Chapter 13 - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
Jacquelin Perry, M.D.
Walking depends on the repeated performance by the lower limbs of a sequence of motions that simultaneously advances the body along the desired line of progression while also maintaining a stable weight-bearing posture. Effectiveness depends on free joint mobility and muscle action that is selective in both timing and intensity. Normal function is also optimally conservative of physiologic energy. Pathologic conditions alter the mode and efficiency of walking. The loss of some actions necessitates substitution of others if forward progression and stance stability are to be preserved. Through a detailed knowledge of normal function and the types of gait errors that the various pathologic conditions can introduce, the clinician becomes able to define the significant deficits and plan appropriate corrective measures.
Each sequence of limb action (called a gait cycle) involves a period of weight-bearing (stance) and an interval of self-advancement (swing) (Fig 13-1.). During the normal gait cycle approximately 60% of the time is spent in stance and 40% in swing. The exact duration of these intervals varies with the walking speed. There also are minor differences among individuals.
The reciprocal action of the two limbs is timed to trade their weight-bearing responsibility during a period of double stance (i.e., when both feet are in contact with the ground) and usually involves the initial and terminal 10% intervals of stance. The middle 40% is a period of single stance (single-limb support). During this time the opposite limb is in swing.
The three components of walking-progression, standing stability, and energy conservation-involve distinct functional patterns. These need to be understood for an appropriate interpretation of some of the limitations displayed by patients. Although for this description they will be separated, during walking all three action patterns are intertwined throughout each stride.
There are two main progressional forces:
- 1.The primary one is forward fall of the body weight (Fig 13-2.). This begins in single stance as the ankle dorsiflexes beyond neutral and accelerates with heel rise.
- The second, which is generated by the contralateral swinging limb (Fig 13-3.), starts with the onset of single-limb support. This action is particularly important before the body is aligned for an effective forward fall.
The momentum generated by these two actions is optimally preserved at the onset of the next stance phase by floor contact with the heel. As the foot drops toward the floor, the pretibial muscles draw the tibia forward (Fig 13-4.). At the same time the quadriceps ties the femur to the leg so that the thigh also advances (although at a slightly slower rate). Thus, throughout the stance period the heel, ankle, and forefoot serially serve as a rocker that allows the body to advance over the supporting foot (Fig 13-5.). For this to occur the foot must be appropriately positioned by the end of swing and controlled as weight is applied. Then, during the support phases there must be adequate passive mobility at the ankle. This necessitates both a free joint range and ability of the calf muscles to yield as they provide tibial stability.
Balance is challenged by two factors. The body is top-heavy, and walking continually alters segment alignment. During walking the body divides itself into two functional units-passenger and locomotor.
The head, arms, and trunk are the passenger unit because they are carried rather than directly contributing to the act of walking. Muscle action within the neck and trunk serves only to maintain neutral vertebral alignment. There is minimal postural change occurring during normal gait. Arm swing is primarily a passive reaction to the momentum generated. The small amount of active control has not proved essential, as evidenced by the ease with which one carries packages. Also, experimental restraint of arm swing registers no change in the energy cost of walking. The composite mass center of these segments is just anterior to the tenth thoracic vertebra and thus lies well above the hip joints. This long lever (33 cm in an average adult) makes balance of the passenger unit very sensitive to alignment changes of the supporting limbs.
The locomotor unit consists of two limbs joined by the intervening pelvis. This makes the pelvis an element of both the passenger and the locomotor units, with two highly mobile junction sites, the lumbar spine and hip joints.
Theoretically, weight-bearing stability of the limb is maximal when its three components (thigh, leg, and foot) are vertically aligned so that one is directly over the center of the other. If these segments were square blocks, there would be a broad shoulder to allow considerable tilting before balance was lost. Instead, the femur and tibia are tall narrow bones. Additionally, the articular surfaces are segments of a circle, so there are no restraining rims available in the sagittal plane and, consequently, no intrinsic stability. The skeletal architecture is designed for mobility. This means that other stabilizing mechanisms are needed. At the hip anteriorly and the knee posteriorly a strong ligament stabilizes one side of the joint. By using hyperextension to align body weight on the opposite side, the person is able to attain passive stability. No similar mechanism exists at the ankle or subtalar joints, however. Instead, here a free range of dorsiflexion-plantar flexion and in-version-eversion exists. Thus, only through direct muscular control is the tibia stabilized over the foot. Passive stability is further challenged by the fact that the foot does not provide equal areas of support anterior and posterior to the ankle axis. Posteriorly the weight-bearing segment of the heel is little more than 1 cm, for the significant factor is the rounded contour of the tuberosities and not the full length of the os calcis. By contrast, the anterior (forefoot) lever that extends to the weight-bearing surface of the metatarsal heads averages 10 cm in an adult. Thus, for optimum foot support (equal anterior and posterior leverages), the body weight line (vector) must be anterior to the ankle joint. This increases the demand for active control of the ankle and subtalar joints.
