Summary
A prosthetic knee replaces the mechanical function of the natural knee for many above-knee and knee-disarticulation users. Knee systems range from simple single-axis hinges to polycentric, pneumatic, hydraulic and microprocessor-controlled designs. Each type balances stance stability, swing control, cadence, terrain adaptation, weight, maintenance and cost differently.
The natural knee does far more than bend. It supports body weight during stance, flexes for toe clearance, extends before the next step, absorbs part of the load during movement and adapts to changes in speed. A prosthetic knee cannot reproduce every biological function, but modern mechanical and electronic systems can manage many of the same walking demands. Understanding the major categories makes it easier for patients, students and clinicians to discuss prescription intelligently.
Who Needs a Prosthetic Knee?
A prosthetic knee is used when the natural knee joint is absent or cannot provide the required function, most commonly in transfemoral or above-knee prostheses and some knee-disarticulation fittings.
Below-knee users retain their biological knee and therefore do not use a prosthetic knee joint.
What Does a Prosthetic Knee Need to Control?
During stance phase, the knee must resist unwanted flexion while the user loads the prosthesis. During swing phase, it must bend enough for toe clearance and then extend in preparation for the next step.
Knee selection therefore depends on both stance stability and swing behaviour. The Protech Ortho knee range includes mechanical, pneumatic, hydraulic, polycentric and microprocessor-controlled options for different functional needs.
Single-Axis Prosthetic Knees
A single-axis knee rotates around one main hinge. The design is mechanically straightforward and can be combined with friction, locks, brakes, pneumatic control or hydraulic control.
Simple single-axis knees are commonly used where predictability, durability and uncomplicated servicing are priorities.
Manual-Locking Knees
A manual-locking knee remains locked in extension during standing and walking until the user releases it for sitting. This creates strong stance security.
The trade-off is a less natural gait because the user may need compensatory movement to clear the foot during swing.
Weight-Activated or Stance-Control Knees
These knees increase resistance or braking when the prosthesis is loaded. They provide additional stance security while allowing the knee to flex during swing when loading decreases.
They can be useful for users who need more security than a free mechanical knee but want greater mobility than a permanently locked system.
Polycentric Prosthetic Knees
Polycentric knees use multiple linked axes rather than one fixed hinge. The changing centre of rotation can improve toe clearance and create favourable stance geometry.
They are often called four-bar or multi-axis knees. Polycentric geometry can be combined with friction, pneumatic or hydraulic swing control.
Pneumatic Knees
Pneumatic knees use compressed air moving through adjustable valves to control flexion and extension. Air resistance can provide smoother swing and support moderate changes in cadence.
They are generally used by community walkers who need more variable swing control than a basic constant-friction system.
Hydraulic Knees
Hydraulic knees use fluid moving through valves to create resistance. Fluid damping can offer refined control of swing and, in some designs, stance flexion or braking.
These knees can be useful for users who walk at variable speeds, negotiate ramps and spend significant time in the community.
Microprocessor-Controlled Knees
Microprocessor knees combine sensors, software and electronically controlled resistance. They monitor loading, position or movement and adjust the knee’s behaviour during walking.
Clinical guidelines and reviews report benefits for appropriate users in areas such as variable cadence, stumble recovery and stability. These systems are also heavier, more complex, higher in cost and require charging.
Mechanical vs Fluid vs Microprocessor Control
Mechanical systems prioritise simplicity and predictable behaviour. Pneumatic and hydraulic knees add fluid resistance for more adaptive swing. Microprocessor systems continuously modify resistance using sensor data.
These are not simply three price levels. Each category suits different users, and a simpler knee can be the better prescription when stability, servicing or budget are the main priorities.
How Activity Level Influences Knee Selection
A household walker may need strong stability and straightforward control. A limited community walker may benefit from stance-control features. A variable-cadence community walker may benefit from pneumatic or hydraulic control. Highly active users may need more sophisticated systems.
Activity level should be assessed by a prosthetist rather than assumed from age alone.
