Of all prosthetic components, the knee system is arguably the most complex. It must provide reliable support when standing, allow smooth, controlled motion when walking, and permit unrestricted movement for sitting, bending and kneeling.
For the transfemoral amputee (above-knee, including hip and knee disarticulation), successful function depends on selecting the correct knee to fit the person's age, health, activity level and lifestyle. For example, some amputees are particularly vulnerable to falls. For them, safety and stability are often more important than functional performance. Active amputees, on the other hand, prefer a knee that will give them a higher level of function even if it requires greater control.
A key way to evaluate an individual's prosthetic needs is to observe his or her walking cycle, which can be divided into two parts: “stance phase” (when the leg is on the ground, supporting the body) and “swing phase” (when the leg is off the ground, also referred to as “extension”). The happy medium between these two extremes (stance, or stability, vs. ease of swing, or flexion) is different for each individual.
Prosthetic knees have evolved greatly over time, from the simple pendulum of the 1600s to those regulated by rubber bands and springs or pneumatic or hydraulic components. Now, some knee units have advanced motion control modulated through microprocessors. Unfortunately, a practical knee unit with the power to raise and lower an individual has yet to be built. There simply isn't a motor that's both small enough and strong enough to do the job – for now.
Although over 100 individual knee mechanisms are commercially available, they can be divided into two major classifications: mechanical and computerized. Mechanical knees can be further separated into two groups: single-axis knees and polycentric, or multi-axis, knees. All knee units, regardless of their level of complexity, require additional mechanisms for stability (manual or weight-activated locking systems) and control of motion (constant or variable friction, and “fluid” pneumatic or hydraulic control). Prosthetic knees incorporating the features of two or more basic designs are referred to as hybrid designs.
Single-Axis Vs. Polycentric Knees
The single-axis knee, essentially a simple hinge, is generally considered the “workhorse” of the basic knee classes, due to its relative simplicity, which makes it the most economical, most durable, and lightest option available. For these reasons, it's used most often for children's prostheses, particularly since they outgrow their prostheses almost as quickly as their clothes. It's also ideal for people who live in remote areas or have limited access to prosthetic care. Single-axis knees do have limitations, however. By virtue of their simplicity, the knees are free-swinging and have no stance control; amputees must use their own muscle power to keep them stable when standing. Young children with their limitless energy shouldn't have a problem with this, but older adults might. To compensate for this, the single-axis knee often incorporates a constant friction control and a manual lock. The friction keeps the leg from swinging forward too quickly as it swings through to the next step. The biggest drawback of this kind of knee is that it can only be set up to walk optimally at one speed.
Polycentric knees, also referred to as “four-bar” knees, are more complex in design, with multiple axes of rotation. Their biomechanical versatility is the primary reason for their popularity. They can be set up to be very stable during early stance phase, yet easy to bend to initiate the swing phase or to sit down. Another popular feature of the knee's design is that the leg's overall length shortens when a step is initiated, reducing the risk of stumbling. Polycentric knees are suitable for a wide range of amputees. Various versions are ideal for amputees who can't walk securely with other knees, have knee disarticulation or bilateral leg amputations, or have long residual limbs.
A standard polycentric knee has a simple mechanical swing control that provides an optimal single walking speed; however, many polycentric knees incorporate fluid (pneumatic or hydraulic) swing control to permit variable walking speeds. The most common limitation of the polycentric design is that the range of motion about the knee may be restricted to some degree, though usually not enough to pose a significant problem.
Ossur's polycentric Total Knee 2000 features a unique seven-axis design. Recently upgraded with needle bearings, retaining rings, and an improved extension assist to enhance durability, the Total Knee 2000 also includes geometric locking for maximum security; three-phase hydraulics to enable smooth changes in walking speed; and stance flex, which acts as a shock absorber to reduce strain and simulate the natural flexing action of the knee. Ossur's Total Knee Junior has undergone similar enhancements for increased durability.
Stability Options
Manual Vs. Weight-Activated Locking Systems
Some amputees need or desire the security of a knee joint that locks in extension to prevent buckling. One option is the manual locking knee, which incorporates an automatic lock that can be unlocked voluntarily. This is the most stable knee available. Walking is possible with the lock either engaged or disengaged, although the locked knee requires excessive energy to use and produces a stiff, awkward gait. The manual locking knee is appropriate for weak or unstable patients as well as more active individuals who frequently walk on unstable terrain.
