Role of biomechanics in the understanding of normal, injured, and healing ligaments and tendons

http://www.ncbi.nlm.nih.gov/pubmed/19457264

• WHY NEED TISSUE ENGINEERING- "It is also known that a midsubstance anterior cruciate ligament (ACL) tear has limited healing capability, and reconstruction by soft tissue grafts has been regularly performed to regain knee function. However, long term follow-up studies have revealed that 20–25% of patients experience unsatisfactory results."
• WHY NEED TISSUE ENGINEERING- "Well over 100,000 of these procedures are done per year in the U.S. alone [13]. However, long term follow-up studies have revealed that 20–25% of patients experience unsatisfactory results at 7 to 10 years following ACL reconstruction"
• FUNCTION- "Because the primary function of ligaments and tendons is
  to transmit tensile forces, experimental studies of the biomechanical
  properties of these tissues are generally performed
  in uniaxial tension... tensile tests have been performed with the ligament
  or tendon insertions to bone left anatomically intact"
• HOW TO MEASURE AREA- "developed to measure both the cross-sectional area
  and the shape of soft tissues with high accuracy and precision"
• FUNCTION- "Additionally, since ligaments and tendons are highly
  organized fibrous tissues, their mechanical properties are
  directionally dependent (anisotropic)."
• FUNCTION-"Mathematical models have been made to describe the viscoelastic
  properties of ligaments and tendons."
• PROPERTY- "Age-related changes are one biological factor worth discussing"
• FUNCTION- "The properties of ligaments and tendons also can change
  with advancing age as well as with activity level"
• WHY ACL DOES NOT HEAL- "Unlike extraarticular
  ligaments, there exist several well known factors that
  limit the ACL from healing [178,179]. The thin synovium
  surrounding the ACL, which has been shown to play an
  important role in providing a vascular supply to the relatively
  avascular ACL as well as to protect it from the harsh
  synovial fluid [180], is disrupted and not regenerated
  until 1 to 2 months following ACL injury [175,181-183].
  Histological examination of the human ACL reveals that
  it is also retracted following rupture and that clots formed
  are insufficient to fill the open gap"
• CURRENT OPTIONS- "Surgical replacements have been done
  for a large percentage of patients to help maintain knee
  stability [196-200]. Autografts, such as the bone-patellar tendon-bone (BPTB) and hamstring tendons, and allografts
  are the graft of choice... 20–25% unsatisfactory results, with complaints
  such as knee pain and extension deficits [78,201-204],
  and most concerning, many of these cases had progressed
  to knee osteoarthritis"
• TEST- "To assess the function of the ACL under multiple degree of
  freedom (DOF) knee motion, our research center has
  developed a robotic/universal force moment sensor (UFS)
  testing system since 1993 (Figure 4) [218]. This novel testing
  system can control and reproduce the multiple DOF
  knee motion [38,40,41,219-221] and is also capable of
  applying external loads to knees. By operating in both force
  and position control modes, the robot can apply a predetermined
  external load to the specimen, such as those
  used for the diagnosis of ACL deficiency"
• COMPUTER MODELS- "Computational finite element models are also valuable for
  studying for the complex function of the ACL and its bundles.
  Once such models are validated with experimental
  data (e.g., knee kinematics or in-situ forces as determined
  using the robotic/UFS testing system [227,228]), they can
  be used to calculate the stress and strain distribution in the
  ACL, by incorporating its non-uniform geometry (in which
  the midsubstance cross-sectional area is one-third that of
  the insertions) and its twisting fiber orientation."
• TISSUE ENGINEERING COULD LEAD TO BETTER RESULTS- "The use of growth factors, gene
  therapy, cell therapy, and biological scaffolds to enhance
  healing certainly will result in improved outcomes
  [61,242-247]. We believe the ECM-derived bioscaffolds
  will play a significant role because it could accelerate the
  healing process and establish a bridge between the torn
  ends."