On May 31, 2004, about 1413 Alaska daylight time (ADT), a cabin crewmember received a minor injury during the cabin door-opening procedure of a Boeing 757-251 airplane, N550NW, after parking at Gate B-10 of the Ted Stevens Anchorage International Airport, Anchorage, Alaska. The airplane was being operated as an instrument flight rules (IFR) scheduled domestic passenger flight under Title 14, CFR Part 121, when the incident occurred. The airplane, operated as Flight 843 by Northwest Airlines Inc., St. Paul, Minnesota, was not damaged. The 2 airline transport certificated pilots, 4 other cabin crewmembers, and the 181 passengers, were not injured. Visual meteorological conditions prevailed. An IFR flight plan was filed. The flight originated at the Minneapolis International Airport, Minneapolis, Minnesota, at 0850 ADT. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on June 1, 2004, an air safety investigator with Northwest Airlines reported that after the airplane parked at the gate, the flight attendant was positioned at door 2L. The customer service agent (CSA) positioned outside the airplane on the jetway knocked on the door. This is the signal to the flight attendant that the door will be opened, if the flight attendant provides a visible "thumbs-up" signal to the agent on the jetway. The flight attendant reported that in response to the knock, she gave a thumbs-up, and stepped away from the door. The door did not open, but she heard a second knock. The flight attendant gave a second thumbs-up through the cabin door window. As she was giving the signal, the door handle came down on her wrist. The jetway gate agent reported that after his first knock, he did not receive a thumbs-up, and knocked a second time. The flight attendant inside the airplane then gave a thumbs-up signal. The agent then opened the airplane door from the outside. During the door opening procedure, the inside handle of the door struck the flight attendant on the forearm. The flight attendant was taken to a local hospital in Anchorage, where she was initally diagnosed with a fractured wrist. Her wrist was placed in a temporary splint. On June 23, the Northwest Airlines investigator informed the NTSB IIC that the injured flight attendant had a follow-up examination with her own physician in Minnesota. The physician took additional x-rays of the injured wrist and determined that it was not broken, but had sustained a sprain. The flight attendant continued to have pain, and had a magnetic resonance image (MRI) of her wrist on July 2, 2004, which was normal. The flight attendant did not return to work until July 11, 2004. Northwest Airlines personnel provided a copy of their cabin operating standards, which states, in part: "To prepare cabin door for opening; 1. CSA knocks on door and awaits "thumbs-up" signal from FA (flight attendant). 2. FA gives thumbs-up signal to CSA to indicate that door is disarmed and safe to open. 3. CSA opens cabin door after FA moves hands clear of door." Additional information provided by Northwest Airlines indicated that the door opening procedure precludes a second thumbs-up signal unless the CSA knocks a second time.