Effects of hindfoot constraint on syndesmotic displacement

The effects of altered hindfoot kinematics on the syndesmosis have not been previously studied. Our purpose was to test how the magnitude of displacement across the syndesmosis changes under simulated subtalar (ST) and/or talonavicular (TN) fusion and with altered hindfoot position in a cadaveric model.

Six cadaveric specimens (three matched pairs) age 33 to 43 years were disarticulated at the knee and mounted into a custom six-degree-of-freedom testing frame with a simulated ground reaction force of 700 N and a tensile Achilles load of 500 N. Specimens were then tested through four cycles of internal and external rotation under four conditions: simulated combined ST + TN fusion, ST fusion alone, TN fusion alone and no fusion. Each condition was tested in the neutral coronal position and 9 degrees of inversion and eversion. Infrared light emitting diode (irLED) marker arrays were used to track displacement across the anterior tibiofibular ligament (ATiFL) in order to assess displacement across the syndesmosis.

Without fusion, displacement across the ATiFL in inversion is greater than that in neutral (p = 0.015). With ST, the measured ATiFL displacement in inversion is greater than that in neutral (p = 0.042). In neutral, the combined ST + TN significantly increased ATiFL displacement when compared to no fusion (p = 0.0043). Increased displacement was seen in inversion compared to eversion in all testing conditions.

Simulated ST and TN fusion increases displacement across the ATiFL during simulated physiologic loading. Hindfoot inversion also increases displacement of the ATiFL.

These observations may have clinical implications with respect to syndesmotic injury and total ankle arthroplasty.