Key Insights On Digital Casting Techniques

Author(s): 
Bruce Williams, DPM, FACFAS

   Most of the scanners mentioned utilize either a non- or partial weightbearing position when scanning the foot. The scanners allow for a true 3D representation of the plantar foot and, to some extent, the sides of the foot and heel in that either the laser light, the pressure pins or the 3D image reconstruction are able to wrap around most of the architecture of the foot during scanning.

   Pressure mat scanning uses static or functional evaluation of the full weightbearing foot. In most instances, pressure mat scanning cannot scan or come into contact with the complete architecture of the plantar surface of the foot. This form of scanning also cannot make a sufficient outline of the sides of the foot or the posterior heel.

   Several studies have attempted to prove that a three-dimensional image can be constructed with the patient walking over a two-dimensional pressure mat.2,3 While static and functional pressure images of the foot can provide much beneficial information, the ability to create a truly custom conforming, three-dimensional image from the patient simply standing or walking over a two-dimensional pressure mat is still questionable.

   Studies comparing plaster impression casting, digital impression scanning and foam box impression casting have attempted to make some sort of determination on reproducibility and repeatability of the processes (i.e. plaster technique, digital imaging or foam box casting).4-8 The studies focused on arch height comparisons and comparisons of the forefoot to rearfoot relationship and heel width to an independent assessment of the patient’s foot in non-weightbearing neutral position. It is not surprising to learn from the studies that the non-weightbearing casting, whether the researchers employed digital scanning or plaster, were most consistent to independent practitioner evaluation of the non-weightbearing foot.

   In many of the aforementioned studies, feet cast in partial weightbearing often had lower arches, both medial and lateral, as well as wider heels. The forefoot to rearfoot repeatability often had little if any difference whether the patient was in a non-weightbearing or partial weightbearing position.

   Many practitioners have found the partial weightbearing technique easier to learn. This is apparently due to the reference of the foot to the flat plane of the scanner or foam box. It also may be due to the positioning of the hip, knee and ankle all being at 90 degrees during casting. With experience using the partial weightbearing technique, I have been able to reproduce almost exact devices from either technique, non-weightbearing or partial weightbearing scanning or casting.

   There are definitely problems with some of these studies in relation to their performance of partial weightbearing casting and scanning. First of all, most of the practitioners who were involved in the casting studies were already familiar with non-weightbearing plaster casting. However, most of them had little, if any, experience with partial weightbearing casting of any type.

   When the studies looked at partial weightbearing casting, using digital scanning or foam boxes, there was a very glaring error in the technique utilized because researchers made no attempt to “lock” the midtarsal joint during the partial weightbearing casting technique.

Key Biomechanical Considerations In Addressing The Forefoot Component

   As stated in the Root text, “With the subtalar joint in its neutral position, and with the forefoot pronated to its maximum about both axes of the midtarsal joint, the plantar plane of the forefoot parallels the plantar plane of the rearfoot. In this position, the forefoot is locked upon the rearfoot so as to resist any further motion of the forefoot in response to any dorsiflexion or eversion forces directed against the planter surface of the forefoot by ground reaction.”9

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