Weight-bearing CT Scans to Evaluate the Syndesmosis
WBCT scans have also been used to examine motion and investigate biomechanics at the distal tibiofibular syndesmosis in both injured and uninjured patients.[7,37–40] Motion at the syndesmosis under weight-bearing conditions was quantified in one study on 32 healthy control patients. Using WBCT scans, they found that the fibula is located anterior in the tibial incisura in 88% of patients. Physiologic motion of the incisura also occurs as the fibula moves, on average, 1.5 mm in the anterior-posterior direction and 3° in external rotation as the foot is moved from maximal internal to external rotation. Another retrospective study found similar results in a group of 26 patients who had undergone both nonweight-bearing and weight-bearing CT scans with foot and ankle diagnoses that were not believed to affect the syndesmosis. They found that the fibula externally rotates and translates posteriorly and laterally during weight-bearing (Figure 6).
Weight-bearing CT scan image demonstrating syndesmotic widening. This bilateral axial weight-bearing CT scan demonstrates the widening of the syndesmosis on the left side (dashed line) compared with the normal right syndesmosis (solid line).
In patients with syndesmotic injuries, there is high interobserver and intraobserver reliability of distal tibiofibular WBCT scan measurements. However, the utility of WBCT scans in the diagnosis of syndesmotic injury is unclear. Burssens et al compared WBCT scan measurements in 12 patients with syndesmotic injuries and seven normal control patients. In the patients with syndesmotic injuries, they found increased movement of 1.4 mm in mediolateral direction of the lateral malleolus in the incisura. They also reported increased external rotation by 4.5° in the injured cohort. In contrast, Hamard et al found that WBCT scans were less effective at distinguishing pathologic syndesmotic injury than conventional nonweight-bearing multiplanar CT scans. They used nonweight-bearing multiplanar CT scans to analyze 11 ankles with suspected syndesmotic injury and used WBCT scans to investigate an additional of eight ankles with suspected syndesmotic injury. True syndesmotic instability in all ankles was determined using ankle arthroscopy. The results of this study suggested that conventional nonweight-bearing multiplanar CT scans were more accurate in determining syndesmotic injury. The authors hypothesized that physiologic widening of the distal tibiofibular syndesmosis during weight-bearing in the upright position may account for these results. Consequently, at this time, WBCT scans have been more important in advancing the understanding of physiologic motion at the syndesmosis than in the diagnosis of syndesmotic instability. Future work could help to identify appropriate reduction of the syndesmosis after injury.
J Am Acad Orthop Surg. 2020;28(14):e595-e603. © 2020 American Academy of Orthopaedic Surgeons