Negative Cast Pouring Position: 3 Typical Scenarios

This evening I am pouring plaster of Paris liquid into the negative cast to make a positive cast. Tomorrow I will begin the process of correcting that positive to a shape I desire to help with the patient’s pronation pattern. I have 5 pairs of casts to deal with this weekend, and this left foot, shown in 3 possible positions, is my most pronated foot of the 10. The RCSP, or resting heel position, in the patient is 12 degrees everted to the ground. The forefoot to rear foot measurement during my examination was over 10 degrees fore foot varus. The first method of cast correction is an ASIS correction. Therefore, you pour the cast at this 12 degree everted position. Although this is a very pronated subluxed position, it is assumed the foot pronates even further when standing. You are pouring, or correcting, this foot in a position of severely pronated, but you are not attempting to create a new arch (sometimes a great challenge), and you are stabilizing this foot with arch re-enforcement, deep heel cups, posts to add motion and stability, etc. This is a viable method especially with shoe fit issues, or previous arch intolerance. More in later posts. 

This same foot can be poured vertical, the classic Root technique, attempting a 12 degree change (12 degree everted to vertical). Of course this does not happen, but the orthotic made off this Root Balanced device, will be be changing and stabilizing the foot. I typically found a great need for orthotic adjustments with dispensed these for this foot, as the force of correction was too much in the soft tissue of the arch and forefoot. I began expert in making them comfortable with compromise, thus needing a new orthotic within the year as the forefoot varus (supinatus) begun less. Again, more in a later post. 

Here, this same left foot, which stands 12 degrees everted in resting position, is being poured at 20 degrees inverted to make the Inverted Orthotic Technique. With this technique, the arch is filled in to not block first ray plantar flexion vital for propulsion (per Dr Dananberg). 20 degree correction will typically change the heel valgus position from 12 everted to 8 everted. All the other good orthotic device stability measures: plastic thickness, deep heel cups, full shoe width, zero post motion, all help in making this a very stable device. Again, more in a later post. 

     Here I summarize 3 orthotic negative cast pouring positions that I have used to help with pronatory symptoms and stability. 3 different pouring positions from the lab are used for this severely pronated left foot where the fore foot varus equals the RCSP (not an uncommon issue). I propose that good/great stability to the foot will be accomplished by all techniques. I recommend re-casting a foot like this 6 months to a year after dispensing the orthotic devices (you should get a reduction in fore foot varus angulation). I firmly believe that if I can produce angular change (say 12 degrees everted resting position towards a vertical heel), overall the patient will be better off unless they have rearfoot valgus (like genu valgum). 

Thursday's Orthotic Discussion of the Week: Pouring the Negative Cast and Building the Anterior Platform for the Inverted Technique

Dear Dr. Blake,

Thank you very much for your blog and for this post ! I've read with great interest your post from Podiatry Arena and, as a self educated person [in my country doesn't exist podiatric schools] I want to tell you that your post is one of the best, expecially from a practical point of view !

 I have a question which maybe seems stupid for you and I apologies for this. I personnaly have difficulties with anterior platform building so I kindly ask you to give me some instructions regarding the composition of the pink plaster from photos. In my practice I make it either too fluid or too rigid and simple I didn't succeed to build the anterior platform in the way you have described here !

I'm expecting with great interest March, when you'll post your manual !

Thank you for your kindness !
Respectfully,

Robert (name changed)
Romania

Dear Robert,
     I am always honored by getting emails and compliments, and especially from so far. Thank you so very much and I hope I can help. Here are some photos that I hope will help. More questions are encouraged.

Here is the initial set up for pouring of the negative casts to make them a solid positive cast. The most important part of the photo is my coffee cup since it is 6:30 am on a Saturday morning. One basin is for the soap solution and one basin is for the casting plaster to be used in making the positive cast. 

For 2 feet I am pouring the plaster to make a positive cast I use 5 cups of plaster and 3 cups of water. There is the 1 cup volume rubber pouring bowl. For 2 pairs of feet, 4 total, I will use 10 cups of plaster and 6 cups of water. The basin I use holds up to 15 cups of plaster. 

This is the Casting Plaster I use with 30 minute Set Time. 

It is also called Red Tag 30. 

I use a one cup bowl, but you can buy these bowls that hold 5 or 10 cups at once to make life easier at times. Here the plaster is in the bowl and will be emptied into the basin to begin the process. 

