Know About The 'Forces' That Cause Pressure Ulcers

Shear and friction are often cited in the development and progression of pressure ulcers (commonly known as bedsores) along with pressure, which can act in conjunction with pressure to cause pressure ulcers. Pressure acting on soft tissues, especially bony prominences, can cause tissue deformation and result in a certain degree of shearing force.

What is shear force?

Shear force occurs when an object is held stationary and a parallel force acts on the surface. Shear force causes an object to deform, and the degree of deformation can be quantified by shear tension.

What is friction?

Friction is the force, measured in Newtons (N), that prevents the relative displacement of two objects in contact with each other.

When the patient rests on a surface, gravity creates a force that pulls the patient downward. The force generated by the interaction between the patient and the support surface is divided into two parts: pressure (vertical part) and shear force (parallel part).

When gravity moves the patient toward the far end of the bed or the edge of the sitting position, the frictional force keeps the skin in contact with the support surface, so frictional force plays an important role in the generation of shearing forces. The relative displacement of the skin and deep tissue contributes to the occurrence and development of shear forces in soft tissues, especially at bony prominences such as the sacrococcygeal.

The angle of the bed, seat, and wheelchair backrest has a large impact on the level of shear stress experienced by the tissue. At any angle between sitting upright and lying flat, shearing forces are created by the body sliding down the incline. When the backrest is at 45 degrees, there is a higher combination of shear and pressure on the hips and sacrococcygeal region because the weight of the upper body is divided equally between vertical and horizontal forces.

How does shear force play a role in the development of pressure ulcers?

The shear force, together with the pressure, causes skin and deep tissue damage and ischemia, ultimately leading to the appearance of PI. Mechanisms include tissue deformation, capillary squeeze closure, reduced blood flow, and physical destruction of tissue or blood vessels.

What factors affect friction?

The friction between the patient and the support surface is determined by the vertical force and the coefficient of friction between the skin and the contact surface. The greater the vertical force, the greater the frictional force. Likewise, the greater the coefficient of friction, the greater the friction force and the greater the force required to move the patient.

Principles for minimizing the effects of shear and friction include:

1. Reduce shearing force: when lying down, reduce the height of the head of the bed as much as possible; when sitting, avoid sliding forward.

2. Avoid actions that cause tissue deformation: avoid sliding or dragging, ensure that the patient's body position and posture will not allow it to slide easily, and ensure that the patient's body is not dragged or deformed after changing the body position;

3. Increase the contact area with the support surface: distribute the vertical force, parallel force and friction force over a larger area to reduce local pressure and shear force.

Shear force and friction force are important external factors in the occurrence and development of pressure ulcers. A clear understanding of the mechanism of shear force and friction force can help prevent pressure ulcers.