tendons and ligaments

load-deformation curve

· unique shape

· 4 regions

Toe region

· concave region

· minimal load to produce deformation

· represents change in collagen fibres from wavy to straight

· strain at end of toe region is 1.5%

Linear region

· progressive elongation of collagen fibres with increasing load

· end marked by yield point

Failure region

· sequential failure of some of the structural units

· represented by small dips

Ultimate load

· complete failure of structure

· marked by ultimate failure point

loading

· can divide area under curve into

· early physiological loading zone

· late injury zone

· transition occurs in linear region toward yield point

· microfailure occurs in region between transition and yield point

Physiological loading

· normal physiological loads are only about 1/4 of ultimate tensile strength

· upper limit for physiological strain is 5%

Injury

· due to loads exceeding physiological range

· microfailure occurs before yield point reached

· when yield point exceeded, ligament begins to undergo gross failure

· injury load may be achieved by maximal muscle contraction coupled with external force

· strength of tendon proportional to cross-sectional area

rate dependency

· tendons and ligaments are viscoelastic structures

· during cyclic testing, stress-strain curve

· displaced to right (indicates some plastic component)

· becomes steeper (increased stiffness from plastic deformation)

· creep response used clinically to lengthen tendons and ligaments

· eg. serial casting in club foot