Chapter 18B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
Transtibial Amputation: Prosthetic Management
Susan Kapp, C.P.
Donald Cummings, C.P.
Once the patient has completed the postoperative phase of treatment and adequate wound healing is established, the goals of rehabilitation become limb maturation and return to normal activity. These goals are often accomplished with the help of an intermediate (or preparatory) prosthesis coupled with gait training, a residual limb shrinkage program, and close supervision by the clinic team.
An intermediate prosthesis consists of a socket, a pylon, a foot, and a method of suspension. It is usually applied when edema is diminished and the patient's residual limb has atrophied sufficiently to allow independent donning and doffing of a prosthetic socket. To accomplish this, the intermediate prosthesis is often preceded by rigid dressings or immediate postsurgical fittings. The intermediate may be worn for a few months or as long as a year, depending upon the pace of residual limb atrophy, before it is replaced with a definitive prosthesis.
Even after initial prosthetic fitting, elastic bandages and residual limb shrinkers still play an important role in the conditioning of an amputated limb. For patients with wound complications, delayed healing, or other circumstances that delay prosthetic fitting, an elastic bandage or shrinker may be the most practical and economical form of residual limb conditioning. Bandaging is beneficial whenever the new amputee is not wearing the prosthesis. It aids in residual limb shrinkage through pressure atrophy of subcutaneous fat and by externally supporting veins and lymphatic channels to allow blood return through muscular contractions. Residual limb shrinkers, which are composed of a series of elastic bands sewn together to form a cylinder with a padded distal end, may be used in place of elastic bandages. For the recent amputee, shrinkers or elastic bandages should be worn whenever the patient is not wearing a prosthesis, rigid dressing, or some other compressive device.
An intermediate prosthesis is generally constructed on an endoskeletal pylon, which ensures that alignment changes can be made as needed throughout the intermediate period. This is a considerable advantage since the needs of the patient can be constantly reassessed and accommodated as his ability to use the prosthesis improves.
Once fitted with an intermediate prosthesis, the patient may progress in physical therapy to full weight bearing. In addition to gait training, it is recommended that the patient be instructed in the use of prosthetic socks, the application of shrinkers or elastic wraps, residual-limb hygiene, and regular inspection of the limb for any sign of excessive pressure. Alignment and socket fit are adjusted by the prosthetist as needed. It is not unusual for the patient to progress through several intermediate sockets within the first year following amputation.
A patient's readiness for a definitive fitting varies depending on his activity level, weight-bearing tolerance, and limb shrinkage. Atrophic changes may stabilize after only 4 months. More likely, however, the process continues for 12 months or more.
The decision to proceed with definitive fitting, largely subjective, is based on the overall perception that the patient has reached a plateau in activity level, prosthetic wearing time, and residual-limb volume. For example, a young active amputee who has worn an intermediate prosthesis is probably ready for definitive fitting when he can tolerate full weight bearing, wears the prosthesis all day, and for a period of perhaps 1 month has not had to add prosthetic socks to accommodate limb shrinkage. In contrast, an elderly patient with other health problems may use a walker and wear the prosthesis only 4 to 5 hours daily, but is nevertheless ready for definitive fitting because a degree of con-sistancy in activity level and residual-limb condition has been achieved.
While preparatory systems are intended to accommodate the multiple changes experienced by a recent amputee, a definitive prosthesis differs primarily in that its design and components are geared toward the goals of the patient after activity levels, prosthetic wearing schedules, and residual-limb changes have all stabilized. A definitive prosthesis may closely resemble the intermediate prosthesis that preceded it, or it may differ dramatically depending upon the goals that have emerged during rehabilitation.
When deciding upon an appropriate prosthesis, the patient and the clinic team is faced with a wide variety of choices due to numerous innovations in prosthetic components, materials, and techniques during the last several decades. Each technique, socket configuration, suspension system, alignment, and component has specific advantages and disadvantages that can be balanced to provide the optimum combination for the patient's unique needs.
THE PATELLAR TENDON-BEARING SOCKET
The patellar tendon-bearing (PTB) socket (Fig 18B-1.) consists of a laminated or molded plastic socket. To make a PTB socket, an impression taken of the patient's residual limb is modified to achieve an intimate, total-contact fit over the entire surface of the residual limb. It can be suspended in multiple ways, which are discussed later.
The anterior wall of the socket usually extends proxi-mally to encapsulate the distal third of the patella. Just below the patella located at the middle of the patellar ligament is an inward contour or "bar" that utilizes the patellar ligament of the residual limb as a major weight-bearing surface. The term patellar tendon bearing can be misleading, however, because the patellar ligament is not the only major weight-bearing surface utilized by the PTB socket.
The medial and lateral socket walls extend proximally to about the level of the adductor tubercle of the femur. Together they control rotation, contain soft tissue, and may provide some mediolateral knee stability. The medial wall is modified with a slight undercut in the area of the pes anserinus on the medial flare of the tibia, thus utilizing another major pressure-tolerant surface. The lateral wall provides a relief for the head of the fibula and supports the fibular shaft. This wall also acts as a counterpressure to the medial wall.
The posterior wall is usually designed to apply an anteriorly directed force to maintain the patellar ligament on the bar. The posterior wall is flared proximally to allow comfortable knee flexion and to prevent excessive pressure on the hamstring tendons.
The distal portion of the PTB socket may incorporate a soft pad that in theory prevents distal edema by aiding venous and lymphatic return from the distal part of the residual limb. A soft socket liner may be used for added protection or comfort.
The PTB total-contact socket is appropriate for virtually all transtibial (below-knee) amputations, except in some postoperative prostheses or when pathologic conditions require an alternative socket.
The PTB Hard Socket
The hard socket (Fig 18B-2.) is rigid plastic and therefore has specific advantages and disadvantages when compared with a socket with a soft liner or distal pad. This style of socket is primarily indicated for a residual limb with good soft-tissue coverage and no sharp bony prominences. It is not recommended for residual limbs with thin skin coverage, scarring, skin grafts, or a predisposition to breakdown.
- Perspiration does not corrode the socket.
- Less bulky at the knee than with an insert.
- Easy to keep clean.
- Contours within the socket do not compress or pack down with use.
- Reliefs or modifications can be located with exactness.
