A Brief Introduction to Lower Limb Prosthetics
Created | Updated Mar 26, 2009
Last year in the United Kingdom around 6,5001 new lower limb amputations were performed. This figure doesn't include amputations of the transtarsal/partial foot, the hallux (big toe) or other toes. Although no one knows for certain the exact number of amputees within the British population, the best estimates based on similar populations2 and large scale monitoring groups3 suggest a figure of one in 1,500. Amputation is a massively destructive procedure, especially the more proximal variants, with very high post-surgical morbidity rates. It is therefore generally undertaken as a last therapeutic measure. The majority of the operations carried out in the UK are because of dysvascularity in the lower limbs; this in itself is generally caused by atherosclerosis and a loss of viable blood supply in the limb. The other main reasons are trauma (road traffic accidents being the most common), infection, neurological disorders (including spina bifida, but predominantly diabetic neuropathy) and tumours4, mostly high-level femoral osteo- and chondrosarcomas.
The first group of patients, the dysvascular category, are responsible for over 70% of new amputations carried out in the UK. Around two thirds of these patients are aged over 65 and a third are diabetics. It is this group which has the poorest post-amputation survival statistics. Due to associated medical problems and poor general health, it is estimated that 40% of this group will die within two years of the operation.
Classification of Amputations and Prostheses
The ISO classification system for amputations is based around the site of the operation. Pre-ISO types of lower limb prosthesis fell into two categories - the above-knee prosthesis and the below-knee prosthesis. However, this simplification meant that very different styles of prosthesis were put together in the same category (the knee joint not being the only articulation in the leg!). Now that prosthesis categories refer to the actual amputation site5, we have a slightly broader range which more accurately places the device.
These are in increasing magnitude of loss:
- The Symes6, or ankle disarticulation prosthesis
- The transtibial prosthesis
- The knee disarticulation prosthesis
- The transfemoral prosthesis
- The hip disarticulation/hemipelvectomy prosthesis
The amputation/removal of the entire pelvis and both legs (hemicorporectomy) has not been included as this is incredibly rare, neither have the varied categories of toe and partial foot (eg, Chopart and Lizfrac) amputations as many of these never require direct prosthetic management and are often treated with footwear adaptations or orthotic intervention.
Function and Functional Loss
Regardless of its type, the prosthesis serves two basic functions. The first is to replace the cosmetic appearance of the leg. For certain users this is the overriding function of the prosthesis. The second basic function is to transfer the patient's weight to the ground when they stand or walk.
Cosmetic Appearance
An aesthetic, acceptable limb not only restores a certain level of self-esteem but also motivates the patient, thus enabling quicker and sometimes more successful rehabilitation. This profile of user generally tends to be younger and female, although nowadays more and more men are placing a higher emphasis on how the prosthesis looks. Unfortunately an early, cosmetically-pleasing appearance is often impossible due to residual swelling in the stump post-op. A larger socket is required to house this. The swelling will reduce over time and allow for smaller/slimmer sockets.
To help attain a more realistic effect many manufacturers (such as Blatchfords, Ossur and Otto Bock) are bringing out high-definition silicon and PVC covers. Certainly, at the upper end of this range, bespoke silicon finishes now look undistinguishable from human skin. However, silicon does have a very different texture and feel to skin; it also reacts differently to heat and light, but solutions to these problems are already being researched and will eventually be found.
Unfortunately, some patients are unable to rehabilitate successfully after their amputation. Generally these patients are much older and have other associated conditions; many are of very limited mobility or even non-ambulatory before the operation. For these cases, there is a separate category of artificial limbs, commonly called wheelchair or cosmetic limbs. These are usually little more than shaped blocks of foam with a soft socket attachment and cosmetic cover to finish. Although functionless in the ambulatory sense they provide the wheelchair-bound patient with the appearance of a limb.
The Human Gait Cycle
To understand the transfer of the patient's weight to the ground when they stand or walk, it is helpful to consider the normal human gait cycle, which is split into two different phases.
Stance phase is when the foot is in contact with the ground and swing phase is the point when the limb has been raised from the ground and is travelling toward its next episode of stance phase. To make this possible, the prosthesis must comfortably allow the patient to apply all their body weight through the socket/interface while the limb is in contact with the ground (stance phase). The limb must then stay attached to the person while it is 'swung' (swing phase).
