Creating Soft Sockets and 3D Scans from Plaster Casts

For more pictures of soft socket creation, check out the gallery.

For more pictures of creating and post-processing plaster casts, check out the gallery.

For more pictures of 3D scanning, check out the gallery.

During my time at Lake Victoria Disability Centre and HVP-Gatagara, I had the opportunity to see and take part in the many steps of making traditional prosthetic devices. This article aims to give an overview of how plaster casts of residual limbs are produced, how traditional soft sockets are made, and how through the use of 3D scanning technology, digital models of these casts and sockets are created.

Creating a Plaster Cast

The process of creating a plaster cast begins with draping the patient with a plastic tarp to prevent plaster from getting onto their clothes, or onto unintended parts of their body. Petroleum jelly is applied lightly to the residual limb to aid in release of the cast after the plaster bandages have cured. In certain cases, the limb may need to be shaved to further aid in the release of the cast. The prepared limb is wrapped with water-soaked plaster bandages, often with a rubber tube or other piece of material (a piece of metal or plastic) against the skin to protect the limb when cutting the cured cast away from the body in a later step. During application of the water-soaked plaster bandages, the bandages are massaged against the residual limb to smooth any seams or ripples in the material and to conform the bandages to the residual limb as best as possible. The plaster is allowed to set, taking approximately 15-20 minutes. The negative casts are removed from the limb by carefully cutting down one side with a razor, following the underlying rubber tube or other piece of protective material, perpendicular to a series of horizontal lines drawn in skin-marking pencil. The horizontal lines are drawn for reference to ensure the seam of the cast can be closed and aligned properly later.

Upper Left: Kyle Reeser wraps a patient’s torso and lap in a plastic drape. Upper Middle: Water-soaked plaster bandages are wrapped around the patient’s residual limb, with a piece of rubber tubing to cut along when removing the cast later. Upper Right: Kyle Reeser wraps a patient’s residual limb with water-soaked plaster bandages. Lower Left: Rajab Hamis wraps a water-soaked plaster bandage around the patient’s residual limb, with a piece of rubber tubing to cut along when removing the cast later. Lower Middle: Water-soaked plaster bandages are wrapped around the patient’s residual limb, with a piece of rubber tubing to cut along when removing the cast later. Lower Right: Cutting the cured plaster cast with a razor blade along the underlying rubber hose.

Contours and measurements taken from the patient are sketched into the inside of the plaster cast with a skin-marking pencil, followed by skirting the cast with additional plaster bandage to make the rim of the opening even.

Views of a negative plaster cast of a patient’s residual limb (left limb). Note the markings in skin-marking pencil (blue).

Petroleum jelly is applied lightly to the inside of the negative cast, and a liquid plaster mixture is poured in. A metal rod is positioned within the cast, now serving as a mold, for ease of handling after the plaster positive cures and is removed from the mold. The plaster positive takes approximately 45-60 minutes to fully cure, and is removed by cutting away the negative mold that surrounded it.

Views of a negative plaster cast being filled with liquid plaster and a metal rod, and allowed to set. The negative plaster cast ‘mold’ is later removed to reveal a positive plaster cast.

The result is a positive cast that closely approximates the shape and dimensions of the patient’s residual limb. This positive cast requires significant post-processing before being used later in the process of creating a thermoformed soft socket.

A positive plaster cast after removal from the negative plaster cast ‘mold’, with a tack indicating the height to which more plaster should be added along the bone.

As foam material will later be thermoformed around the positive plaster cast to create a soft socket:

1. Adding material to the positive plaster cast will result in a larger void inside of the final soft socket, and
2. Removing material from the positive plaster cast will result in a smaller void in the final soft socket

For a comfortable and proper lower limb soft socket fit, the soft socket should be be slightly tighter around soft tissue (muscle) and leave a little more room for bony areas of the residual limb. Small tacks are hammered into the positive cast along bone lines, and allowed to stick up from the cast 2-3 mm. These tacks act as guides and anchoring points for the addition of plaster material in these areas. Being faithful to the original shape of the limb, plaster is added to the cast to meet these tacks. Plaster to be added to the original positive cast is tinted a slight blue by adding methylene blue. Due to this light blue tint, there is a blue-to-white transition at the boundary of the original plaster and the plaster added during post-processing. As rasps are used to remove material during shaping and processing of the cast, the technician knows when they have reached this layer that they are now removing original plaster. The final, processed positive plaster cast should be smooth and free from defects, with slightly-altered dimensions provide a more snug and comfortable fit.

Plaster (tinted slightly blue with methylene blue) is added to the positive plaster cast in post-processing.

