Subtalar Joint Axis Variations

Subtalar Joint Axis

Normally considered being angulated 16 degrees from the sagittal plane and 42 degrees from the transverse plane – these are the averages reported in the literature, but a wide standard deviation about these has also been reported.

Clinical techniques to determine STJ axis position may not be determining STJ axis, but a composite ‘rearfoot’ axis.

Two variations have clinical implications:
1) Transverse plane variations:
• axis may be more medial or more lateral than the assumed normal position
• ground reactions force are assumed to be equal on both sides of the ‘normal’ axis
• if the axis is more medial, more of the foot (and hence ground reaction forces) are lateral to the axis  excessive pronatory moments/forces
• if the axis is more lateral (less common), more of the foot (and hence ground reaction forces) are medial to the axis  greater supinatory moments to the foot (ankle sprains are more common)
• if axis is medial, a greater force from an orthoses will be needed to supinate the pronated foot
• if the axis is lateral, too much force from an orthoses can predispose to too great a supinatory force  predisposes to ankle sprains

2) Sagittal plane variations:
• as the axis is angled approximately 45 degrees to the sagittal plane, approximately one degree of calcaneal motion in the frontal plane will equal one degree of motion in the transverse plane
• if the axis is more vertical than the 45 degrees, there will be more rotation of the leg with less motion of the foot – research has shown that patients with this deviation get more pronatory related leg than foot symptoms
• if the axis is more horizontal than the 45 degrees, there will be more foot motion than rotation of the leg – these people tend to have more leg than foot symptoms
• those with a more vertical axis will demonstrate more transverse plane motion of the foot during gait, especially and abductory twist
• those with a more horizontal axis will demonstrate more frontal plane motion during gait

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