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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 108-1).

FIGURE 108-1

MRI of anterior cruciate ligament (ACL) tear in the frontal view. Note the normal menisci, which are black throughout. (Reproduced with permission from 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 or meniscus. Patellar subluxation/dislocation is becoming an increasingly recognized mechanism of injury, especially in young athletes. 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 sometime requires referral to an orthopedic surgeon.


  • Knees were the most commonly injured body part in studies of high school1 and collegiate athletes,2 comprising one-third of injuries.

  • In patients presenting with knee injuries with hemarthrosis, over 70% occurred during sports; 52% ACL rupture, 41% meniscal tear, and 17% lateral patella dislocation.3

  • ACL injuries occur at an annual incidence of 77/100,000.3 The annual incidence is higher in children and adolescents (121/100,000), peaking at 392/100,000 in 16-year-old girls and 422/100,000 in 17-year-old boys.4

  • 88% of ACL tears are accompanied by another injury, most commonly a meniscal tear.3

  • Meniscal tears are seen on MRI in 91% of patients with symptomatic osteoarthritis arthritis, but are also seen in 76% of age-matched controls without knee pain.5

  • Patellar dislocations accounted for 36% of knee effusions in 10- to 14-year-olds and 28% of knee effusions in 15- to 18-year-olds in a series of 131 patients who presented to a pediatric emergency department.6

Figure 108-2 shows the normal anatomy of the knee.

FIGURE 108-2

Anatomy of a normal knee. (Reproduced with permission from ...

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