Trans-Tibial Biomechanics

What are areas are typically pressure sensitive (TTA)?

Fibular head

Tibial crest / tubercle

Terminal bone ends

Common fibular nerve

Tibial artery

When can the anterior distal part of the tibial crest (above the cut end) press against the socket, causing pain?

Presses upon socket during initial contact or loading 2/2 Quad Activation causing knee extension

What are areas are typically pressure tolerant (TTA)?

Patellar ligament

Medial tibial flare

Gastroc muscle flap

Tibialis anterior m.

What does total contact do?

Distributes forces throughout surface area

Allows for no movement of tissue (unless there is a change in girth 2/2 decreasing edema, which occurs often!)

Provides sensory feedback for movement = Bone exerts force on socket which causes movement

What happens if total contact is lost?

Limb tissues expand toward socket wall

→ Causes shear forces around area (Limb & socket exert equal opposing forces) if motion is allowed

= Pressure Ulcers / Infection

How should you reduce pressure on sensitive areas?

DO NOT "Cushion the Pain"

= Results in same force applied to a smaller area, increasing pressure

DO "Build Up" Pressure Tolerant Areas

= Raise or build-up surrounding areas, which consequently relieves sensitive areas

What is the effect of aligning the prosthetic so forces are perpendicular vs. parallel to the surface?

Force perpendicular to surface = Most efficient to support weight

Force parallel to surface = More force required to support (squeeze) WB

In the sagittal plane, how far forward should the socket be flexed & why?

5º Anterior Angulation → Shifts weight to patellar ligament

+ Simulates knee position in midstance + reduces knee hyperextension in terminal stance

+ Center of socket is closer to middle of the feet (anterior to angle) → Shorter lever arm

In the sagittal plane, at the ankle, what moment (flexion vs. extension) is safest for the patient?

Anterior = DF Moment

If there was a posterior moment (PF), without the tibialis anterior m., the patient will fall backward

In the sagittal plane, what moment is created at the knee joint if the socket is positioned anteriorly?

GRF Posterior to Knee = Knee Forward Flexion Moment

In the sagittal plane, what are the risks (at the knee joint) of the socket being positioned anteriorly?

Shortened Toe Lever

→ Knee buckling upon initial loading / foot flat, especially if pain inhibits the quads **Quadriceps need to exert an equal extension moment to prevent

→ Reduced resistance to knee flexion after midstance

In the sagittal plane, if the socket is positioned anteriorly, where is the pressure around the knee?

Pressure on proximal posterior/distal anterior limb terminus (Pressure Sensitive) that ↑ with quad firing

In the sagittal plane, what moment is created at the knee if the socket is positioned posteriorly?

GRF Anterior to Knee = Backward Extension Moment

Lengthens Lever Arm = From weight of body to the toe lever

In the sagittal plane, if the socket is positioned posteriorly, where is the pressure around the knee?

Pressure on pressure tolerant areas → Proximal anterior & distal posterior limb

If the socket is positioned posteriorly in the sagittal plane, how will gait be affected?

↑ Resistance to knee flexion after foot flat

Knee hyperextension just before heel-off through stance = Knee more secure / stable but less mobile (↑ Effort ie. walking up a hill)

In the sagittal plane, if the body weight (GRF) is behind the hip joint, what is the resultant moment?

Hip Extension Moment: GRF Posterior to Hip

**Hip will not flex until GRF is anterior to hip joint

In the frontal plane, how does an outset foot affect gait? Where is the pressure?

Creates Medial Moment of the Socket *Pressure on Lateral Fibular Head = Sensitive

Widens BOS + Long lever arm demands hip ABD strength & hip ADD ROM

In the frontal plane, how does an inset foot affect gait? Where is the pressure?

Creates Lateral Moment *Pressure on Medial Tibial Flare = Tolerant

Narrows BOS + Shorter lever arm requires less forceful hip ABD to maintain single support

= More forward movement, but more concerns for balance

Based on lateral & medial moments, what is the ideal alignment in the frontal plane?

Inset Foot (Bench socket center 1-cm lateral)

Creates Lateral Moment *Pressure on Medial Tibial Flare (Tolerant)

What occurs if an individual is fitted for a prosthesis in supine/sitting ie. Bench Fitting (without considering frontal plane alignment), then it is found that the patient has Genu Valgum in WB-ing?

Results in laterally tilted, adducted socket

+ Widens BOS

+ ↑ Valgus Force

Pressure is exacerbated on the lateral side (fibular head) ie. Amplifies the affects of genu valgum

For what knee deformity, does an adducted socket restore a normal base of support?

Genu Varum

What is the optimal transverse plane alignment at the foot?

5-10º Foot ER = Normal

*Usually matched to sound foot

In the transverse plane, what happens if you create toeing-in (IR at tibia) of the prosthesis foot?

Narrows BOS

Functionally lengthens toe lever = Enhances stability

In the transverse plane, what happens if you create toeing-out (> 10º ER at tibia) of the prosthesis foot?

Widens BOS = Enhanced M-L balance & stability

+ Sound foot often seen in ER

Functionally shortens toe lever = Reduces resistance to knee flexion & buckling

How does toe-in / toe-out alter frontal plane alignment at the knee?

ER (Toe-out) = Increases Valgus

IR (Toe-in) = Increases Varus

If you have a TTA, how much more energy are you expending while walking?

25% more energy required to walk, at approx. 25% slower

What is the effect of a short TTA length on energy efficiency?

Short TT Lengths < 20% of Tibia (7-cm if 5’9”) limb spared

= Suspension difficult

*More energy to walk slower

If an individual with a TTA, has >20% of their tibia length (>8-cm), how is energy efficiency impact?

> 20% limb length has no impact on energy or walking speed

BUT < 15-cm = Weaker Quads & Hamstrings

What is the effect of a long TTA length on energy efficiency?

Long TT Lengths: > 50%

= Need ~ 20-cm (55% if 5’9”) below amputation for posterior leaf leg

*Produce faster walk speed + need less energy

What are kinematic gait-related impairments to consider?

ROM – Knee Flexion Contractures (Prognostic for ambulation after VA)

Strength – Knee (Quadriceps) Weakness + Hip (Glutes) Weakness

What are possible patient causes of excess foot rotation during IC & Loading?

Strength/Length long (IR) or short (ER)

Limited hip IR/ER ROM + SIJ Hypomobility

Weak hip IR/ER → Pelvic Control

Insufficient socket allows rotation

What are possible prosthetic causes of excess foot rotation during IC & Loading?

Suspension insufficient

Heel cushion too hard

Excessive toe in/out

What are possible patient causes of knee buckling during IC & Loading?

Weak Ecc Quads & Glutes

Impaired kinesthesia / proprioception

Pain/pressure anterior-distal tibia

What are possible prosthetic causes of knee buckling during IC & Loading?

Anything that ↑ DF moment

+ Foot DF

+ Shoe heel too high

+ Excess socket flexion

+ Hard heel cushion

Anything that shortens toe lever

+ Socket too anterior

+ Foot / Keel too short

What are possible patient causes of wide BOS ie. medial thrust during Midstance?

Hip joint or ABD/TFL contracture

Anything that ↓ Stance time

+ Lack of weight acceptance

+ Pain on proximal-lateral area (fibular head)

+ Weakness / poor balance

What are possible prosthetic causes of wide BOS ie. medial thrust during Midstance?

Swinging prosthesis is too long

= Non-total contact, poor suspension

Anything Causing Medial Moment

+ Foot outset

+ Lateral socket tilt

What are the possible patient causes of lateral trunk lean ie. Trendelenburg during Midstance?

Hip joint / TFL ABD contracture (Assess gluteal crease relation to foot)

Weak Gluteus Medius

Pain / pressure proximal-lateral *Fibular Head

What are the possible prosthetic causes of lateral trunk lean ie. Trendelenburg during Midstance?

Prosthesis is too short

Anything Causing Medial Moment

+ Foot outset

+ Lateral socket tilt

What are the possible patient causes of IL pelvic retraction ie. insufficient hip extension in Midstance?

Hip / SIJ Flexion Contractures

Gluteal weakness

Poor weight acceptance

What are the possible prosthetic causes of IL pelvic retraction ie. insufficient hip extension in Midstance?

Anything that ↑ PF Moment

+ Foot PF = Long Lever

+ Socket posterior or not flexed

+ Non-articulated ankle