Midtarsal Joint ‘Oblique’ Axis (MTJOA)
The traditional model of the MTJ has an oblique axis that is angled 52 degrees from the transverse plane and 57 degrees from the sagittal plane. This model has been shown to be flawed, but accepting that the MTJOA exists for learning purposes, it is a useful way to understand foot function.
Clinically when the subtalar and ankle joints are held by one hand and then the navicular and cuboid gripped by the other hand and dorsiflexed, a wide variation of abduction with dorsiflexion from person to person is clinically observable. Understanding this variation is simplified if it is assumed as moving about an assumed MTJOA. If the axis is angled greater than the assumed 57 degrees from the transverse plane (ie more vertical), there will be greater abduction of the forefoot with MTJ pronation. If the axis is less vertical, there will be less abduction. Clinically a more vertical axis is seen as a greater amount of relative abduction of the forefoot on the rearfoot (or midfoot medial bulging). This has implications for the use of foot orthoses in that the foot may not be able to stay on top of an orthoses due to the excessive amount of transverse plane motion.
Nothing is really known about deviations of the axis from the sagittal plane or about deviations of the assumed longitudinal axis.