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The Primus Metatarsus Supinatus foot type is the end result of a failed or incomplete unwinding of the talar bone during its embryological development (Rothbart 2002 JBMT). The above animated foot model demonstrates (1) the position of the talar bone in the foot and (2) a frontal plane perspective of the talar head unwinding.
The unwinding of the talus is torsional. As the talar head unwinds (untwists), the entire medial column of the foot unwinds (which includes the medial portion of the navicular, medial cuneiform, first metatarsal and hallux). Failure of the talar head to completely unwind results in the failure of the navicular, medial cuneiform, 1st metatarsal and hallux to completely unwind (referred to clinically as the Primus Metatarsus Supinatus foot type - PMs).