During walking the trunk and limb segments are continually moving from behind to ahead of the supporting foot. Thus passive stability is a fleeting experience. At the onset of stance, flexion torques are created at the hip and knee that must be restrained by active muscular effort. As body weight moves forward, this demand is gradually replaced by passive support from tense fasciae. Conversely, the demand for active ankle restraint (a plantar flexion force) does not begin until body weight moves forward of that joint axis. Once the forefoot becomes the major area of support, muscular response must increase rapidly. Thus the ever-changing alignment of body weight is stabilized by selective muscular control.
The timing and intensity of each muscle's activity are dictated by the relationship of body weight to the center of the joint that muscle controls. This is the torque created, a product of force times leverage. The length of the lever (moment arm) is the perpendicular distance between the body weight line and each joint center. Body weight is the basic force, but its effect is modified by the direction in which it is moving. The composite effect is determined by measuring the instantaneous ground reaction forces. At the beginning and end of stance, body weight drops rapidly toward the floor. The resulting accelerations increase the ground reaction force to a value greater than the body weight. During the midstance period the body rises slightly as the limb becomes more vertical. This somewhat reduces the weight directed toward the ground. Consequently, the force demands presented to the muscles vary by both the loading experience and the alignment of body weight over the joints. Selective neural control and proprioception as well as adequate strength are needed for appropriate muscle response.
The basic measure of efficiency is energy expenditure per task performed. For walking, this is oxygen used per meter traveled. Oxygen is consumed as the muscles contract. Thus efficiency is improved by reducing the amount of muscular effort required to walk. This normally is accomplished by two mechanisms: momentum is substituted for muscle action wherever possible, and displacement of the body from the line of progression is minimized.
- Optimum use of momentum occurs during the person's natural gait velocity. That motion pattern requires the least energy expenditure per meter traveled. Both a slower and a faster pace increase the energy cost.
- Minimization of body displacement from the line of progression is accomplished by coordinating pelvic, knee, and ankle motion to keep the relative limb length fairly constant throughout stance. At the onset of a double stance, body height is lowest because the two limbs are diagonal. Conversely, the highest position occurs in the middle of single stance, when the supporting limb is vertical.
For a normal adult step length of 70 cm the height loss would be abrupt, and ankle dorsiflexion of 22 degrees would be needed by the trailing limb. To reduce these extremes, normal gait involves three pelvic motions: lateral drop, transverse rotation, and anterior tilt. All three approximate 4 degrees and follow the swing limb. The stance limb contributes by two actions. At the beginning of the weight-bearing period a reciprocal relationship between knee flexion and ankle dorsiflexion adjusts limb length as it moves from a diagonal to a vertical alignment. Also, heel rise in the latter half of stance adds relative length to the trailing limb. Through these actions the potential 7-cm displacement is reduced to 5 cm, a 30% saving.
The interplay of progression, standing stability, and energy conservation results in a complex and continually changing relationship among the various limb segments as the body advances over the supporting foot and the toe is lifted to clear the ground. Each joint performs a representative pattern of motion. During stance the postural changes are induced passively by the influence of body weight. Swing-phase motion depends on muscle action.
Ankle.-Two periods of plantar flexion and dorsiflexion are experienced in each gait cycle (Fig 13-6.). At the onset of stance the ankle has a 90-degree position. As the heel is loaded, the foot drops into 10 degrees of plantar flexion. Then the action reverses and gradually reaches 10 degrees of dorsiflexion. At this time plantar flexion is resumed and reaches 20 degrees by the end of stance, although the latter arc of motion occurs in the double-stance period when the limb is being rapidly unloaded. With toe-off the foot is quickly raised to neutral dorsiflexion and maintained in this position throughout swing.
