Biomechanics of Trans-tibial Prosthesis

Key Factors Determining Prosthetic Use

  • The individual: prior ability, health status, motivation, training and education

  • The socket: fit and comfort, total contact, type of socket

  • Alignment: influenced by biomechanical principles

Prosthesis Goals

  1. Comfort: Must provide comfort whether the user is standing or sitting. If uncomfortable, the user will likely not wear the prosthesis.

  2. Energy Efficiency: Design should allow the user to perform daily tasks with minimal energy consumption.

  3. Cosmesis: Aesthetic appearance of the prosthetic limb is important for the user’s self-esteem and social acceptance.

The Socket

Fit and Comfort:

  • The fit of the socket is paramount for comfort and functionality; poor fit can lead to discomfort.

Total Contact:

  • Modern designs strive for total contact to distribute pressure evenly and enhance comfort.

  • Prevents oedema and aids venous return

  • Provides greater area to distribute the load

  • Provides greater contact to provide sensory feedback

  • Accommodating for differences in tissues firmness - ie. building up more pressure around soft tissues to compress them so firm tissues not taking full load

  • Pressure tolerance - ie. building up more pressure around pressure tolerant areas than pressure sensitive

    • Pressure sensitive areas: spine of tibia, head of fibula, distal end of the tibia and fibula, hamstring tendons

    • Selective loading - unable to weight bear on distal end

Patella Tendon Bearing Prosthesis

  • Patellar Tendon Shelf: Accepts a majority of body weight (approx. 2/3) and guides correct socket positioning, diameter ~20mm

  • High Posterior Rim: Allows knee flexion, maximises surface area, relief for hamstrings, rounded brim, minimises discomfort while sitting.

  • Anterior Border: mid patella level, triangular shape, liner

Alignment

Socket Flexion

  • At least 5 degrees of flexion is recommended to prevent hyperextension during stance phase. Opens up patella tendon shelf.

  • No contracture → 5 degrees of flexion

  • 15degrees flexion contracture → 15 degrees socket flexion

  • 5degrees flexion contracture → 5 degrees of flexion

Counterforce Alignment:

  • Must ensure that forces acting on the socket are evenly distributed to prevent rotational issues at the stump

    • Socket angle = 5 degrees adduction / lateral flexion

      If opposing forces out of alignment -> rotational issues at socket -> pressure areas

    • Mediolateral alignment: foot directly under socket (<5mm inset or outset)

      Foot Inset => pressure lat/distal + med/proximal Foot outset => pressure med/distal + lat/proximal

    • Anteroposterior alignment: 1/3 posterior, 2/3 anterior (~9cm form heel)

      Socket forward => P ant/distal + post/prox Socket backward => P ant/prox + post/distal

Foot Levers

  • 2/3 toe lever, 1/3 heel lever

  • Dorsiflexion → inc. heel lever, dec. toe lever

  • Plantarflexion → inc. toe lever, dec. heel lever

Observations

  • Foot rotated outwards

  • Height - up to 12mm shorter

  • Standing alignment

    • Front - height

    • Side - socket flexion