The Fibular Collateral Ligament Is a More Important Restraint to Varus Laxity Compared to the Anterolateral Complex in the Anterior Cruciate Ligament-Deficient Knee in a Cadaveric Biomechanical Study.

Purpose: To compare the influence of the fibular collateral ligament (FCL) and the anterolateral complex (ALC) on varus knee laxity in paired anterior cruciate ligament (ACL)-deficient cadaveric knees using varus stress radiographs.

Methods: Varus laxity in 9 paired (N = 18, mean age 73.8 years) human cadaveric knees was assessed using varus stress radiographs with a 12-Nm varus stress applied at 20° of knee flexion. All knees underwent testing in the intact state and following ACL sectioning. One knee of each pair was randomly assigned to undergo FCL sectioning, and the contralateral knee was assigned to undergo ALC sectioning (anterolateral ligament [ALL] followed by the Kaplan fibers).

Results: Both FCL sectioning and ALC (ALL and the Kaplan fibers) sectioning resulted in increased lateral compartment gapping compared to the intact state, 2.44 mm and 1.13 mm, respectively. ALL sectioning with intact Kaplan fibers did not result in increased lateral compartment gapping. Paired knee comparison revealed a significantly greater influence of the FCL than the ALC in restraining lateral compartment gapping under an applied varus stress (P = .0003).

Conclusions: Sectioning the FCL resulted in significantly greater lateral compartment gapping under a varus stress than combined sectioning of the ALL and Kaplan fibers in an ACL-deficient knee, although both scenarios resulted in significantly increased gapping compared to the intact state. Sectioning of the ALL with intact Kaplan fibers did not result in increased lateral compartment gapping.

Clinical relevance: The FCL is the most important structure in restraining varus laxity in the ACL-deficient knee, and the ALC is of secondary importance in restraining varus laxity. In ACL-deficient patients with a high-grade pivot shift, mild varus laxity on clinical examination, and an intact FCL on magnetic resonance imaging, injury to the anterolateral complex should be considered and may be evaluated with varus stress radiographs. This study validates prior biomechanical studies of FCL deficiency and demonstrates that an approximately 1-mm increase in lateral compartment gapping on varus stress radiographs may occur secondary to ALC injury, and clinicians should be aware of this when considering treatment for ACL-deficient patients with high-grade anterolateral laxity.