NEGATIVE CAST CORRECTIONS
It's best to cast a patient in weight bearing, because that's the position the patient will most likely be in when wearing the orthosis. But, that's not always possible. Often the patient is spastic, in pain, or just plain uncooperative. In these situations a negative cast modification is in order. All too often, the negative cast correction is accomplished by cutting a wedge from the ankle crease, across the ankle center and to the apex of the heel. This method of correcting the planter\dorsi flexion at the ankle unnecessarily removes the vital ankle m\l measurement and does not accurately reflect the actual pivot point. (See fig. 1)
The best way to correct plantar\dorsi flexion at the ankle uses the anatomical ankle center (or it's approximate). Find the ankle height and draw a line completely around the cast. Then pick out ankle center (without tibial torsion), and circle it. This area will act as a hinge when the cast has been cut. In the case of too much plantar flexion, the line drawn across the Achilles tendon should be cut from pivot point to pivot point. Then a wedge should be cut out of the ankle crease area. Don't cut through the circles at ankle center. The cast should pivot freely at the ankle. Set it in the proper position and seal it together with plaster bandage. The same process is used in cases of dorsi-flexion, only cutting the wedge out of the Achilles tendon instead of at the crease. (See Fig. 2).
If the cast is in valgus\varus as well as plantar flexion, the pivot point can be rotated before being cut (fig.3). The shift in axis is measured by eye, but with some practice can be easily estimated. This method should only be used when the amount of correction is mild, as it does not reflect true ankle motion. In more severe cases, you must make a second correction after the first one has been sealed-up and set-up. This second cut will occur with the pivot points being placed at the crease and the Achilles tendon.
Cast supination\pronation can be corrected in a like manner, using only one pivot point. The location of the pivot to correct supination, for example, is located at the 5th metatarsal. A cut is made completely around the dorsal and plantar aspect of the foot, from pivot to pivot (once again, don't cut pivot point) and behind the metatarsal bone (fig.4a). Rotate the forefoot section downward (fig. 4b). In mild cases position the forefoot level with the heel. In more extreme cases, rotate the forefoot only half way . The cast should then be sealed up. When modifying the mold, the gap from the cut and some of the arch must be filled in to accommodate the correction.