Equis ISSN 2398-2977

Musculoskeletal: fracture

Contributor(s): Stephen Adams, Patrick Colahan, Bud G E Fackelman, Vetstream Ltd, Chris Whitton

Bone biomechanics

Biomechanical behavior of bone

  • The response of bone to applied forces depends on its material properties, geometry, loading mode, loading rate and the frequency of loading.
  • An understanding of biomechanical forces at work on bone is essential for correct surgical fixation.

Tension

  • Distracting loads are applied at the ends of the bone.
  • Maximal tensile stress occurs on a plane perpendicular to the applied load   →   creating transversely orientated fracture lines.
  • The bone lengthens and narrows under tensile stress and failure occurs   →   debonding of cement, osteons are pulled out.
  • Tensile fractures tend to occur in proximal ulna   Ulna: fracture  , proximal sesamoid bone   Proximal sesamoid: fracture  , the patella   Patella: fracture  , the calcaneus   Tarsus: fracture.

Compression

  • Bone is strongest under compressive loads, compared to other loading modes.
  • Equal and opposite loads are applied toward each other at the ends of the bone.
  • With compression, bone shortens and widens and failure occurs obliquely through osteons.
  • This oblique line corresponds to the plane of maximum shear stress to which bone is less resistant than compressive stress.
  • Y-shaped fractures on the distal humerus   Humerus: fracture  and femur   Femur: physeal fracture  are the result of compressive forces.

Bending

  • Load is applied such that the bone bends on its axis   →   combination of tension and compression forces on opposite sides of the bone.
  • The bone fails firstly on the tension side and the fracture line travels toward the side of compression.
  • Shear forces then act in 45° direction on compressive side   →   butterfly fracture.

Torsion

  • Bone is forced to twist around its axis   →   torque produced within bone.
  • Shear stresses applied over the whole bone but the size of the stresses increases with increasing distance from the neutral axis (usually axis of rotation)   →   periosteal shear stresses are greatest parallel and perpendicular to the axis.
  • Bone first fails along the shear line, parallel with the long axis.
  • Second fracture line occurs along line of tensile stress   →   spiral fracture.

Rate dependency

  • Bone absorbs more energy when loads are applied at higher rates   →   horses training at slow speeds tend to sustain simple fractures but horses running at high speeds sustain comminuted fractures and more tissue trauma due to release of absorbed energy at the time of fracture.

Bone fatigue

  • With cyclic loading of bone, there is a release of strain energy that can cause microcracks along cement lines.
  • If cyclic loading is maintained there may be progressive microdamage in cortical bone.
  • In addition, bone microdamage is more likely to occur when there is muscle fatigue, ie the muscle is less able to absorb concussive stresses.
  • Repair processes may not be able to keep up with level of repeated microdamage even at low loads and may contribute to failure due to rapid resorption of bone.
  • The majority of fractures in racehorses are fatigue fractures.
  • Fracture on tensile surface propagates rapidly transversely   →   complete fracture.
  • Fracture on compressive surface   →   slow propagation of fracture   →   remodeling may occur before it becomes complete.
Print off the Owner factsheet on Fractures to give to your clients.

Fracture healing

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Assessing fracture healing

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Fixation methods

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Case selection

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Complications of fracture management

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Further Reading

Publications

Refereed papers
  • Recent references fromPubMed.
  • Misheff M M, Alexander G R & Hirst G R (2010)Management of fractures in endurance horses. Equine Vet Educ22(12), 623-630.
  • Hesse K L & Verheyen K L P (2010)Associations between physiotherapy findings and subsequent diagnosis of pelvic or hindlimb fracture in racing Thoroughbreds. Equine Vet J42(3), 234-239PubMed.
  • Martens A & Declercq J (2006)Fracture fixation in the standing horse: for surgeons who dare. Equine Vet Educ18(6), 314-315.

Other sources of information

  • Bramlage L (2004)Development of Fracture Management in the Horse.In: Proc BEVA Congress Handbook of Presentations. pp 27-28.
  • Auer J A (1999) EdEquine Surgery. W B Saunders, Philadelphia.
  • Nixon A J (1996) EdEquine Fracture Repair. W B Saunders, Philadelphia. ISBN: 0-7216-6754-6.
  • White N A & Moore J N (1990) Eds Lippincott, Philadelphia. ISBN: 0-397-50937-5.


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