Normal Kager's Triangle Size on Radiography
Kager's triangle is a radiolucent fat-filled space visible on the lateral ankle radiograph, bounded anteriorly by the flexor hallucis longus, posteriorly by the Achilles tendon, and inferiorly by the superior calcaneus. Its clear triangular lucency serves as an indirect marker of soft-tissue integrity in the posterior ankle compartment. Obliteration or distortion of this space is a recognized radiographic sign of underlying pathology, making its assessment an important part of routine lateral ankle film interpretation.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | Table |
Clinical Significance
Under normal conditions, Kager's triangle appears as a well-defined, homogeneously lucent triangular fat pad on the lateral projection. Any increase in soft-tissue density, loss of the sharp borders, or complete obliteration of this space should prompt further investigation. Because the Achilles tendon forms the posterior boundary, tendon thickening, rupture, or peritendinous edema are among the most common causes of disruption.
Obliteration or haziness of Kager's triangle can be seen acutely following trauma or tendon rupture, where hemorrhage and edema fill the pre-Achilles fat pad. Chronic changes may reflect tendinopathy, calcific tendinitis, or inflammatory arthropathy. A subtle convex anterior bulge of the posterior tendon outline within the triangle may indicate focal tendon thickening before frank rupture.
- Achilles tendon rupture — acute obliteration with loss of posterior tendon margin
- Achilles tendinopathy — chronic irregularity or thickening of the posterior boundary
- Retrocalcaneal bursitis — soft-tissue density in the inferior angle of the triangle
- Calcific tendinitis — calcific deposits within or adjacent to the triangle
- Inflammatory arthropathy (e.g., rheumatoid arthritis, seronegative spondyloarthropathy) — diffuse soft-tissue swelling effacing the fat pad
Reference: Moeller T. Normal Findings in Radiography. TFL. (2000).
Imaging Notes
Assessment of Kager's triangle is performed on a true lateral radiograph of the ankle with the foot in neutral or slight plantar flexion. The X-ray beam should be centered over the lateral malleolus, and strict lateral positioning is essential to avoid rotational overlap that can artificially obscure the fat pad. The triangle is best evaluated by tracing the three borders: the posterior margin of the flexor hallucis longus tendon anteriorly, the anterior margin of the Achilles tendon posteriorly, and the superior cortex of the calcaneus inferiorly.
When reviewing the lateral ankle film, systematically compare the density within Kager's triangle to the adjacent subcutaneous fat. Any asymmetric increased opacity, convexity of the Achilles tendon outline, or inferior recess soft-tissue fullness should be documented and correlated with clinical findings. MRI remains the gold standard for characterizing internal tendon signal, but radiographic evaluation of Kager's triangle provides a rapid, readily available first-line assessment.