Two people can walk out of the same clinic on the same afternoon, each carrying a custom device for their shoes, and the two devices can have almost nothing in common. One barely flexes when you press on it. The other gives under a thumb. Both were made from a cast of that person's foot. Both were prescribed by a chiropodist. And each would be the wrong device in the other person's shoe.

Prescribed inserts fall into two broad families: functional and accommodative. Which family yours belongs to explains most of what you will notice about it, from the pressure under your arch in the first week to how often it needs replacing.

One device corrects, the other protects

A functional device works on position and motion. It holds the foot closer to a neutral alignment through the parts of the step where the joints would otherwise collapse or roll too far. That means the shell has to be stiff enough to resist body weight without deforming.

An accommodative device works on pressure. It makes no attempt to reposition anything. It takes load that was concentrated on one small area and spreads it across a wider surface, so the material has to be soft enough to deform and contour.

Stiff and soft are not quality grades. They are two different clinical goals, and the rest of the prescription follows from whichever goal applies to your foot. If you are still at the earlier question of whether a prescribed device belongs in your treatment at all, our guide to custom orthotics covers that ground first.

Inside a functional prescription

The core is a semi-rigid shell, usually polypropylene or a carbon composite, cast or scanned to your foot and then modified. Three things in particular get adjusted: the depth of the heel cup, which governs how firmly the heel bone is held; the angles built into the rear or the forefoot, called posting, which change how the foot loads at specific moments in the step; and the stiffness of the shell itself.

Feet that tend to suit this approach include:

• flexible flat feet, where an arch is visible when seated and flattens on standing

• excess pronation showing up as inner knee ache or shin pain after activity

• plantar fascia pain where the tissue is being repeatedly overloaded

• early strain in the tendon running behind the inner ankle

• sport, where one foot has to tolerate thousands of near identical repetitions

The common thread is a foot that can still be moved. If the joints have the range to be repositioned, there is something to correct. Two of those presentations have their own write-ups, on flat feet and high arches in adults and on heel pain and plantar fasciitis, and gait analysis is often what reveals the motion a standing exam misses.

Inside an accommodative prescription

Here the materials do the work. Layered foams, usually EVA, Plastazote, Poron or something comparable, are stacked in different densities with the softest layer against the skin. The build is total contact: it fills the space under the whole sole so no single point carries a disproportionate share of the load.

Relief for a specific spot is often created by taking material away rather than adding it. A well under a painful sesamoid or a healed ulcer site lets that point sit lower than the tissue around it, so the surrounding area picks up the pressure instead.

Feet that tend to suit this approach include:

• rigid or arthritic feet whose joints no longer have the range to be realigned

• neuropathy, where sensation is reduced and a pressure point can do damage before it is felt

• a history of ulceration, where the whole point is keeping load off a vulnerable site

• thinning of the fat pad under the ball of the foot or the heel, which is common with age

• fixed forefoot changes, including long standing bunions and toes that no longer straighten

• a painful callus or corn that keeps returning under one metatarsal head

Reduced sensation raises the stakes. A device that conceals a pressure problem rather than relieving it can let skin break down quietly, which is why accommodative work for someone living with diabetes belongs alongside regular skin and sensation checks rather than on its own. Our diabetic foot care page and the post on warning signs worth acting on describe what that monitoring involves.

How the choice actually gets made

The deciding test takes seconds. A chiropodist moves the relevant joints by hand to see whether the foot can be brought toward a corrected position without force. A foot that moves can be worked with. A foot that resists needs protecting where it already sits.

Other findings feed in. Where the callus has formed tells you where load is going. Sensation testing changes the risk calculation. Body weight and activity level affect how fast a soft material will bottom out. Footwear counts too, since a device can only perform inside a shoe able to hold it. Gathering all of that is the job of the biomechanical assessment, which forms part of a standard chiropody foot assessment.

Imaging rarely enters this particular decision. It gets ordered when there is a question about a bone or a joint itself, not to classify a device.

Most real prescriptions sit between the two

Strictly functional and strictly accommodative devices both exist, and plenty of prescriptions borrow from each. A semi-rigid shell can carry a soft top cover and a metatarsal pad. A controlling rear section can be paired with a cushioned forefoot extension for a stiff big toe joint. Someone with a flexible arch and an arthritic big toe joint needs motion control behind and protection in front, in the same device.

This is also where the comparison with shelf products gets sharper. A store bought insert is one fixed shape in a handful of sizes, so it cannot be built around a particular painful spot or a measured joint angle. That distinction has its own post, on custom devices versus store bought insoles, and the custom foot orthotics page sets out how casting and manufacturing work at our clinic.

What you will notice day to day

The two types announce themselves differently. A functional shell is usually felt under the arch for the first week or two, and chiropodists typically start you at a few hours a day and build from there. Muscles and joints are being asked to work in a slightly new position, so some aching while that settles is expected rather than a warning sign.

A soft device tends to feel comfortable from the first wearing. The trade off shows up later, because foams compress, and a device that depends on its thickness loses effect as it thins. Rigid shells generally outlast soft builds by a wide margin, though top covers and pads on either type are consumable. Knowing when a device needs refreshing and when it needs replacing is the subject of our refit and replacement guide.

Shoes follow the device. Firm shells want a stable heel counter and a removable factory insole. Thick accommodative builds want depth and volume, and for some feet that means a shoe sold specifically as extra depth. If your week moves between activities, sport specific prescriptions is worth reading, because a device that fits a running shoe may not fit a cleat or a dress shoe.

Does the type affect your insurance claim

Usually not directly. Extended health plans tend to pay on the basis of a custom made device dispensed by a qualified provider with a prescription and supporting documentation, rather than on which family the device belongs to. What does matter is that the paperwork describes clearly what was made and why. The steps are laid out in our post on the Ontario orthotics claim process, and plan rules such as frequency limits are covered in extended health benefits for orthotics.

Frequently Asked Questions

My feet are stiff and arthritic. Does that rule out a corrective device?

It usually rules out a strongly corrective one, since correction depends on available joint motion. It does not rule out a prescribed device. A stiff foot often does well with a contoured build that protects the sore areas, sometimes with a small amount of control where some motion remains. A chiropodist can tell you after testing the joints by hand.

Could my prescription switch from one type to the other over time?

Yes, and sometimes it should. Feet change. Joints stiffen, fat pads thin, arthritis progresses and sensation can decline. A device that suited you a decade ago may not match the foot you have now, which is one reason a reassessment before a replacement is more useful than reordering the old specification.

Is the softer device the more comfortable one?

Early on, often yes. Across a full wear cycle it depends on the problem. When pain comes from tissue being overloaded by motion, cushioning can feel pleasant without reducing that overload, and the symptom tends to persist. Comfort in week one is a poor predictor of how a device performs by month three.

Do I need a scan or an X ray before this can be decided?

Not for the device type. That call comes from a hands on exam, your history, the pattern of skin changes and how you move. Imaging answers a specific question about a bone or joint, which is a separate matter from the prescription.

Can each foot get a different type?

They can. Feet are frequently not symmetrical, particularly after an injury, a surgery, or years of uneven loading. It is entirely reasonable for one side to be built for control and the other for protection.

Book an assessment

If you are wondering which kind of device your own feet call for, a chiropodist at Donwood Foot Clinic in North York can test the joints, read the pressure pattern and talk through the reasoning before anything is cast. Book an appointment online.