From the research , we can summarize that it is the clinician ’ s job to master a few casting techniques to provide the desired level of care for their clients , whether that be in subtalar neutral or not , using full-weight bearing , semi-weight bearing , or a non-weight bearing casting method . The type of weight-bearing casting method ( full , semi , non ) is in large part easier to break down the casting techniques as opposed to what medium is being used . The different mediums being direct mould , foam box , wax and sand , computer-generated contact digitizer , plaster casting , plaster sock and computer-generated laser foot scan .
In this article , I review the different methods and the mediums of foot orthoses casting . Each has pros and cons . It is up to the clinician to decide which of these techniques best suit their clientele and to master them .
Full-weight bearing and semi-weight bearing casting
Full-weight bearing casting is achieved in a standing position with the client using their own body weight to produce the cast . This method produces a fully compensated , closed kinetic chain cast impression in which the foot is not in subtalar neutral . It is used when cushioning is the goal and motion control is not of much concern . Examples of when this method could be chosen are a client with diabetes who has a Charcot Joint or a client with severe rheumatoid arthritis .
Semi-weight bearing casting is achieved in a seated position with the hip , knee , and ankle all at 90 ° angles . This is still a closed kinetic chain , semi-compensated position . The most common method is to cast the foot in subtalar neutral . This is achieved by the clinician holding subtalar neutral by using one hand to landmark while the other hand externally rotates the client ’ s tibia . Semi-weight bearing casting allows the clinician more control in the foot positioning , which allows for an orthosis with more components of motion control . Semi-weight bearing casting has been used for clientele where the amount of correction has to do more with foot position , shell material selection and posting principles .
To capture both a full- and semi-weight bearing cast , a direct mould , foam box , wax and sand or computer-generated contact digitizer can be used ; which medium a clinician uses depends on the criteria .
Direct mould casting
In the direct mould process , the heatmouldable material used to cast the foot becomes the shell of the orthosis . The weight-bearing client may wear an insulated sock to protect the foot from the heat .
The material is heated and placed on a foam block or in some sort of footwear . The client holds the foot in the same position until the material has cooled . For semi-weight bearing direct moulding , the big difference is that the clinician compresses the cast / shell material into the desired shape . This is more labour intensive and has a greater margin of error for optimal foot positioning .
Options of making a more controlling shell are very limited due to the nature of the heat-mouldable materials that can be heated directly against the skin . Higher durometer materials need to be heated at too high of a temperature to allow direct molding against the foot . Moreover , common posting materials would also be too high of a durometer to be used in conjunction with the materials used to make a direct mould cast / shell . This technique is used extensively in the retail industry , such as in ski shops .
Advantages of direct mould :
• Quick method of orthotic manufacturing .
• Good technique if a flexible device is required .
• Very clean cast ; not as messy as plaster .
• Fewer supplies are required .
• Can still work with the shell when it comes off the foot .
• Inexpensive .
Disadvantages of direct mould :
• Plantar pressure of the foam tends to supinate the foot around the longitudinal mid-tarsal joint axis , levating the first and exaggerating a forefoot varus .
• First and fifth rays can both “ float ” if the casting foam is too rigid .
• Difficult to get the material tight to the foot , allowing too much tissue expansion .
• Difficult to know if the mid-foot is locked and subtalar neutral has been obtained .
• Must be aware of the temperature of the material before placing against foot .
Foam box casting
The foam box is one of the most common casting mediums in pedorthic clinics . When the clinician uses foam box in a full-weight bearing cast , the client places a foot lightly on top of the foam and slowly applies weight as the foot sinks into the foam . The clinician attempts to hold subtalar neutral in this process ; however , this is difficult to achieve due to lack of strength to hold the desired position while the client is applying pressure . The full-weight bearing foam box method works great for rigid foot structures .
Semi-weight bearing foam box is the more common of the two weight bearing techniques . Even though it takes the strength of the clinician to push the foot into the foam , they can use different techniques to achieve their desired cast results . One of these techniques is the gait referenced casting , which was developed by Edward Glaser , DPM . 3
For both weight-bearing methods , it is common to use “ foam art ” to modify the cast . For example , the clinician uses a finger to take away foam from an area of a bony prominence to increase offloading .
Current Pedorthics March | April 2012 11