negative casting

negative casting

; negative mould mould of foot/part of foot, formed using low-loss plaster of Paris (PoP) bandage, dental impression material, casting foam or putty, from which a positive cast is made (Box 1)
Box 1: Negative (slipper) casting technique
  • Patient lying prone or supine with foot projecting beyond end of couch foot rest

  • Mark the vertical bisection of the posterior aspects of the lower leg and heel; check the position of the foot is in subtalar joint (STJ)-neutral

  • Cut three strips of plaster of Paris (PoP) bandage, each twice as long as the distance from the medial side of the 1 MTPJ - heel - lateral side of the 5 MTPJ

  • Lay the strips of PoP bandage on top of one another and wet them in a water bath

  • Lift PoP bandage out of the water bath: smooth the wet bandage between the index and long fingers of one hand to squeeze off the excess water and form one thick layer of PoP bandage

  • Fold over 2cm along one long side to form the upper edge of the PoP negative cast of the foot

  • Place the damp bandage around the foot from medial 1 MPTJ to lateral 5 MTPJ so that the sides of the foot are covered with the bandage and the folded upper edge of the bandage reaches up the sides of the foot, as far as the inferior margins of the malleoli

  • Carefully mould the damp bandage around and under the heel, into the instep and across the sides and plantar aspect of the foot, carefully smoothing out all creases; the PoP resembles a slipper that does not cover the toes

  • Ensure, by thoroughly smoothing the outer surface of the bandage, that there are no air pockets between the bandage and the skin and that there is perfect skin contact between all the covered areas of the foot and the inner surface of the bandage

  • Place the foot in STJ-neutral and maximally pronate the midtarsal joint (MTJ):

    • Prone patient: either align the foot on the leg and apply gentle traction to the lesser toes whilst keeping the MTPJs in their neutral position

    • Prone patient: or apply thumb pressure over the plantar aspect of the 4/5 MTPJs to lock the metatarsal joint in neutral

    • Supine patient: align the foot by placing the thumb of one hand along the plantar aspect of the lesser toes (from 2 to 5 toes) whilst holding the dorsa of the toes and keeping the MTPJs in their neutral position; the foot is maintained in STJ-neutral position as the weight of the patient's foot is suspended from the toes

  • Maintain the foot in STJ-neutral until the bandage has fully set

  • Carefully separate the skin from the negative cast, so that the cast can be eased off the foot, as if it were a slipper being removed from the foot

  • Check the inner aspect of the negative cast: normal skin creases should be visible on the whole of the inner surface; the cast should be an exact model of the foot in the STJ-neutral position and should have captured all pathomechanical features of the foot

  • If the negative cast has been taken with the patient in position 10.2, the hollow overlying the 4/5 MTPJs, created by the pressure of the clinician's thumb, should be gently eased out so that the negative cast recaptures the true contours of the plantar aspect of the foot