Knee.-The knee also experiences two phases of flexion and extension in each gait cycle (Fig 13-7.). Beginning in full extension (or flexed 5 degrees), it rapidly flexes to 15 degrees. Then it progressively extends to neutral. With the onset of double stance, it again begins to flex. This action continues in swing to reach 60 degrees before extension is resumed.
Hip.-Only a single arc of hip extension and flexion occurs in each gait cycle (Fig 13-8.). As the foot strikes the ground, the hip is in 30 degrees of flexion. Throughout stance there is progressive extension into 10 degrees of hyperextension. Then flexion begins in terminal double stance and continues through most of swing. When the 30-degree posture is reached, it is maintained until stance resumes.
PHASES OF GAIT
The functional significance of each joint's motion pattern at any point in the gait cycle is dependent on the total limb requirements for effective progression and stance stability. This is identified by subdividing the continuum of limb action according to the tasks that must be accomplished. The result is eight functional units. Each constitutes one phase of gait.
Normal events have customarily been used to designate the different gait phases. Although this was adequate for amputees, many other types of disability prevented the patients from accomplishing some of the key actions. The terms thus became meaningless. To avoid such confusion, a generic terminology was developed. Because it has proved universally applicable to normal, amputee, arthritic, and paralytic patients, it is the system that will be used in the ensuing discussion.
Because one gait cycle blends into the next in an endless fashion, any event can be selected as the starting point. Initial floor contact is the most consistent event in both normal and pathologic gait and, thus, will serve as the first gait phase.
Normal function is the model for judging a patient's gait. Disability reduces the limbs' effectiveness by altering their pattern of motion. The significance of the observed deviations relates to the changes in total limb posture that occur during the individual gait phases. Thus the basis of gait analysis is to have a firm concept of limb function during each gait phase, to know the purpose of the normal motion patterns, and to appreciate the penalties that disabled performance imposes.
To allow progression while also maintaining weight-bearing stability, the limb performs five distinct tasks that define the phases of stance.
Of primary concern is the way the foot strikes the floor. Although this is a momentary posture, it is significant because of its influence on subsequent knee action.
Heel strike with the foot at a 25-degree angle to the floor is the normal occurrence. The ankle is in an approximately neutral position (perhaps plantar-flexed 3 degrees). The knee is extended between 0 and 5 degrees of flexion, and the hip is flexed 30 degrees.
At this moment the free drop of body weight creates a vertical vector passing through the heel that is anterior to both the knee and the hip. Three torques are generated: ankle plantar flexion, knee extension, and hip flexion (Fig 13-9.).
Control of both knee and ankle is critical to having normal heel strike. Dorsiflexion of the ankle to neutral is dependent on free joint mobility and active control by the pretibial muscles (tibialis anterior, long extensors of the great and common toes, and peroneus ter-tius). Knee extension is accomplished by quadriceps action.
Hip position does not influence the mode of floor contact but does determine the angle between foot and floor.
Acceptance of body weight in a manner that ensures limb stability and still permits progression is the goal at this time.
As weight is dropped onto the limb, a heel rocker action is initiated that leads to two significant motion patterns. The action at the ankle precedes and contributes to that occurring at the knee.
Ankle.-After floor contact by the heel, the foot quickly drops into 10 degrees of plantar flexion in a controlled manner. The motion is initiated when body weight is applied to the foot at the dome of the talus while floor contact is still at the tip of the os calcis. An unstable lever results from differences in bone length between these points. Strong action by the pretibial muscles retards the terminal arc of ankle plantar flexion, so forefoot contact is gradual. Thus, heel strike is heard but no foot slap. This action dominates the first 6% to 8% of the gait cycle.
While these sagittal motions are occurring, there is also transverse rotation at the subtalar joint. The point of heel contact is lateral to the middle of the ankle joint (Fig 13-10.). This creates a valgus thrust on the subtalar joint. As the foot responds, support for the talus is reduced. The talus falls into internal rotation. The strong ankle ligaments carry the tibia and fibula with the talus so that the entire ankle joint turns inward.
