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Prevent excess posterior translation of the tibia relative to the femur
7%
12/166
Prevent excess anterior translation of the tibia relative to the femur
80%
133/166
Resist excess varus force on the knee
2%
4/166
Resist excess valgus force on the knee
3%
5/166
Provide a cushion between the lateral tibial and femoral condyles
4%
6/166
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This patient has positive anterior drawer and Lachman’s tests which are indicative of an anterior cruciate ligament (ACL) tear. The function of the ACL is to prevent excess anterior translation of the tibia relative to the femur. ACL tears typically present in athletes following a knee twisting injury followed by pain and swelling. The swelling that occurs after injury is due to tearing of the vessels running along the ACL. Patients may feel a "pop" when the injury occurs. Patients are often able to bear weight but feel unsteady. The functions of the ACL are to (1) restrict excess anterior translation of the tibia relative to the femur, (2) prevent hyperextension of the knee, and (3) control rotational stability of the tibia. One of the primary functions of the ACL is to prevent excess anterior translation of the tibia relative to the femur. Thus, physical exam maneuvers that test the integrity of the ACL typically apply an anteriorly directed force to assess for excess laxity in anterior translation of the tibia. Two of these tests are the anterior drawer test and Lachman’s test (which is the most sensitive). The major difference between these two tests is the degree of knee flexion at which the test is performed: the anterior drawer test is performed with the knee in 90 degrees of flexion while Lachman's test is performed with the knee in 20 degrees of flexion. Although MRI is the definitive diagnostic test, arthrocentesis is often performed as an adjunct because hemarthrosis is usually present. Incorrect Answers: Answer 1: The posterior cruciate ligament (PCL) functions to prevent excess posterior translation of the tibia relative to the femur. Answer 3: The lateral collateral ligament (LCL) functions to resist excess varus force on the knee. Answer 4: The medial collateral ligament (LCL) functions to resist excess valgus force on the knee. Answer 5: The lateral meniscus functions to disperse weight and enhance stability between the articular surfaces of the lateral tibial and femoral condyles. Bullet Summary: The ACL is often torn in young athletes following a twisting mechanism and an audible “pop.” Anterior drawer and Lachman’s tests are physical exam maneuvers that assess for an ACL tear by demonstrating excess anterior translation of the tibia relative to the femur.
5.0
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