- Requires extra skill in casting and modification.
- Difficult to fit bony or sensitive residual limbs.
- Not as easily modified as a socket with a liner.
Soft liners (Fig 18B-3.) are fabricated over the modified cast to fit inside the socket. They act as an interface between the limb and socket to provide added comfort and protection for the residual limb by moderating impact and shear. They are often fabricated from a 5-mm polyethylene foam material. Occasionally silicone gel is used for the more sensitive residual limb. Silicone and similar materials may also be used to create an air seal against the patient's skin so that the liner can provide suction suspension.
Soft liners are recommended for patients with peripheral vascular disease; for thin, sensitive, or scarred skin and sharp bony prominences; and for patients with peripheral neuropathy. The bilateral transtibial amputee may prefer liners to protect the distal portion of the tibia when rising from a chair or during stair and incline climbing. The added protection of a soft liner may also benefit the highly active patient.
- Provides a soft, protective socket interface.
- Is appropriate for the majority of residual limbs.
- Rebound in the liner may aid circulation by providing a "pumping action" and by providing intermittent pressure over bony prominences.
- Is easily modified.
- Materials may deteriorate over time.
- Not as sanitary as a hard socket because liners tend to absorb fluids.
- Increases bulk around the knee and proximal circumference of the prosthesis.
- The liner may compress over time with resultant loss of intimate fit.
- Increases the weight of the prosthesis.
To improve overall comfort and to help prevent edema, the distal portion of PTB sockets generally incorporate a soft pad. A few special instances may not call for these protective pads, but most often they are standard.
- May aid in venous and lymphatic return.
- Provides increased comfort.
- Protects the distal portion of the residual limb when it settles into the socket as a result of volume loss.
- Facilitates future modifications of the distal end of the socket.
- Added fabrication time.
- Increased weight.
- May be less hygienic due to absorption of fluids.
Flexible Sockets With Rigid External Fra
New plastics and materials have enabled prosthetists to offer patients the benefits of a flexible inner socket that is inserted into a rigid frame (Fig 18B-4.). The inner socket is fabricated from polyethylene or a similar material and the frame from laminated plastic or thermoplastic material. The frame provides coverage only over the primary weight-bearing areas, while the more pressure-sensitive areas, i.e., bony prominences and soft tissues not requiring rigid support, are enclosed only in the flexible socket. This technique often results in a more comfortable socket and can be utilized in en-doskeletal or exoskeletal systems.
- Decreased weight.
- Increased comfort.
- Improved heat dissipation.
- The inner socket may be replaced to accommodate anatomic changes.
- More difficult and time-consuming to fabricate.
- May not be as cosmetic as conventional prostheses.
The cuff (Fig 18B-5.) is generally fabricated from da-cron and lined with leather. It encircles the thigh and purchases over the femoral condyles and proximal part of the patella. Attachment points on the socket are slightly posterior to the sagittal midline in order to resist hyperextension forces at the knee and to allow the limb to withdraw slightly from the socket during knee flexion.
Cuff suspension is appropriate for average-length residual limbs with good knee stability. It is not recommended for short residual limbs since they generally require increased surface area contact and rotational control through more proximal trim lines. Excessive scarring or sensitive skin in the area in contact with the cuff may be another contraindication.
- Ease of donning and doffing by the patient.
- Adequate suspension for the majority of transtibial amputees.
- Provides moderate control of knee extension.
- Easily replaced.
- Cannot completely eliminate socket pistoning.
- During knee flexion, may pinch soft tissue between the posterior proximal end of the socket brim and the cuff.
- May restrict circulation.
- Provides no added mediolateral stability.
PTB Supracondylar Suprapatellar Suspension
The PTB supracondylar, suprapatellar (PTB-SCSP) socket (Fig 18B-6.) was designed originally as an alternative suspension and as a means of providing increased mediolateral and anteroposterior stability of the residual limb. This socket differs from the standard PTB socket in that the medial, lateral, and anterior walls extend higher and fully encompass the femoral condyles and the patella. The posterior wall is unchanged.
During casting and modification, the proximal mediolateral dimension just superior to the femoral condyles is reduced to provide purchase over the femoral condyles, particularly the medial condyle. The area just proximal to the patella may also be contoured inward to create a "quadriceps bar," which provides added suspension over the patella and resists recurvatum.
This type of suspension is particularly recommended for patients with short residual limbs since it encompasses more surface area to share weight bearing and resist torsional forces. Patients with mild mediolateral knee instability or those who cannot tolerate a supracondylar cuff can also benefit from PTB-SCSP suspension. This socket style generally cannot provide adequate purchase over the femoral condyles for obese or very muscular patients. Patients with moderate to severe ligamentous laxity usually require the added stability of metal joints and a thigh corset rather than supracondylar suspension alone.
- Suspension is an inherent part of the socket.
- Is less restrictive to circulation than a cuff or thigh corset.
- Aids in knee stability, rotational control, and pressure distribution.
- Reduces pistoning.
- Modifications over the patella and femoral condyles must be precisely located.
- Enclosure of the patella can inhibit comfortable kneeling.
- May be less cosmetic and more destructive to clothing because higher trim lines protrude when the knee is flexed.
PTB Supracondylar Suspension
The major difference between this and the PTB-SCSP socket is that the patella is not enclosed (Fig 18B-7.). The medial and lateral brims purchase over the femoral condyles, but anteriorly they dip downward to form a more traditional trim line near the distal end of the patella. The quadriceps bar and its knee extension control are thus eliminated.
This suspension may be indicated when a patient wishes to do a lot of kneeling or cannot tolerate the quadriceps bar or encapsulation of the patella. The patient must have a stable cruciate ligament with no need for an extension stop at the knee.
It is contraindicated, as is the PTB-SCSP socket, for patients with moderate to severe ligamentous laxity who require the added stability of metal joints and a thigh corset.
Advantages (over the PTB-SCSP socket)
- May make kneeling easier.
- May be more cosmetic.
Disadvantages (as compared with the PTB-SCSP socket)
- Does not provide a knee extension stop.
- May provide less effective suspension than a PTB-SCSP since there is no suprapatellar purchase and because the absence of patellar encapsulation makes the medial and lateral walls more flexible.
- Less mediolateral stability than the PTB-SCSP.