I have fillled the basin with 10 cups of plaster good enough for 2 averaged sized pairs of feet. If both your casts are for sizes 12 or greater, consider 12 or 13 cups of plaster. The water ratio is still 60% approximately the plaster amount.

I buy a soapy concentrate and dilute with water 50%. This is poured into the negative cast to saturate the bottom and sides. Any extra soap is allowed to drip out and returned to the bottle. This makes it so much easier to get off the negative casting material once the positive is dried. 

Here are the negative casts after having been soaked thoroughly with the soapy solution and then being tipped over to allow the soap to drain. 

The negative casts are then leveled so that the heel bisections stand perfectly vertical when poured. This is done with every cast so that the top of the cast represents a parallel with the ground. 

Here my talented brother Bob is pouring water in a 60% ratio to the plaster with some colored dye to make the plaster solution for pouring. One pair of casts typically need 5 cups of plaster and 3 cups of water. 

The plaster (mud!!) is mixed thoroughly to get out any clumps of plaster so that the solution is a uniform consistency. 

Plaster clumps are completely broken apart

Then the very smooth consistent liquid plaster is poured into the balanced to heel vertical negative casts.

My great brother Bob skillfully pours the plaster to make the positive casts set a heel vertical.

Here the negative casts sit after the plaster is poured

Typically you can pour up to 3 pairs of casts in one basin. That basin would have 15 cups of plaster and 9 cups of water.

Once the negative casts are poured, it is important to recheck the heel bisection to make sure it has remained vertical. 

After pouring, sticks are placed into positives to minimize breakage with the high pressure vacuum press. 

Once you remove the casting plaster, the first and fifth metatarsal areas are marked to define the anterior platform borders. You find the contact point on the first metatarsal (lowest point) and then place the line 15 mm closer to the arch. I like to then even the 5th metatarsal platform with that line, but the fifth metatarsal is normally a little shorter. The vertical lines are in the space between the first and second metatarsals and the 4th/5th metatarsals.

Here you can see the proximal line 15 mm from the contact point. This will be where the plastic of the orthotic device ends. You do not want the plastic to run under the weight bearing surface when walking or running. 

Here the nails are placed on the platforms to make the angle for Fettig Modification of the Inverted Technique. Here the medial nail under the big toe joint will set the overall inversion and lateral nail under the fifth metatarsal head for the forefoot valgus correction.

When I am working with plaster, I typically have 2 or 3 bowls going at once. The plaster has to be right, not too runny and not to solid, to apply and shape. It takes time to learn your plaster. I try not to stir it much, for stirring will make the plaster harden faster. I try to pour off the excess water once the bubbles stop, for too long with excess water makes the plaster to take forever to dry. 

I use wooden sticks soaked for 10 minutes or so for making the transition from anterior platform to the medial arch fill more solid during vacuum press. The high pressures can break the platform away from the medial arch without the sticks in place.

After the nails are in place, and the plaster the right consistency, place a piece of paper down to make the anterior platform. 

Gently stir the plaster to check the consistency

It is so important to play with the plaster and know it's consistency

and play some more!

And more

I place dye into the plaster to make the various parts of the positive cast stand out

Mix it in well

When the plaster is ready, place it on a piece of paper and press the positive cast gently down. It is important not to push too hard to distort the nails. It is important not to push too lightly and lose the angle set by the nails.

Here the anterior platform area of the positive cast is coming in contact with the plaster mound

Gently, but firmly, I press the positive down into the mud

Then I use a spatula to make the medial and lateral edges straight up and down.

It is important to check your angles, if off from what was ordered, immediately knock off the platform and start all over again.

This photo is out of order but shows the positive casts, after being poured, drying for 1 hour in the sun. 

Here the wooden stick is placed into the anterior platform to set further strengthen the area before pressing. The total length of the stick coming out of the platform and into the arch will be 1 and 1/2 inches in general.

Another view of the stick

The platform is being formed following the lines on the positive cast.

When cutting the anterior platform, keep the spatula moist with water to allow easier trimming.

Do not lose focus on the shape of the platform desired and outlined by those lines 

The medial and lateral sides will still need to be trimmed to go straight up and down from the positive cast.

Another view of the 20 degrees Inversion with 6 degree forefoot valgus in the Fettig modification of the Inverted Orthotic Technique.

Trimming of the sides with scrapper

Final product with anterior platform with medial and lateral expansions.