Lower limb prosthetics

Author(s): Bradeigh S. Godfrey, DO

Originally published:09/20/2013

Last updated:09/20/2013


Definition of a prosthesis

  1. An artificial substitute for a missing body part, used to restore the functional or cosmetic purposes of that body part. The goals of a prosthetic limb may be cosmetic or functional.

Evaluation shuld include the following:

  1. The patient’s prior level of function and activity level, including level of independence of activities of daily living (ADLs), assistive devices used
  2. The patient’s geographical location and proximity to medical care and prosthetic lab •Etiology and time since amputation
  3. General medical condition, including comorbidities such as heart and lung disease, diabetes, vascular disease, and polyneuropathy
  4. Employment
  5. Recreational pursuits and sports
  6. Goals of patient and family
  7. Family and caregiver support

Characteristics of the patient must also be evaluated to determine the appropriate prosthetic prescription, including:

  1. Assessment of cognitive function necessary to care for and don/doff the prosthesis•Function of the upper limbs
  2. Function of the intact lower limb
  3. Residual limb strength, shape, length, and condition, including skin condition, sensation, and pulses
  4. Stability of joints and ligaments of the residual limb
  5. Presence or absence of any joint contractures
  6. Weight of the patient, as some prosthetic components have weight limits

Functional levels: In 1994 the federal government adopted a system that attempts to clarify which lower limb prosthetic components (knee, foot, and ankle) should be used for patients depending on their functional levels.1 These functional levels are based on the patient’s potential, not on the current level of function.

K level Description
K0 Does not have the ability or potential to ambulate or transfer safely with or without assistance, and a prosthesis does not enhance quality of life or mobility.
K1 Has the ability or potential to use a prosthesis for transfers or ambulation on level surfaces at fixed cadence. Typical of the limited and unlimited household ambulator.
K2 Has the ability or potential for ambulation with low-level environmental barriers such as curbs, stairs, and uneven surfaces. Typical of the limited community ambulator.
K3 Has the ability or potential for ambulation with variable cadence. Typical of the community ambulator who can traverse most environmental barriers and has vocational, therapeutic, or exercise activity that demands prosthetic utilization beyond simple locomotion.
K4 Has the ability or potential for prosthetic ambulation that exceeds basic ambulation skills, exhibiting high-impact, stress, or energy levels. Typical of the prosthetic demands of the child, active adult, or athlete.


  1. The prosthetic prescription

    The patient with a lower-limb amputation is typically first prescribed a preparatory prosthesis which is designed to be strong and relatively adjustable. This is worn for the first few months as the residual limb decreases in volume and develops a more desirable shape. When needed, a definitive prosthesis is prescribed. The major components of a lower limb prosthesis include the socket, suspension, knee unit (if applicable), foot/ankle complex, and any extras.

    Transtibial Socket Designs

    1. The patellar tendon-bearing socket has an inward contour that uses the patellar ligament as a partial weight-bearing surface. Despite the name, this socket design aims for a total-contact fit and involves weight-bearing throughout the pressure-tolerant areas of the residual limb.
    2. The total surface-bearing socket is designed to distribute pressure more equally across the entire surface of the residual limb, including carrying some of the load on pressure-sensitive areas.
    3. Other socket variants include flexible inner sockets with a rigid external frame; the rigid frame supports the pressure-tolerant areas while the flexible inner socket encloses the more pressure-sensitive areas. Another option is to use soft inserts to fit inside the socket to act as an interface between the limb and the socket. Elastomeric liners, such as silicone and other materials, can also be used as an interface between the socket and the patient’s skin. Socks of various ply can be added or subtracted to adjust for volume fluctuations experienced by the patient.

