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The Reproducibility and Repeatability of Varus Stress Radiographs in the Assessment of Isolated Fibular Collateral Ligament and Grade-III Posterolateral Knee Injuries

An in Vitro Biomechanical Study

^a Sports Medicine and Shoulder Divisions, Department of Orthopaedic Surgery, University of Minnesota, 2450 Riverside Avenue, R-200, Minneapolis, MN 55454. E-mail address for R.F. LaPrade: lapra001{at}umn.edu
^b Department of Orthopaedic Surgery, Ullevaal University Hospital, University of Oslo, N-0407 Oslo, Norway
Investigation performed at the University of Minnesota Biomechanics Laboratory, Minneapolis, Minnesota

Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

Methods: Ten nonpaired fresh-frozen cadaver lower extremities were used. Two varus loads, a 12-Nm moment and a clinician-applied varus stress, were applied to the intact knees and after sequential sectioning of the fibular collateral ligament, popliteus tendon, popliteofibular ligament, and anterior and posterior cruciate ligaments to simulate degrees of posterolateral knee and associated combined cruciate ligament injuries. The shortest distance between the most distal subchondral bone surface of the lateral femoral condyle and the corresponding lateral tibial plateau was measured to quantify lateral compartment opening and was analyzed on digital radiographs. Three observers were used to determine interobserver reproducibility and intraobserver repeatability.

Results: In the intact knee, the mean lateral compartment gapping due to a 12-Nm moment and a clinician-applied varus stress was 8.9 and 9.7 mm, respectively. Lateral gapping significantly increased by 2.1 and 2.7 mm in association with sectioning of the fibular collateral ligament and by 3.4 and 4.0 mm in knees with a simulated posterolateral corner injury for each respective load-application technique (p < 0.0001 for all comparisons). Intraobserver repeatability was high, with all observers independently obtaining an intraclass correlation coefficient of 0.99, whereas the analysis of interobserver reproducibility demonstrated an intraclass correlation coefficient of 0.97.

Conclusions: Measurements with use of current clinical digital imaging systems can be used to quantify the amount of lateral compartment knee opening. Clinicians should suspect an isolated fibular collateral ligament injury if opening on clinician-applied varus stress radiographs increases by approximately 2.7 mm and a grade-III posterolateral corner injury if values increase by approximately 4.0 mm.

Clinical Relevance: Varus stress radiographs appear to provide an objective and reproducible measure of lateral compartment gapping that should prove useful for the diagnosis, management, and postoperative follow-up of patients with fibular collateral ligament and posterolateral knee injuries.

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