Another approach is the weight-activated stance-control knee. This knee is very stable and is often prescribed for a first prosthesis. When weight is placed on the prosthesis, the knee will not bend until the weight is displaced. The system functions as a constant-friction knee during leg swing but is held in extension by a braking mechanism as weight is applied during stance phase. This knee is a common choice for older and less active amputees.
Ossur's TKO 1500 is designed to overcome the primary disadvantage of existing mechanical friction locking knees. Unlike previous stance-control mechanisms, it enables the amputee to initiate knee flexion while the foot is still on the ground, without unweighting the prosthesis.
The Swing Phase Lock stancecontrol knee is Fillauer's newest product. It locks automatically prior to stance phase, closely replicates normal walking patterns, allows free flexion and toe clearance during swing phase, requires less energy output, offers three modes of control, and eliminates cables and special heel connections.
Motion Control Options
Constant Friction Vs. Variable Friction
All knee systems require some degree of swing control to maintain a consistent gait. In many cases, this control is provided by mechanical friction at the axis of rotation and adjusted to match the normal cadence of the opposite leg. Constant-friction knee units are simple, lightweight and dependable. Their main disadvantage is that the amputee is limited to a single walking speed.
Variable friction provides increased resistance as the knee bends from full extension. This provides “cadence response,” allowing variable walking speeds; however, this system requires frequent adjustment and replacement of moving parts and is considered less advanced than fluid control knee systems.
Fluid Control Systems: Pneumatic Vs. Hydraulic
Advanced swing control for prosthetic knees uses fluid dynamics to provide variable resistance, enabling amputees to walk comfortably at different speeds. These units consist of pistons inside cylinders containing air (pneumatic) or fluid (hydraulic). Pneumatic control compresses air as the knee is flexed, storing the energy, then returns the energy as the knee moves into extension. Gait control can be further enhanced with the addition of a spring coil. Pneumatic systems are generally considered to to friction knees but to be less effective than hydraulic systems.
For active amputees, hydraulic systems provide the closest thing to normal knee function. Hydraulic systems use a liquid medium (usually silicone oil) instead of air to respond to a wide range of walking speeds. Although hydraulic knees provide a smoother gait, in comparison with other knee systems hydraulic units are heavier, require more maintenance, and cost more.
Jim Smith Sales, Inc., offers the recently improved Ultimate Knee, which has the unique ability to be a manual lock knee, a stance lock knee, and a stance yield knee, all within the same unit. The hydraulic system has been updated to achieve higher function and require less maintenance; a unique feature is a hydraulic terminal impact dampener, which provides a gentle end to the swing phase, demanding less energy of the user.
Microprocessor Knees
Microprocessor, or computerized, knees are a relatively new development in prosthetic technology. An onboard sensor detects movement and timing and then adjusts a control cylinder accordingly. The real-time data collected by the microprocessor determines which setting to use. The microprocessor control knee lowers the amount of effort amputees must use to control their timing, resulting in a more natural gait.
Otto Bock's C-Leg has a hydraulic knee that has both swing and stance controlled by a microprocessor taking readings 50 times per second. In addition, the C-leg can be programmed for two modes of settings that can be changed by simply tapping the toe. Typically, one mode is set for everyday use; the second can be anything from a locked knee for standing in line for concert tickets to a free-swinging knee to limit resistance while biking. There's also a shorter version available for knee disarticulation amputees. Walter Reed Army Medical Center is now using the C-Leg for veterans returning from Operation Iraqi Freedom.
The Adaptive Prosthesis is the third generation of microprocessor control in Endolite's history. This single-axis knee features a hydraulic and pneumatic hybrid cylinder controlled by microprocessors through time, force, and swing sensors that detect various changes in the gait cycle at 62.5 times a second. It can be programmed for stumble recovery, stairs, slopes, ramps and variable walking speeds. The Adaptive Prosthesis does not require a second mode for biking; just get on it and go.
In spite of all of the amazing inventions and constant tweaks and improvements, the perfect prosthetic knee has yet to be invented; otherwise, there wouldn't be over 100 different designs on the market. As advanced as the technology seems today compared to the earliest designs of the 1600s, one can only imagine the developments that will eventually result as researchers further explore the potential of mechanical, hydraulic, computerized, and – eventually – “bionic,” or neuroprosthetic, technology.
