A 6-year-old girl was playing with another youngster in her District Heights home in July when the two of them found an M80 firecracker. She lit the large firecracker on the kitchen stove, and it exploded in her hands, ripping the tip off of a finger and tearing open her hands.

Rescue workers acted quickly. Within minutes, Karen L. Hough was placed in a state police helicopter and flown 30 miles north to the Raymond M. Curtis Hand Center at Baltimore's Union Memorial Hospital.

There, in three hours of delicate surgery, doctors sutured the severe lacerations on her hands. A team of two surgeons worked in a facility designed specifically for injuries like Karen's. The child was in the hospital for two days, and doctors expect her to recover full use of her hands.

Karen was lucky. A scrapbook kept by the hospital, and used for instruction purposes, contains pictures of other hands injured in similar accidents and shattered beyond recognition.

Dr. Gaylord L. Clark Jr., one of 15 surgeons at the center and one of its founders, noted: "We had a child who lost the entire hand because of an M80 explosion in here two to three years ago."

An M80, also known as an ash can or two-inch salute, is powerful enough "to blow a mailbox apart," said a spokesman for the Prince George's County Fire Department, which aided Karen.

"Microsurgery a process of using microscopes to perform intricate surgery on tiny veins and arteries allows surgeons to see better. And if you can see better, you can fix it better," Clark said. The surgeons at the center have had at least seven years each of post-medical school training in microsurgery.

Clark said the process of reattaching a severed limb or extremity, a procedure called replantation, requires at least two doctors and takes at least 12 hours to perform. He remembered that the longest surgery he ever performed was a replantation that took 27 hours.

Although some injuries the center treats are caused by explosives and home repair accidents, the vast majority occur on the job, usually while workers are using heavy equipment in such places as steel mills, printing plants and farms. Some patients come to the center hoping doctors can correct problems caused by congenital defects and age.

The Hand Center at Union Memorial Hospital was founded in 1975 by Dr. Raymond M. Curtis, who had an interest in hand surgery because of his World War II Army experience in treating soldiers with hand-grenade injuries.

About 1,500 operations are performed each year, a hospital spokesman said. A third of those are emergencies; the remainder are follow-up and nonemergency surgery, he said.

The center, the first in the United States to provide microsurgery and hand rehabilitation in the same facility, specializes in replantation of amputated upper extremities. Its success is made possible by recent technological developments, ranging from sophisticated equipment for performing replantation microsurgery to computerized machinery used in rebuilding and strengthening the muscles in the upper body after severe shock and injury.

The center has worked closely with regional paramedics to develop a procedure for saving limbs severed in accidents. In the past, it was often assumed that an amputated limb should be put on ice to preserve it. But in 1975, the hospital distributed literature that showed why a limb should be wrapped in any fabric available and then put on ice, saving it from frostbite before its replantation.

But it is a work simulator, which has radically altered the science of hand rehabilitation, that is the center's real claim to fame.

Located in the center's carpentry shop, the work simulator has been used since it was made in 1979 from an old dentist's chair by one of Curtis' friends. A Baltimore company now manufactures the work simulator.

True to its name, the machine simulates experiences, such as grasping a golf club or a pile of plates, by using different attachments that are connected to a computer. The computer measures and records the patient's resistance, endurance and pace.

"The work simulator brought therapeutic exercises out of the dark ages," said Pat Baxter of the Philadelphia Hand Center, one of many hospitals that rents a similar machine for what she calls a "very reasonable" $500 a month.

"You can simulate the very heavy resistive work. Before, for example, when we had someone who had to operate a heavy piece of equipment, like a bulldozer, there was no way that we could simulate the steering and gears. . . . [The simulator] has really filled a gap that is very hard to fill."

The Philadelphia Hand Center, modeled after the Curtis Hand Center, sees about 100 cases each day. Baxter, director of the Return to Work Program, estimated that 99 percent of her patients have been able to return to normal lives.

At the Curtis Hand Center, Carolyn Handy of west Baltimore, a patient, was a machine operator at Pittsburg Paint and Glass Co. when she injured her right hand in February 1981. Handy said the rag she was using to clean a fiber mixing machine was caught in a moving mechanism, dragging her hand into the equipment before she could pull it free.

As Handy remembers, "It mangled the palm of my hand, and my tendons flew all the way back to my elbow. So they took three tendons from my leg and took two nerves from there."

Handy, who stayed in Union Memorial for two weeks, has had three operations since the accident. While shellacking a table in the rehabilitation unit, she said the most important part of her treatment took place in the rehabilitation unit where she learned to use her hand again and regained confidence. She has been learning carpentry skills in the hospital shop, which looks just like a clean and well-organized professional woodworking shop.

Handy said the biggest problem with her injury was psychological. Her family treated her like an invalid, and her sisters tried to do everything for her, she said. Now, she's looking forward to taking home a wooden stool she built, with a tan cover that she wove on the hospital loom.