Support the foot. This is where you start, and your efforts here will affect everything that follows.
A more thorough dissertation on the topic:
Most feet collapse under load to the medial side. Fewer feet roll to the outside. Fewer still roll not at all. You wish you were in the last category, despite what you read in the funny papers.
Preliminary foot exam, and why I don’t do them.
Used to be, I would look a foot over before taking an impression. At some point I realized that the casting process was far more objective than I was, so far as identifying the need and extent of posting, and so I quit with the exam, so as to avoid assumptions. Accuracy improved, and so that was that.
Footbeds come in a variety of types and grades of quality. Anything that provides better contour underfoot without discomfort is a move in the right direction. Better contour provides more data points, and that helps the brain ‘see’ what is going on down there.
Moving beyond contour, there is the subject of support. This may be defined in as many ways as there are different fabricators/installers. The evaluation of the need for support will vary as well.
Back when, I was taught that if I could identify less than 6 degrees of variance in the plane of the forefoot relative to the hindfoot, I should do nothing to address it. Since then, I have learned that if you work to the operating tolerances of the CNS (central nervous system), you get far better results in terms of effect, and the product (footbed) will last for quite a long time, as well.
If you do not account for the mobility of the bones in the forefoot, your client will need to adapt constantly to instability. This usually appears as lost range of movement, excess muscle activity and fatigue, discomfort, and inopportune input directed to the ski.
There are 3 primary methods for taking an impression, or mold, of the foot: Weighted, semi weighted, and non-weighted. Each has advantages and disadvantages, some of which favor the fabricator, and others, which favor the end user.
For a ‘solid’ foot (in the distinct minority) any method will work.
The weighted impression has the advantage for the busy retail environment, in that the mold is of the collapsed foot, and the odds of discomfort are lower. However, the only real gain here is contouring, as actual support of the mobile bones will be minimal. The other thing to bear in mind is that whenever a foot is weighted, even if only slightly, the CNS will try to use that foot for stability, and the associated muscle activity will color the contour to some extent.
A semi-weighted cast can be very accurate with regard to both support and comfort, but requires the most of the fabricator to avoid customer complaint. Not well suited to retail, due to time constraints and the cost of a skilled fabricator.
Non-weighted casts tend to be the most complete with regard to contouring, but they also tend to provide too much arch fill with too little metatarsal support, and thus many find them unsuitable from a comfort standpoint.
‘Posting’ refers to material added to the underside of the contour, for support and/or structural integrity. Posting varies considerably in density, depending on the fabricators goals and the reality of their work environment. Firm posting lasts longer and provides better support, but requires more operator skill, while softer posting foam is more forgiving of molding/trimming mistakes, but will not last as long.
Word has it, that the foot changes shape roughly every 4 years. More likely, the average footbed has a ‘4 year’ lifespan; by which point the materials have broken down to the point of discomfort.
The foot adequately supported, you can move on with confidence to the next adjustment in the sequence.