Background: Arthroscopic criteria for identifying syndesmotic disruption have been variable and subjective. We aimed to quantify syndesmotic disruption arthroscopically using a standardized measurement device.
Methods: Ten cadaveric lower extremity specimens were tested in intact state and after serial sectioning of the syndesmotic structures (anterior inferior tibiofibular ligament [AiTFL], interosseous ligament [IOL], posterior inferior tibiofibular ligament [PiTFL], deltoid). Diagnostic ankle arthroscopy was performed after each sectioning. Manual external rotational stress was applied across the tibiofibular joint. Custom-manufactured spherical balls of increasing diameter mounted on the end of an arthroscopic probe were inserted into the tibiofibular space to determine the degree of diastasis of the tibiofibular joint under each condition.
Results: A ball 3 mm in diameter reliably indicated a high likelihood of combined disruption of the AiTFL and IOL. Disruption of the AiTFL alone could not be reliably distinguished from the intact state.
Conclusion: Use of a spherical probe placed into the tibiofibular space during manual external rotation of the ankle provided an objective measure of syndesmotic instability. Passage of a 2.5-mm probe indicated some disruption of the syndesmosis, but the test had poor negative predictive value. Passage of a 3.0-mm spherical probe indicated very high likelihood of disruption of both the AiTFL and the IOL.
Clinical relevance: The findings challenge the previously used but unsupported standard of a 2-mm diastasis of the tibiofibular articulation for diagnosis of subtle syndesmotic instability.
Keywords: ankle sprain; arthroscopy; syndesmosis.