Lower Limb Prosthesis 101: Knowledge is Power

By: Amanda Lyons, PT Clinical Leader, Sheltering Arms Institute Multispecialty Unit

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Being fitted with a new prosthetic limb or learning to walk in a prosthesis can be a very challenging time in someone’s life. In this article, we’ll discuss what K levels are and how we establish them, the different components of a prosthetic lower limb, and how to properly use prosthetic socks.

We’ll also be answering common questions around amputation and prostheses then sharing some helpful community resources to guide you along the way.

What are K Levels?

K levels were established in 1995 by Medicare to better quantify the need and potential benefit of prosthetic devices for individuals that have experienced a lower limb amputation. This rating system is still used by insurance companies, Medicaid, and Medicare to determine eligibility for payment or reimbursement of funds.

The good news is that K levels can change as you continue to progress and accept new challenges, qualifying you for different components of your prosthetic. The main way to calculate your K level is through the Amputee Mobility Predictor (AMP). We do this at both our inpatient and outpatient settings, Sheltering Arms Institute and Sheltering Arms Outpatient Physical Rehabilitation Centers.
 
To learn more about the Amputee Mobility Predictor, please visit the Physical Medicine and Rehabilitation (ACRM) website.

What are the Different K Levels?

The information below describes what each K level is, pulled directly from The Centers for Medicare & Medicaid Services.

  • K-Level 0: Does not have the ability or potential to ambulate or transfer safely with or without assistance, and a prosthesis does not enhance quality of life or mobility.
  • K-Level 1: Has the ability or potential to use a prosthesis for transfers or ambulation in level surfaces at a fixed cadence. Typical of the limited and unlimited household ambulator.
  • K-Level 2: Has the ability or potential for ambulation with the ability to transverse low-level environmental barriers such as curbs, stairs, or uneven surfaces. Typical of the limited community ambulator.
  • K-Level 3: Has the ability or potential for ambulation with variable cadence. Typical of the community ambulator who has the ability to transverse most environmental barriers and may have vocational, therapeutic, or exercise activity that demands prosthetic use beyond simple locomotion.
  • K-Level 4: Has the ability or potential for prosthetic ambulation that exceeds basic ambulation skills, exhibiting high impact, stress, or energy levels. Typical of the prosthetic demands of the child, active adult, or athlete.

What are the Different Components of a Lower Limb Prosthetic?

Components and parts of lower limb prosthesis
The different parts of a prosthetic leg (socket, knee, pylon, and foot)

A lower limb prosthetic is composed of 4 main components which include:

1. Socket

The socket component is created when the prosthetist takes a cast of your residual limb, usually 4 months after your amputation when the residual limb has had time to heal. It is positioned between the residual limb and the actual prosthesis.

They are customized and fitted for comfort based on the structure of the residual limb. These are used for both above-the-knee amputation (AKA) [image A] and below-the-knee amputation (BKA) [image B].
 

Above the knee lower limb prosthesis versus below the knee prosthesis
Above the knee lower limb prosthesis [A] versus below the knee prosthesis [B]

2. Knee Joint Component (Only for AKA)

 
There are different knee joint components based on your K level.

  • K1 Rated Individuals – Most simple type of knee with either a single or multi axis joint with constant friction. It is very common to have a manual locking feature and is most frequently used for transfers and around the house (short distances).
  • K2 Rated Individuals – Designed for people who are out and about. It uses a multi axis, constant friction joint. There is an extension assist and a storing stance which also helps with flexion. It is a little higher level than the K1 rated knees.
  • K3 & K4 Rated Individuals – The three different options are hydraulic, pneumatic, and microprocessor knee joints. Both the hydraulic and pneumatic knees consist of pistons inside cylinders containing air (pneumatic) or fluid (hydraulic), which allows for walking at various speeds.
     
    The microprocessor knee allows for increased ease when navigating stairs and uneven terrain. Sensors in the knee detect movement and timing and then adjust a fluid/air control cylinder accordingly. They lower the amount of effort amputees must use to control their timing, resulting in a more natural gait.

