Mastering the ACL Clinical Exam

The Anterior Cruciate Ligament (ACL) rupture is the quintessential sports injury. With over 200,000 reconstructions performed annually in the US alone, diagnosing it accurately is a core competency for any orthopaedic surgeon. While MRI is the gold standard for imaging, the clinical examination remains the cornerstone of diagnosis, particularly for assessing the functional status of the knee.

An MRI tells you the ligament is torn; the exam tells you if the knee is unstable. This guide dissects the biomechanics, technique, and interpretation of the two most critical tests: the Lachman and the Pivot Shift.

Visual Element: Anatomical diagram of the ACL's two bundles: the Anteromedial (AM) bundle (tight in flexion) and the Posterolateral (PL) bundle (tight in extension).

Anatomy: What Are We Testing?

The ACL is not a single cord; it is a complex ribbon of collagen with two functional bundles.

1. Anteromedial (AM) Bundle: Controls anterior translation.
2. Posterolateral (PL) Bundle: Controls rotational stability.

• Lachman Test: Primarily assesses the AM bundle (anterior translation).
• Pivot Shift Test: Assesses the PL bundle and the Anterolateral Ligament (ALL) complex (rotational stability).

The Lachman Test: The Sensory King

The Lachman test is the most sensitive clinical test for ACL rupture (Sensitivity ~85-95%). If the Lachman is negative, the ACL is likely intact.

Technique

1. Position: Patient supine. Knee flexed to 20-30 degrees.
2. Stabilization: Use your non-dominant hand to stabilize the distal femur.
3. Translation: Use your dominant hand to grasp the proximal tibia (thumb on the tibial tuberosity). Apply a brisk anterior force.
4. Assessment: You are feeling for two things:
   • Amount of Translation: Compared to the normal side.
   • The End Point: This is critical. A "firm" end point (thud) indicates an intact ACL. A "soft" or "mushy" end point indicates rupture.

Grading

• Grade 1: <5mm translation (with firm end point).
• Grade 2: 5-10mm translation (soft end point).
• Grade 3: >10mm translation (soft end point).

The Pivot Shift: The Specificity Queen

The Pivot Shift is the most specific test (Specificity ~98%). It is pathognomonic for ACL insufficiency. It is also the test that best correlates with patient symptoms ("giving way") and satisfaction after surgery.

The Phenomenon

The Pivot Shift reproduces the subluxation event.

• In Extension: The IT Band acts as an extensor. In an ACL-deficient knee, the tibia subluxates anteriorly.
• In Flexion (>30°): The IT Band passes behind the axis of rotation and becomes a flexor. It pulls the subluxated tibia back (reduces) into position.
• The "Clunk": The sensation you feel is the tibia reducing from a subluxated position.

Technique

1. Start: Leg in full extension. The tibia is subluxated anteriorly (though you can't see it).
2. Forces: Apply a Valgus stress (to compress the lateral compartment) and Internal Rotation torque.
3. Motion: Slowly flex the knee while maintaining these forces.
4. Result: At approximately 20-40 degrees of flexion, you will feel a sudden "clunk" or "shift" as the tibia reduces.

Grading (International Knee Documentation Committee - IKDC)

• Grade 0 (Normal): No shift.
• Grade 1 (Glide): Smooth gliding reduction (no clunk).
• Grade 2 (Clunk): Abrupt reduction / clunk.
• Grade 3 (Gross): The tibia locks out or the shift is visibly profound (often indicates concomitant meniscal or ALL injury).

The Anterior Drawer: The "Historic" Test

The Anterior Drawer is performed at 90 degrees of flexion.

• Sensitivity: Low (~50% in acute injuries).
• Issues:
  • Hamstring spasm prevents translation.
  • Meniscal wedging blocks translation.
  • Hemarthrosis makes 90° flexion painful.
• Utility: It is mostly useful for assessing chronic insufficiency or associated capsular laxity, not acute diagnosis.

The Lever Sign (Lelli Test)

A newer test gaining popularity.

1. Technique: Patient supine. Place your fist under the proximal calf (acting as a fulcrum/lever).
2. Action: Push down on the distal quadriceps.
3. Result:
   • Intact ACL: The heel rises off the table (the ACL tethers the tibia to the femur).
   • Ruptured ACL: The heel stays on the table (the tibia slides anteriorly instead of lifting the leg).
4. Utility: Excellent for patients with large legs or those who are guarding, as it relies on gravity and leverage rather than brute force.

Instrumental Laxity Testing (KT-1000 / Rolimeter)

While clinical exam is subjective, arthrometers quantify the laxity.

• Significance: A side-to-side difference of >3mm is diagnostic of an ACL tear.

Visual Element: Chart comparing the Sensitivity and Specificity of Lachman, Pivot Shift, and Anterior Drawer tests side-by-side.

Summary

• Screening: Use the Lachman (high sensitivity). "Is it torn?"
• Confirmation: Use the Pivot Shift (high specificity). "How unstable is it?"
• Verification: Check PCL (Sag sign) to ensure your starting point is correct.

Clinical Examination Video Library

Watch high-definition videos of the Lachman, Pivot Shift, and Lever Sign being performed on positive and negative knees.