Patterns of injury mechanism observed in terrible triad

The purpose of this study was to classify the injury mechanism of terrible triad (TT) and suggest a treatment method according to the mechanism. Forty TTs with magnetic resonance imaging (MRI) scans and 3-dimensional computed tomography (3D CT) were enrolled. 3D CT scans of coronoid fractures were used for classifying into O'Driscoll type representing injury mechanisms and measuring sizes. In MRI scans, lateral collateral ligament complex (LCLC) injuries were classified into distraction (D) type caused by varus force and stripping (S) type caused by forearm external rotation force. Using these findings, possible injury mechanisms were speculated and classified into groups. Characteristic soft tissue injury patterns of collateral ligaments and overlying muscles, direction of dislocation in simple radiographs, and the amount of involvement of radial head fracture were investigated. Ulnar- and radial-side instabilities of soft tissues were newly defined and investigated. There were 29 (73%) cases by posterolateral external rotation (PLER), 5 (13%) cases by posteromedial external rotation (PMER), and 6 (14%) cases by posteromedial internal rotation (PMIR). Simple radiographs showed all posterolateral dislocations in PLER TT compared to posteromedial or pure posterior dislocations in PMER or PMIR TT. Regarding LCLC and overlying extensor muscle, they were all completely torn with D type in all PLER compared to D type in PMER or PMIR. The ulnar collateral ligament was spared in 5 (17%) cases among 29 PLER in contrast to complete rupture in all PMER and PMIR. In PLER, PMER, and PMIR, involvement ratios of radial head fracture were 82% (range, 27%-100%), 61% (range, 25%-100%), and 61% (range, 25%-100%), respectively, and sizes of coronoid fractures were 7 mm (range, 1-14 mm), 6 mm (range, 2-11 mm), and 10 mm (range, 2-16 mm), respectively. In PLER, PMER, and PMIR, percentages of ulnar-side instabilities were 17%, 20%, and 17%, respectively, and those of radial-side instabilities were 59%, 60%, and 83%, respectively. TT is caused by at least 3 mechanisms (PLER, PMER, and PMIR) with characteristic soft tissue injuries and fracture patterns. PLER is the main mechanism of injury. It is always observed in the form of posterolateral dislocation on simple radiographs compared with pure posterior or posteromedial dislocation of PMER or PMIR. It should be managed individually based on injury mechanisms presenting different instability patterns.