctors found various new devices, mostly less reliable and unapproved by the U.S. government.

So Herzenberg, director of LifeBridge Health's Rubin Institute's International Center for Limb Lengthening, and Standard, the center's head of pediatric orthopedics, worked with an Irvine, Calif.-based company called Ellipse Technologies and another doctor in California to develop a new system.

Doctors surgically insert a titanium rod inside the thigh or shin bone, or both. The rod contains a small magnetic motor and gear box that is used to slowly lengthen the rod. It's powered by a hand-held magnetic field generator placed atop the skin at the site of the magnet four times a day for about a minute.

Like the external fixator, when the bone is pulled apart — at a rate of about a millimeter a day — tissue and eventually new bone fills the gaps. The procedure can stretch each part of a limb about three inches. A bone typically takes at least two months to lengthen and three months to solidify. The rod is removed about a year later.

Herzenberg, who has been working in orthopedics for 35 years, called the innovation a "game changer" because of the simplicity for patients. He and Standard are among a handful of doctors using the Precise rods and have inserted them in about 100 patients, out of about 800 or 900 who have gotten them worldwide. Other hospitals using the system include St. Mary's Medical Center in West Palm Beach, Fla., and Shriners for Children in St. Louis.

The Precise system was first launched in 2012, and the Food and Drug Administration approved a revamped version in February that is stronger so patients can bear weight on their legs earlier. It also can be used on smaller patients.

The Sinai doctors have used the Precise system on patients ages 69 to 7, when limbs are large enough for the rod.

"This is more comfortable and safe and precise," Herzenberg said. "But it's still limb lengthening and doesn't solve every problem."

Surgery still is needed to correct malformed legs, and another procedure is done to insert the rods, which must be threaded inside the bone without causing new problems. Patients can't walk during the lengthening period, and bone doesn't always grow as planned. Patients need close monitoring for months, so they need to stick close to the hospital.

And the system is expensive. The rod alone costs $13,000, or about twice the external fixator's price. All told the tab can reach $100,000, though the doctors said it can be covered by insurance.

Crews endured external fixators during a first round of leg lengthening four years ago, which gave her about 6 inches, in addition to about 2 inches she gained when her bowed legs were straightened. She also used that method to lengthen her arms in between leg treatments.