The Use of Diastasis to Determine Lisfranc Instability

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Category: Midfoot/Forefoot; Trauma Introduction/Purpose: Lisfranc injuries have a reported incidence of 1 in 55,000 people. However, given the subtle displacement present in some of these injuries, up to one third are missed on initial diagnosis. Weightbearing imaging is the gold standard for evaluation and diagnosis of the Lisfranc complex, and diastasis between the medial cuneiform and second metatarsal (C1-M2) is the most commonly discussed parameter. However, no explicit consensus on precisely where within the Lisfranc complex to measure the diastasis has been established. Therefore, the aim of this study was to determine what specific diastasis measurement is most sensitive in diagnosing Lisfranc instability using weightbearing radiographs and computed tomography (WBCT). Methods: A total of 159 adult patients with bilateral foot WBCTs were included: 44 with confirmed Lisfranc instability and 115 controls. A subset had unilateral weightbearing X-rays (38 and 83 patients, respectively). Lisfranc instability was confirmed intraoperatively or by physical exam. Exclusion criteria were: prior midfoot surgery, Charcot arthropathy, severe midfoot arthropathy, cuneiform fractures, and significantly displaced 1st-3rd metatarsal base fractures. In the axial plane, C1-M2 diastasis was measured on weightbearing radiograph and WBCT. In the coronal plane, diastasis was measured at different levels between C1-M2 (dorsal, interosseus, plantar, planter apex) and C1-M3 (plantar apex) on WBCT. For WBCT measurements, percent difference between feet was calculated. Case and control measurements were compared with Mann-Whitney U tests (p < 0.05). Diagnostic cutoffs were determined using receiver operator curves (ROC) and minimum distance to the corner. Sensitivity, specificity, negative and positive predictive values, and area under the curve were calculated for each cutoff value. Results: A significant portion of the subjects were female (Lisfranc instability: 60% and controls: 61%). Average age was 39±17 years and 44±17 years for the Lisfranc instability and control groups, respectively. Mann-Whitney U testing revealed a significant difference between groups for all measurements except plantar C1-M2 (p=0.13), C1-M2 plantar apex (p=0.69) and C1-M3 plantar apex (p=0.07). Figure 1 summarizes the diagnostic cutoff values and the associated efficacy measures. Axial WBCT diastasis was the most effective diagnostic measure, followed by dorsal coronal WBCT diastasis. Using a percent difference cutoff of 34%, axial WBCT diastasis (AUC=0.90) demonstrated outstanding diagnostic capability. Dorsal coronal WBCT diastasis with a percent difference cutoff of 46% (AUC=0.80) demonstrated excellent capability. The remaining measurements provided less reliable diagnostic capability, including diastasis on radiograph (AUC=0.77). Conclusion: Axial WBCT diastasis and dorsal coronal WBCT diastasis both effectively diagnose Lisfranc instability on WBCT, with axial WBCT diastasis demonstrating outstanding diagnostic capability. The traditional method of C1-M2 diastasis on weightbearing radiograph is not as effective as either of these WBCT measurements and only provides adequate diagnostic capability. Our results emphasize the value of diastasis measurements using bilateral WBCT imaging to diagnose Lisfranc instability. These results also suggest that diastasis measurements should be performed in the axial plane and on the dorsal aspect of the Lisfranc complex.