Knee.-Flexion of the knee to 15 degrees is initiated by the heel rocker action. As the pretibial muscles contract to restrain ankle plantar flexion, they also draw the tibia forward. This is a rapid action, and it advances the leg faster than the thigh and trunk can follow. As a result the body weight line shifts posterior to the knee, and a flexion torque (moment) is induced (Fig 13-11.).
Two types of muscle action result. There is increased quadriceps activity to restrain the rate of knee flexion. Conversely, no longer are the hamstring muscles needed to prevent knee hyperextension. Marked reduction of semimembranosus and semitendinosus activity is the response. Persistence of the biceps femoris (long head) probably relates to its external rotation action at the knee.
Hip.-Little change in thigh position occurs during the loading response. The large hip flexion moment present with the impact of initial floor contact is reduced by two passive actions. Rapid realignment of the vector to the body center brings the weight line close to the hip joint axis (see Fig 13-11.). An extensor thrust created by propulsion of the limb through the heel rocker advances the femur as well as the tibia.
Active muscular restraint of the hip flexion moments is provided primarily by the gluteus maximus and adductor magnus. Both these muscles markedly increase their activity after initial contact. Participation by the hamstrings is reduced because of their actions at the knee.
Advancement of the body and limb over a stationary foot is the functional objective of this gait phase.
As the other foot is lifted for swing, a period of single-limb support begins. Maximum stability is gained by having the foot stationary and in total contact with the floor (heel and first and fifth metatarsal heads). Ankle rocker action allows progression to continue, and this occurs through residual momentum and that generated by the contralateral swinging limb. As body weight advances across the arch, the base of the vector moves from the heel to the forefoot. This alters vector alignment at the ankle, knee, and hip, with resulting changes in joint posture and muscle control (Fig 13-12.).
Ankle.-At the onset of single stance the ankle is still slightly plantar-flexed (5 degrees). From this position there is gradual dorsiflexion. The basic arc is from -5 to +5, with 10 degrees of dorsiflexion being attained just as the heel rises to initiate terminal stance. Thus an ankle rocker is created for body progression. Allowing the tibia to advance beyond the neutral position so that body weight moves over the forefoot is the critical action.
As the body vector moves anterior to the ankle, a dorsiflexion moment is created that would accelerate the rate of limb advancement if it were not controlled. Judicious restraint is an essential component of knee stability.
The soleus responds by restraining the tibia's progression. Assistance by the gastrocnemius reduces the effort that the soleus must exert, but it also induces a flexion torque at the knee. While these two muscles are acting to avoid excessive tibial advancement, they are doing so in a yielding manner; thus the desired degree of ankle dorsiflexion is attained.
An available range and timely tibial restraint are the critical events during midstance.
Knee.-Knee flexion induced during the loading response increases slightly and reaches its maximum of 18 degrees just as single-limb support is initiated. Motion of the knee then reverses to progressive extension, which depends on the tibia's being actively restrained by the soleus so that the femur can advance at a relatively faster rate. At approximately the middle of the midstance phase the body vector moves anterior to the joint center, and an extensor moment is begun. The need for active muscular control is terminated. Quadriceps action is maximal at the onset of midstance. It then progressively declines as the knee extends over the vertical tibia, and advancement of the body vector lessens the flexor torque. Once the vector becomes anterior to the knee axis, extension stability is provided passively, and the quadriceps relaxes.
Hip.-Progressive decline in hip flexion and entry into extension allow the trunk to remain erect while the limb becomes more vertical. These motions are accomplished passively.
The onset of single stance, however, creates new demands in the coronal plane. Lifting the opposite limb removes support for that side of the body (Fig 13-13.). The unsupported pelvis falls and creates quick hip adduction in the stance limb. This is rapidly limited to 4 degrees and then reversed by active abduction to the extent of slight over correction.
Hip abductor muscle activity is intense throughout midstance while the extensors are quiet.
Forward fall to generate a propulsive force is the primary objective. Heel rise signifies the onset of this second phase of single stance. Now the forefoot serves as the progressional rocker, with the body falling forward of its area of support. This creates the primary propulsive force for walking. With the limb acting like a spoke in a wheel, the body's center of gravity drops slightly during advancement. Throughout terminal stance the ankle and foot are the critical sites of action.
