In today’s prosthetic industry there are a large range of prosthetic knee units available. In this article, rather than talking about the functionality and features of each knee unit, we will try to categorise and explain the ideas behind the different types of knees. We will look at how combinations of different functional mechanisms can create a complex and highly functional knee unit with a predictive functionality.
Single axis (Monocentric) knee units
The monocentric mechanical joint is probably the simplest design of knee unit. It provides a basic hinge function allowing the knee to bend and extend. All monocentric knee units are highly dependent on prosthetic alignment and without additional functional mechanisms, are currently very limited in their area of application. For safety, this style of knee is usually combined with a mechanical manually activated lock. In this case, the wearer walks with a fixed straight prosthesis and unlocks it to sit down.
The photo above demonstrates a monocentric knee unit with a simple mechanical lock.
To add functionality to this base design, a spring can be added to provide assisted extension (straightening) of the knee. This can be considered a safer device as the wearer will have a lower risk of standing on a bent unstable knee.
But what happens if, even with the spring, the wearer stands on this knee when it is slightly bent? The knee will immediately collapse underneath them. To avoid this situation, a mechanical friction break can be added which will prevent a loaded knee from bending. However, it is important to note that a friction brake will only work when the knee is bent at a very slight angle. Because the break is a mechanical friction device that utilises the contact of two surfaces, it will wear out overtime and require a service to maintain proper functional sensitivity.
The photo above demonstrates a monocentric knee unit with breaks and a spring assist.
To further improve the functionality of a single axis knee unit, a pneumatic system can be installed. Pneumatic systems are designed to control the knee unit’s flexion and extension speed during the swing phase of gait (when the prosthesis is off the ground during walking). This system allows the prosthetist to adjust the knee to the wearers general walking pace. The result is a smoother movement of the prosthesis. Pneumatic systems usually consist of a cylinder and piston with manually adjustable valves. As the knee bends, the piston in the cylinder compresses air which escapes past it through the valves. By adjusting the aperture of these valves, the prosthetist adjusts how easy it is to bend and extend the knee.
The photo above demonstrates a monocentric knee unit with a pneumatic system and friction breaks.
Hydraulics, in general, work similarly to a pneumatic system, in that they consists of a cylinder and piston with adjustable valves. However, hydraulic units utilise a liquid instead of air. Due to the nature of liquid (compared to air it is not a compressible substance) a hydraulic system can provide higher resistance in comparison with pneumatics which can be beneficial for fine tuning. A simple hydraulic knee unit, similarly to a pneumatic system, can be adjusted to an individuals general walking pace. More sophisticated hydraulic knee units allow the control of both the swing and stance phases of gait independently. In this situation, the wearer will be able to walk both downstairs and slopes using the resistance of the knee.
The photo above demonstrates a monocentric hydraulic knee unit.
Multiaxial (Polycentric)
Due to the geometry of the multiaxial knee units, they can provide higher stability during the stance phase of gait (when the prosthesis is on the ground). Often this stability is referred to as a geometric lock. Compared to the friction brakes found in some single axis knees, the geometric lock does not use a friction force and will not wear out over time. Multiaxial knee units with more advanced geometric construction allow for slight flexion of the knee without collapsing which improves the functionality of the prosthesis (dampening of undesirable forces, etc).
Similar to single axis knee units, multiaxial knees can be further enhanced with springs to provide additional extension assistance as well as pneumatics or hydraulics and allow the prosthetist to further adjust the flexion and extension of the knee during swing.
The photo above demonstrates a multi axis pneumatic knee unit.
The photo above demonstrates a multi axis hydraulic knee unit with advanced geometric construction allow for slight flexion of the knee without collapsing.
Adaptive knee units
Compared to the knees described above, adaptive units can dynamically change their adjustments in real time to meet the amputees gait requirements. The simplest example of this can be when the wearer needs to walk quicker, the knee unit will change it’s swing resistance to adapt to this new speed.
Monocentric pneumatic microprocessor knee units were historically the first conception of this design. These knee units can adapt only to walking speed. Usually these knees were combined with mechanical breaks to provide stance phase stability. Also there is a combination of these designs with a non adaptive hydraulic system to control stance phase, called a hybrid system.
Polycentric pneumatic microprocessor knee units are similar to the early designs, can only adapt to changes in walking speed and use the geometric lock concept to control stance phase stability.
Monocentric fluidic knee units utilise the physical properties of the liquid to dynamically change the resistance of the knee unit without the need for microprocessors, thus not requiring an electric power source. This design controls both stance and swing phases similar to the most sophisticated hydraulic knee units; the wearer will be able to walk both downstairs and slopes using the resistance of the knee. However the knee unit with fluidic control will also adapt to inclinations of slopes and stairs, additional weight, speed of walking, etc.
The photo above demonstrates a monocentric fluidic knee unit.
Monocentric hydraulic microprocessor knee units similar to fluidic knee units, control both the swing and stance phases of gait providing adaptation to speed changes, slopes, stairs etc. Advanced microprocessor knee units can be programmed to provide additional functions such as specific recreational or working activity, walking upstairs, etc.
The photo above demonstrates a monocentric hydraulic microprocessor knee unit.
Special knee units
All knee units designs we have discussed above are developed mostly for everyday walking. Unfortunately, mechanical devices cannot fully replace the anatomical knee in terms of multipurpose without undesirable side effects.
To meet this specific requirements there are some special models designed to be used in certain situations.
The picture above shows a hydraulic knee unit designed specifically for running. This knee unit has a limited range of motion that is controlled by hydraulics to reduce energy lost and provide a faster cycle of flexion and return.
The photo above shows a knee unit designed for specific sport activity (snow boarding, etc) and provides the wearer with the ability to maintain a posture with flexed knees.
The photo above shows a knee unit specifically designed for people with short trans-femoral amputations and hip dis-articulations to locate the heavier parts of the prosthesis closer to the body. As a result, this design significantly reduces the so called “hammer effect” and improves the wearer’s ability to control the limb.
The video above explains the hammer effect.
Conclusion
There are lots of different knee units available that range from very basic to the very advanced in functional terms and structural design. However, very often the price of the advanced technology is heavier units of higher cost and more specific maintenance regime. For this reason it is important to install the knee unit in accordance to the indications, contraindications and the individual users actual prosthetic needs.
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