OVERVIEW OF CASTING TECHNIQUES
Advantages of foam box casting :
• Fast and clean .
• Easily transportable .
• Adequate soft tissue displacement .
• Good pressure from the foam helps load the lateral column .
• Allows soft tissue expansion to occur .
• Client can remain in chair for casting and does not have to get on a plinth .
Disadvantages of foam box casting :
• Any positioning is usually lost with even partial weight bearing .
• Force of foam can distort hyper-mobile forefoot alignment .
• Poor visual confirmation of the foot position .
• Too much help from the patient can distort the impression .
Sand and wax casting
The wax and sand medium is used in some pedorthic clinics . A thin sheet of wax is heated in water and placed on a metal tray that holds levelled sand . The client then stands or sits in front of the casting box and places the foot in the box until the wax has cooled . Once the wax hardens and becomes brittle , the cast is removed from the sand box and the client ’ s foot . 4 This is a form of direct moulding .
Advantages of sand and wax casting :
• Allows soft tissue expansion to occur .
• Yields a positive that is extremely clean .
• No clean-up on the patient .
Disadvantages of wax and sand casting :
• Can be time-consuming .
• Requires storage of the sand box and materials .
• Clean-up is needed for any spilt sand .
• Any positioning is usually lost with partial weight bearing .
• Poor visual confirmation of the foot position .
Contact digitizer casting
The computer-generated contact digitizer is the technologically advanced casting method for full- and semi-weight bearing . This technique uses CAD / CAM software to obtain the three-dimensional cast . With the contact digitizer , the client stands on or sits in front of the unit and the digitizer elevates numerous pistons that contact the foot ’ s plantar surface . The 3-D image is then transferred electronically and viewed on a computer , where the image can be modified on the screen . This system is commonly used in pedorthic clinics .
Plaster casting allows a lot of control and flexibility in making the negative cast .
Advantages of contact digitizer casting :
• Accurate plantar contour measurement .
• On-screen cast modifications with software .
• No distortion by operator or materials between scan and fabrication of orthosis device .
• Quick and clean scanning and manufacturing .
• Cast storage and duplicate orthoses greatly simplified using available software .
• Positive perception by client of the procedure .
Disadvantages of contact digitzer casting :
• What you cast is what is manufactured ; a poor cast is a poor orthosis .
• Systems are quite heavy , making mobility difficult .
• Expensive , especially if equipment fails .
Misconceptions :
• Office scanners provide diagnostic information .
• Result is more exact because it ’ s done with computers .
• Technology replaces the skill of the practitioner .
• CAD / CAM technology is only for large institutions due to cost .
Non-weight bearing casting
Non-weight bearing casting is achieved with the client lying prone , supine or in a seated position . Whatever position is used , the most common method is to cast in a subtalar neutral position . This is an open kinetic chain , in an uncompensated position . To achieve subtalar neutral , the clinician places their thumb between the sustentaculum tali and the medial malleolus with the index finger in the sinus tarsi cavity on the lateral portion of the talus . The thumb of the other hand locks the calcaneocuboid joint by applying a dorsiflexion force to the fourth and fifth metatarsal heads until resistance is felt in the ankle joint . When the client lies in a prone position , the clinician is best able to see the forefoot and rear foot position . However , prone position is harder for the client to manoeuvre into .
The other method of casting in nonweight bearing is the hang technique , 5 which is not used very often . With this method , the client lies supine or prone and the cast is taken with no loading of the forefoot , and subtalar neutral positioning does not occur . All types of orthoses can be produced from non-weight bearing casts . This method of casting is most used when motion control is the primary objective .
Plaster casting
Along with foam box casting , plaster casting is the other most common casting medium in pedorthic clinics and the medium used in the pedorthic certification exam . It requires the most skill in how to apply and position the
12 Pedorthic Footcare Association www . pedorthics . org