Ankle.-At the onset of heel rise the ankle drops into the maximum dorsiflexion (10 degrees) occurring in stance. Motion then reverses to reach 5 degrees of plantar flexion by the end of single-limb support. Hence stabilization in a relatively neutral position is the objective so that the forefoot can act as the propulsive rocker.
Stability is accomplished by the triceps surae. With heel rise the body vector is concentrated at the forefoot (Fig 13-14.). The distance between the vector and ankle joint axis generates a maximal dorsiflexion torque. This must be restrained if the ankle is to maintain its neutral position. Both the soleus and the gastrocnemius respond vigorously and cease abruptly as the single-stance period is terminated and body weight is rapidly transferred to the other foot.
Foot.-With body weight being applied at the ankle (talus) and supported by the forefoot, a strong dorsiflexion torque is generated through the midfoot. Stability is gained by inversion. This posture is initiated by the obliquity of metatarsal support. The line between the second and fifth metatarsal heads is 28 degrees anterior to the coronal plane. Inversion of the subtalar joint so changes the axes of the talonavicular and calcaneocuboid joints that they cease to be parallel. This locks all the midfoot joints (transverse tarsal, intercuneiform, cuneiform-cuboid, and metatarsal bases).
Heel rise also initiates dorsiflexion at the metatarsophalangeal (MP) joint. While most of body weight is concentrated on the metatarsal heads, the bases of the proximal phalanges contribute by enlarging the support area. At the end of terminal stance the body is well forward, and the MP joint dorsiflexes approximately 20 degrees.
All the foot and toe muscles are active. Inversion is preserved by the tibialis posterior and soleus. At the same time it is restrained by the peroneals (longus and brevis). The longand short-toe flexors also support the arch by the compressive force that their longitudinal alignment provides. Passive arch support is gained from the plantar fascia, which is tensed as the MP joint dorsiflexes.
Compression from the toe flexors and tension from the plantar fascia also stabilize the MP joints so that the phalangeal bases can add to the base of support.
Knee.-With the tibia stabilized on the foot, forward alignment of body weight passively extends the knee. Maximum extension varies between 0 and -5 degrees. At the end of terminal stance when the vector is at the margin of MP support, the knee begins to flex. Body weight is rapidly falling toward the other limb. There is no quadriceps action at this time, for knee extension stability is gained from the body vector's continually being anterior to the knee joint axis.
Hip.-Passive extension of the hip joint continues as body weight advances beyond the supporting foot and the trunk remains erect. By the end of stance there are 10 degrees of hyperextension. The anterior joint structures are maximally stretched by falling body weight. This commonly stimulates the iliacus to provide a restraining force. There is no hip extensor activity during terminal stance. As body weight begins to fall toward the other limb, the hip abductors terminate their action, and passive abduction is induced.
Preparation of the limb for swing is the purpose of the actions that occur during the preswing phase. Floor contact by the other foot initiates this interval of terminal double support. Rapid transfer of body weight to that limb allows the desired actions to follow. The critical area of response is the knee.
Knee.-There is rapid passive flexion to 40 degrees, which occurs because the body weight has rolled so far forward on the forefoot rocker that the tibia no longer is stable. As the tibia advances, the knee joint axis is moved anterior to the body vector. A flexion torque is created (Fig 13-15.).
There is no flexor muscle action. The quadriceps (mainly the rectus femoris) may react briefly to restrain the rate of passive knee flexion if needed.
Hip.-Flexion of the hip joint is initiated with the recovery from hyperextension to neutral that occurs during this phase. The iliacus, often accompanied by the rectus femoris, is active.
Ankle and Foot.-There is rapid ankle plantar flexion to a 20-degree position, a passive event since all the significant musculature is relaxed at the time of contralateral foot contact. Only the flexor hallucis longus remains active. It sustains MP joint compression and restrains dorsiflexion while floor contact is maintained. This is not a major weight-bearing obligation, for the load is primarily on the other foot.
Lifting the foot from the ground and limb advancement followed by preparation for stance are the objectives of the three phases of swing.
Recovery from a trailing posture is the task that is accomplished. This involves two critical actions: flexion of both the hip and the knee (Fig 13-16.).
Hip.-From the neutral position attained at toe-off the hip rapidly flexes to 20 degrees. Although the iliacus is the major force advancing the thigh, the need for speed generates assistance from the sartorius, gracilis, and adductor longus.
