Mechanism of Supination External Rotation Short Oblique Ankle Fractures Revisited: A Cadaveric Study

PURPOSE: The Lauge-Hansen classification system for ankle fractures has been the most commonly used system because it explained mechanism of injury of several common fracture patterns. However, there are limitations to the original Lauge-Hansen experiments and we have chosen to focus our central objective of this study on the biomechanical mechanisms behind stage 1 & 2 supination-external rotation (SER) ankle fractures in a cadaveric model. METHODS: 5 Fresh frozen cadaveric specimens were mounted into a custom made ankle rig with the tibia held rigid using half pins while allowing free movement of the fibula. The foot was secured to a wheel with a torque sensor attached to record examiner external rotational stress application. An electromagnetic tracking system was used to track the motion of the specimen with 6 degrees of freedom at each segment. A control arm was used to hold the foot in dorsiflexion while all other rotations were held in neutral. An ultrasound probe was used to monitor tibiofibular space as the examiner applied a controlled 100N maximal external rotational torque. Specimens were first tested with all ligaments intact prior to incremental resection of the anterior inferior tibiofibular ligament (AITFL) with repeat stress examination after each change. RESULTS: All specimens withstood the normal state testing of up to 100N of external rotational force without any injury. However, 4 out of 5 specimens received short oblique fracture patterns to the distal fibula after partial (75%) or full AITFL resection. Comparison of pre and post radiographs, visual observation via dissection, and live ultrasound video confirmed these results. 3D kinematics were recorded and analyzed as well to determine bone movement and fracture timing and compared to ultrasound video of the tibiofibular space. CONCLUSION: Prior studies have used unmeasured forces, non-physiological ligament strain rates, and poor alignment techniques. We sought to exclude the ligament strain rate and other design issues from our study by performing incremental resection of the AITFL as a synthetic mechanism for stage 1 SER ankle injuries and focusing on the reproducibility of the stage 2 fibula fracture in an SER injury. Our study demonstrated: 1. A 100N external rotational force did not result in an AITFL injury 2. Partial sectioning of the AITFL alongside a 100N external rotational force led to a reproducible oblique distal fibula fracture in a cadaveric ankle model.