Using Preoperative Molds to Decrease Operating Room Time When Applying External Fixation Devices
Mark F. Devens, CO
ABSTRACT
The purpose of this article is to convey the method and materials used to assist the surgeon in perioperative construction of various external fixation devices. The use of these devices resulted in a decrease in operative time, efficiency in constructing the fixation apparatus, and the provision of a teaching tool for medical personnel along with family and patient. The material and device allows the surgeon to fabricate various constructs, cut and alter angulation, and interchange parts, all of which are done non-operatively to determine the potential clinical outcomes of the surgical application of the fixator.
Key Words: external fixator, foam mold, preoperative planning
The previous decade has seen a significant increase in applications of external fixation devices such as the EBI, Orthofix, and Ilizarov systems. Although there is significant documentation of the procedures and results of these devices, there has been no documentation of the use of foam molds of the affected extremity before surgery that prove beneficial in preoperative alignment and prefabrication of the external fixator. In those extremities with severe deformities, the surgical team must take significant time to "build" the fixator around the extremity, increasing surgical exposure and anesthesia time.
To meet the needs of the pediatric orthopedic physicians at Children's Memorial Hospital, I have developed a foam mold duplicate of the involved extremity that is used preoperatively in the construction of the external fixator. This preoperative construction using the duplicate foam extremity avoids prolonged operative and anesthetic exposure to the patient.
Methods
During preoperative evaluation, a standard plaster wrap is taken of the patient with only a thin cotton sock as an interface (Figure 1
, Figure 2
, and Figure 3
). Care must be taken to incorporate enough of the extremity proximal and distal to the involved site, allowing the surgeon adequate landmarks in deciding construction and placement of the fixation device. In Figure 3, the cast incorporated the knee and distal femur, even though the deformity was in the distal one-third of the tibia. This allows alignment of the knee and ankle joints when constructing the fixator. It is essential to contour the cast when it is drying to accentuate appropriate bony landmarks such as the malleoli and patella. The cast is carefully removed and sealed with plaster bandage.
After setting, a separating agent such as talcum powder is sprinkled into the cast and the negative cast is then filled with standard molding plaster. No pipe is placed in the cast. The cast is stripped and smoothed before fabrication of a plastic negative mold. No modifications are made to the cast (Figure 4
).
The plastic negative is fabricated in the usual fashion by vacuum forming 3/16 of an inch of polyethylene over the cast, using only thin nylon for an interface, making sure to keep as straight a seam as possible (Figure 5
).
After cooling, the plastic is cut down the middle of the seam, keeping 1/4 to 3/8 of an inch of the seam sticking out to facilitate resealing the negative plastic mold (Figure 6
).
The negative plastic mold is then sealed along the seam with duct tape, and extra strips of duct tape are cut for sealing the top of the negative mold after the foam is poured. The inside of the negative mold is sprayed with silicone spray to facilitate separation (Figure 7
).
Currently, we are using Rigid Urethane Foam (Parts A and B), which is mixed in equal parts and poured into the negative mold. Before the foam starts to expand, the top of the mold is sealed with the precut strips of duct tape and holes are poked in the duct tape to allow air to escape as the foam rises (Figure 8
and Figure 9
).
After the foam is cured, the negative mold is removed. If enough pressure was obtained in the negative, the foam will have a smooth "skin" (Figure 10
). Although a smooth "skin" is desirable, it is not essential for a successful mold.
Results
An inexpensive time-saving device can easily be fabricated for those patients for whom the surgeon feels preoperative construction would be of benefit (Figure 11
). There should be no complications from the casting for the mold, and the materials are readily available commercially.
The greater the deformity with a need for multiple fixator components is, the greater the interoperative time saved when the mold is used preoperatively. It would be difficult to quantify the timesaving involved because of the learning curve of the medical personnel, support staff, etc. That said, many physicians have found it advantageous to use the preoperative molds and, as their skills progress, they use it only for extreme cases.
Discussion
The mold is rigid and sturdy, yet it allows easy penetration of the external fixator wires and pins. The surgeon has an exact replica of the extremity that is light and contains the landmarks necessary to align and prefabricate the external fixator prior to surgery.
The mold has also been beneficial as a teaching device to residents and staff who are unfamiliar with the procedures of external fixation use, and it can easily be cut to change angulation to obtain the desired extremity position. Previous molds with an attached external fixator have been kept in clinic to show prospective patients and family the type of device and attachments to the extremity, greatly assisting them in understanding this complex surgery. Surgery, anesthesia, and tourniquet time are greatly reduced, and the surgeon is allowed to construct the device without the pressure of time constraints.
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