Prosthetics
Levels of amputations
Upper extremity
- Finger or partial hand: Removal of one or more fingers or part of the hand
- Wrist disarticulation: Removal of the hand at the wrist joint
- Transradial (below elbow): Removal of the forearm below the elbow joint
- Elbow disarticulation: Removal of the arm at the elbow joint
- Transhumeral (above elbow): Removal of the arm above the elbow joint
- Shoulder disarticulation: Removal of the arm at the shoulder joint
Lower extremity
- Toe or partial foot: Removal of one or more toes or part of the foot
- Ankle disarticulation (Syme’s amputation): Removal of the foot at the ankle joint, keeping the heel pad so the residual limb can bear weight at its end
- Transtibial (below knee): Removal of the leg below the knee joint
- Knee disarticulation: Removal of the leg at the knee joint
- Transfemoral (above knee): Removal of the leg above the knee joint
- Hip disarticulation: Removal of the leg at the hip joint
Focus of NPTE is on lower extremity prosthesis
Components of prosthesis
Primary components
- Sockets
- custom-molded prosthetic that will have total contact with residual limb; goal is equal distribution of tissue load throughout the socket
- Liners
- Silicone barrier between residual limb and socket to decrease shear forces
- Socks
- Used to aid in changing the size of the residual limb
- Terminal device
- Interface between prosthetic limb and external environment
- Upper limb: the terminal device is a hand or a hook
- Lower limb: the terminal device is the foot
- Interface between prosthetic limb and external environment
Components of a prosthesis specifically for transtibial amputation
Ankle-foot assembly
- SACH foot (solid ankle cushioned heel):
- A simple, durable, and lightweight foot with a solid ankle joint
- Single-axis foot:
- Features a hinged ankle joint that allows for up-and-down movement, enhancing knee stability.
- Multi-axis foot:
- Similar to a single-axis foot, but also allows for side-to-side movement, making it better suited for uneven terrain
- Microprocessor-controlled (MPC) feet:
- These feet use sensors and a computer to adjust the ankle’s position and resistance based on the user’s movements and the environment
Shank
- Endoskeleton prosthesis:
- Structure: Features a tubular structure connecting the primary components, like the socket, suspension system, foot, and knee unit
- Appearance: Often covered with cosmetic foam
- Advantages: Endoskeletal prostheses are adjustable and lighter than exoskeletal prostheses.
- Disadvantages: Less durable than exoskeletal prostheses
- Exoskeleton prosthesis:
- Structure: Has a hard outer shell that provides structural strength and cosmetic shape
- Materials: Traditionally constructed from wood or plastic
- Durability: More durable than endoskeletal prostheses
- Advantages: May be preferred by people who do physical labor or are in harsh environments
- Disadvantages: Less customizable than endoskeletal prostheses.
Socket
- Patellar tendon bearing (PTB) sockets:
- Focus: PTB sockets, often used for below-knee amputations, focus weight-bearing stress on pressure-tolerant structures like the patellar tendon and medial tibia flare
- Advantages:
- Can be a good choice for patients with shorter or bony residual limbs
- Easier to don/doff for amputees with poor hand dexterity, poor eyesight, or hemiparesis
- Can be modified to accommodate changes in the residual limb after amputation
- Disadvantages:
- May not be ideal for patients with residual limb scar tissue or chronic skin breakdown
- Advantages:
- Focus: PTB sockets, often used for below-knee amputations, focus weight-bearing stress on pressure-tolerant structures like the patellar tendon and medial tibia flare
- Total surface bearing (TSB) sockets:
- Focus: TSB sockets, also used for below-knee amputations, distribute weight evenly across the entire limb surface, reducing pressure points
- Advantages:
- Often used with gel liners, which cushion the limb and protect the skin, making them great for amputees with scar tissue or sensitive skin
- Offer better suspension than PTB designs, often using locking pins or suction systems
- Disadvantages:
- Can be more difficult to don/doff than PTB sockets
- Advantages:
- Focus: TSB sockets, also used for below-knee amputations, distribute weight evenly across the entire limb surface, reducing pressure points
Components of prosthesis specifically for transfemoral amputation
Knee unit
- Single-axis knees:
- Mechanism: A single pivot point, resembling a simple hinge
- Stability: Offers good stability during the stance (weight-bearing) phase but may require more effort for the swing (forward movement) phase
- Suitable for: Individuals with lower activity levels, those who can voluntarily stabilize the knee
- Types:
- Manual locking: The knee can be locked in extension for stability, and unlocked manually for sitting
- Constant friction: Provide a constant resistance to knee flexion during the swing phase, aiding in stability
- Polycentric (multiaxis) knees:
- Mechanism: Multiple pivot points, allowing for a more natural and efficient gait, particularly during the swing phase
- Stability: Shorten the prosthesis during swing, reducing the risk of tripping and improving stability
- Suitable for: Active individuals, those with longer residual limbs, or those who require a more natural gait
- Types:
- Hydraulic/pneumatic: Use fluid or air to control the swing phase, allowing for smoother and more natural movement
- Microprocessor knees:
- Mechanism: Use of sensors and electronics to adjust the resistance to knee flexion and extension based on the user’s gait and activity level
- Benefits: Improve stability, reduce energy expenditure, and allow for a more natural and comfortable gait
- Suitable for: Highly active individuals, those who require a high level of stability and control, or those who want to maximize their mobility
Socket
- Ischial containment (IC) sockets
- These designs feature a narrow medial-lateral dimension with the ischium (part of the pelvic bone) encapsulated within the socket, rather than sitting on the brim
- Sub-ischial sockets
- These sockets have a lower proximal trim line than IC sockets, and are often used with vacuum-assisted suspension
- Quadrilateral (quad) sockets
- These were historically the most common type, but are less common now, although still used for some patients
- Direct socket
- A novel method of fabricating a laminated interface directly onto the residual limb
Keys for transfemoral socket trim lines
- In a transfemoral socket, the posterior and medial walls are lower than the anterior and lateral walls
- This allows room for the groin and sacrum
Prosthetic gait deviations
Prosthetic gait deviations refer to any variations from a normal walking pattern that occur in individuals using prosthetic limbs, often due to compensatory strategies or issues with the prosthesis itself.
Vaulting
- Definition: An attempt to lengthen the stance phase on the intact limb by knee extension and ankle plantar flexion during mid-stance
- Causes:
- Limb too long
- Poorly suspended prosthesis
- Excessive plantarflexion
Circumduction
- Definition: The prosthetic limb travels in a lateral arch during swing.
- Causes:
- Excessive abduction of the prosthesis
- Prosthesis too long
- Inadequate suspension
- Excessive plantarflexion of the prosthetic foot
- Excessive knee friction or a locked knee unit
Abducted gait pattern
- Definition: The prosthetic limb is carried in an abducted position throughout the swing and stance phase
- Causes:
- Outset prosthetic foot
- Prosthesis too long
- Medially placed intact foot
Knee instability
- Definition: Excessive knee flexion on the prosthetic side in early stance
- Causes:
- Excessive foot dorsiflexion
- Excessive socket flexion
- Posterior translation of foot/pylon
- Excessively hard heel cushion on prosthetic heel
- Weakness of the knee extensors
Reduced toe clearance
- Definition: Prosthetic toe drags or catches during the swing phase
- Causes:
- Prosthetic limb too long
- Inadequate suspension
- Residual limb not getting into prosthetic socket all the way
- Excessive plantarflexion of the prosthetic foot
Pelvic drop
- Definition: The pelvis on the prosthetic side drops on initial contact as if “stepping into a hole.”
- Causes:
- Prosthetic limb too short
- Residual limb has shrunk relative to the socket
- Excessively compliant heel cushion
Lateral trunk lean over prosthesis (Trendelenburg)
- Definition: Trunk bends laterally over the prosthesis (compensated Trendelenburg) during stance
- Causes:
- Inadequate adduction of the socket
- Prosthesis too short
- Outset foot
- Medial wall too high, causing pain
- Gapping at the lateral wall of the socket
Medial/lateral whip
- Definition: Abrupt medial or lateral movement of the prosthetic heel during swing
- Causes:
- Excessive external or internal rotation of the socket
- Socket too tight
- Inadequate suspension
- Excessive valgus of prosthetic knee
Increased knee flexion
- Definition: Long prosthetic step (decreased stance time on prosthesis)
- Causes:
- Painful socket - residual limb; painful
- Prosthesis too long
- Excessive foot dorsiflexion
- Excessively hard heel cushion on prosthetic heel