Variants of the PTB Supracondylar, Suprapatellar and PTB Supracondylar Sockets
The PTB-SCSP socket usually incorporates a removable soft liner with a foam wedge buildup over the medial femoral condyle that compresses sufficiently to allow the amputee to push his residual limb past the supracondylar undercuts and into the prosthesis.
The socket with a removable medial brim (Fig 18B-8.) is another type of supracondylar suspension. As the name implies, the proximal medial brim is removed to allow the residual limb to be inserted into the socket. It is then replaced to provide purchase over the medial femoral condyle. Laminated into the proximal brim is a steel bar that fits into a channel on the medial aspect of the socket, thus allowing the medial brim to be removed and then replaced in its original position. The anteroproximal part of the socket brim can also be made to encompass the patella in the fashion of a PTB-SCSP socket.
A third variant is the removable medial wedge (Fig 18B-9.). Once the patient has inserted his residual limb into the socket, the supracondylar area between the limb and the medial socket wall is filled by a Plastisol or crepe wedge that "keys into" the proximomedial portion of the socket brim.
The selection of supracondylar methods depends upon patient needs such as cosmesis, durability, ease of donning and doffing, the need for a soft liner, and patient preference.
The suspension sleeve (Fig 18B-10.) has been in use since 1968 when it was introduced at the University of Michigan, Ann Arbor. Sleeves are prefabricated from thin latex rubber or neoprene and come in a variety of sizes. They fit snugly over the proximal aspect of the prosthesis and are rolled up over the patient's thigh 2 or 3 in. proximal to the prosthetic socks. By making contact with the patient's skin, the sleeve converts the socket into a sealed chamber. Three suspensory forces attributed to the sleeve are described by Chino (1975): negative pressure created during the swing phase, friction between the residual limb and the socket, and longitudinal tension in the sleeve. Sleeves may serve as the sole means of suspending a prosthesis, but they also provide excellent auxiliary suspension during sports and recreational activities for those who normally use supracondylar suspension.
Sleeves are contraindicated as the sole suspension for very short limbs or those that require more proximal trim lines for added knee stability. Patients whose activities require kneeling may find the sleeve less durable and may need to replace them frequently due to punctures of the sleeve material. Some patients living in hot, humid climates may find that the sleeve creates perspiration and hygiene problems.
- Simple and effective means of suspension.
- Helps minimize socket pistoning.
- Good auxiliary suspension.
- Does not create proximal constriction.
- Provides no added knee stability.
- Suspension is greatly decreased if the sleeve is punctured.
- Perspiration may build up under the sleeve and create skin irritation or hygiene problems.
- Must be replaced regularly.
- Sleeves may restrict full knee flexion and require good hand function to don and doff.
Silicone Suction Socket
The silicone suction socket (3-S) (Fig 18B-11.) or Icelandic roll-on suction socket (ICEROSS) was conceptualized and developed by Ossur Kristinsson with further development by Durr-Fillauer Orthopedic, Inc. The socket utilizes a "silicone liner" either custom-made or prefabricated. This liner is worn directly against the skin and dramatically reduces shear forces created by socket pistoning. Suspension is achieved by the inherent suction capabilities of a silicone material against skin and a shuttle lock mechanism at the distal end of both liner and socket. The silicone liner is used strictly to provide suspension. It may be used with a hard socket or with a soft, lined socket depending on the amputee's needs.
- Improved suspension.
- Increased range of motion in flexion.
- Decreased shear on residual limb.
- Some patients may have difficulty in donning the liner.
- Punctures or tears in the silicone can dramatically decrease suction suspension.
Joints and Thigh Corset
The thigh corset (Fig 18B-12.), traditionally made of leather, is fastened snugly around the distal two thirds of the patient's thigh and is attached to the socket by metal joints with vertical support bars. Although joints and a corset may be the sole form of suspension, they are often combined with waist belt suspension. In order to prevent the joints from reaching full extension, which is noisy and soon wears the joints down, a posterior "check strap" is usually added between the corset and socket. The check strap can be adjusted to limit knee extension to varying degrees, depending upon the patient's needs.
Prior to 1958, the thigh corset combined with a waist belt was probably the most common form of transtibial prosthetic suspension. Since sockets were open ended and did not fit as intimately as today's total-contact sockets, the main purpose of the thigh corset was to utilize the thigh to share weight bearing and reduce torque forces about the residual limb. The extended lever arm of the joints and corset provided maximum me-diolateral stability, and the use of a hyperextension check strap, if adjusted properly, could effectively prevent recurvatum. The trade-off, however, was that the lack of total contact combined with a tightly laced corset often resulted in chronic distal edema. Also, because the corset bound the thigh tightly and reduced muscular demands, it frequently contributed to marked atrophy of the thigh musculature.
While modern total-contact socket designs (and suspension systems) have greatly reduced the need for joints and thigh lacers, they are still appropriate when maximum mediolateral or anteroposterior stability is required. Knee joint instability is a common indication. The corset also provides some degree of shared weight bearing and is useful when partial unloading of the residual limb is necessary. Patients who perform exceptionally heavy-duty work may benefit from the added stability of joints and corset suspension.
- Provides maximum mediolateral stability.
- Can provide maximum prevention of recurvatum.
- Redistributes some weight bearing and torque forces to the thigh.
- Increases proprioceptive feedback.
- Can contribute to distal edema.
- Tends to atrophy thigh musculature.
- Leather is not very hygienic.
- Joint centers must be precisely located to minimize motion between the leg and the prosthesis.
- Adds weight and bulk to the prosthesis.
- Not very cosmetic.
- Requires more fabrication time.
- Usually requires additional suspension of a fork strap and waist belt.
Waist Belt Suspension
The waist belt (Fig 18B-13.) can be used as an auxiliary suspension or as a sole means of suspension. The standard system consists of a belt situated above the iliac crests or between the iliac crests and the greater trochanters. On the amputated side, an elastic strap extends distally to a buckle at midthigh. Fastened to this buckle is a strap that attaches to a PTB cuff or inverted "Y" strap connected to the prosthesis.
Waist belt suspension is often used on postoperative or intermediate prostheses because it maintains suspension regardless of residual-limb volume changes. It is also recommended when all proximal constriction must be eliminated due to skin or vascular conditions. The elderly or debilitated patient may prefer the added security of a waist belt. It also affords good auxiliary suspension for sports prostheses.