All prosthetic limbs do the first (transfer of body weight) in the same way, regardless of style or site. The patient wears a prosthetic socket which generally encompasses their residual limb/stump7. The socket is always custom-made for the patient; it has to be an intimate fit to transfer the body's weight through structures that are not designed for that purpose. The weight is then transferred through an endoskeletal (or in some instances an exoskeletal8) frame towards the foot. Differing amputation levels will have more or less componentry - eg, knee or hip joints. Torque absorbers, shock absorbers and turntables9 may also be included in the limb build. Regardless of component choice, the patient's weight is transferred from user to ground via the socket and then down the structure to the foot.
The second issue - that of keeping the limb attached during swing phase - has a much larger variety of solutions, many specific to certain amputation levels. For instance, some transfemoral limbs are held on by creating a partial vacuum in the socket, thus using suction to hold the socket to the stump. Here, the soft fleshy tissue of the residual thigh is drawn into the socket, any remaining air is expelled through a one-way expulsion valve and tissue around the distal pelvis creates a lock sealing the stump in. However, to achieve a 'suction-suspended' limb for transtibial prosthesis we first require some form of cushion gel liner (usually silicon or urethane) to interface between the user and the socket. This is because, unlike the thigh, the front of the shin has very little soft tissue and the underlying bony anatomy is very close to the surface. Without the liner the tight intimate suction socket would prove to be unbearably uncomfortable.
Another way of suspending the limb is to use a similar gel liner with a locking spike built into it; this attaches to a ratchet or shuttle lock in the bottom of the socket anchoring the user. There are also various types of belt, cuff and strap arrangements sill in use by many patients. With certain levels, particularly disarticulations, it is possible to make a socket which, when the stump is pushed in, locks around the natural bony prominences. As a result, it requires nothing more than a well-shaped socket!
Individuality and Componentry
As touched upon earlier, there are a wide variety of prosthetic components used to aid the function, comfort and cosmesis of the prosthesis. Matching the correct componentry to the person is a key challenge to the prosthetist, not least due to the fact that there are many hundreds of different types of feet, knees and other devices, but also the fact that every patient requires different functions from their prosthesis. The frail little ten-stone dysvacular lady who only ambulates ten yards at a time to get to her toilet requires a substantially different limb from the fourteen-stone ex-serviceman intent on running his next marathon10.
One of the first things people often associate with amputees is the paralympics, in particular the sprinters11. The feet used by these athletes are manufactured from carbon fibre composite and are essentially large springs. The purpose of these is not just to transfer the user's weight to the ground but also to store as much of this energy as possible (think of a compressed spring) so that it can return it to the athlete, helping to propel them to the finish line (imagine the spring uncoiling). An example at the other end of the spectrum is the single axis foot. Here, the reason for prescribing is one of stability. With many of the more unsteady users and those very high amputations (hip disarticulation and hemipelvectomy), getting the foot flat and stable on the ground quickly is more important than energy return or the ability to adapt to uneven surfaces. The single axis foot does this by rapidly planter-flexing at 'heel strike' and quickly getting to 'foot-flat'. To explain planter flexion is the downward motion of the foot from the ankle joint12 and heel strike is the initial part of the stance phase in the gait cycle, when the foot begins contact with the ground. As the name suggests this initial contact is with the heel. Foot flat is when the entire surface of the foot contacts the ground, this is when the limb is most stable and for some users it is preferable to get to this state as quickly as possible.
Now, this is one contrasting example, using just one section of the prosthesis (the foot). There are many other feet with markedly different functions: multiaxial to allow a range of motion in all directions thus helping with less even surfaces, feet with inbuilt shock absorbers to reduce the force transmitted back through the limb at heel strike (good for heavy users), feet with torsion adapters which many golfers find aid their swing and even feet where the patient can self-adjust the pitch the foot sits at allowing them to change from flat shoes to higher heels (or even outrageously high heels!).
The same variation and choice also applies to knee joints. There are SAKLs (Semi-Automatic Knee Lock) that stay locked as the patient stands and walks but can be released to flex for sitting, and then automatically lock again as the patient stands. Additionally, there are friction-controlled knee locks, which swing freely when lifted from the ground, but become locked and stable as the patient transfers weight through them, as well as yielding knees with hydraulic chambers that bend gradually and allow the user to descend the stairs 'leg over leg'. There are also 'intelligent' pneumatic dampening systems that adapt to the speed of the patient and vary the dampening effect on the limb as it swings through allowing for a variety of cadences and, at the highest end of the market, programmable knees that constantly sense the knee's motion and position in space, automatically adjusting for inclines or stumbles.
Needless to say, there are also many hip joints and ankle mechanisms, as well as different interface materials and socket constructions, adapters and...well, the list goes on. All this and we haven't even started on upper limb prosthetics!