    Transtibial Suspension

    1. A supracondylar cuff or strap may be helpful for patients with very short residual limbs or may be used as auxiliary suspension in some patients.
    2. Another option is suspension through the distal end of the socket, such as with a pin-locking suspension or a lanyard system. This type of suspension provides a very secure fit. However, “pistoning” of the lower limb soft tissues can occur during gait, which can lead to discomfort and chronic skin changes.
    3. Suction in various forms is also used commonly for suspension, either through a one-way valve or electric vacuum pumps. Vacuum suspension may decrease the pistoning of the lower limb that is commonly seen with pin-locking suspensions.2 With either of these systems, the patient must wear a suspension sleeve over the top of the socket and over the knee and thigh to maintain the knee. Suction creates a very secure fit but can be compromised by small holes in the suspension sleeve; the suspension sleeve also increases bulk behind the knee, especially in positions of knee flexion. Another option is a special liner with several concentric rings to create the seal; this eliminates the need for an additional suspension sleeve.
    4. Supracondylar suspension involves encompassing the medial and lateral femoral epicondyles with the socket, which may provide some additional mediolateral stability in some patients with very short residual limbs. An older mode of suspension is the thigh corset with side joints, which may be considered in long-time prosthetic wearers who prefer this style, or those with poor mediolateral stability due to derangement of the knee ligaments.

    Transfemoral Socket Designs

    1. The quadrilateral socket is an older socket design that is relatively narrow anterior-to-posterior, with a posterior shelf to enable weight bearing on the ischium.
    2. The ischial containment socket is more ovoid in shape, with a smaller mediolateral dimension. The posterior and medial walls encase the ischial tuberosity. When compared with the quadrilateral design, the ischial containment socket may distribute pressures more evenly.3 There are also several variations of this socket, including a flexible inner socket within a rigid frame.

    Transfemoral Suspension

    1. Suction is a common choice for transfemoral suspension, utilizing a one-way valve and liner with concentric rings. Distal suspension with a pin or lanyard is another option. A pelvic band or silesian belt may be used as the primary suspension or as auxiliary suspension in some patients.

    Prosthetic Knees

    1. Manual-locking knees can be locked for patients who require the most stability, and must be unlocked for the patient to sit.
    2. Single-axis knees have a single axis of rotation, and stability is achieved thru alignment and the patient’s voluntary control.
    3. The polycentric knee consists typically of 4 bars that pivot during flexion, which allows for a changing axis as the patient progresses through the gait cycle. This allows a great deal of knee stability and reduces the protrusion of the knee unit when the patient sits.
    4. There are several options for knees that utilize hydraulics or pneumatics to provide resistance to swing and/or stance phase. These can be adjusted for the patient’s unique needs.
    5. Microprocessor-controlled knee units utilize a microprocessor to control the pneumatics or hydraulics throughout the gait cycle. The microprocessors gather information on the position in the gait cycle and process electronically to adjust the resistance of the knee. Some allow the individual to change the mode of the knee to suit the intended activity. These knees are appropriate for active individuals at the K3 or K4 level. However, some patients at K2 level may benefit from a microprocessor stance-controlled knee, to improve their function and balance.4

    Prosthetic Foot/Ankle

    1. The SACH (solid-ankle cushion heel) foot does not have an articulated ankle but allows for simulation of plantar flexion when the heel cushion is compressed during initial contact. This foot is inexpensive, durable, and low-maintenance. A major disadvantage is that it does not accommodate for walking on uneven surfaces.
    2. A single-axis foot has an articulated ankle with one axis of rotation. It allows the prosthetic foot to reach foot-flat easily in early stance phase, which enhances knee stability.
    3. A multi-axis foot allows movement in all three planes and allows the user to accommodate for uneven surfaces.
    4. Dynamic response feet have a flexible keel that “stores” potential energy during early stance phase that is then “released” through recoil of the material in late stance and early swing phase. This imitates the function of the gastrocnemius-soleus group. These feet are appropriate for active community ambulators who change their cadence, and for athletes.
    5. Other ankle components may include vertical shock pylons to decrease the impact of initial contact, or torsional adapters to allow motion in the transverse plane to simulate tibial rotation.
    6. Hip disarticulation and hemipelvectomy prostheses
    7. The socket encases bilateral iliac crests in the hip disarticulation patient, and often utilizes abdominal compression in the hemipelvectomy patient. This prostheses must include a hip joint, which may be a ball-and-socket joint or single-axis.
    8. Prosthetic stability is accomplished by aligning the hip joint and knee axis relatively posterior.

    Knee disarticulation prostheses

    1. This level of amputation preserves the femoral condyles, which allows for supracondylar suspension and a long level arm. However, the functional length of the thigh becomes much longer when the socket and prosthetic knee are added. This can be partially compensated for by utilizing a low-profile polycentric knee.

    Symes and ankle disarticulation prostheses

    1. This amputation level has the advantage of a long lever arm and allows some distal weight-bearing without a prosthesis. However, the distal residual limb is bulbous due to presence of the malleoli and the socket must have either an opening or elastic liner. Several low-profile foot options are available.

    Partial foot prostheses

    1. The various levels of foot amputations may require various options, such as a toe filler with or without an ankle-foot orthosis and shoe modifications.


Although microprocessor knees have been available for over 15 years, a new knee unit (the Genium, by Otto Bock) has several features that may have biomechanical advantages over the older microprocessor knee designs, including the ability to ascend stairs step over step.5 A military-grade version of this knee joint, the X3, will likely be released in 2013 and will be the first fully submersible prosthetic knee joint.

Microprocessor feet are becoming more widely used, as well as those that are powered. Combining a powered ankle and microprocessor knee unit is being investigated.

Osseointegration is the practice of implanting a titanium implant directly into the long bone after an amputation, extending outside of the skin. The prosthetic limb can attach directly to it, eliminating the socket-skin interface. This is still in the development stage in the United States.

Specialized sports prostheses can include specialized components for running, cycling, swimming, and many other activities. Prescription and fabrication of these prostheses requires close collaboration between the physiatrist, prosthetist, and patient, in order to meet the demands of the athletic activity.


It is important for physicians caring for patients with amputations to understand the evidence base for their prescriptions, but unfortunately there is a paucity of high-quality clinical trials in this area. Many decisions are made based on expert opinion and the best practice of the experienced prosthetic team. It will be important for physicians to be involved in continued research to demonstrate the advantages and disadvantages of various prosthetic components, especially from a functional and clinical perspective.


1. Durable medical equipment regional committees. Region A Medicare News. 1994; Dec (14).

2. Klute GK, Berge JS, Biggs W, Pongnumkul S, Popovic Z, Curless B. Vacuum-assisted socket suspension compared with pin suspension for lower extremity amputees: effect on fit, activity, and limb volume. Arch Phys Med Rehabil. 2011;92(10):1570-1575.

3. Lee VS, Solomonidis SE, Spence WE. Stump-socket interface pressure as an aid to socket design in prostheses for transfemoral amputees: a preliminary study. Proc Ins Mech Eng H. 1997;211(2):167-180.

4. Burnfield JM, Elberly VJ, Gronely JK, et al. Impact of stance phase microprocessor-controlled knee prosthesis on ramp negotiation and community walking function in K2 level transfemoral amputees. Prosthet Orthot Int. 2010;36(1):95-104.

5. Bellman M, Schmalz T, Ludwigs E, Blumentritt S. Immediate effects of a new microprocessor-controlled prosthetic knee joint: a comparative biomechanical evaluation. Arch Phys Med Rehabil. 2012;93(3):541-549.


Edelstein JE, Moroz A. Lower-Limb Prosthetics and Orthotics: Clinical Concepts. Thorofare, NJ: SLACK Incorporated; 2011.

Smith DG, Michael JW, Bowker JH. Atlas of Amputations and Limb Deficiencies: Surgical, Prosthetic, and Rehabilitation Principles. 3rd Ed. Rosemont, IL: American Academy of Orthopedic Surgeons; 2004: 503-505, 541-555.

Author Disclosure

Bradeigh S. Godfrey, DO
Nothing to Disclose

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