Cast Correction & Mold Design
Our lab is able to process digital foot scans, plaster negative casts, STS Slipper Sock negatives and Bio Foam impressions to create images that are used to design the positive molds or direct mill EVA orthotics.
The CAD/CAM application provides a method to correct imperfections and distortions in the impressions.
Using the scanned images, positive molds are designed and produced which are then used for thermoforming the orthotic shells.
Our standard is to balance the negative casts or images by either inverting or everting them to the position that the bisection of the posterior aspect of the heel is perpendicular to the weight bearing surface. This is known as "balancing" and in effect provides an intrinsic forefoot post. If you have a reason not to balance the casts, indicate that on the Rx.
It is essential to specify the amount of arch fill of the positive cast. Each lab will have its own standard amounts offered on the Rx form. Arch fill is the addition of material to the positive cast or mold in the medial arch area. The less fill added to the positive mold, the tighter the orthotic will contact the medial aspect of the foot. This helps to provide more control of a flexible flat foot and helps to off load the forefoot in a cavus foot. Some pathological conditions will require a moderate or maximal amount of fill in the arch of the positive mold in order to provide an orthotic that is tolerated by the patient.
The amount of arch fill of the positive that you specify will also depend on the impression technique used. Each person develops his or her own method of taking impressions so the resulting shapes of the impressions will vary from one practitioner to the other.
The standards for positive cast arch fill for our lab are: No fill, Minimal fill ~ 1/16", Standard fill ~ 1/8", and Maximum fill ~ 1/4".
When designing the positive molds, the amount of heel expansion fill should also be considered. Because most impressions are taken non-weight bearing, the soft tissue around the heel at the area where the top edge of the heel cup contacts the foot is narrower (on average 4 mm) than when the foot is weight bearing. By expanding the heel area of the mold relative to the impression of the foot, there is less soft tissue irritation caused by the concentrated force of the orthotic shell in this area. Using a caliper, you can measure the width of the heel with the patient weight bearing. This measurement can be used by the lab to ensure that the width of the heel cup is correct.
The Blake inverted cast technique has been reported to be very helpful in controlling hypermobility of the foot. We are able to make these modifications to the positives in any amount up to about 30 degrees. There are several references in the podiatric literature which will provide a full explanation of this technique.
The Kirby medial heel skive is another modification that can be done to the positive molds which is effective in controlling the rearfoot by providing an intrinsic "medial wedge" or lift in the medial aspect of the heel cup. The amount of skive is usually specified as either 2 mm, 4 mm or 6 mm. The measurement of the skive refers to the thickness of material that is removed from the positive mold under the medial aspect of the heel. When the orthotic shell material is formed to the mold, a medial wedge is created. If the heel cup is too narrow or too shallow, lateral heel irritation can occur. Therefore, practitioners should order deeper heel cups when ordering heel skives.
A lateral heel skive can be made on the positive in order to help evert the rearfoot in the treatment of lateral ankle instability.
Plantar fascia grooves (PFGs) are concave intrinsic accommodations that are placed in the orthotic shells corresponding with the medial aspect of the plantar fascia. They may also be filled with Poron to cushion this ligament.
When specifying the heel cup depth, the main consideration should be the amount of rearfoot control desired. The deeper the heel cup the more surface area of the orthotic is in contact with the heel. A very shallow heel cup can be prescribed if the patient will be wearing the orthotics in dress shoes. Heel cup depths for custom foot orthotics can be ordered from between 4 mm to 25 mm depending on the type of shell material ordered. TL 2100 can only be formed with a heel cup less than 15 mm deep. A "standard" heel cup depth for an average size adult foot (size 7 to 12) is 14 mm. For adult sizes under 7 a 10 mm or 12 mm could be considered "standard" and for adult sizes over 12 an 18 mm heel cup depth could be "standard".
The orthotic width refers to the width of the shell at the distal aspect. However, as the front is made wider or narrower, it also slightly widens or narrows the device at the medial arch area. The wider the device, the more surface area will be provided for more control and off loading. A narrow device may be ordered when there are issues with shoe gear.
Definitions of Orthotic Widths as listed on Cox Orthotics Rx form are as follows:
Normal Width: the anterior edge of the device extends from the lateral aspect of the 5th metatarsal head to the bisection (center) of the 1st metatarsal head.
Narrow Width: the anterior edge of the device extends from the lateral aspect of the 5th metatarsal head to a point 3 mm lateral to the bisection (center) of the 1st metatarsal head.
Wide Width: the anterior edge of the device extends from the lateral aspect of the 5th metatarsal head to a point 3 mm medial to the bisection (center) of the 1st metatarsal head.
Extra Wide Arch: the medial aspect of the arch area is made approximately 1 cm wider than normal on an average size foot, and the anterior edge of the device is wide. This provides more surface area for more control.
If you need orthotics shaped to a width other than those listed above, you can request specific dimensions.
Medial Flanges A medial flange is an extension of the arch of the shell which projects upward with the apex at the area of the navicular tuberosity. The purpose of the flange is to provide a more direct force at the medial-plantar aspect of the foot in order to offer more control. A sweet spot accommodation is added when there is a need to dissipate point pressure at the navicular. For some patients, fitting a device with a flange in their shoes can become a problem due to the added bulk of the device.
Polypropylene is the material of choice when flanges are required. We cannot fabricate orthotics with flanges using Carboplast or TL 2100.