Position of oblique axis of the midtarsal joint:
This “convenient theoretical fiction” can be considered to be, on average, angle 52 degrees from the transverse plane and 57 degrees from the sagittal plane. Large deviation from this can be detected clinically.
• one hand is used to lock up the ankle joint and subtalar joint in about its neutral position by gripping the posterior aspect of the calcaneus
• as a pure dorsiflexory and plantarflexory force is applied to the forefoot with the other hand, the amount of adduction and abduction is noted
• if the amount of adduction/abduction is greater than the amount of plantarflexion/dorsiflexion axis can be assumed to be more vertical
• if the amount of adduction/abduction is less than the amount of plantarflexion/dorsiflexion axis can be assumed to be more horizontal
The findings of this method should theoretically match the findings of the navicular drift and drop observations. For example if the oblique axis is considered more vertical, then the amount that the navicular drifts should be greater than drift.
• the orientation of the axis can be used to predict the direction that the midfoot will move in if there is compensatory or excessive motion of the midtarsal joint
• if axis is more vertical (navicular drift > navicular drop) more compensation in the transverse plane during gait (observed as medial midfoot bulging)
• if the axis is more horizontal (navicular drift < navicular drop) more compensation in the sagittal plane during gait (observed as more collapse of the medial longitudinal arch)