Prosthetics

Components of prosthesis specifically for transfemoral amputation

Knee unit

• Single-axis knees:
  • Mechanism: A single pivot point, resembling a simple hinge
  • Stability: Little built-in stance stability; the user keeps the knee from buckling through alignment and voluntary hip extensor control. The manual-locking type (below) adds stance stability by locking the knee in extension
  • 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: Provides constant resistance to knee flexion/extension throughout swing, controlling swing speed at a single walking cadence - it does not add stance-phase stability
• Polycentric (multiaxis) knees:
  • Mechanism: Multiple pivot points, allowing for a more natural and efficient gait, particularly during the swing phase
  • Stability: High inherent stance stability from the shifting center of rotation; also shortens the shank during swing, improving toe clearance
  • Suitable for: Active individuals, those with knee disarticulation or long residual limbs, or those with weak hip extensors who need added stance support
• Fluid-controlled (hydraulic/pneumatic) swing control:
  • Mechanism: Uses fluid or air resistance to control the speed of knee swing, automatically adjusting to the user’s walking cadence (unlike constant friction, which only controls one speed)
  • Suitable for: Can be paired with single-axis or polycentric knee units for a smoother, more natural, variable-speed gait
• 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
• Knee axis alignment: in a transfemoral prosthesis, a knee axis placed behind the weight line (the trochanter-knee-ankle, or TKA, line) is stable in stance because body weight pushes the knee into extension, but a knee placed too far posterior is so stable that it is hard to initiate knee flexion for swing

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

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/adductors (medial) and the gluteals and for sitting (posterior)

Residual limb management and prosthetic training

• Shrinking the limb: Wrap with a figure-eight elastic bandage or shrinker sock, distal-to-proximal (never circular wrapping, which can act like a tourniquet).
• Preventing contractures: Transfemoral: avoid prolonged hip flexion/abduction - encourage prone lying, no pillow under the hip. Transtibial: avoid prolonged knee flexion - no pillow under the knee, encourage prone lying.
• Sock-ply management: Add plies as the limb shrinks or loses volume; remove plies if the socket feels tight or skin shows pressure marks.
• Skin checks: Inspect the residual limb before and after each wearing session for redness, blistering, or breakdown.
• Suspension options: pin/lock, suction, vacuum-assisted, sleeve, supracondylar cuff, belt/strap.

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 (stance phase, intact limb)

• Definition: The patient rises up on the toes (plantarflexes) of the intact limb during its mid-stance to functionally lengthen it, so the prosthetic limb can clear the ground during swing.
• Causes:
  • Limb too long
  • Poorly suspended prosthesis
  • Excessive plantarflexion

Example: Vaulting during gait

A patient with a transfemoral prosthesis rises onto the toes of the intact limb during prosthetic swing. What should the PTA do?

Answer: Document the deviation and consider likely prosthetic causes - the prosthesis may be too long, poorly suspended, or set in excessive plantarflexion. Report these to the supervising PT, who coordinates any adjustment with the prosthetist; the PTA does not adjust the prosthesis directly.

Circumduction (swing phase, mainly transfemoral)

• 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 (swing and stance)

• 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 (early stance)

• Definition: Excessive knee flexion on the prosthetic side from initial contact through 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
  • Pain in the residual limb or socket

Reduced toe clearance (swing)

• 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 (initial contact)

• 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

Lateral trunk lean over prosthesis (Trendelenburg) (stance)

• 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 (swing)

• 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