Plaster Impression Casting

Impression casts for custom fabricated orthoses is taken with messy Plaster-of-Paris.  Not only is plaster messy, it is usually quite fun for kids.   It takes several minutes for the plaster to harden while the practitioner positions and maintains the lower limb over the ankle and then the joints of the foot.  The contours are molded while the wet plaster dries firmly in the desired shape.  There are many techniques in removing the impression model without having to use a noisy, vibrating cast saw.

 

Plaster impression casting for AFO. The cast will need to extend more proximal than the AFO trimlines and more distal to end of toes even though the distal trimlines will most often extend to the MPJs
After gently cutting along the metal protective strip the negative plaster model can be gently opened and slipped off of the foot-ankle.
The negative model is closed and set aside to dry
Position leg over ankle and capture off weight midtarsal joint contours; emphasis lateral-plantar column rather than medial long arch
Rolling plaster bandage around foot

 

The negative plaster impression model is then used to create a positive statue/model of the leg and foot once a thick mixture of plaster is poured into the negative model and allowed to dry.

Others advocate using thin fiberglass rolled as the plaster is rolled to conform to the lower leg, ankle and foot.  Fiberglass tape never forms as well as plaster.

THE CONCERN:                If a similar technique is used to pour plaster into the fiberglass negative model, the fiberglass becomes embedded into the plaster slurry as it then hardens.  It is doubtful that a true custom orthosis are fabricated from any fiberglass impression cast.

Instead, the fiberglass models are more often sent to a Central Fabrication facility where the cast is simply used as a starting guide.  A pre-made orthosis is selected that matches the size of the cast reasonably well and the orthosis can be altered later with customizable parts.   Unfortunately the parents believe that it is custom-made for their child;  it is NOT.

Many professionals ask when a pre-fabricated orthosis is useful or beneficial.  The answer is simple.  If the child can potentially benefit from an orthosis, then Never try to make them fit into an orthosis not designed specifically for them.  If they are in need and could potentially benefit from an orthosis of any type, it must be custom designed, fabricated. and fit.