Background <p>Essex-Lopresti injuries, defined by a combination of radial head fracture, interosseous membrane (IOM) rupture, and distal radioulnar joint (DRUJ) disruption, result in forearm longitudinal instability, ulnar-positive variance, pain, and loss of rotation. Although radial head replacement (RHR) is commonly performed, the biomechanical contribution of IOM reconstruction remains controversial. This study quantified the relationship between sequential disruption of forearm stabilizers and resultant longitudinal and rotational instability, and evaluated the extent to which RHR with IOM reconstruction restores forearm stability.</p> Methods <p>Ten fresh-frozen cadaveric forearms underwent sequential sectioning of the proximal radioulnar joint (PRUJ), DRUJ, partial and complete IOM, and radial head, followed by anatomic, overstuffed, and understuffed RHR with IOM reconstruction. Rotational motion was assessed using a custom jig and goniometer, while longitudinal displacement under axial load was measured using a materials testing machine. Data were analyzed with paired t-tests and repeated-measures ANOVA.</p> Results <p>Sequential sectioning significantly increased both rotation and longitudinal translation. Total forearm rotation expanded from 84° (intact) to 171° (complete injury; <i>p</i> &lt; 0.001), primarily due to increased supination. Longitudinal displacement rose by ~ 30% after PRUJ/DRUJ injury, 100% after partial IOM sectioning, 200% after complete IOM disruption, and 435% following radial head removal (<i>p</i> &lt; 0.001). RHR with IOM reconstruction restored near-normal rotation (90°, <i>p</i> = 0.518 vs. intact) and axial displacement (neutral, 4.37&#xa0;mm; supination, 6.34&#xa0;mm; <i>p</i> = 1.000 vs. intact). Overstuffed RHR restricted rotation, while understuffed RHR showed no significant difference from intact.</p> Conclusions <p>RHR combined with IOM reconstruction effectively restores forearm rotational and longitudinal stability in Essex-Lopresti injuries.</p>