Medial Arch Fill In to Positive Casts

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I'm a student and I'm a bit confused with a step in the positive cast preparation. I'm sure this is very fundamental: What is functional purpose of the medial arch fill in?

This plaster addition obviously alters the height of the medial longitudinal arch, however how does the height of the orthotic's medial arch affect the midtarsal joints? I'm guessing that a small medial arch fill in (say 1/8) will decrease midtarsal joint range of motion due to the STJ supinatory moment that an increased arch height would impart.

A quick explanation from someone in the know would be appreciated.

 

This depends on what you are trying to do with the orthoses. With a conventional "arch Support" you would have minimal arch fill to allow the orthoses to physically support the arch. However, you have to decide if the arch is actually falling in stance or if the first ray is being pushed by the ground dorsally. If the subject has an obvious plantar flexed first ray then when standing on a hard flat surface the ground will dorsally displace the distal end of the ray to the same level as the lesser metatarsals. You can do this in the non weight bearing assessment and see the arch apparently flattening. In this case the less arch fill used will support this dorsally displaced position and limit hallux extension into propulsion.

As a sagittal guy I nearly always maximally fill the arch adding an FHL/first ray cut away to allow first ray plantar flexion and iniating hallux extension. For feet with obvious mid tarsal instability I'll throw in a medial heel skive and deeper heel cup.

Hope this helps

 

Welcome Christie,

The purpose of the medial expansion is to prevent pain. If you stand with your foot hanging over the enge of a piece of plastic it hurts right at the edge. The purpose of the expansion is to flare the edge of the orthotic away from the foot. Jeff Root gave that back in 1984 at CCPM. That is true if you use the classic Root approach which is minimal fill. If you use minimal fill the arch of the device will often be uncomfortably high and labs realized this and started using more arch fill to make the arch height easier to tolerate.

As an aside the oblique and longitudenal axis of the MTJ are fictional conveniences. (Sorry Craig, for bothching the quote.) An axis of a joint is an imaginary line that describes motion and not a rigid hinge. The motion of the joint determines the axis and not vice versa. You can move a midtarsal joint in pure dorsiflexion plantarflexion. It is best to consider the MTJ as a planar joint with an infinite number of possible axes of motion.

Regards,

Eric

 

Thanks Eric and Graham. Your explanations make sense. I thought i'd fabricate a moulded orthotic from a positive cast with NO medial arch fill. I tried it on for a while but couldn't withstand wearing them. I've now realised first hand what you guys were talking about: medial arch fills do prevent pain!

 

Welcome to Sole Support orthoses! No medial arch fill....lots of discomfort...but biomechanically correct....or so the company says!:bang: