Prosthetic Knee Options

  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.

 

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.

 

Manual Locking Knee

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 Constant Friction Knee

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.

 

Weight Activated Stance Control Knee

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

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.

 

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