The waist belt is not recommended for patients with severe scarring or sensitive skin in the regions in contact with the belt.
- Much of the weight of the prosthesis is distributed proximally over the iliac crests.
- Enables patients to loosen the supracondylar cuff or other form of suspension.
- Good auxiliary aid when other types of suspension are inadequate.
- The elastic strap provides some knee extension assistance.
- Discomfort of wearing a belt.
- Does not provide even suspension through swing phase (the tension of the elastic is proportional to the degree of knee flexion).
- The fork strap does not provide any resistance to knee extension.
- No mediolateral stability is provided by waist belt suspension alone.
The prosthetic foot is an important, multifaceted component of the transtibial prosthesis. The primary purpose of the prosthetic foot is to serve in place of the anatomic foot and ankle. In doing this, the prosthetic foot should provide the following functions:
- Joint simulation.-In the normal human foot and ankle, the talocrural joint allows plantar flexion and dorsiflexion, the subtalar joint allows inversion and eversion, and the other joints of the foot (in particular, the metatarsophalangeal joints) allow smooth rollover during heel-off and toe-off. These motions are vital to normal energy-efficient gait and are particularly important during ambulation on uneven ground. A successful, energy-efficient gait with a prosthetic foot is therefore largely dependent upon the ability of the foot to compensate for the absence of normal joint function.
- Shock absorption.-The foot must absorb the impact of heel strike and weight acceptance without transmitting excessive forces to the residual limb. Too much shock absorption, in contrast, might fail to generate the normal knee flexion moment when the foot is flat and result in an unacceptable gait pattern.
- A stable weight-bearing base of support.-This is important during stance phase or when the amputee is standing.
- Muscle simulation.-In normal human gait, the dorsiflexors eccentrically lengthen to prevent foot slap after heel strike. During midstance and heel-off, the plantar flexors stabilize the ankle joint and resist the powerful dorsiflexion moment that occurs during these phases of gait. During running or rapid walking, the plantar flexors may actually "push off" and assist in propelling the weight of the body forward. The primary way in which a prosthetic foot substitutes for muscle activity is through stance-phase stability (substitution for the plantar flexors). In addition, some prosthetic feet allow controlled plantar flexion and dorsiflexion, thus simulating both dorsiflexors and plantar flexors. Through dynamic response principles, a few specialized feet actually provide some degree of dynamic "push-off" during late stance.
- Cosmesis.-While function of the prosthetic foot is of primary concern to the prosthetist, the importance of cosmesis cannot be overlooked. The design of a particular foot may enhance or diminish its cosmetic appeal.
There are essentially four different designs of prosthetic feet available for use with transtibial prostheses. They are SACH (solid-ankle, cushion-heel) feet, single-axis feet, multiaxis feet, and flexible-keel-dynamic-response feet. Each will be discussed in detail.
The light weight, durability, low cost, and cosmesis of the SACH foot make it the single most frequently recommended prosthetic foot. Although recent innovations in prosthetic foot design may change this, the SACH foot has been the traditional foot of choice for children and for the majority of adult patients with transtibial or ankle disarticulation amputations. They are available for multiple shoe styles and heel heights, postoperative uses, Syme's fittings, external-keel "waterproof' fittings, and pediatric sizes.
Standard SACH Feet
Internal-keel SACH feet (Fig 18B-14.) include a solid wood or aluminum internal keel that extends to the toe break and is surrounded by a molded external foam foot with cosmetic toes and a cushioned heel wedge available in different densities.
The SACH foot has no movable components, so joint motion is simulated by the rubber surrounding the keel. Plantar flexion is replaced by the compression of the heel wedge. Ankle dorsiflexion is not available in the SACH foot. Neither inversion nor eversion of the ankle is provided, although forces in the coronal plane are dampened by compression of the rubber sole. Forefoot dorsiflexion is simulated by the flexible toe portion distal to the end of the internal keel.
Shock absorption at heel strike in the SACH foot is good. It is dependent upon the density of the heel cushion and the superincumbent weight of the patient. Heavier patients require firmer heel cushions.
The SACH foot has excellent stability due to several determinants. During standing activities, the heel cushion provides resistance when the patient "rocks" backward on the heel. Softer heel cushions produce less resistance to this motion and may diminish the stability of the weight-bearing base in the sagittal plane. This factor is probably more significant for bilateral amputees who lack a sound foot for balance control and proprioception. The second determining factor is keel length. Resistance to dorsiflexion in the SACH foot is proportional to the length of the keel. A longer keel will increase the toe lever arm but may result in excessive hyperextension forces at the knee during late stance. Use of a keel that is too short will reduce stability and lead to early heel rise and a shortened stance phase on the prosthesis. The third determining factor is keel width. The wider the keel, the more stable the base of support. External-keel SACH feet are more stable in the coronal plane because of the widened keel.
Plantar flexor muscle simulation in the SACH foot is accomplished by the presence of the solid keel and ankle, which prevents ankle dorsiflexion. The dorsiflexors are simulated by the cushion heel, which absorbs plantar flexion forces during heel strike and foot flat. The invertors and evertors are simulated to a small degree by compression of the rubber sole.
Cosmesis of the SACH foot is good. Since there is no motion in the ankle, the junction between the foot and shank can be reduced to a barely perceptible line. However, a difference between the materials of the shank and foot is often still visible.
The postoperative SACH foot (Fig 18B-15.) is designed so that the patient can walk without shoes or in slippers. As the name implies, its primary use is on postoperative or temporary prostheses. It has no heel rise, and since no shoes are worn, the postoperative foot has a wider sole than a standard SACH foot to provide more stability. The molded rubber foot and heel are softer, which makes the postoperative foot very shock absorbent.
The SACH foot is indicated for virtually all patients, young and old, wearing temporary, intermediate, or definitive transtibial prostheses. The standard SACH foot is contraindicated for ankle disarticulation amputees. This level requires a special design.
- Moderate weight.
- Good durability.
- No moving components.
- Minimal maintenance.
- Good shock absorption for moderately active patients.
- Limited plantar flexion and dorsiflexion adjustability.
- The heel cushion deteriorates over time.
