Understanding & Managing Shear
- By Lois Brown
- Nov 01, 2010
Mobility Management first highlighted tilt in its February 2010 issue. This article expands on MM’s August 2010 issue, which featured the story “The Recline of Civilization” and gave light to recline as a distinct positioning option with its own physics and clinical application.
As Stephanie Tanguay — OTR, ATP, clinical education specialist for Motion Concepts — explained in the article, most manual and power chairs utilized a low pivot point from which the back canes moved to open (recline) and close (sit upright) and that was level with the seat rail. The shearing effects were created due to the pivot point moving further away from the hip joint. Since seat cushions further raised the hip joint relative to the pivot point, the shear effects become even more prominent.
Efforts to reduce shear have primarily been in the design of power tilt and recline systems. The clinical application and benefits are clearly outlined in the RESNA position paper on power tilt, recline and elevating legrests and can be equally applied within manual mobility systems, but with attention to reducing the effects of shear.
This can be accomplished through the frame design and careful attention to matching the design of the seating system contours. A manual mechanical shear component on a tilt-in-space chair can open up the possibility of expanding the use of back options with reduced shear, such as trunk lateral supports or more customized contoured backs when usually this would be a contraindication.
Defining the Forces at Work
Let’s understand the physics at play and apply the terminology.
Reenalda, et al, 2009, defined the following: Stress is the force that tends to deform the tissue. Pressure and shear both contribute. Let’s define friction first, as shear is a byproduct of friction.
Friction is the force that tries to prevent the movement between two surfaces. Friction is the mechanism by which shear forces are applied to the skin. This gives rise to shear stresses and shear strains within the tissues. Friction is always opposite to the direction of movement or intended movement. It is friction that prevents movement when an external force is applied, and it is friction that causes the heat and abrasions that result from that movement. Friction is a function of the normal force and the coefficient of friction. Therefore, shear can be highest where the normal forces are highest (i.e., highest potential for double the damage).
Shear occurs when two forces are in opposing directions such that there is a deformation of the tissue in parallel planes. Horizontal shear stress (more appropriately called “parallel shear stress”) results from frictional forces. Ferguson-Pell (1990) and Noble (1977) used the term “normal stress” for those forces acting perpendicular to the surface, and “shear stress” for those acting parallel to the skin. Parallel shear stress is a force that exists whenever there is sliding or the potential for relative motion (sliding) between two surfaces.
Parallel shear stress has in the past been referred to as horizontal shear, but since this shear can occur in the vertical direction — for example, along the backrest of a wheelchair — it is more appropriately referred to as parallel shear stress.
Shear is difficult to measure and elusive in that it is not dependent on the position or posture, but is dependent on the direction and extent of the movement just prior to getting to that position. Shear/friction are necessary to keep the person from sliding out of the chair — the key is to avoid having that shear/friction where normal forces are highest (i.e., not on the bony prominences). The solution is to make the coefficient lowest where the pressures are highest, and highest where the pressures are lowest.
The Effects of Controlling Shear
Taylor found that when you control shear, you can significantly reduce pressure at a decubitus ulcer by a factor of six. Hobson in 1992 found that a full body tilt to 25° decreased surface shear force to nearly zero, but with the backrest only reclined to 20° caused a 25-percent increase in surface shear force.
A study was done by Aissaoui (2001) to investigate the effect of system tilt and back recline angles on sliding and pressure distribution of seated subjects. Ten able-bodied subjects adapted successively to 12 postures on a multi-adjustable simulator chair. The system tilt angle was varied from 0° to 45° posterior tilt, while the seat-to-back angle varied from 90° to 120°.
A maximum of 40.2 percent of weight shift was found when combining a system tilt angle of 45° with a seat-to-back angle of 120°. A maximum value of 74mm of sliding was observed for the acromion marker during repositioning. Significant weight shift at the level of the seat is obtained only when the system tilt angle exceeds 15° in a posterior direction. We can put forward here that a small tilt of ≤15° can be used to adjust back pressure distribution, whereas large posterior tilts are used for an effective weight shift at the seat level. The peak pressure gradient remains in general in the interval of ±30 percent from the neutral posture for the able-bodied subjects and is fairly constant at 15° of tilt. A significant amount of displacement along the back and seat reference plane was found for the shoulder and hip markers, but this displacement does not necessarily correspond to a pure translation motion of the pelvic segment.
We can see by the results of this study that while the tilt/recline angles can produce increased pressure distribution, we have shear forces to contend with, as seen by the amount of change of the acromion marker during repositioning. And while small tilts of less than or equal to 15° adjusted back pressure distribution, it has functionally been noted in clinical practice to hold the pelvis back in the seating system to reduce some of the parallel shear forces between the user and the back seating surface.
Applying shear reduction mechanisms to manual mobility systems follows evidenced-based practice to further reduce parallel shear forces at the user/seating surface. These equally apply to the frame design and how biomechanics has been taken into consideration to reduce shear forces in manual and power mobility systems. Again, it is imperative to consider the backrest material and cover, contours and external support components that will work together to minimize shear when providing a comprehensive seating solution to reduce pressure and shear for that particular client. When you think about recline with power tilt, don’t forget to consider when prescribing with weight-shifting tilt-in-space to minimize the shear forces for the patient.
Aissaoui R, Lacoste M, Dansereau J. (2001). Analysis of sliding and pressure during a repositioning of persons in a simulator chair. IEEE Transactions on Neural Systems & Rehabilitation Engineering, Vol. 9, No. 2, pp. 215-224.
Ferguson-Pell, M. (1990). Seat cushion selection. Journal of Rehabilitation Research and Development, Clinical supplement # 2, 49-73.
Hobson, D.A. (1992). Comparative effects of posture on pressure and shear at the body-seat interface.
Journal of Rehabilitation Research and Development, Vol. 29, No. 4, 21-31.
Reenalda, Jasper; Van Geffen, Paul; Nederhand, Marc; Jannink, Michiel; IJzerman, Maarten; and Rietman, Hans (2009). Analysis of healthy sitting behavior: Interface pressure distribution and subcutaneous tissue oxygenation. Journal of Rehabilitation Research & Development, 46 (5). pp. 577-586. ISSN 0748-7711.
Taylor V. Pressure Mapping Clinical Protocol. 1999 ISS Handbook.
This article originally appeared in the November 2010 issue of Mobility Management.
Lois Brown, MPT, ATP, is the rehab clinical education specialist for Invacare Corp., Elyria, Ohio. She is a frequent presenter on seating & mobility topics at industry events.