Foams

Material properties and characteristics relevant to foam selection and fabrication in P&O.

Foams are used in P&O for skin protection and for posting alignment. They may be adhered (i.e., glued) to the orthosis/prosthesis or formed over a model. Examples include prosthetic foam liners (i.e., pelite liners), foot orthoses, fracture orthoses, Charcot boots (CROW), and spinal orthoses. When selecting foams, consider the following:

• Cellular Structure Properties
  • Cell Type and Size Open-cell foams allow air and fluid flow through interconnected pores, making them breathable. Closed-cell foams trap gas in isolated cells, providing buoyancy, thermal insulation, and moisture resistance. Cell size affects surface area, permeability, and mechanical properties. Most foams used in P&O are closed cell foams.
    • Pros: do not soak up body fluids and provide more resistance to compression than open cell foams.
    • Cons: not breathable, i.e., hot
• Mechanical Properties
  • Durometer and compression deflection
    • Durometer: The most commonly used mechanical property used to select foams in P&O.
    • Compression deflection
  • Compression set and resilience
    • Compression set: Low compression set better for long term use. Especially for weight bearing materials, we want the material to return to its original shape after prolonged compression.
    • Resilience: Another way to think about compressibility, measured differently.
  • Elongation, tensile strength, and tear strength
    • Elongation: Consider the type of load that the foam will experience. Elongation is a tensile measurement.
    • Tensile strength is also measured during a tensile test, where the maximum stress the material can endure before breaking is recorded.
    • Tear strength is an outcome of the tensile strength test.
  • Abrasion resistance and coefficient of friction
    • Abrasion Resistance: Important in selecting materials for high-wear environments, such as foot orthoses and areas of donning doffing.
    • Coefficient of friction: Crucial in designing systems where sliding contact occurs, such as over scar tissue or immobile skin.
  • Fatigue Resistance Critical for weight bearing applications.
  • Hysteresis and Energy Absorption The difference between loading and unloading curves indicates energy dissipation capability, important for impact protection and vibration control.
• Processing and Fabrication Properties
  • Adhesion Properties Bonding capability to substrates or other foam layers affects assembly methods and joint reliability.
  • Machinability Ability to be cut, shaped, or machined without cell structure damage or excessive dust generation.
    • Grindability: refers to the ease of modifying the foam on a grinder.
  • Thermoformability: thermoplastic foams can be formed over a mold. Thermoset foams are not reformable.
    • Molding temperature: the temperature that the foam can be reformed
•  Physical and Other Properties
  • Density
  • Thickness Typical thickness of foams inside an orthosis or prosthesis are 3-5mm or 3/6″ thick. Thicker foams (1/2-1″) are use for posting or other buildups. Iron is a very old, yet traditional measurement in the shoe industry to measure the thickness of materials. One iron is equal to 1/48 inch (0.5292 millimeters), so, for example, a sole 1/4 inch thick is described as “12 iron”.
  • Biocompatibility Designed to interact safely and effectively with living tissues