JD

Loading and Material Properties

Loading and Material Properties: An Introduction

  • Human joints withstand forces during daily activities for support and protection.
  • Stress, strain, failure, and stiffness are concepts used to understand the mechanical behavior of structures.

Load

  • Load is defined as force applied to a structure.
  • Gravity and external forces apply loads to body tissues.
  • Immobilization, inactivity, or improper training can affect load.
  • Tissues respond to loading or lack of loading, modifying their health (structure, composition, and function).
  • Loads can be manipulated during rehabilitation to optimize function and structure of tissues like bones, capsules, ligaments, muscles, tendons, and cartilage.

Loading and Response to Loading: Deformation

  • Deformation refers to changes in length or shape when a load is applied.

Loading and Response: Load-Deformation Curve

  • Y-axis: Applied load.
  • X-axis: Amount of deformation due to the load.
  • Toe Region (A): Initial load to take up slack in the tissue.
  • Elasticity (A → B): Ability to return to the original state after deformation.
  • Yield Point (B):
    • End of the elastic region.
    • The material may not immediately return to its original state after load removal.
    • It might return with time.
  • Plasticity (B → C ish):
    • Loading great enough to cause permanent deformation after load removal.
    • Microfailure occurs.
    • Useful for lengthening tissues.
  • A – B : Load applied/released → no deformation
  • B – C : Load applied/released → deformation

Ultimate Failure Point (C)

  • Continued loading in the plastic region leads to overt failure.
  • Rupture: Failure within the structure of connective tissue.
  • Avulsion: Failure at the tendon/ligament attachment to bone.
  • Fracture: Failure within bony tissue.

Load-Deformation Curve Usage

  • Used to determine the strength and stiffness of structures.
  • Examines elasticity, plasticity, ultimate strength, and stiffness of a material.
  • Indicates the amount of energy storage before failure.

Loading and Response: Stress-Strain Curve

  • Stress: Measure of load in an object, expressed as force per unit area.
  • Strain: Percent change in length or cross-section of material.
  • Both stress and strain are calculated, not measured directly.
  • Stress-Strain curve: Amount of deformation (x) vs. load per unit area (y).
  • Examines material properties of substances, including tissues.

Stress-Strain Curve - Material Properties

  • Strength:
    • Load sustained before failure.
    • Deformation sustained before failure.
    • Energy it can store before failure.
  • Stiffness: Resistance to external loads as a specimen deforms, indicated by the slope of the load-deformation curve in the elastic region.

Modulus of Elasticity (Young’s Modulus)

  • Value obtained by dividing stress by strain in the elastic region of the stress-strain curve.
  • Examines stiffness of a material.
  • Higher stiffness corresponds to lower compliance, and vice-versa.
  • Young’s Modulus values for common orthopedic materials (GPa):
    • Stainless steel: 200
    • Titanium: 100
    • Cortical bone: 7-21
    • Cement: 2.5-3.5
    • Cancellous bone: 0.7-4.9

Stress-Strain Curve Influences

  • The type of stress and strain developed in human tissues depends on:
    • Material.
    • Type of load applied.
    • Point of load application.
    • Direction and magnitude of load.
    • Rate of loading.
    • Duration of loading.
  • Stress-strain curves compare:
    • Strength properties of different materials.
    • The same tissue under different conditions (e.g., ligaments before and after immobilization).

Material Stress-Strain Behavior

  • Idealized stress-strain behavior for metal, glass, and bone.
  • Bone exhibits some plastic deformation but behaves more like brittle glass than ductile metal.
  • Anisotropic behavior of cortical bone is influenced by the orientation of load application (longitudinal, tilted, transverse).

Viscosity

  • A material’s resistance to flow.
  • A fluid property.
  • High viscosity (e.g., honey) → high resistance to deformation.
  • Low viscosity (e.g., water) → low resistance to deformation.
  • Viscosity decreases with slow loading and increases with rapid loading.

Viscoelastic Materials

  • Connective tissues are viscoelastic.
  • They combine elasticity and viscosity.
  • Exhibit time-, rate-, and history-dependent behavior.

Creep

  • Constant low loading over an extended period results in slow deformation of soft tissues.
  • Clinical Application: Stretching a shortened tissue by applying constant force and gradually elongating the tissue.
  • Friend: preventing foot drop, weight-bearing using tilt table, dynamic splints for increasing ROM
  • Foe: re-loading too soon.

Stress-Relaxation

  • The ability of tissues to gradually need less stress (force) to maintain the same elongation over time.
  • Resistance to stretch decreases over time, allowing for more movement.
  • Clinical Applications: Muscle stretching, joint mobilization, static progressive splint.

Hysteresis

  • Loss of energy demonstrated by a viscoelastic material.
  • It is the difference between energy expended when loaded and energy regained when unloaded.
  • Seen by the path of the load-deformation curve.
  • No change in the path indicates no hysteresis.
  • A change in path indicates some hysteresis.
  • Little-to-no energy loss means low hysteresis.
  • High energy loss means high hysteresis.

Strain-Rate Sensitivity

  • Most tissues behave differently when loaded rapidly versus slowly.
  • Rapid loading: tissue is stiffer, larger peak force.
  • Subsequent stress-relaxation will be larger with rapid loading.
  • Creep will take longer under rapid loading conditions.

Clinical Implications: Creep, Stress-Relaxation and Strain-rate sensitivity

  • To increase connective tissue length with minimal injury risk, apply a slow load to the maximum tolerable level and maintain it while creep occurs.
  • If stress-relaxation occurs, return the force to the original level and maintain further creep.
  • Total length change should be no more than 2 to 6% strain to avoid injury.
  • Larger loads over short periods produce high stresses and low strains (strain-rate sensitivity).
  • Loads over longer periods produce lower stresses and higher strains (creep and stress-relaxation).

Isotropy and Anisotropy

  • Isotropic: Properties do not vary regardless of force application direction (e.g., glass).
  • Anisotropic: Strength and elasticity vary when loaded in different directions (e.g., most human tissues like bone).

Forces – Loading Modes

  • Compression / Joint Compression / Compressive Forces
  • Tension / Tensile / Joint Distraction / Distractive
  • Bending
  • Shear
  • Torsion
  • Combined loading