Variations in the human subtalar joint.

Clinical observations reveal that high-arched cavus feet are rigid while low-arched planus feet are hypermobile. One reason may be osseous variations in the human subtalar joint (STJ). This article reviews STJ anatomy, biomechanics, and pathomechanics and describes STJ variations found in 32 adult human calcanei. In the sample, 20 bones had two STJ facets and 12 bones had three STJ facets. A oneway Student t-test showed that the bones with two STJ facets had significantly greater (t = 2.23) facet surface area than the bones with three STJ facets. A second finding concerned three sagittal plane approximations of arch height and STJ axis angle. Calcanei with two STJ facets had significantly lower angles (tA = -1.99, tB = -1.82, and tC = -2.18) than three STJ facet calcanei. To the extent that osseous factors represent joint range of motion, this study shows that STJ facet number is a factor in foot mobility. J Orthop Sports Phy Ther 1987;8(10):489-494.