3. Pylon

The pylon is the component that provides the connection between the residual limb (leg stump) and the prosthetic foot. For AKA, it is between the knee joint and the foot, and for BKA, it is between the socket and the foot. It allows for shock absorption and helps for building the length of the leg.

4. Foot

There are different foot components based on your K level. This is appropriate for both above the knee and below the knee amputations.

  • K1 Rated Individuals – SACH foot
  • K2 Rated Individuals – Single-axis, flexible keel
  • K3 Rated Individuals – Dynamic response, shock absorber, torque adaptor, multi-axis
  • K4 Rated Individuals – dynamic response, shock absorber, torque adaptor, multi-axis, flex-foot cheetah

What are Prosthetic Socks and How Do You Use Them?

Prosthetic socks come in different thicknesses (1-ply to 5-ply), with 1-ply being the thinnest and 5-ply the thickest. It is used to help accommodate for the shrinking of the residual limb and the ply in the morning may differ from the ply used in the evening due to swelling.

They are worn between the socket and the gel liner on the residual limb. The ply count can go above 5-ply but if you are starting to need 10-ply or above, it may be a sign that you should visit your prosthetist to be fitted for a new socket.

Prosthetic Limb Troubleshooting

When Should You Make an Appointment to See the Prosthetist?

  • If it is painful, you should see your prosthetist as soon as possible.
  • If it is uncomfortable and persists, no matter what sock combinations you use, you should consider seeing your prosthetist for an adjustment.

What Can Cause Your Residual Limb to Become Swollen?

There are many reasons why your amputated limb may be swelling. One of the major causes of swelling in your residual limb is taking in too much salt. A simple change in your diet can help here. If needed, contact a dietician to help you create a meal plan that is right for you.

Another common reason for swelling to occur is not wearing a shrinker sock at night. A shrinker helps make sure your residual limb stays a nice consistent shape and size.

A more serious reason swelling could occur is an infection. If you get a cut or your incision opens up and becomes infected, swelling will be apparent.

There are also certain medical conditions like heart or kidney disease, impaired circulation, and uncontrolled diabetes that could be a cause of the swelling. Make an appointment with your doctor if your swelling does not go down. 

What Does it Feel Like When Wearing Too Many Ply Socks?

  • Residual limb throbs, feels constricted and chocked
  • Uncomfortable pressure
  • End of limb becomes weepy (moist) and red and/or purple in color.
  • Feeling that the prosthesis is too tall

What Happens When You Do Not Wear Enough Ply Socks?

When you do not wear enough ply socks, the prosthetic may move around too much. This can cause the foot to turn in or out and could cause a fall. You may also feel tightness at the bottom from dropping too deep into the socket.

Others have expressed the feeling of the prosthesis being too short and, in some instances, skin breakdown occurs at the bottom of the residual limb from friction and rubbing.

When is it Time to Replace Your Prosthesis?

Below is a list of different scenarios and reasons that may indicate the need to replace your prosthesis. 

  • Your weight is no longer within the range of safety of the components.
  • The components are no longer working to the specifications of the manufacturer.
  • Your activity level is no longer compatible with the components used, such that they are increasing the individual’s net energy cost rather than decreasing it.
  • A specific component or module needs replacing, but the replacement is not compatible with the rest of the existing components.
  • So many changes or alterations have been made to materials that their structural integrity has been compromised.
  • It is impossible to increase or decrease the size of the socket and or frame without rebuilding the whole prosthesis.

Want to Learn More About Amputation and Prosthetics?

Check out The Amputee Coalition by visiting https://www.amputee-coalition.org/. Their mission is to reach out to and empower people affected by limb loss to achieve their full potential through education, support & advocacy, and to promote limb loss prevention.

Sheltering Arms Institute and Sheltering Arms Outpatient Rehabilitation Centers also host a local support group, held virtually on Zoom, every first Wednesday of the month from 6-7 p.m. For more information and to RSVP for the next meeting, please email Sarah Davis at sdavis1@shelteringarms.com.

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