Knee.-The amount of knee flexion required for toe clearance of the floor (60 degrees) is attained by adding 20 degrees to the 40-degree posture acquired during preswing. Despite the fact that this is a critical event, there is not a dominant flexion force. Momentum from the advancing thigh is supplemented by action of the short head of the biceps femoris and the sartorius and gracilis.
Ankle.-Dorsiflexion of the ankle is initiated, but only half the 20-degree plantar flexion present at toe-off is recovered in this brief time. Thus toe clearance of the floor is not dependent on ankle dorsiflexion during the initial phase of swing. The muscles contracting quickly to lift the foot are the tibialis anterior, long-toe extensors, and peroneus tertius.
As limb advancement continues, the changes in the tibial alignment make foot control critical for floor clearance (Fig 13-17.).
Hip.-Maximum flexion to 30 degrees is reached by continued iliacus action.
Knee.-Relaxation of the flexor muscles allows the knee to extend passively. This accelerates advancement of the leg and foot. By the end of midswing, knee flexion (30 degrees) equals that of the hip, and the tibia is vertical.
Ankle.-Dorsiflexion to neutral is accomplished and then maintained. Verticality of the tibia continues the need for active control of the foot. The tibialis anterior and other pretibial muscles respond accordingly.
Advancement is terminated, and the limb is prepared for stance. The critical event is complete knee extension (Fig 13-18.).
Hip.-The 30 degrees of flexion attained in midswing is maintained. For this purpose momentum supports limb weight while the hamstrings prevent further motion. All three muscles-semimembranosus, semi-tendinosus, and biceps femoris long head-exhibit peak activity at this time.
Knee.-Because gravity must be opposed and only a brief time is available for this, extension of the knee to neutral (0 or -5 degrees) continues under active control. The quadriceps provides the needed extensor force. All heads remain active. Simultaneous contraction of the hamstrings to decelerate hip flexion also prevents hyperextension of the knee.
Ankle.-Continued neutral dorsiflexion is the basic posture, but the foot may drop into slight plantar flexion (3 to 5 degrees) at the end of the phase. The pretibial muscles tend to diminish their intensity of action as the semivertical position of the foot presents a less-demanding torque.
Five joints in each limb (hip, knee, ankle, subtalar, and metatarsophalangeal), under the selective control of 28 major muscles, coordinate their actions to provide continual progression and weight-bearing stability with minimal displacement of the body's center of gravity. To meet these demands each limb performs eight motion patterns that have been identified as the phases of gait.
The sequence of muscle action relates to three major tasks being accomplished during each gait cycle: weight acceptance, single-limb support, and limb advancement. Preparation of the limb for weight acceptance begins in terminal swing when the hip extensors and quadriceps are activated. Following initial floor contact, the loading response is an increase in the intensity of these hip and knee extensor muscles to stabilize the trunk and limb against the rapid transfer of body weight. To ensure knee stability, the hamstrings and single joint hip extensors exchange their intensities. Continuation of the swing-phase ankle dorsiflexor control provides a heel rocker for progression and shock-absorbing knee flexion. Ankle plantar flexion modulates the heel rocker to protect the quadriceps from overde-mand.
Stability for the phases of single-limb support (mid and terminal stance) depends almost entirely on timely and graded action of the soleus aided by the gastrocnemius. These muscles increase their intensity as progression increases the demand torque at the junction of foot and leg (the ankle). Knee and hip extensor stability is provided passively by body alignment. Activation of the hip abductors (which actually began in the loading response) ensures lateral stability of the pelvis and trunk on the supporting limb. Subtalar and midfoot stability is gained from the perimalleolar muscles. Progression of the limb and body over the stationary foot depends on graduated ankle dorsiflexion to advance the tibia. Further progression of the tibia is gained by heel rise and forward roll over the forefoot in terminal stance.
Preparation for limb advancement begins in the final phase of stance (preswing) as the limb is unloaded by the rapid transfer of body weight onto the other limb. The knee and hip are unlocked from their extended positions, and flexion is initiated. Active continuation of these events in initial swing and midswing lifts the toe to clear the floor and advances the limb. Dynamic dorsiflexion provides the necessary foot control. At the end of midswing, limb advancement ceases, and the preparations for stance begin.
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Chapter 13 - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles