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A 33-year-old woman felt a pop in her knee while skiing around a tree. She felt immediate pain and had difficulty walking when paramedics removed her from the slopes. Within a couple of hours, her knee was swollen. On examination the next day, she was able to walk 4 steps with pain. She had a moderate effusion without gross deformity and full range of motion. She had no tenderness at the joint line, the head of the fibula, over the patella, or over the medial or lateral collateral ligaments. She had a positive Lachman test, a negative McMurray test, and no increased laxity with valgus or varus stress. The physician suspected an anterior cruciate ligament (ACL) tear, placed her in a long leg range of motion brace, and advised her to use crutches until an evaluation by her physician within the next several days. She was treated with acetaminophen for pain and advised to rest, apply ice, and keep her leg elevated. Later, an MRI confirmed an ACL tear (Figure 106-1).

Figure 106-1

MRI of ACL tear in the frontal view. Note the normal menisci, which are black throughout. (Courtesy of John E. Delzell, Jr., MD, MSPH.)

Knee injuries are common, especially in adolescents. Women have a greater risk of knee injuries because of body mechanics. Most knee injuries involve the ACL, meniscus, or medial or lateral collateral ligaments. The mechanism of injury and physical examination findings suggest the type of injury, which can be confirmed by MRI. Treatment includes rest, ice, compression, elevation, and referral to an orthopedic surgeon.

  • Knee injuries are the second most common adolescent sporting injury (after ankle injuries)1 and typically are seen in sports requiring pivoting, such as basketball or football. Figure 106-2 shows the normal anatomy of the knee.
  • The risk of ACL injury while playing soccer, is 2 to 3 times higher in females than in males after the age of 12 years.2
  • The risk of ACL injury was 3.79 times greater in girls in a prospective study of boy and girl high school basketball players in Texas.1
  • Incidence of ACL injuries was approximately 3 per 1000 person-years in United States active military personnel, with no difference in gender.3
  • Meniscal injuries commonly occur with ACL tears (23% to 65%).4
  • Meniscal tears were seen on MRI in 91% of patients with symptomatic osteoarthritis, but also seen in 76% of age-matched controls without knee pain.5
  • Collateral ligament injuries account for approximately 25% of acute knee injuries.

Figure 106-2

Anatomy of a normal knee. (From Simon RR, Sherman SC, Koenigsknecht SJ. Emergency Orthopedics, The Extremities, 5th ed. New York, NY: McGraw-Hill; 2007:392, Fig. 15-5. Copyright 2007.)

  • ACL injuries occur with sudden deceleration ...

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