The type of knee used on an above-knee prosthesis depends on the patient’s activity level, the patient’s weight, the patient’s strength and ability to control the knee, residual limb length, funding, and patient preference.
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Friction is used in the knees in order to control the knee joint during walking. Friction controls how far and how fast the knee bends and straightens during gait. Some knees have mechanical friction while others have hydraulic resistance. Computerized knees are also available that control the knee speed based on the person’s gait. Mechanical knees provide constant friction where the hydraulic knees and computerized knees change the knee speed depending on how fast the person is walking.
The manual locking knee is the most stable knee used in prosthetics. The knee is locked during gait and the patient releases the lock mechanism in order to sit down. Manual locking knees are primarily used with patients who have very short residual limbs and/or poor hip strength and are unable to control the knee.
Single Axis Prosthetic Knee Joint
Single-axis knees are basic knees that bend freely. The amputee must rely on his own muscle control for stability. The single-axis constant friction knee is generally used by children who have a lower center of gravity or for patients with excellent musculature control that walk at a single speed. Friction in the knee can be adjusted by tightening a bolt. For exoskeletal knees, an extension strap made of elastic may be added to the front of the prosthesis to aid the knee in kicking forward. This knee is very durable and is easy to maintain and repair.
Single-axis knees can be exoskeletal (hard plastic) or endoskeletal (metal components). Oftentimes, hydraulic or pneumatic controls are added to single axis knees to allow for variable speed walking. Stance control may also be added to improve stability.
The weight-activated stance control knee is one of the most widely used knees in prosthetics. This knee is a single axis constant friction knee with a braking mechanism. When weight is put on the knee during gait, a braking mechanism is applied and the knee will not buckle. Using this knee, the patient must unload or take weight off of the prosthesis in order for the knee to bend. The wearer will need to unload the knee to sit or to initiate the swing phase of gait. This knee is sometimes referred to as the “safety” knee.
Four-Bar Pneumatic Knee Joint
The polycentric knee has a variable center of rotation allowing for stability at all phases of gait. The 4 bar linkage also allows the knee to collapse better during the swing phase of gait, essentially shortening the shin and allowing the foot to clear the ground easier. This collapsing feature also allows the knee to bend easier for sitting and is the ideal knee for knee disarticulation or long above knee amputees. The swing phase control can be either mechanical friction or hydraulic resistance. There are many manufacturers of polycentric knees.
Hydraulic and pneumatic knees allow adjustment of walking speed by the use of hydraulics (either liquid or air) within the knee. As a person’s walking speed increases or decreases, the hydraulics adjust to control the speed at which the shin of the prosthesis swings forward and bends backward. This type of knee is often used for more active patients who vary their walking speeds and do not need assistive walking devices. Hydraulics can be used with single axis or polycentric knees. The following knees are just a few of the various hydraulic knees that are now available.
Have a question about Prosthetic Knee Options?
A prosthetic leg, or lower limb prosthesis, is an artificial replacement for part or all of your leg. If you can’t use your leg, either because of a birth defect or amputation, you might use a prosthetic leg to walk.
A leg prosthesis can replace as much of your leg as you need, from your foot, ankle and shin to your knee, thigh and hip. It can offer you greater mobility and the chance to live a more active lifestyle.
Prosthetic limbs are assistive devices to enhance your mobility and independence. They mimic the functions of a human limb to help you do certain things better than you could without them.
Using a prosthetic limb can also help you maintain your overall musculoskeletal health. Making your different side more usable allows you to distribute your body’s workload more evenly across both sides.
Modern prosthetic legs are made of strong but lightweight materials, like carbon fiber or fiberglass, titanium or aluminum. They also have some softer parts, like foam cushioning and silicone sleeves.
The moving parts at the joints might include mechanical springs and gears or computer microprocessors. Some prosthetic legs include hydraulic or pneumatic cylinders, which make movement smoother.
Prosthetic legs can look different ways. Some are high-tech and look like robot limbs. Others are more low-tech and skeletal-looking. You can also get different types of “skins” to cover your prosthesis.
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Some people customize their prosthetic legs with fun fabrics, logos or tattoos. Others prefer a low-profile look that’s custom-painted to look like their natural legs. Most cosmetic covers are removable.
Different types of prosthetic legs replace different parts of your leg. You might need a prosthesis with an ankle joint, a knee joint and/or a hip joint. Different types also have different mechanisms at the joints.
Some prosthetics are purely mechanical, while others have computerized parts. Some move in limited ways, and others move in many ways. Some are designed for particular sports or other activities.
Different types of prostheses are available for different types of lower limb amputations, including:
Your limb deficit or amputation level will determine the level of prosthesis you need. In general, an above-the-knee prosthetic leg is a more complex device than a below-the-knee prosthetic leg.
One difference is the knee joint. If your prosthesis is below the knee, it only needs a shin, ankle and foot, which may or may not be flexible. If it’s above the knee, it’ll need to include a functioning knee joint.
Your knee joint is central to the mechanics of walking. It bears much of your weight and affects your stability and balance on various terrains. It also determines whether your leg can run, bike or swim.
The attachment part of the prosthesis also needs to be more sophisticated, as the prosthesis must attach to a shorter lever arm. Generally, the higher the amputation level, the trickier it is to attach the prosthesis.
The joints in your prosthetic leg may be mechanical or bionic, which means they include computer microprocessors. These microprocessors enable the joints to automatically adjust to different terrain.
Both types have pros and cons. Mechanical legs tend to be more durable, lightweight and affordable than bionic legs. They aren’t “smart,” but you can use them for basic walking and sitting down.
Bionic legs use microprocessors to provide real-time feedback about how you’re using them. They adjust to different activities and environments more like a natural leg would. Some are also programmable.
Both mechanical and bionic legs may include pneumatic or hydraulic components (compressed air or water). These provide adjustable resistance while walking, which allows for a more natural gait pattern.
Prosthetic legs have several components, which are sometimes interchangeable. When you and your prosthetist design your custom prosthesis, you’ll discuss each of these components.
You might benefit from a prosthetic leg if you’ve had a lower limb amputation or have a congenital limb difference that makes walking difficult. However, not everyone benefits equally from a prosthetic leg.
Many factors can influence how much you’re likely to benefit from a prosthetic leg, and what complications you may face along the way. Before recommending one, your provider will want to know:
Getting a new prosthetic limb and learning to use it is a huge undertaking. It will take time, patience and practice on your part. It may be frustrating at times, and you’ll need to manage your expectations.
Here’s a brief breakdown of the steps involved:
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