Biomechanics in prosthetic rehabilitation

Original Editor - Greg Halford and Hmayak Tarakhchyan as part of the WCPT Network for Amputee Rehabilitation Project

Top Contributors - Admin, Sheik Abdul Khadir, Kim Jackson, Rachael Lowe, Tony Lowe, Evan Thomas, Aicha Benyaich, Claire Knott and Amanda Ager  

Introduction

An understanding of biomechanics is important when working with amputees and people with prosthetic limbs. It is especially relevant to understanding how gait deviations occur and how they can be remedied. An understanding of biomechanics is essential for Prosthetists as it influences so many aspects of their work during prosthetic rehabilitation. The following is a brief introduction to the basics of Biomechanics for Lower limb Prosthetics that can help people who are not Prosthetists better understand the area.

A force is any interaction which tends to change the motion of an object. Forces are important in Prosthetic devices. A prosthetic device applys forces to the human body and can change the way forces interact with the body. When force is applied to the human body it is done over an area of skin. This produces pressure with the following equation.

Pressure equals force over Area (P=F/A)

When the area over which a force is distributed is made larger the pressure is reduced. If someone steps on your foot in a stiletto (a fine point high heeled woman’s shoe) there is a lot of pressure (and pain).

The same person standing on your foot in a heel that is larger it would cause less pressure (and pain) because the area is bigger. The fact that it is the same person means that the Force is constant in this scenario.

The heel of a stiletto shoe would cause large pressure per unit area. When the area is increased (in the picture on the right) the pressure is decreased.

In the below example we see the pressure also reduced by demonstrating the effect of increasing the area of contact under the foot.

In this take on the classic “bed of nails” scenario we see that the Pressure is reduced as the area is increased. The force again remains constant.

This idea is used in prosthetic socket design to attempt to spread force over a large enough area to reduce pressure to an acceptable amount. Often patients might ask for an uncomfortable part of a prosthesis to be removed. Sometimes when this is done the area that force is spread over is reduced, hence by the above equation increasing the pressure. This can lead to increased pain after some time.

  Centre of Mass A general biomechanical principle is understanding the Centre of Mass, sometimes called the Centre of Gravity. It is the middle of the mass of an object and the force of gravity can be assumed to act on the object from this point. It is also the place that rotations will occur around. See here an explanation of the Centre of Mass, and some links on how to determine the Centre of Mass of an object. Its not necessary to get into understanding the mathematics and calculations for this course. http://hypertextbook.com/facts/2006/centerofmass.shtml https://www.boundless.com/physics/textbooks/boundless-physics-textbook/linear-momentum-and-collisions-7/center-of-mass-72/center-of-mass-of-the-human-body-305-1641/ In an amputee because some mass is excised the center of the mass of the person changes.

Note the center of mass has to be more clearly added to this picture. GH

 

The Ground Reaction Force The Ground Reaction Force occurs when contact occurs of a support surface, it is equalling and opposing the force due to body mass passing through the foot to the ground surface. Addapted from Ref : http://medical-dictionary.thefreedictionary.com/ground+reaction+force It should be noted that the ‘force due to body mass’ includes the momentum and accelerations of body mass as well as just the effect of gravity on it. Hence it is different and yet similar to use of the weight line.

A look at the difference between how the GRF influences gait in people without a prosthesis and with it is in the following web page. http://moon.ouhsc.edu/dthompso/gait/pobmk/frames.htm

The GRF is not used clinically in many places due to difficulty obtaining it in real time. Clinicians can try to understand the movements they see by understanding the effect the GRF might be having on the human body and prosthesis.

Moments or Torques If a Force does not act directly on the COM of an object it will rotate the object. If a force does not act through the middle of a joint with another segment it will change the angle of the joint. In the below example the foot and the leg should be considered as two segments, joined at the Talo-Crural Joint. As the GRF ‘pushes’ up on the foot it will tend to move the foot into dorsiflexion. The force produces a Torque that is the size of the force times the length of the lever arm (distance from the line of action of the force to the joint center)

This picture needs to be changed, I don’t have the correct software to do it correctly right now, the posterior arrow will be removed, the anterior one moved distally and the line of action and lever arm drawn on.

In normal human movement the moment created by an externally applied Ground Reaction Force is balanced (for the most part) by a internally generated muscle force which provides an opposing moment.

This Paragraph could be a footnote **** This general rule is not always 100% true but is useful approximation. In the case of rapid or forceful motion these moments are not balanced. Also in the case of the knee reaching full extension in standing it is tension in the tissues of the posterior compartment of the knee which provide the balancing moment.

In this example the dorsiflexion moment created by the anterior GRF is balanced by the planter flexors through tension in the Achilles Tendon. This muscle has a fixed lever arm.

