Biomechanical basis of bone fracture and fracture repair

Andy P. Moores

doi:10.22233/9781910443279.4

British Small Animal Veterinary Association , 20 (2016); https://doi.org/10.22233/9781910443279.4

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Biomechanical basis of bone fracture and fracture repair

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Author: Andy P. Moores
From: BSAVA Manual of Canine and Feline Fracture Repair and Management
Item: Chapter 4, pp 20 - 31
DOI: 10.22233/9781910443279.4
Copyright: © 2016 British Small Animal Veterinary Association
Publication Date: January 2016

Abstract

Fracture patterns vary hugely between patients: one dog’s tibial fracture may be different to another’s. Even if the fracture pattern is similar, the options for fracture stabilization and how a fixation system is implemented are likely to differ between patients depending on factors such as the age, size and temperament of the dog or cat. This chapter looks at mechanical concepts; forces acting on bones; fracture of bone; mechanics of bone healing; mechanics of fixation systems.

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Figures

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4.1

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4.1 Load–deformation curve. The gradient of the line represents the stiffness of the structure.

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4.2

The effect of a longer plate in resisting a bending force. The medial force on the distal fracture fragment will create a bending moment acting about the distal edge of the proximal fracture fragment. This moment must be resisted by the opposite moment (FPUd). The longer the distance between the fracture and first screw (d) is, the greater FPUd will be. FPU = screw pull-out force. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2016 British Small Animal Veterinary Association

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4.2 The effect of a longer plate in resisting a bending force. The medial force on the distal fracture fragment will create a bending moment acting about the distal edge of the proximal fracture fragment. This moment must be resisted by the opposite moment (FPUd). The longer the distance between the fracture and first screw (d) is, the greater FPUd will be. FPU = screw pull-out force. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.

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4.3

Diagrammatic representation of the force categories considered when evaluating a fracture, a fixation method or a repaired fracture. Weight bearing and muscle contractions contribute to compressive forces down the long axis. When the bone is at an angle to the ground or when the muscles pull more on one side than on the other, bending will be induced. This may be in any direction. Torsion will occur when the mass of the body changes direction while the limb is bearing weight. Tensile forces exist where soft tissues such as tendons or ligaments attach to the bone. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2016 British Small Animal Veterinary Association

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4.3 Diagrammatic representation of the force categories considered when evaluating a fracture, a fixation method or a repaired fracture. Weight bearing and muscle contractions contribute to compressive forces down the long axis. When the bone is at an angle to the ground or when the muscles pull more on one side than on the other, bending will be induced. This may be in any direction. Torsion will occur when the mass of the body changes direction while the limb is bearing weight. Tensile forces exist where soft tissues such as tendons or ligaments attach to the bone. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.

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4.4

Fracture patterns of bone. (a) Axial compression results in an oblique fracture. (b) Pure tension results in a transverse fracture. (c) Torsion results in a spiral fracture. (d) Bending results in a transverse fracture with or without a butterfly fragment. (e) Bending in combination with compression will result in a transverse fracture with a larger butterfly fragment. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2016 British Small Animal Veterinary Association

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4.4 Fracture patterns of bone. (a) Axial compression results in an oblique fracture. (b) Pure tension results in a transverse fracture. (c) Torsion results in a spiral fracture. (d) Bending results in a transverse fracture with or without a butterfly fragment. (e) Bending in combination with compression will result in a transverse fracture with a larger butterfly fragment. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.

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4.5

Types of pins for external fixators. A: Smooth pins rely on friction with the bone or bracing against other pins in the frame. B: Negative-profile threaded pins engage the bone more securely but are susceptible to failure at the shaft–thread junction. C: Ellis pins have a small length of negative-profile thread, designed to engage only one cortex. The weak point of the pin is protected from bending forces. D: Positive-profile threaded pins have a larger major diameter, so holding strength is increased. Because the shaft diameter is not reduced at the start of the thread, they are better able to resist the cyclic bending forces associated with weight bearing. Drawn by Vicki Martin Design, Cambridge, UK and reproduced with her permission. © 2016 British Small Animal Veterinary Association

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4.5 Types of pins for external fixators. A: Smooth pins rely on friction with the bone or bracing against other pins in the frame. B: Negative-profile threaded pins engage the bone more securely but are susceptible to failure at the shaft–thread junction. C: Ellis pins have a small length of negative-profile thread, designed to engage only one cortex. The weak point of the pin is protected from bending forces. D: Positive-profile threaded pins have a larger major diameter, so holding strength is increased. Because the shaft diameter is not reduced at the start of the thread, they are better able to resist the cyclic bending forces associated with weight bearing. Drawn by Vicki Martin Design, Cambridge, UK and reproduced with her permission.

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4.6

External skeletal fixator frame designs. (a) Type 1a frame (unilateral, uniplanar frame). (b) Type 1b frame (unilateral, biplanar frame). (c) Modified type 2 (or type 2b) frame (bilateral, uniplanar frame). (d) Type 2 (or type 2a) frame (bilateral, uniplanar frame). (e) Type 3 frame (bilateral, biplanar frame). Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2016 British Small Animal Veterinary Association

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4.6 External skeletal fixator frame designs. (a) Type 1a frame (unilateral, uniplanar frame). (b) Type 1b frame (unilateral, biplanar frame). (c) Modified type 2 (or type 2b) frame (bilateral, uniplanar frame). (d) Type 2 (or type 2a) frame (bilateral, uniplanar frame). (e) Type 3 frame (bilateral, biplanar frame). Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.

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4.7

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4.7 Different screw types. From left to right: 3.5 mm cortical screw; 3.5 mm shaft screw; 3.5 mm locking screw; 4 mm partially threaded cancellous screw.

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4.8

Plate functions. (a) Compression plate. (b) Neutralization plate. (c) Bridging plate. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2016 British Small Animal Veterinary Association

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4.8 Plate functions. (a) Compression plate. (b) Neutralization plate. (c) Bridging plate. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.

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4.9

(a) Mediolateral postoperative radiograph of a distal radial fracture in a Maltese stabilized with a cranial T-plate. (b) The caudal cortex was not compressed, increasing the stress on the plate, which resulted in fatigue failure of the plate in the postoperative period. © 2016 British Small Animal Veterinary Association

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4.9 (a) Mediolateral postoperative radiograph of a distal radial fracture in a Maltese stabilized with a cranial T-plate. (b) The caudal cortex was not compressed, increasing the stress on the plate, which resulted in fatigue failure of the plate in the postoperative period.

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4.10

Cerclage wire tightening methods. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. A: Twist method. B: Single loop method. C: Double loop method. © 2016 British Small Animal Veterinary Association

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4.10 Cerclage wire tightening methods. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. A: Twist method. B: Single loop method. C: Double loop method.

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4.11

Tension-band wire. The pull of the gluteal muscles is resisted by the tension-band wire. The two forces combined result in a compressive force at the fracture (large pale arrow). Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. © 2016 British Small Animal Veterinary Association

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4.11 Tension-band wire. The pull of the gluteal muscles is resisted by the tension-band wire. The two forces combined result in a compressive force at the fracture (large pale arrow). Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.