SLR - October 2015 - Lisa Zhang

Preoperative Radiographic and CT Findings Predicting Syndesmotic Injuries in Supination-External Rotation-Type Ankle Fractures

Reference: Choi Y, Kwon SS, Chung CY, Park MS, Lee SY, Lee KM. Preoperative Radiographic and CT Findings Predicting Syndesmotic Injuries in Supination-External-Rotation Type Ankle Fractures. J Bone Joint Surg Am. 2014 Jul 16; 96(14):1161–1167.

Scientific Literature Review

Reviewed By: Lisa Zhang, DPM
Residency Program: University Hospital

Podiatric Relevance:
Distal tibiofibular syndesmotic ligament injuries present a challenging aspect of ankle fracture treatment for the podiatric surgeon. Not only do these injuries increase the instability of the fractures by rendering stabilization more difficult, chronic untreated syndesmotic injuries may lead to early degenerative arthritis for the patient later in life. The current standard of care or evaluating syndesmotic injuries is the intra-operative Cotton/Hook test, or the lateral stress test. While some podiatric and orthopedic surgeons may see little value in preoperative radiographic measures of syndesmotic injury, the authors in this study posit that CT imaging may identify factors associated with syndesmotic injuries requiring operative fixation in SER (Supination-external rotation) fractures. Specifically, the authors hypothesized that fibular bone mineral density, fibular fracture height from ankle joint line, and concurrent deltoid ligament injuries would contribute to syndesmotic injuries.

Methods:
A total of 660 patients with ankle fractures who underwent operative treatment from 2009-2012 were enrolled. Inclusion criteria was that the ankle fracture was classified as an SER-type injury with a lateral malleolar fracture. Exclusion criteria were patients with a CT scan done at outside hospital, an ankle fracture resulting from a direct blow, and abnormal ankle anatomy. The patients underwent operative reduction and internal fixation. An unstable syndesmotic injury was evaluated on a fluoroscopic lateral stress test (Cotton test) intra-operatively after all fractures were internally fixed, and was defined as a tibiofibular clear space of > 5mm. Syndesmotic screw fixation was performed using a 5.0 mm cannulated screw when unstable syndesmotic injuries were diagnosed. Then the patients were assigned to either of two groups, the unstable syndesmotic group or the stable syndesmotic group. 5 relevant CT indices were identified: fracture height, fracture length, medial joint space, extent of fracture, and osseous attenuation. Two relevant indices were identified on preoperative ankle mortise radiographs: fracture height and medial joint space. Interobserver and intraobserver reliability was done. Lastly, the CT measurements between the unstable and stable syndesmtoic groups were compared.

Results: Of the 660 patients enrolled, 191 had an SER type of ankle fracture. Of those, thirty-eight patients (19.9 percent) had a syndesmotic injury and underwent syndesmotic fixation along with reduction and internal fixation. In comparison, between the unstable and stable syndesmotic groups, fracture height on CT, medial joint space on CT, and bone attention of the medial malleolus and lateral malleolus were significantly different (p < 0.05 for all). The fracture height (p <0.05) and medial joint space (p<0.05) on radiographs were also significantly different between the two groups. When fracture height, defined as the vertical height between distal tibial articular surface and the lowest point of the lateral malleolus, of >3 mm on CT scan was used as a threshold, a sensitivity of 63.2 percent and a specificity of 92.8% was seen in determining unstable syndemostic injuries in SER type injuries. For CT measures of fracture height, medial joint space, and bone attenuation, an overall sensitivity of 68.4 percent and 86.7 percent were achieved.

Conclusion: Although the gold standard of evaluating for syndesmotic injuries has long been an intraoperative assessment, this study highlighted that preoperative CT and radiographic imaging in SER type ankle fractures may prove of diagnostic and therapeutic value. Several explanations may account for the above findings. For example, bone strength of the lateral malleolus – measured by bone attenuation on CT – is important, because if the lateral malleolus is stronger (high bone attenuation on CT), then the syndesmotic ligament is disrupted first, and fractures occur below the ankle joint. However, if the lateral malleolus is weaker (low bone attenuation on CT), then the weak lateral malleolus distal the ankle joint breaks first, leaving the syndesmosis intact. A heightened suspicion for syndesmotic injuries in SER fractures preoperatively may be useful for the podiatric surgeon in the effective treatment of these injuries.  

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