People with Amputations have a lack of direct muscle control over some joints. People with amputations have missing body parts so there are cases where the muscle action is not available to act as a counter to the GRF as the muscle and joint are no longer present. If the GRF passes anterior to the ankle in a Trans Tibial Amputee then the ankle is permanently stiff enough to resist deforming into dorsiflexion. *

• This is not the case for powered prosthetic feet like the Propio foot from Ossur.

In the case of a person with a TransFemoral amputation with free knee (with no locking mechanism) the GRF should be placed anterior to the knee for the duration of single limb support. This is done in the following manner through static alignment using the more identifiable Trochanter Hip and Ankle (TKA) line.

In this scenario the person with the amputation continues to have indirect control of the knee though the ability to extend the hip joint. When unwanted biomechanical situations present themselves Prosthetist may have the opportunity to alter the biomechanical situation, by fixing a joint to movement or by manipulating and moving the GRF to a more advantageous position. This will be talked more about in the alignment section. Check that it is talked about in the alignment section GH

Axial Loading of a Prosthesis The forces of the body are normally transmitted from the skeletal system to the ground through the structures of the plantar aspect of the foot. In a lower limb amputee this structure may be missing. One of the main jobs of the lower limb prosthesis is to provide a medium for axial loading (otherwise known as vertical force transmission). In other words it must hold up the body weight in standing. The force produced by body weight `pushes` down on the prosthesis. The force passes through the prosthesis and down into the ground. The prosthesis must be strong enough to push up enough to hold this weight. Prosthetic components from manufacturers are rated to tolerate various weight tolerances. Custom made parts should be strong enough to bear the weight and can be made to recommendations or at the discretion of the Prosthetist.

Axial loading is achieved by providing enough counter force to resist the effect of gravity. This force must be distributed across pressure tolerant areas. Good understanding of this is needed for prosthetic socket design.

I NEED TO FIGURE OUT HOW TO GET A CELANER VERSION INTO HERE AND ADD THE TF ONE AS WELL.

When we think of forces in a prosthesis the prosthetic socket provides the medium for forces to pass from 1. ground to person

AND

2. from person to the ground.

The prosthetist loads skin with pressure in order to transmit forces from the prosthesis to the skeletal system of the amputee. If an area is ideal for axial loading then a large amount of force can be transmitted through it. Forces are generally provided parallel to the skin, reducing the amount of sheer forces. Changing the alignment (into some flexion) of the limb can affect the available area for vertical force transmission. The vertical part of these forces are shown. In the below diagrams. Putting more flexion in the socket can have long term effects on joint range of motion and should be carefully considered as a solution to axial loading difficulties.

The picture I did to illustrate this didn’t work out. Ill have to do it again when I get back to the office on Friday the 13th Feb. GH

Prosthetic Coupling The coupling between the prosthetic socket and the amputee’s residual limb is not completely stiff. This coupling can be thought of as being ‘a bit like a joint’ between two body segments.

In this picture we see that the GRF of the person with the amputation does not ‘push’ directly up on the prosthesis. Rather it `pushes` up more medially. This creates a torque or moment around the coupling between the socket and the amputee. Soft tissue is compressed by the rotation, leading to pressure inside the prosthetic socket. The pressure that is developed as a result of the torque is shown by the red arrows below, proximal medially and distal laterally.

I need to add an arrow to this picture still to put the GRF in Medially (to the right) also add Medial and Lateral markers .Perhaps Ill also exaggerate the problem of the medially set foot. GH This torque is resisted by soft tissue compression, in other words the socket torque causes pressure in specific parts of the prosthetic socket in a predictable way. Socket pressure issues should always be dealt with by a Prosthetist who understands the importance of alignment. Moving too quickly to altering the socket is not always a good solution in the clinic. Alignment changes are normally rapid and readily reversible with modern components.

Main points of this section. Forces are applied to the human body in Prosthetics, this will invariably lead to pressure. Pressure can be managed by increasing the area it is applied over and distributing it thoughtfully to pressure tolerant areas. Alignment of a lower limb prosthesis can have an affect on socket pressures in a predictable way Changes to a prosthesis can alter the prevailing biomechanical situation through limiting ranges of motion or moving the GRF

Extra Reading http://www.oandp.org/jpo/library/1997_02_049.asp http://moon.ouhsc.edu/dthompso/gait/kinetics/GRFBKGND.HTM http://courses.washington.edu/anatomy/KinesiologySyllabus/LE-Prosthetics-1.pdf

I don’t know how this isnt breaking copyright but here is a link to a full book on the topic http://physiosite.weebly.com/uploads/1/0/4/7/10478019/biomechanics_of_lower_limb_prosthetics.pdf

References

References will automatically be added here, see adding references tutorial.