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