Transverse plane postion of the subtalar joint axis:
• patient prone, leg rotated so that foot is vertical
• the plantar surface of the calcaneus is palpated with the thumb to invert and evert the foot along a medial/lateral line near the posterior aspect of the calcaneus – the point at which the foot does not invert or evert is considered to be the location of the axis
• the above is repeated more distally on to obtain several more points, so a line can be drawn on the plantar aspect of the foot, that represents the axis
• the process is more difficult when attempts are made to determine the axis distal to the midtarsal joint (it an still be done, but with caution)
Normally, the axis should pass from about the centre of the heel to about the first metatarsal head. Wide variations between individuals are noted.
• position of axis is correlated to the amount of force needed to supinate the foot (ref)
• if a foot is pronating excessively at the subtalar joint, a foot orthoses that is to be used to counter this must apply its force medial to the axis – determination of the position of the axis can be used to help decide where the orthoses can apply its force
Terminology – the literature widely refers to this as the subtalar joint axis, however, as the calcaneus is inverted/everted on the talus at the subtalar joint, a greater range of motion is occurring between the navicular and talus at the talonavicular joint. It may be more appropriate to refer to this clinical technique as finding a ‘rearfoot axis’.