What is anterior laxity of the tibia (anterior cruciate ligament instability)?

What is Anterior Laxity of the Tibia?

Anterior laxity of the tibia refers to excessive forward (anterior) movement of the tibia relative to the femur, most commonly caused by injury or rupture of the anterior cruciate ligament (ACL), which is the primary restraint preventing this abnormal motion. 1

Anatomical Basis

• The ACL is the primary structure that prevents anterior translation (forward sliding) of the tibia on the fixed femur 2
• When the ACL is torn or deficient, the tibia can slide forward excessively during physical examination maneuvers and functional activities 3
• The lateral compartment of the knee contributes more to anterior tibial translation than the medial compartment, with ACL rupture causing increased laxity in both compartments but significantly greater contribution from the lateral side 3

Clinical Assessment

The Lachman test is the most reliable clinical examination for detecting anterior tibial laxity and assessing ACL integrity 4, 3

• During the Lachman test, the examiner applies anterior force to the proximal tibia with the knee flexed at 20-30 degrees 5
• ACL rupture leads to increased anterior tibial translation coupled with tibial internal rotation 3
• The degree of anterior translation can be graded as mild (1+), moderate (2+), or severe (3+) based on the amount of forward movement and the quality of the endpoint 6

Measurement and Quantification

• Anterior laxity can be objectively measured using instrumented testing devices like the KT-1000 arthrometer, which applies standardized anterior forces (typically 67-89 N) 5
• Stress radiography using the Telos device at 250 N force can measure anterior drawer values, though it has limited diagnostic sensitivity (59%) and is better used for prognostic or therapeutic purposes rather than initial diagnosis 7
• MRI findings such as anterior translation of the tibia relative to the femur (≥7 mm) can predict the degree of knee joint laxity, particularly when MRI is performed within 3 months of injury 6

Rotational Influence

• The degree of anterior laxity varies with tibial rotation position 5
• External rotation of the tibia (15 degrees) demonstrates greater anterior translation than neutral or internal rotation positions 5
• Fixing the tibia in an externally rotated position during examination may decrease the effect of secondary restraints and improve sensitivity in testing for ACL laxity 5

Associated Findings

• ACL deficiency can also produce mild knee hyperextension (typically less than 5 degrees), though this varies between patients 2
• Bone marrow contusions visible on MRI after acute knee trauma are highly predictive of focal osteoarthritis development within one year 8
• The presence and severity of lateral bone contusions are associated with high-grade pivot-shift instability and concomitant lateral meniscal lesions 1

Clinical Significance

Anterior tibial laxity represents functional knee instability that can lead to secondary injuries including meniscal tears and progressive cartilage damage, with the risk beginning within 3 months after ACL rupture 8

• Patients may experience episodes of the knee "giving way" during pivoting or cutting activities 4
• The degree of laxity influences treatment decisions, with greater instability favoring surgical reconstruction in active individuals 1
• In older patients (typically >40 years) with lower activity levels, anterior laxity may be well-tolerated without reconstruction through activity modification and rehabilitation 1