Dime Sign
Definition and Clinical Significance
The dime sign is a radiographic indicator used to assess fibular length and ankle mortise alignment on anteroposterior (AP) ankle radiographs, where the lateral border of the distal fibula should overlap the lateral talar dome by approximately the width of a dime (approximately 1-2 mm). 1, 2
Radiographic Assessment
The dime sign serves as one of three key radiographic parameters for evaluating fibular length and ankle mortise reduction:
- Dime sign: The distal fibula should overlap the lateral talar dome by approximately 1-2 mm on a properly aligned mortise view 1, 2
- Shenton line: A smooth arc should be present along the medial border of the talus and the medial malleolus 1
- Talocrural angle (TCA): Normally measures approximately 78 degrees (95% CI ± 1.6 degrees) on a true mortise projection with anatomic fibular length 2
Clinical Application in Ankle Fractures
The dime sign is particularly valuable in assessing:
- Lateral malleolar fracture reduction: Used to verify adequate fibular length restoration after operative fixation 1
- Syndesmotic injury evaluation: Loss of the normal dime sign overlap suggests fibular shortening and potential syndesmotic disruption 1, 2
- Post-reduction quality assessment: Confirms anatomic restoration of the ankle mortise 1
Critical Technical Considerations and Pitfalls
Image Positioning Effects
Sagittal plane rotation significantly impacts the radiographic appearance of fibular length and can create false-positive interpretations of the dime sign. 2
- Caudal rotation artifact: Approximately 14.7 degrees of caudal rotation can obscure 2 mm of fibular shortening by producing the same talocrural angle as a true mortise view with anatomic fibular length 2
- Axial rotation: Has a comparatively smaller effect on the dime sign appearance compared to sagittal plane rotation 2
- Observer reliability: Intraobserver agreement for assessing the dime sign ranges from poor to moderate (mean 0.47, range 0.13-0.59), and interobserver agreement is uniformly poor (mean 0.08, range 0.01-0.20) 2
Clinical Implications
- Always verify mortise view alignment: Ensure a true mortise projection before interpreting the dime sign, as malpositioned radiographs can mask fibular shortening 2
- Use multiple parameters: The dime sign should be assessed in conjunction with Shenton line and talocrural angle measurements, not in isolation 1, 2
- Fibular shortening consequences: Loss of the normal dime sign indicates fibular shortening, which compromises ankle stability and force transmission, potentially impacting clinical outcomes 2
Comparative Effectiveness in Fracture Fixation
In lateral malleolar fractures treated with plate osteosynthesis versus pin/cerclage fixation, the dime sign showed no significant difference between techniques (p>0.05), though Shenton line was better preserved with plate fixation (90.3% vs 73.2%, p=0.02) 1