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Forced Active Buckling Sign

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Steps to create the forced active buckling (FAB) sign in a left knee with: (1) hip flexion and internal rotation and (2) active knee extension with mild valgus stressing[1]
  • Forced Active Buckling-Sign Test
  • FAB-sign
  • Active Buckling Test
  • Active Giving-Way Test
  • Resisted Knee Extension Buckling Sign
  • Active Instability Test
  • Functional Buckling Sign

Purpose

Description

  • The patient is supine
  • Flex the patient hip until the knee is in approximately 20-30 degrees of flexion
  • Grab the leg and applie some internal rotation
  • The patient is asked to extend their knee against resistance
  • While doing this, the examiner applies a valgus force to the knee
  • Positive Test
    • Sudden buckling or giving away of the knee

Pathology

Evidence

ACL Injury

  • Blanke et al[1]
    • Sensitivity: 78%
    • Specificity: 95%
    • PPV: 94%
    • NPV: 81%
    • LR+: 15.6
    • LR-: 0.23

See Also

References

  1. ↑ 1.0 1.1 Blanke, Fabian, et al. "The forced active buckling sign: a new clinical test for the diagnosis of ACL insufficiency." The journal of knee surgery 33.01 (2020): 042-047.
Created by:
John Kiel on 28 January 2025 14:54:29
Authors:
Last edited:
10 November 2025 20:35:26
Category:
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