METHODS Fourteen cadaveri specimens were sectioned to analyze the internal architecture of the human calcaneus. We described the arrangement and orientation of trabecular patterns within the calcaneus and made multiple measurements of its cortical thickness. OBJECTIVE To characterize the internal architecture of the calcaneus and correlate these findings with well-described patterns of calcaneus fracture in order to better understand the fracture mechanics of this common fracture. METHODS Fourteen dry, frozen, human calcanei were sectioned using a saw. In each the coronal, sagittal and axial planes, we sectioned separate specimens into slices of 0.5 mm thickness. High-resolution radiographic images were taken of the sectioned specimens. The internal trabecular arrays were described and measurements of cortical thickness were recorded. The correlation between these findings and the known pattern of calcaneal fractures was analyzed. RESULTS A dominant trabecular pattern running antero-posteriorly along the long axis of the calcaneus was observed. In the posterior tuberosity the trabeculae were arranged parallel to the posterior border. There was an area of sparse or absent mineralization in the anterior part of the calcaneus corresponding to the "neutral triangle" described by Wood and Harty 10, 23. The thickest sites of the calcaneal cortex were the lower pole of the posterior tuberosity, the upper surface at the angle of Gissane, and the lateral surface below the anterior portion of the posterior facet. CONCLUSIONS The trabecular architecture of the calcaneus is created by applied stress in concordance with Wolff's law. The weakest plane of resistance to stress is parallel to these organized trabeculae or through areas lacking trabeculae. This study demonstrates that the primary and secondary fracture lines commonly encountered in calcaneus fractures correlates with the internal architectural map of the calcaneal trabecular patterns.