BackgroundThe anterior bundle of the medial collateral ligament (AMCL) of the elbow has been shown to be the most important valgus stabilizer of the elbow. However, the isometry of this band has not been quantified.HypothesisIsometric fibers exist within the AMCL, and these fibers are located within its central region.Study DesignControlled laboratory study.MethodsTwelve cadaveric elbow specimens were mounted in a testing apparatus in a valgus gravity-loaded orientation. Passive supinated flexion was performed and the motion recorded using an electromagnetic tracking device. Hundreds of attachment points for the AMCL of the elbow were recorded on the medial epicondyle and ulna. The overall change in length between each point on the ulna to every humeral point, throughout the arc of motion, was quantified (. L = Lmax – Lmin). The locations of the smallest. L values were determined relative to the attachment site of the AMCL on the medial epicondyle.ResultsTrue isometry was not found throughout the arc of flexion. The smallest. L values averaged 2.8 ± 1.2 mm (range: 0.7 mm to 5.2 mm). Isometric fibers do not exist within the AMCL; however, “nearly” isometric areas are located on the lateral aspect of the attachment site of the AMCL on the medial epicondyle, near the anatomic axis of rotation.ConclusionsWe postulate that these nearly isometric areas would be the most ideal location for graft attachment during reconstruction of the AMCL.