Creating a Soft Socket

Traditionally-made soft sockets at HVP-Gatagara were created from sheets of 6 mm thickness polyethylene foam. The post-processed positive plaster cast is used as a form for thermoforming under vacuum, so a polyethylene foam tube is first fashioned with inner diameter just large enough to slip over the plaster cast. The technician measures the proximal and distal diameters of the plaster cast and transfers these measurements (plus some percentage increase) to a sheet of polyethylene foam, tracing out a trapezoidal shape. The technician uses a belt sander to trim the long edges of the foam trapezoid at an angle (as shown in the figure below) to increase the surface area for later bonding of those long ends with contact adhesive together to form a tube. HVP-Gatagara uses Premia Bond contact adhesive for this purpose, and I quote information from the Premia Bond packaging below:

Premia Bond contact adhesive, manufactured by: Dynamic Chemicals Ltd. (Nairobi)
Suitable for bonding leather, rubber, plastic, floor tiles, ceramic, wood, Formica, glass, metal, and textiles

1. Ensure all surfaces to be bonded are free from grease [and] dust, and are dry.
2. Apply glue to both surfaces and let it dry for 8-10 minutes.
3. Bring the two surfaces together and apply pressure to ensure a firm bond.
4. With time, the bond between the surfaces will become stronger.

Roberto Postelmans took the “apply pressure to ensure a firm bond” direction a little further, by using a rubber mallet to pound the seam against a workbench for 30-45 seconds to maximize this bond.

Upper Left: A P&O technician at HVP-Gatagara cuts out a trapezoidal shape that, when rolled into a tube, would be the approximate shape needed for a soft socket. Upper Middle: The technician uses a power sander to sand the long edges of the trapezoid at an angle. Upper Right: The technician applies contact adhesive to the long edges of the trapezoid and folds it into a tube. Lower Left: A diagram showing how the foam material is folded into a tube. Lower Right: Roberto Postelmans uses a rubber mallet to pound the seam of the foam tube to ensure strong adhesion at the seam.

The positive plaster cast is held in place near the head of a specially-made vacuum head by inserting the metal rod of the cast into the large hold of the vacuum source (see figure below). The polyethylene foam tube is heated in an oven at 80 degrees Celsius for several minutes to soften the material. The hot, foam tube is quickly slid over the plaster cast, followed by a thick plastic bag. Vacuum is applied, sucking the air out of the bag and causing it to compress the foam tube against the underlying plaster cast. Vacuum is applied for approximately 1 minute as the foam returns to room temperature, with the interior of the foam tube now taking the shape of the positive plaster cast of the residual limb.

Upper Left: Thermoforming of a soft socket under vacuum. Upper Right: The result or thermoforming, prior to being trimmer away. Lower Left and Lower Right: The blunt head of a vacuum system for thermoforming soft sockets. The blunt head has a large central hole through which the metal rod of the positive plaster cast can be inserted. The large and small holes of the blunt head provide vacuum.

The thermoformed form tube — hereafter referred to as the soft socket — is kept on the plaster cast for stability as it is post-processed. Material is easily subtracted from the soft socket by sanding the polyethylene form, and is easily added by gluing strips of the foam to itself using contact adhesive. In this way, a cap of foam is added to the distal side of the soft socket, sealing the otherwise open-end left from the original foam tube. With the interior of the soft socket complimenting the plaster cast (and therefore the patient’s residual leg) perfectly, the technician can add material to the exterior of the soft socket to achieve a good fit with the hard socket that it will eventually mater with.

Roberto Postelmans modifying the exterior of a soft socket with the layered buildup of foam material.

Once post-processing of the exterior of the soft socket is complete, the soft socket can be wrapped in PVC leather for aesthetics and as a washable external protective barrier. The PVC leather is glued to the exterior of the soft socket using the same Premia Bond contact adhesive.

Roberto Postelmans wrapping a soft socket with PVC leather using contact adhesive.

The resultant soft socket can then be tried on by the patient to ensure a snug and comfortable fit. Any complaints by the patient can be rectified by modifying the positive plaster cast and beginning the thermoforming and shaping process over again.

Views of the completed soft socket, wrapped with PVC leather. Note the slit cut vertically into the side of the soft socket, as strain relief when the patient slips the socket on.

3D Scanning Plaster Casts and Soft Sockets

Roberto Postelmans of ORTHOLAB uses a Sense 3D scanner paired with

Isaac Rukundo, director of the P&O department at HVP-Gatagara, 3D scanning a plaster cast of a patient’s residual limb.

Left: A plaster cast of a patient’s residual limb. Scanning a plaster cast yields a digital model that can be used to 3D print a form-fitting soft socket. Right: A traditionally-manufactured soft socket, formed over the plaster cast from the left panel. Scanning a soft socket yields a digital model that can be used to 3D print a matching hard socket.

Notes on 3D Scanning

Left: Roberto Postelmans using a Sense 3D scanner to scan Kyle Reeser’s hand. Right: A 3D model resulting from scanning a hand. Note the material in between the fingers of the 3D scan, a deficiency in the 3D scanner’s ability to create an accurate model to track it’s registration points on the fingers.

Takeaways

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