- The heel cushion may loose elasticity.
- The rigid forefoot provides poor shock absorption for high-output activities.
Other SACH Foot Types
The Syme SACH foot (Fig 18B-16.) was designed to provide the ankle disarticulation amputee with the advantages of a SACH foot. Since an ankle disarticulation in an adult results in only an average I½ in. shortening of the leg, the space available for the addition of a prosthetic foot is limited. The Syme foot, therefore, is lower in height than the SACH foot in order to accommodate this minimal ground clearance. Important to note is the thinner heel cushion. Because the foot height is reduced, the heel cushion is also reduced and therefore less shock absorbent.
In the external-keel SACH foot (Fig 18B-17.) the keel portion is not incorporated within the rubber foot. Instead, the rubber portion of the foot is affixed to the keel. It is used for exoskeletal prostheses only. This foot is recommended for a Syme prosthesis when an internal-keel foot will not accommodate a minimal leg length discrepancy. The need for optimum cosmesis at the ankle on an exoskeletal prosthesis may also be an indication for an external-keel SACH foot. The foot's wider keel makes it ideal for patients who require added stability, although an attempt should be made to gain stability through prosthetic alignment first. The external-keel SACH foot also permits the prosthesis to be made waterproof.
The single-axis foot (Fig 18B-18.) is available for exoskeletal or endoskeletal prostheses. Its components include a solid wood internal keel, a molded foam rubber shell, a metal single-axis joint, a rubber plantar flexion bumper, and a dorsiflexion stop.
Ankle plantar flexion and dorsiflexion are provided in a limited way by rotation about the ankle joint. Minimal inversion and eversion occur through the flexibility of the rubber sole. Toe dorsiflexion is simulated by the flexibility of the rubber toe section.
The single-axis foot offers shock absorption at heel strike through the plantar flexion bumper, which is available in multiple durometers. Because the foot plantar-flexes after heel strike, thus dampening knee flexion moments, and since it is in contact with the ground for a longer period of time, stance-phase stability is excellent.
Single-axis feet have specific application in transfem-oral (above-knee) prosthetics and are rarely necessary for transtibial amputees, although some amputees prefer the sensation of ankle motion.
- The plantar flexion capability provides increased knee stability at heel strike and foot flat and may lessen the difficulty of descending inclines.
- Plantar flexion resistance can be varied.
- Relatively high maintenance due to moving components.
- Increased weight.
- Less cosmetic.
- Tendency to "squeak."
This foot (Fig 18B-19.) provides more ankle motion than any other prosthetic foot. Available for endoskeletal and exoskeletal prostheses, it provides motion in all three planes, which makes it particularly suitable for patients who walk on uneven terrain.
Its components include a solid-wood internal keel, a molded rubber foot, a central rubber rocker block that allows sagittal-plane motion, and a transverse ankle joint that provides inversion, eversion, and transverse rotation.
Joint simulation is achieved by the various bumpers. Although transverse rotation is not truly an anatomic ankle joint motion, it reduces shear forces transmitted to the residual limb and is an alternative to a rotation unit.
Shock absorption is excellent in the multiaxis foot because of the many bumpers. The degree of compressibility and rebound of these individual components determines the degree of shock absorption during various gait phases.
Because of the many motions it allows, the foot may be considered less stable statically. However, because of its ability to absorb forces in all planes the multiaxis foot reduces torque on the residual limb that might occur on uneven terrain.
Muscle simulation is the same as that of the single-axis foot, with added true simulation of the invertors and evertors through the corresponding rubber bumpers. Cosmesis is comparable to the single-axis type due to the space required between the ankle block and foot.
It is a good option for patients who traverse frequently over uneven terrain, but its increased weight and maintenance may overshadow its advantages. It is not recommended for patients who are weak and debilitated, those for which cosmesis is a priority, or those with limited access to prosthetic follow-up.
- Allows motion in all planes.
- Reduces torque on the residual limb.
- Increased weight.
- Increased maintenance.
- Decreased cosmesis.
- May provide less stability than other feet on smooth surfaces.
Flexible-Keel- Dynamic-Response Feet
Prosthetic feet are primarily designed for walking, yet many lower-limb amputees have the desire to be more active and therefore require the use of a prosthetic foot that will allow them increased activity. This need has promoted research and resulted in a new generation of feet (Fig 18B-20. and Fig 18B-21.) that aid the more active amputee.
These feet incorporate a shock absorption mechanism in the form of a flexible keel that dissipates energy, provides a smoother gait, and gives some degree of push-off that the rigid keel cannot provide. As a patient's cadence increases, the amount of time spent on the heel decreases, while the amount of time spent on the forefoot increases. Since relatively more time is spent, and considerably more forces are exerted on the forefoot, there is an increase in the dorsiflexion moment. Through the use of new designs and materials, this dorsiflexion moment allows the keel to compress or distort, thereby absorbing energy that is released during push-off, and aids in propelling the patient forward. Some of the materials currently in use include graphite composite, Delrin, Kevlar, polyurethane elastomer, and flexible rubber, which generally result in a lighter-weight foot. In addition, the feet allow a more fluid motion, which produces a more normal gait.
BIOMECHANICAL VARIABLES IN TRANSTIBIAL PROSTHETICS
The successful fitting of a transtibial prosthesis requires a thorough understanding of the biomechanical variables involved and the ability to achieve an appropriate compromise between these variables to meet the unique needs of each patient. Biomechanical factors in transtibial prosthetics can be divided into four broad categories: socket fit, alignment, foot function, and suspension (Fig 18B-22.).
Biomechanics of Transtibial Socket Fit
The prosthetic socket is the primary connection between the patient's residual limb and the prosthesis. As such it must provide comfort and function to the patient under the duress of two force systems: the weight of the body due to gravity and the forces applied to the residual limb through contact with the socket. These forces are continually changing during both static and dynamic use of the prosthesis. The successful resolution of these forces can come about only through careful attention during patient evaluation, casting, and socket modification to the following details.
Displacement and Pressure Tolerance of Residual-Limb Tissues (Total Contact)
In theory, residual-limb/socket pressures could be maximally reduced by ensuring that every square centimeter of the residual limb is in contact with the socket and is sharing an equal portion of the load. In actual practice this is complicated by differences in tissue displacement and tissue pressure tolerances. For example, some bony portions of the residual limb like the distal part of the tibia or the head of the fibula cannot be compressed as much as soft-tissue areas.
