Question
What does the term ‘horizontal stiffness’ refer to when selecting a cushion for a client’s wheelchair? Why is it important to consider?
Answer
During cushion selection, the key concept to keep in mind is "horizontal stiffness". Horizontal stiffness is important to address because it will determine which type of cushion will be the most appropriate for your client. Horizontal stiffness is essentially defined as how stiff or stable the cushion is in the horizontal plane while sitting on top of it. For example, if you attempt to wiggle when sitting on top of a hard, wooden surface, it will not move in a lateral direction. This wooden surface is considered to have a very high degree of horizontal stiffness. A dense foam cushion would also have a high degree of horizontal stiffness because it will not move much laterally while a person is sitting on it and moving around. In contrast, the air cushion has a low degree of horizontal stiffness.
The figure below shows a cushion with a low degree of horizontal stiffness on top and a cushion with a high degree of horizontal stiffness on the bottom. If you have a cushion with a high degree of horizontal stiffness, the good news is that it will provide a very high degree of stability. It will give your client a good solid base of support. However, clients with bony prominences or low body mass indices may slide around on top of cushions with high horizontal stiffness increasing friction and shear forces against their skin. In contrast, when a client sits on a cushion with a very low degree of horizontal stiffness such as an air cushion, there are less shear and friction forces against the skin. When the client wiggles around on top of the air cushion, the air that is under her/him will move around as well. The good news is the risk of skin breakdown due to shear and friction is going to be reduced; however the stability and support will be reduced. When selecting the best cushion option for your client, it is important to determine how much stability does the client need versus how much are they at risk for skin breakdown due to shear and friction forces? Fortunately, there are a lot of different types of cushion options available today. Many will have mixed medium or different layering options where the cushion starts with a very firm base, then a middle layer with less horizontal stiffness and an air bladder or gel inlay under the actual contact area. The foam base will take care of the stability factor and the gel or air inlay under the actual contact area will take care of the shear risk factor.
Editor’s note: This Ask the Expert was adapted from the article ‘Selecting the Ideal Wheelchair Seating System’. The complete article can be accessed here.
