The most responsive foot position for non-invasive detection of isolated unstable syndesmotic injuries - a 3D analysis

Abstract

Background: The aim of this study was to identify the most responsive foot position for detection of isolated unstable syndesmotic injury.

Methods: Fourteen paired human cadaveric lower legs were positioned in a pressure-controlled radiolucent frame and loaded under 700 N. Computed tomography scans were performed in neutral position, 15° internal / external rotation, and 20° dorsal / plantar flexion of the foot before and after cutting all syndesmotic ligaments. For each position, generated 3D models of the intact and injured distal tibiofibular joints were matched and analyzed by calculating three parameters: diastasis, anteroposterior displacement, and shortening of the fibula.

Results: Transection of syndesmotic ligaments resulted in significant posterior translation of the fibula (4.34°, SD 1.63°, p < 0.01) compared to uninjured state for external rotation, significant anterior translation (-2.08°, SD 1.65°, p < 0.01) for internal rotation, and significant posterior translation (1.32°, SD 1.16°, p = 0.01) for dorsiflexion. Furthermore, the syndesmotic injury led to significantly increased clear space (0.46 mm, SD 0.46 mm, p = 0.03) in external rotation of the foot.

Conclusion: External rotation of the foot under loading seems to be the most responsive position for detection of isolated syndesmotic instability. Under external rotational stress, anteroposterior instability and increased clear space resulting from a complete isolated unstable syndesmotic lesion were most evident.

Keywords: 3D measurement; Syndesmosis; computed tomography; foot position; syndesmotic injury; weightbearing.

Fig. 1

Custom-made loading frame with an artificial specimen mounted for CT scanning under weightbearing. The distal end of the frame is made out of radiolucent composite material, the main part – out of aluminum. A pneumatic cylinder is connected to a compressed air system at the proximal end of the frame for specimen’s loading

Fig. 2

Parameter calculation based on matched DTFJs. One ankle joint with intact syndesmotic ligaments (nat) in external rotation was matched (based only on tibial geometry) to the same ankle joint in external rotation with complete injury to all parts of the syndesmotic ligaments (inj). Clear space difference (∆CS), vertical offset (∆z) and translation angle (∆α) are determined by the vectors connecting the tibial and fibular centroids in each separate state (→). For calculation of the centers of volume, the tibia and fibula were virtually cut 20 mm proximally to the articular surface. The schematic illustration was made using Geomagic Design X software (3D Systems, Rock Hill, SC, USA) and Microsoft PowerPoint (Microsoft, Redmond, WA, USA)

Fig. 3

Resulting 3D parameters for each foot position. Clear space difference (∆CS), vertical offset (∆z), and translation angle (∆α) for external/internal rotation (ER/IR), dorsal/plantar flexion (DF/PF) and neutral foot position (NP) in loaded condition (700 N) show changes in configuration of the DTFJ due to the syndesmotic ligament injury (Grade 3 according to West Point Ankle Grading System). Stars indicate values significantly different from zero. Axes are scaled to corresponding thresholds regarding native anatomy (∆CS: ±2 mm, ∆z: ±3 mm, ∆α: ±5°) to visualize better the effect size of syndesmotic ligament injury on the 3D measurements

Fig. 4

Differences in detection of unstable syndesmotic injuries between neutral foot position and external rotation. Left) Visualization of the neutral foot positioning in the CT frame for a native ankle and the corresponding injured unstable ankle after transection of all syndesmotic ligaments (top) together with the corresponding 3D bony configuration of the DTFJ after best-fit matching of the bony tibial geometry (middle) and the configuration of the unstable injured (red) DTFJ as compared to the native (blue) DTFJ and revealing minimal differences (bottom). Right) Visualization of the foot positioning of the same specimen in 15° external rotation for a native ankle and the corresponding injured unstable ankle after transection of all syndesmotic ligaments (top) together with the corresponding 3D bony configuration of the DTFJ after best-fit matching of the bony tibial geometry (middle) and the configuration of the unstable injured (red) DTFJ as compared to the native (blue) DTFJ and revealing significant differences (bottom)