Body Weight, Build Height and Residual-Limb Length
Every knee has a weight rating and physical dimensions. Long residual limbs may leave less vertical space for the knee and foot, making build height a major selection factor.
A component cannot be chosen safely if its weight limit or structural dimensions do not match the user.
The Knee Must Work With the Socket and Foot
A knee does not function independently. Socket stability influences how confidently the user loads the knee, while foot stiffness and alignment affect the ground reaction forces acting through the system.
This is why custom prosthetic fitting and gait rehabilitation should evaluate the complete prosthesis rather than only one component.
Applications Beyond Basic Walking
Different knees can support work, commuting, ramps, stairs, uneven terrain and recreational activity to different degrees. Some knees also provide special modes or resistance for activities.
A component should be chosen for the user’s normal routine, not for one rare situation.
Maintenance and Service
Mechanical knees may need periodic inspection of friction, bearings or fasteners. Pneumatic and hydraulic systems can require specialist servicing. Microprocessor knees add charging, software and electronic maintenance.
Before buying, ask where the knee can be serviced and what the expected turnaround is.
Single-Axis vs Polycentric: Which Geometry Is Better?
Neither geometry is automatically superior. A single-axis knee is mechanically simple and its hinge location is easy to understand during alignment. A polycentric knee can create useful stance stability and toe clearance because its instantaneous centre of rotation changes as the knee flexes.
The prosthetist chooses geometry alongside the control system. A polycentric knee can still use simple friction, while a single-axis knee can incorporate sophisticated hydraulic or microprocessor control.
How Stance Flexion Can Improve Walking Comfort
Some knee systems allow a controlled amount of flexion while the limb is loaded. This can soften loading response and may make ramps or uneven surfaces feel less rigid. The feature must be matched to the user’s balance and loading technique because uncontrolled flexion would reduce stability.
What to Ask During a Prosthetic Knee Trial
Ask the user to experience slow walking, normal walking, faster cadence, sitting and standing, controlled stopping and ramp use when clinically safe. The team should observe toe clearance, socket movement, confidence and whether the user has to compensate with excessive hip hiking or circumduction.
A trial is not only about whether the knee feels smooth. It should answer whether the user can control it repeatedly and whether the benefits justify the added weight, cost and servicing.
The Best Prosthetic Knee Is the One Matched to the User
Common Mistakes When Choosing a Prosthetic Knee
Common mistakes include selecting a knee only by price, ignoring service access, choosing a model near the edge of its weight rating, and assuming that more technology always means more stability. Another frequent error is upgrading the knee when the real problem is socket fit or alignment.
The correct prescription starts with the user’s mobility needs and then checks whether the complete prosthesis can support those goals safely.
There is no universal best knee. The correct prescription combines stance security, swing control, activity level, strength, environment, servicing and budget.
Patients and clinicians can compare current prosthetic knee joint options with the full prosthetic system during a professional assessment.
Frequently Asked Questions
What is a prosthetic knee?
It is an artificial knee mechanism used in many above-knee prostheses to provide controlled flexion, extension and stance support.
Does every above-knee amputee need the same type of knee?
No. Knee selection depends on activity, strength, balance, body weight, residual limb, environment and budget.
What is a single-axis knee?
It is a knee that rotates around one main mechanical axis.
What is a polycentric knee?
It uses multiple linked axes, creating a changing centre of rotation during flexion.
How does a pneumatic knee work?
It uses compressed air resistance to control swing-phase flexion and extension.
How does a hydraulic knee work?
It uses fluid resistance through valves to control knee movement.
What is a microprocessor knee?
It uses sensors and software to adjust resistance dynamically during movement.
Which knee is most stable?
Stability depends on the specific mechanism and user. Locking, stance-control, hydraulic and microprocessor systems can all provide different forms of security.
Do prosthetic knees require maintenance?
Yes. Maintenance needs vary from basic mechanical inspection to specialist hydraulic or electronic servicing.
Can a knee be upgraded later?
Often yes in modular systems, provided build height, weight rating, alignment and other components remain compatible.
