Biomechanical Loading and the Skeleton

Enthesis (pl. entheses)

A site where a tendon or ligament attaches to bone

• anchors the muscle and dissipates stress associated with the contraction of muscle

Types of Entheseal Changes

• robusticity

• pitting

• ossification exostoses (bone spurs)

Entheseal Robusticity

• variation between size by person

• can change over time due to activity, genetics, etc.

• children might have holes in enthesis (NOT pathology)

Entheseal Pitting

• Microporosity: <1mm

• Macroporosity: >1mm

Ossification Exostoses

• outgrowth of bone

• proliferates past enthesis

Types of Entheses

• Fibrous

• Fibrocartilaginous

Fibrous Entheses

• inserted at a considerable distance from the joint

• virtually no compressive forces because the tendon or ligament is not kinked/bent

Fibrocartilaginous Entheses

• the pulling action along the tendon/ligament creates shearing force at the insertion

• the tendon/ligament is kinked because it is inserted close to the joint space

• the resulting change of angle of the tendon/ligament adjacent to the joint creates pressure on the deeper layer of the enthesis

• more likely to be changed by activity than fibrous entheses

Law of Bone Remodeling

Bone tissue places itself in the direction of functional demand

Bone Loss Process

• coupled action of osteoblasts and osteoclasts is uncoupled

• greater amount of removal than replacement, especially at the endosteal surface

Anisotropic*

Different material properties depending on the direction of loading

Types of Loading

• Tensile

• Compression

• Shearing

• Bending

• Torsion

• Combined loading

Tensile Loading

equal and opposite forces are applied outwardly

• common in the knee

• patella pulled toward quad. contraction and by the tendon*

Compression Loading

equal and opposite forces are directed toward each other

• very common

• pressure (ex. falling & landing on arms)

Shearing

application of forces perpendicular to the long axis of the bone, but opposite and parallel to each other

Bending

• Tension on the convex side

• Compression on the concave side

Torsion

• twisting of the skeletal element about the long axis

• combination of tension, compression, and shear forces

Combined Loading

• multiple forms of loading acting on bone

  • bending and torsion are the most common

Bone Biomechanics

• the application of engineering principles to bone as biological material

• recognizes boen as dynamic tissue that modifies continuously in response to loading and activity levels

Wolff’s Law

Tension and compression cycles create a small electircal potential that stimulates bone deposition and increased density at points of stress

• Loading stress

  • human bipedalism is a common example

• Point of no stress

  • usually inside medullary cavity

Point of No Stress

point where loading stress forces intersect and cancel out; usually inside the medullary cavity

Bone Biology Paradigm

• primary function of bone is biomechanical

  • Goal: remodeling optimizes bone strength given biomechanical forces it must endure

• Osteocytes are sensors for loading stress → send signals to osteoblasts/osteoclasts to steer bone formation, resorption, and repair

  • occurs regionally

  • remodel bone in response to recent mechanical load

  • prepare for future mechanical stress by “toughening” bone

Toughening Mechanisms of Bone

• bone resists temporary (elastic) and permanent (plastic) deformation of bone

• bone dissipates energy through the formation and the targeted repair of damage

Elastic Property

material property of bone that resists temporary deformation

Plastic Property

material property of bone that resists permanent deformation

Toughness

the total energy that a bone can absorb before failure (related to strength)

Strength

the ability of bone to resist failure (fracture) during loading; formed by repair of bone

Factors that Influence Bone Morphology

• stressors

• nutrition

• hormones

• age

• cultural modification

Bone Functional Adaptation

• bone adjusts the amount and distribution of its mass to withstand biomechanical loads

• feedback mechanism

  • strain below optimal threshold results in bone loss

  • strain within the optimal range results in remodeling to repair and maintain bone

  • strain above optimal threshold results in bone modeling

Cross-Sectional Geometric Analysis

• Magnitude of stresses is proportional to the distance from the central or “neutral” axis

• measure geometric properties from cross-sections taken perpendicular to the long axis of the skeletal element

Rigidity

the ability to resist bone deformation during loading

Cortical Bone Thickness

• cortical thickness tells us about axial loading

• long bones are curved and affected by muscle forces (more often by bending and torsion)

• cortical bone thickness alone is not an appropriate indicator of mechanical stress

Axial Loading

• compression

• tension

Second Moments of Area

• geometric properties that are used to measure bending and torsion rigidity

• diaphyseal geometry is assessed at specific percentages of length of the bone

  • (comparison is best using midshaft)