Fibular Collateral Ligament: Varus Stress Radiographic Analysis Using 3 Different Clinical Techniques

Abstract

Background: Fibular collateral ligament (FCL) tears are challenging to diagnose. Left untreated, FCL tears lead to residual ligament instability and increased joint loading on the medial compartment of the knee. Additionally, when a concomitant anterior cruciate ligament (ACL) reconstruction is performed, increased forces on reconstruction grafts occur, which may lead to premature graft failure. Stress radiographs constitute a reliable and validated technique for the objective assessment of a complete grade III FCL tear.

Purpose: To evaluate side-to-side difference (SSD) values of lateral compartment gapping on varus stress radiographs in patients with a grade III injury to the FCL. Additionally, to evaluate the reliability and reproducibility of 3 different measurement techniques that used various radiographic reference points.

Study design: Cohort study (diagnosis); Level of evidence, 2.

Methods: Inclusion criteria were patients who sustained an FCL with or without a concomitant ACL injury and underwent a combined FCL + ACL reconstruction between 2010 and 2016. Patients were excluded if they had a complete posterolateral corner injury, open physes, intra-articular fracture, meniscal root tear, other ligament injury, or prior surgery on either knee. All FCL tears were diagnosed with a clinical varus stress examination at 0° and 20° of knee flexion and varus stress radiographs at 20° of knee flexion measured in 3 different locations. The SSD for lateral compartment gapping was obtained from the varus stress radiographs and then statistically compared for interrater and intrarater reliability.

Results: A total of 98 consecutive patients (50 males, 48 females; 13 isolated FCL injuries, 85 combined ACL + FCL injuries) with mean age 33.6 years (range, 18-69 years) were included. Measurement techniques 1, 2, and 3 had mean ± SD lateral compartment SSDs of 2.4 ± 0.20 mm, 2.2 ± 0.20 mm, and 2.0 ± 0.03 mm, respectively (no significant differences). Interrater reliabilities for the 3 measuring techniques were 0.83, 0.86, and 0.91, respectively, while intrarater reliabilities were 0.99, 0.77, and 0.99, respectively.

Conclusion: This study demonstrated a lower SSD value of 2.2 mm to be consistent with a grade III FCL tear on clinician-applied varus stress radiographs in the clinical setting. Although all SSD measurement locations had excellent reliability, the method using the midpoint of the lateral tibial plateau was found to be the most reproducible.

Keywords: ACL; fibular collateral ligament; posterolateral corner; varus stress radiographs.

Figure 1.

Varus stress radiographic technique performed on patient’s right lower extremity. The patient’s knee is placed in 20° of flexion with the use of a polyurethane wedge underneath the knee. A clinician applies a varus load by placing one hand on the medial femoral condyle and the other hand on the lateral aspect of the foot.

Figure 2.

Varus stress radiographs resulting from measurement technique 1. This measurement technique was performed by extending a vertical line from the most inferior aspect of the lateral femoral condyle to the corresponding point on the lateral tibial plateau. (A) Measurement technique 1 demonstrated on an uninjured right knee. (B) Measurement technique 1 demonstrated on a left knee with a combined anterior cruciate ligament and fibular collateral ligament injury, resulting in a 2.2-mm side-to-side difference.

Figure 3.

Varus stress radiographs resulting from measurement technique 2. This measurement technique was performed by identifying the most distal aspect of the popliteal sulcus and drawing a vertical line down to the corresponding point on the lateral tibial plateau. (A) Measurement technique 2 demonstrated on an uninjured right knee. (B) Measurement technique 2 demonstrated on a left knee with a combined anterior cruciate ligament and fibular collateral ligament injury, resulting in a 2-mm side-to-side difference.

Figure 4.

Varus stress radiographs resulting from measurement technique 3. This measurement technique was performed by measuring the distance from the most lateral aspect of the lateral tibial plateau to the lateral tibial eminence. The midpoint of this line was identified by measuring half the distance of the length of the line. A line from the midpoint was then drawn to the corresponding point on the femoral condyle. (A) Measurement technique 3 demonstrated on an uninjured right knee. (B) Measurement technique 3 demonstrated on a left knee with a combined anterior cruciate ligament and fibular collateral ligament injury, resulting in a 5.8-mm side-to-side difference.