Since soft tissues are displaced during axial loading, a socket that simply makes equal contact with the surface area of the residual limb may cause more pressure over bony anatomy and less pressure over soft tissue. Further, even if pressures are equalized over the surface area of the residual limb, some bony or sensitive areas may be unable to tolerate these forces. The problem is increased when patients have unusually thin skin, sharp bony prominences, scars, or neuromas. In contrast, other areas of the residual limb such as the medial tibial flare or the patellar ligament can tolerate a great deal of pressure with no pain or skin breakdown.
Most fitting problems can be accommodated through appropriate socket design. In order to apply greater pressure to pressure-tolerant areas and less to pressure sensitive areas, tissues are selectively loaded through inward contours over weight-bearing surfaces and reliefs over sensitive areas. Areas within the socket that require relief (Fig 18B-23.) are the tibial crest, tibial tubercle, lateral tibial flare, distal tibia fibular head, peroneal nerve, hamstring tendons, and the patella. Pressure-tolerant areas (Fig 18B-24.) are the patellar ligament, medial tibial flare, medial tibial shaft, lateral fibular shaft, and the anterior and posterior compartments.
Modification for Dynamic Forces
In normal human locomotion, floor reaction forces produce moments at the joints of the lower limb. Similar forces exist during ambulation with a prosthesis, but they are applied through the prosthetic socket to the enclosed residual limb. These forces upon the residual limb must be managed to achieve socket comfort and prevent skin breakdown. The major dynamic forces to be considered are anteroposterior and mediolateral forces. Anteroposterior forces are generated from heel strike to foot flat while a powerful knee flexion moment exists. Knee stability is maintained by contraction of the quadriceps. The resulting forces between the socket and residual limb are concentrated on the an-terodistal portion of the tibia and posteroproximal soft tissue (Fig 18B-25.). The socket, therefore, must provide even pressure distribution in the popliteal area and anterodistal relief coupled with anterior, medial, and lateral counterpressures to prevent excessive pressure over the distal end of the tibia.
Mediolateral forces occur during single-limb support on the prosthetic side when floor reaction forces may result in varus or valgus forces. As a result, the socket tends to change its angular relationship with respect to the residual limb. With normal foot inset, forces are generally increased over the proximomedial and disto-lateral aspects of the residual limb. These forces can be reduced if the foot is moved laterally, but since foot inset is desirable in most cases, the prosthetic socket must accommodate these forces. Proximomedial forces are not a significant problem because they are focused upon the pressure-tolerant medial femoral condyle and medial tibial flare. But distolateral forces can create excessive pressure on the transected end of the fibula. Socket modifications to prevent this include relief for the distolateral aspect of the fibula, lateral stabilizing pressure along the shaft of the fibula, and lateral stabilizing pressure over the anterior compartment (pretibial muscle group) (Fig 18B-26).
Other dynamic forces created within the socket that may present prosthetic problems include torque and shear. If excessive torque exists, the tendency of the socket to rotate in relation to the residual limb may cause discomfort, skin breakdown, or gait deviations. Shear occurs whenever the socket moves in a direction opposite to residual-limb motion. For example, if a patient's suspension is too loose, the prosthesis tends to drop away from the limb during swing phase, only to be driven back to its correct position during heel contact. This proximal and distal motion creates shear forces between the residual limb and the socket. Shear forces can occur in any plane.
A certain amount of shear is unavoidable because some motion between the socket and the underlying tissues will always occur. Excessive shear forces result in discomfort or skin breakdown. Patients with sensitive skin, such as diabetic or burn patients, may be especially susceptible to skin breakdown from shear forces. These forces can be reduced by a soft, socket insert or a nylon sheath worn directly over the skin. Rotation units or "torque absorbers" are another option. They can be used in transtibial prostheses when the patient uses the prosthesis for activities such as golfing that generate an unusually high amount of torque or to protect fragile skin. Proprioception and sensory feedback with the prosthesis are increased when the socket is in intimate contact with the residual limb.
Biomechanics of Transtibial Prosthetic Alignment
This is the second broad category of biomechanical factors in transtibial prosthetics. Alignment refers to the spatial relationship between the prosthetic socket and foot. Correct dynamic alignment may be determined by the prosthetist as the patient ambulates on an adjustable alignment unit. This unit allows for anteroposterior foot positioning, anteroposterior tilting of the socket, mediolateral foot positioning, mediolateral tilting of the socket, height adjustment, and rotation of the prosthetic foot.
Proper anteroposterior foot positioning will result in even weight distribution between the heel and toe portion of the foot statically. Dynamically this will result in a smooth, energy-efficient gait pattern, including controlled knee flexion after heel strike, smooth rollover with a limited recurvatum tendency, and heel-off prior to initial heel contact on the contralateral foot.
Proper anteroposterior socket tilt will result in an attitude of initial flexion statically, thus loading those areas that are pressure tolerant. Dynamically proper flexion not only improves the weight-bearing characteristics of the socket but also allows for a smooth gait pattern, places the quadriceps muscles "on stretch" to give them a mechanical advantage for control of the prosthesis, and limits recurvatum forces during midstance and terminal stance.
Proper mediolateral foot positioning will result in loading of the proximomedial and distolateral aspects of the residual limb statically. Dynamically it will duplicate the normal genu varum moment at midstance and provide optimum loading of the medial tibial flare during stance phase. It is necessary to note that optimum foot inset is related to the length and condition of the residual limb. A short residual limb may require that foot inset be reduced. The appropriate amount of foot inset is determined for each patient with the understanding that there will always be a trade-off between energy expenditure and torque on the residual limb. Foot inset results in appropriate loading of the medial tibial flare, a narrow-based gait pattern, decreased energy expenditure, and increased torque on the residual limb due to the normal genu-varum moment at mid-stance. It also provides for a more cosmetic appearance to the prosthesis. Reduced foot inset results in a wide-based gait pattern, increased energy expenditure, but decreased torque on the residual limb because the genu varum moment is limited or eliminated.
