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.

Fracture healing

Assessing fracture healing

Fixation methods

Case selection

Complications of fracture management

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Donati B, Fürst A E, Del Chicca F & Jackson M A (2020) Plate removal after internal fixation of limb fractures: a retrospective study of indications and complications in 48 horses. Vet Comp Orthop Traumatol PubMed.
• Johnston A S, Sidhu A B S, Riggs C M, et al (2020) The effect of stress fracture occurring within the first 12 months of training on subsequent race performance in Thoroughbreds in Hong Kong. Equine Vet J PubMed.
• Boorman S, Richardson D W, Hogan P M, et al (2020) Racing performance after surgical repair of medial condylar fracture of the third metacarpal/metatarsal bone in thoroughbred racehorses. Vet Surg 49 (4), 648-658 PubMed.
• Martig S, Hitchens P L, Lee P V S & Whitton R C (2020) The relationship between microstructure, stiffness and compressive fatigue life of equine subchondral bone. J Mech Behav Biomed Mater 101 PubMed.
• Johnson K A (2019) Risks and outcomes of equine flat bone fractures. Vet Comp Orthop Traumatol 32 (4) PubMed.
• Whitton R C, Ayodele B A, Hitchens P L & Mackie E J (2018) Subchondral bone microdamage accumulation in distal metacarpus of Thoroughbred racehorses. Equine Vet J 50 (6), 766-773 PubMed.
• Levine D G & Aitken M R (2017) Physeal fractures in foals. Vet Clin North Am Equine Pract 33 (2), 417-430 PubMed.
• Misheff M M, Alexander G R & Hirst G R (2010) Management of fractures in endurance horses. Equine Vet Educ 22 (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 J 42 (3), 234-239 PubMed.
• Martens A & Declercq J (2006) Fracture fixation in the standing horse: for surgeons who dare. Equine Vet Educ 18 (6), 314-315.

Other sources of information

• Auer J A & Stick J (2019) Equine Surgery. 5th edn. W B Saunders, USA.
• Nixon A J (2020) Ed. Equine Fracture Repair. 2nd edn. Wiley-Blackwell, USA. ISBN: 978-0-813-81586-2.