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* [[Diaphyseal Fractures - Small Animal]]
===Causes of Fracture===
* [[Fractures - Bovine]]
* The causes of fracture fit into two distinct categories:
* [[Fractures - Equine]]
*# '''Fracture of trauma'''
* [[Non-diaphyseal Fractures - Small Animal]]
*#* Breakage of normal healthy bone due to excessive stress pressure of short duration.
*# '''Pathological fracture'''
*#* Breakage of bone weakened by some underlying metabolic, inflammatory or neoplastic condition.
===Description of a Fracture===
* There are various terms to describe a fracture's appearance.
* Separation of the ends of the fracture may be '''complete''' or '''incomplete'''.
* When there is no penetration of the overlying skin, a the fracture is described as '''closed'''.
* When the sharp ends penetrate the overlying skin, the fracture is '''compound'''.
** In this scenario there is the danger of introducing infection.
*'''Comminuted''' describes a fracture where there are multiple small fragments of bone at the site of breakage.
* Where the edges of the fracture are impacted into each other, the fracture is said to be '''compressed'''.
* When one side of the fracture is depressed below the plane of the other, the term '''depressed''' is used.
** This occurs in the flat bones of the skull.
* '''Microfractures''' are fractures that are only visible on histological section as cracks in the bone.
** Grossly, there might be evidence of some haemorrhage in the area.
===Fracture Repair===
[[Image:fracture repair.jpg|thumb|right|150px|Fracture repair (Courtesy of BioMed Archive)]]
* On breakage, there is rupture of the periosteal, cortical and medullary vessels, causing:
** A blood clot in the breakage area.
*** Fibrin is the important component.
** Local necrosis of tissue supplied by these vessels.
*** This lowers the local pH.
* The fate of the blood clot depends upon its location.
** The periosteal portion is lysed and disappears;
** The medullary portion is removed by macrophages.
* The necrotic material is removed by phagocytosis.
** Necrotic [[Bone Marrow - Anatomy & Physiology|bone marrow]] is removed by [[Macrophages|macrophages]].
*** This is a fairly rapid process.
** Osteoclasts remove necrotic bone.
*** This is a slow process.
* On the periosteal side, the periosteum proliferates into the clot.
** Forms a fibrous collar around the bone called the soft callus.
* The cells in the inner aspect of the soft callus, particularly those near the fracture fragments, differentiate into osteoblasts. [[Image:fracture callus.jpg|thumb|right|150px|Fracture callus (Courtesy of BioMed Archive)]]
** Grow across the divide between the two fragments, laying down coarse woven bone.
* The woven bone laid is known as the hard callus.
** This periosteal coarse bone is of utmost importance in repair.
*** It is responsible for much of the strength of the fracture repair.
** This is replaced over a period of time by mature compact bone.
*** Aligns itself according to the stresses applied to it.
* Periosteal cells that are further away from the fracture fragments differentiate into cartilage-producing cells.
** Produce a cone of cartilage between the two fragments.
*** As the local pH changes to more alkaline conditions, this cartilage undergoes calcification, with invasion by blood vessels and osteoblasts.
**** The cartilage is replaced by bone - endochondral ossification.
* On the medullary side, the endosteum proliferates and invades the clot, laying down bone.
** This bone may totally occlude the medulla and is later remodelled to reconstitute a patent lumen.
===Complications===
* There are several possible complications that may arise in the repair of bone.
# Inadequate immobilisation of the fractured ends will lead to incomplete repair by callus formation.
#* An intervening mass of fibrocartilage remains, forming a false joint.
#** In some cases the false joint can even appear to form a synovial lining.
#* If the fractured ends are sufficiently far apart, no substantial callus forms.
#** The intervening space is taken up by connective tissue organisation.
# Failure to align the fractured ends in proper apposition to one another will produce excessive callus.
#* This takes a longer time to be remodelled by the adult compact bone.
# [[Healing and Repair - Pathology#Description of a Fracture|Comminution]] delays healing due to persistent irritation.
# Infection delays healing due to the effects of the toxins on theproliferating cells.
#* May give rise to a systemic infection affecting the rest of the body.