The most convenient method to determine the correct height of the prosthesis is through clinical comparison of the iliac crests or the posterior superior iliac spines. This general rule may not apply if the patient exhibits pelvic obliquity, congenital leg length discrepancy, or unilateral femoral shortening. Such cases must be taken individually, and often the best indicator of correct length is through gait analysis and patient comfort. Proper height will result in a smooth and symmetrical gait with no excessive trunk lean to either side.
Proper foot rotation is important both cosmetically and functionally. Prosthetic toe-out refers to the angle between the line of net forward progression and the medial border of the prosthetic foot. A transtibial prosthesis is initially aligned so that the medial border of the foot is parallel to the line of progression. This initial alignment results in a slight external rotation of the prosthetic foot, thereby approximating the 5 to 7 degrees of normal anatomic toe-out. However, this position may need to be altered during static and dynamic alignment so that foot position during ambulation matches that of the sound limb.
Foot rotation can also affect prosthetic function. How this occurs may be understood if the keel of the foot is viewed as a lever arm. During stance phase the tendency of the body to fall over the foot is resisted by the counterforce of this lever arm. Rotation of the foot therefore directly affects the length and the direction of force exerted by the lever arm. The net effect of externally rotating the foot is to increase stability by widening the base of support. There is a cosmetic trade-off, of course, if the toe-out attitude of the prosthesis does not match that of the contralateral limb. Fig 18B-27.1, 27.2, 27.3, 27.4, 27.5, 27.6 summarize prosthetic alignment deviations and their causes and gives corrective measures.
Biomechanics of Prosthetic Feet
It is essential to have a thorough understanding of prosthetic foot biomechanics because often foot selection alone can determine the ultimate success or failure of a prosthesis. There are six possible variables to be considered when choosing a prosthetic foot. They are alignment (previously discussed), length of the toe lever arm, width of the keel, flexibility of the keel, durometer of the heel cushion, and fit of the prosthetic foot within the shoe.
A common misconception when discussing foot function is to confuse toe break location with toe lever-arm length. The toe lever arm diminishes at the toe break but includes the entire length of the foot. This overall lever arm can be shortened or lengthened by moving the foot anterior or posterior in relation to the socket, but the position of the toe break in relation to the foot remains constant. (Some flexible-keel-dynamic-response feet do not have a definite toe break but rather a gradually diminishing resistance from midfoot to the toe tips.) Dynamic alignment is of course still necessary to determine the optimum foot position.
Keel widths are determined by the manufacturers. A wider keel provides greater medial lateral stability during stance phase by widening the base of support. For example, external-keel feet and the Carbon Copy II have wider keels than other feet do. The difference, however, is rarely significant enough to be the sole rationale for prescribing these.
The function of prosthetic feet varies so greatly today that it is important to mention the differences between rigid and flexible keels. Keel flexibility provides for a smoother gait pattern with a less pronounced transition at toe break. To optimize resistance of the forefoot during the late stance phase, the flexible-keel-dynamic-response foot can be moved anteriorly or slightly plantar-flexed during alignment of the prosthesis.
The heel cushion absorbs shock and helps initiate knee flexion during loading response. Increased heel stiffness results in greater knee flexion forces at heel strike and decreased shock absorption. Conversely, lower heel stiffness results in lower knee flexion forces and increased shock absorption.
The selection of heel cushion density or resistance obviously involves a trade-off between shock absorption and forces acting to flex the knee or rotate the socket upon the residual limb. As in similar prosthetic decisions, the choice must be based upon the patient's needs. Heavier patients are more likely to require a firm heel cushion to provide sufficient heel lever-arm force during loading response. In contrast, lighter patients will generally require mediumor soft-density heel cushions to avoid creating an excessive heel lever arm. Very active patients may prefer a firm heel cushion since more rapid cadences increase the net loading on the foot. Geriatric patients or household ambulators often require soft heel cushions to limit knee flexion forces and maximize shock absorption. The more susceptible the residual limb is to pain or skin breakdown, then the greater is the probability that the patient will benefit from a softer heel.
The prosthetic foot is designed to function under the stress of ambulation. It compresses, rebounds, flexes, and extends as it operates throughout the gait cycle. With the exception of postoperative feet and those designed for barefoot ambulation, the prosthetic foot is designed to fit inside a shoe. It should not be surprising, then, that the function of a prosthetic foot can be enhanced or decreased by the shoe within which it is fitted. At times it may be necessary to modify the foot or the shoe configuration to ensure optimum function. Attention must be given to shoe heel height, shoe heel material and shape, and shoe fit as related to foot motion.
Shoe heel height is probably the single most important factor of shoe fit as regards prosthetic foot function. It is essential that shoe heel height match the built-in heel rise of the foot. This will ensure that socket alignment in the sagittal plane is not altered and that the keel of the foot maintains the correct position with respect to the floor. Once a prosthesis has been aligned and fabricated, the patient should not significantly increase shoe heel heights unless an appropriate wedge is added inside the shoe.
The material and contours of the heel of the shoe can make a significant difference in the way the prosthetic wearer ambulates. For example, a soft crepe heel enhances the shock absorption qualities of a SACH foot. In comparison, a hardwood or rubber heel will tend to increase the knee flexion moment during loading response. If such heels present a problem, it is appropriate to round or bevel the posterior corner of the heel, thereby decreasing the knee moment at heel strike. Women's high heels may compromise stance-phase stability and are not recommended for weak, debilitated patients.
When a solid-ankle foot is forced into a tight-fitting shoe, the ability of the foot to compress and bend during ambulation is diminished. It is always better to fit the shoe slightly looser on the foot so that maximum flexibility is achieved.
PRESCRIPTION OF TRANSTIBIAL PROSTHESES
When making decisions about a prosthetic prescription the clinic team must analyze available patient information to formulate a plan for prosthetic treatment. The team should be familiar with socket designs, suspension systems, shanks and feet, as well as the indications for each. The prosthetic prescription should represent a consensus between all the members of the health care team, including the patient. The following factors influence the prescription recommendation.
This can only serve to give a general idea about the patient's activity level. A younger patient tends to be active and therefore usually requires a durable prosthesis that will function for many activities. An elderly patient will often have a lower activity level, may have other concurrent health problems, and will generally require a lightweight prosthesis with a protective socket interface.
