Casting Confusion ?
What to Know Before the Dye is Cast
BY SÉAMUS KENNEDY , C . PED .
One of the most frequent questions that I am asked during an in-service on foot orthotics is , “ What kind of cast do you prefer ?” I have given presentations to many different groups : pedorthists , orthotists and prosthetists , podiatrists , and physical therapists , and yet this same question always comes up .
Caught among the theory of plaster slipper casts ; the practicality of impression foams ; and the new choices with digital scanners , most practitioners are more than a little confused . I know there are strong viewpoints on this topic , so I will humbly offer some observations of my own . ( Please don ’ t write to the editor and complain . But feel free to share your thoughts through Current Pedorthics , e-mail asturman @ pedorthics . org .)
Plaster Cast
The plaster slipper cast is the gold standard of casting methods . It was popularized in the 1960s as Root et al . were developing the field of lowerextremity biomechanics . At that time there was an improved understanding of foot function and orthotics . The emphasis shifted from general arch support to sub-talar joint control . There were also significant changes in orthotic fabricating materials : thermoplastics replaced cork , leather and metal . All this new science required better ways to replicate foot shape . Plaster casting is the optimum method for capturing the foot in sub-talar joint neutral . It is done non-weight bearing with the patient lying either supine or prone , while the thumb and index finger load and control the fourth and fifth met heads holding the midtarsal joint at the end of its range of motion . A good cast cannot be rushed , and the inside surface should reveal some of the lines of the skin . Specific anatomical points or areas in need of relief can be indicated in pencil . Marking calcaneal bisection is also helpful when intrinsic balancing or posting is required . There is also an STS synthetic slipper “ sock ” available that mimics plaster without the delay or mess . The slipper cast is particularly useful when you are designing functional foot orthotics . Their aim is to improve the biomechanical function of the foot . It is beneficial in applications such as sports orthotics , where precision is required in order to obtain maximum benefits . It is also good for flexible foot types , where weight-bearing casts may only capture a fully pronated foot structure . However , after more than a decade of owning a custom foot orthotics lab , I have witnessed some pretty unusual things come through the door that are supposed to represent the shape of a foot . Thus , I give you my first rule of plaster casting : make sure that the final cast is a true image of the patient ’ s foot . Being honest , rather than academic , will be more helpful to all involved . If you lack time , plaster splints or good technique , then it may be best to avoid the plaster slipper cast .
Impression Foam
Impression foam casting has the obvious advantages of being both quicker and cleaner . It offers a good method of creating a negative of the patient ’ s foot . Approximately 75 percent of all casts that we receive are taken with impression foam . I usually recommend that practitioners take a semi-weightbearing cast when using foam . The patient should be seated in a chair with both feet plantargrade . It is important that the cast be taken slowly with the practitioner in control and guiding the descent of
16 Pedorthic Footcare Association www . pedorthics . org