ACL injury

• Injury to the anterior cruciate ligament (ACL) usually results from rotational force on a fixed foot.
• Patients often report the sensation of a "pop" in the joint.
• Pain and swelling are immediate
• Hemarthrosis is clearly evident within 1 to 2 hours.
• The knee generally is held in an extended position, with marked reduction in the range of motion.
• Anteroposterior instability of the knee can be assessed by the "drawer sign."
  • patient in a supine position
  • hip flexed 45 degrees
  • knee flexed 90 degrees with the
  • foot is held in a fixed position on the examination table.
  • abnormal forward excursion of the tibia when force is applied behind the knee by the examiner is highly suggestive of an ACL injury
  • Performing the drawer test in a sitting position may result in a false-positive test result.

The ACL inserts on the tibial spine, inside the knee joint. The tibial tuberosity is the insertion for the quadriceps tendon, outside the knee joint. Tenderness would not be expected over the tibial tuberosity after an ACL injury. There may be associated severe collateral ligament damage, resulting in laxity and tenderness over the medial collateral ligament, although  this is not always present.

References:
Baskin MN. Injury—knee. In: Fleisher GR, Ludwig S, eds. Textbook of
Pediatric Emergency Medicine. 4th ed. Philadelphia, Pa: Lippincott
Williams & Wilkins; 2000:339-347
Mercier LR. The knee. In: Practical Orthopedics. 4th ed. St Louis, Mo:
Mosby-Year Book, Inc; 1995:207-232