Again, this category can only provide a general guideline based on stereotype. A woman with an amputation may place cosmesis at the top of her list of concerns, whereas a male may prefer function even at the expense of cosmesis.
The patient's geographic location may be very important. If the patient lives in an extremely hot, humid climate where perspiration is a chronic problem, leather liners or rubber suspension sleeves may be questionable options since both can create skin or hygiene problems. If the patient lives in a rural community and has difficulty returning for follow-up, components that require frequent maintenance are not practical.
Date of Amputation
When the amputation is recent, the patient's present physical status may give an idea whether he is progressing normally with the prosthesis or whether some problem or complication may be present. If the amputation occurred years ago, the results of any previous prosthetic fittings should be discussed.
The patient's general health and specific medical condition are major factors to consider in the recommendation for a prosthesis. Although prostheses are not prescribed according to disease categories, conditions or complications associated with certain pathologies may influence the choice of components.
The patient's activity level affects the components prescribed. A patient who is very athletic requires a sturdy, durable prosthesis, perhaps with specialized components. In comparison, a household ambulator will require a lightweight prosthesis designed for a less-strenuous activity level.
Type of Employment
If the patient works outdoors or on uneven terrain, an exoskeletal prosthesis with a multiaxis foot may be appropriate. A businesswoman, on the other hand, may prefer the cosmesis of an endoskeletal prosthesis with a high-heel SACH foot and sculpted toes.
As materials and techniques are improved, it is becoming more common for patients to request a prosthesis designed for sports. Transtibial prosthetic components and techniques are available for swimming, skiing, jogging, and other sports. Flexible-keel-dynamic-response feet are an example of the trend toward meeting the desire of patients to return to a more active life-style.
In many cases the patient will already be wearing a prosthesis and should be asked what he likes or dislikes about the present prosthesis. Often, an awareness of problems with the old prosthesis can help avoid difficulties with the new prosthesis.
The patient's personal goals should be taken into consideration and prosthetic design tailored to those goals whenever possible.
Residual Limb Shape
This helps identify potential fitting problems. A bulbous residual limb, which is often present shortly after amputation, has a larger circumference distally than proximally. If the difference between circumferences is large enough, the patient will be unable to don or doff the prosthetic socket. One solution is a temporary, nonremovable plaster of paris prosthesis to be utilized until the distal circumference of the residual limb has decreased enough to allow donning and doffing of the socket. A conical residual limb is characteristic of a long-term prosthetic wearer and should not present a fitting problem. A cylindrical residual limb is probably the ideal shape since pressure and stabilizing forces can be applied evenly.
If distal coverage is thin, the length of the socket and the fit of the distal pad are of critical importance. If distal soft tissue is very heavy, this will probably decrease as the patient uses the prosthesis, and the limb may actually lose contact with the distal pad in the socket. If this occurs, a new distal pad must be fabricated to restore total contact.
Residual limbs with prominent bones and thin subcutaneous tissue will probably require the added protection of a soft liner in the socket. Because of their inherent protective padding, residual limbs with heavy subcutaneous tissue can often be fitted with hard sockets and a distal end pad.
Skin problems such as blisters, ulcerations, cysts, verrucose hyperplasia, and abrasions usually occur as a result of an ill-fitting prosthesis and can generally be resolved by socket or alignment modifications or by a new prosthesis. Allergic skin reactions caused by materials can be remedied by choosing an alternate material.
Condition of Bony Anatomy
A soft liner may be indicated to provide protection to bony prominences. Bone spurs or unbeveled bones that present a fitting problem should be brought to the attention of the physician for possible surgical correction.
Condition of the Knee Joint
The stability of the knee joint is very important to the prosthetic prescription. If ligament laxity is present, supracondylar or joint and corset suspension is recommended.
Condition of the Thigh Musculature
In transtibial prosthetics the quadriceps are probably the most important muscles for a smooth, controlled gait. If these or other muscle groups are weak, the patient may require physical therapy for strengthening.
Range of Motion
The patient should ideally be able to achieve full knee extension and flexion. If an extension contracture is present, a minimum of 35 degrees of knee flexion is necessary for normal ambulation. If the patient has a flexion contracture of greater than 25 degrees, prosthetic fitting will be difficult. When contractures exist, the patient may be referred for physical therapy. Contractures that cannot be reduced will have to be accommodated in the prosthesis.
COMPUTER-AIDED DESIGN, COMPUTER-AIDED MANUFACTURE
Computer-aided design-computer-aided manufacture (CAD-CAM) is beginning to play a role in the practice of prosthetics and orthotics. Transtibial prostheses of today are designed and fabricated by using subjective techniques that can be difficult to teach and reproduce.
The traditional methods are labor intensive and use calipered measurements, palpation, and molded plaster impressions to gain data regarding residual-limb size and shape. This results in a qualitative evaluation of socket fit rather than a more objective quantitative evaluation. Without quantitative information, a well-fitting socket is difficult to produce and analyze. The traditional process is also time-consuming because of the trial-and-error method that even the most experienced prosthetist must use.
As more accurate methods of data collection are developed through CAD, it is hoped that the time and effort of the prosthetist can be greatly reduced. Presently the data collection methods include digitized passive plaster impressions, an optical shape sensor that rotates about the limb to collect data points of the high-contrast silhouette, laser shape sensing, and experimental use of ultrasound to gain information regarding a patient's residual limb. Each of these methods provides residual-limb topography from which residual-limb changes can be monitored and documented. This method of data gathering and documentation can potentially be applied to most levels of amputation as well as to many limbs requiring orthotic care.
Once the data are stored in the computer, the prosthetist makes modifications to the three-dimensional image on screen. Software packages offer a variety of features with which to manipulate shapes. Once the desired shape is complete, the data are then sent to a numerically controlled milling machine where a positive model is carved from a plaster blank. From this point traditional fabrication and fitting techniques are utilized. If the patient should require socket modifications, they are made on the electronic model, and a new socket is fabricated by incorporating the changes.
Data collected by computers could be used to create a data base from which ideal socket shapes might be developed. A clearer understanding of what constitutes a well-fitting socket could have a profound effect on prosthetic practice. Although early CAD systems were greeted with some skepticism, as the technology has evolved, it is now evident that CAD will become a very useful tool for the prosthetist-orthotist.
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Chapter 18B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles