Mechanical injury: energy transfer and tissue deformation
THE CONTEXT
Mechanical impact transfers energy to tissues. The resulting injury depends on how tissues move, deform and resist strain.
Energy transfer and tissue response
•
Impact between a moving force and a counterforce transfers energy to the body, changing the tissues' state of rest or motion.
•
Tissues differ in solidity, fluidity, density and elasticity, so a forceful impact does not affect them uniformly.
•
Some energy moves the body as a whole, while much may produce non-uniform movement of localized parts and subject tissues to compression, traction or both.
•
Except for tissues containing gas, body tissues resist reduction in volume. Mechanical force instead displaces and deforms them, producing traction strains.
Strain and tissue failure
•
Traction strains may arise from simple elongation or from bending, torsion and shearing.
•
Shear strain changes shape without changing volume and tends to make contacting parts slide parallel to their plane of contact.
•
Tissues differ in resistance to traction and rupture when their cohesiveness is exceeded.
•
Rigid tissues such as bone resist deformation but fracture when their elastic limit is exceeded.
•
Soft tissues are plastic. Their change in shape is limited by cohesion between cells and by connective and vascular frameworks and capsules.