38 2 CHAPTER 2: LITERATURE REVIEW 2.1 Introduction to bone biology Bone is a specific type of connective tissue that has remarkable healing capabilities. Its healing process is unique compared to...

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38    2 CHAPTER 2: LITERATURE REVIEW 2.1 Introduction to bone biology Bone is a specific type of connective tissue that has remarkable healing capabilities. Its healing process is unique compared to other tissues whose healing results in scare formation. Bone healing is a complicated process which involves several biological, biophysical and biochemical mechanisms at microscopic and macroscopic levels (Harwood et al., 2010). To have a better understanding of the bone healing mechanisms it is essential to review bone biology. 2.1.1 Bone function Bone, as the main element of the skeleton system, has different functions. It provides a framework for the body and thereby supports different tissues. Most bones are attached to skeletal muscles by tendons and produce body movements by contraction of the muscles. Bones, especially the ribs, chest and skull, also protect vital organs from damage. In addition, bone is the major store for minerals and the marrow within cancellous bone forms red and white blood cells. 2.1.2 Bone tissue morphology At the microscopic level, bones are classified as cortical (compact) or cancellous (trabecular). Cortical bone has a dense and sturdy structure with 5-10% porosity and forms the outer shell of a whole bone. In contrast, cancellous bone has a spongy structure with 50-95% porosity. It is generally bounded within the cortical bone covering. It has a three dimensional interconnected rod or plate of bone called trabeculae. This porous network is filled with bone marrow and blood vessels (Seeley et al., 2002, Wang et al., 2010). Figure 2-1 and Figure 2-2 illustrate the structure of cancellous and compact bone respectively.   Figu Figu bon ure 2-1 Can ure 2-2 Com ne layers (lam ncellous bon trabeculae mpact bone i mellae) whi i ne is formed e. Figure is a is mainly co ich surroun s adapted fr 39  d by 3D inte adapted from omposed of nd a central c rom Seeley erconnected m Seeley et f osteons. O canal conta et al. (2002 d rod or plat t al. (2002) steon consis ining blood 2) te of bone ca sts of conce d vessels. Fi alled entric gure   epiph As sh conn that undif all ca endo struc The shaft hysis or end hown in Fi nective tissu contains b fferentiated avities with osteum. It i cture of peri of a long d of the lon gure 2-3, th ue known as blood vesse d stem cells hin the bone is primarily iosteum (Co Figure Figu bone (diap ng bone is c he external s periosteum els while t . (Cowin, 2 e including y a stem c owin and Do 2-3 Microsc ure is adapte 40  physis) con composed m surface of m. The outer the inner l 2001, Seeley g medullary cell layer a oty, 2007). copic struct ed from See nsists prima mainly of ca all bones is r layer of pe ayer contai y et al., 200 y cavity of t and does no ture of a lon eley et al. (2 arily of cor ancellous bo s covered b eriosteum is ins bone c 02). The in the diaphys ot contain ng bone. 2002) rtical bone one (Figure by a thin lay s a fibrous t cells as we nternal surfa sis is covere the collage . The 2-3). yer of tissue ell as ace of ed by enous 41    2.1.3 Bone matrix Bone is composed of extracellular matrix (ECM), different types of cells and interstitial fluid. The bone ECM consists of organic (mainly collagenous and non- collagenous proteins) as well as mineral (mainly hydroxyapatite crystals) constituents. The unique stiffness and strength of bone stem from its composite structure. The mineral constituent of bone which occupies approximately 65% of the ECM mass is responsible for bone compressive strength. On the other hand its organic constituent provides tensile strength and flexibility for bone (Seeley et al., 2002). 2.1.4 Bone cells There are three major types of bone cells which have different origins and functions. Osteoblasts are the critical bone cell types which are derived from mesenchymal stem cells (MSCs). They synthesize bone matrix through a process called ossification and produce collagenous and other organic components of bone tissue. When the osteoblasts become confined in bone matrix, they are mature cell type called osteocytes which occupy small cavities (lacuna) within the matrix. They are the most abundant bone tissue cell type. Different from osteoblasts, osteocytes are inactive and cannot divide, however they are able to produce components required for the maintenance of bone matrix (Seeley et al., 2002). Osteoclasts are huge and multinuclear cells that are responsible for breakdown of bone matrix. They degrade mineral and organic components of bone matrix through a process of resorption (Seeley et al., 2002). Unlike osteoblasts, osteoclasts are not originated from MSCs and are derived from hematopoietic stem cells in bone marrow (Cowin, 2001). Bone also has a small number of committed osteoprogenitor cells and uncommitted undifferentiated MSCs which have a critical role in bone healing. They are generally located in periosteum (Einhorn, 1998). 2.1.5 Bone development During the fetal development, bone forms through two main processes: intramembranous and endochondral ossification. Bone tissue is synthesised directly by osteoblasts through intramembranous ossification. However in endochondral ossification, cartilage is firstly formed and then is calcified and replaced by bone. Both 42    types of ossification generate woven bone that has randomly oriented collagen fibres. After bone formation, osteoclasts degrade woven bone and osteoblasts generate a new matrix and form lamellar bone through the process of bone remodelling. Lamellar bone has an organized matrix structure which enhances the strength of the bone (Doblaré et al., 2004). Intramembranous ossification is responsible for the formation of flat bones of the skull, the mandibles and the clavicles. It begins at approximately the eighth week of fetal development and continues until two years of age. It is also involved in the fracture healing process. Intramembranous ossification begins when MSCs within embryonic mesenchyme or medullary cavity of a fractured bone differentiate into osteoprogenitor cells which are specialized to become osteoblasts. The osteoblasts form bone matrix and differentiate into osteocytes trapped within the matrix. This results in formation of many tiny trabeculae of woven bone. The trabeculae join together and create trabecular bone. As the trabecular bone develops, the periosteum is formed around it by the differentiated MSCs. The osteoblasts from the periosteum generate the outer surface of the bone. The compact bone is formed when the space between the trabeculae is filled by osteons which are the unit of compact bone. After the remodelling process, the final structure of bone is formed (Cowin, 2001, Seeley et al., 2002). Endochondral ossification forms bones at the base of the skull and most of the remaining skeletal system bones. Cartilage formation begins at fourth week of fetal development. Some parts of this cartilage start to form bone at around eighth week of the embryonic development. However endochondral ossification of some of the cartilages may continue until eighteen years of age. Endochondral ossification starts by aggregation of MSCs. These cells differentiate into chondroblast to form a hyaline cartilage model. As chondroblasts become trapped within the cartilage matrix, they differentiate into chondrocytes. Then, chondrocytes near the centre of cartilage increase in size and the cartilage matrix enlarges and becomes calcified. Since nutrient cannot reach the chondrocytes in the calcified region, they die and leave enlarged lacunae. At the same time, the blood vessels invade the perichondrium around the cartilage and MSCs inside the perichondrium differentiate into osteoblasts. The perichondrium turns to periosteum when osteoblasts start to form compact bone on the surface of the cartilage model. At the next stage, the blood vessels 43    of the periosteum increases and invade the lacunae. The connective tissue of the blood vessels conveys osteoblasts and osteoclasts from the periosteum to form and develop bone matrix in the calcified region. This area of bone development is called primary ossification centre. As bone development progresses, osteoclasts remove the bone in the centre of diaphysis and create medullary cavity. The diaphysis of long bones is the primary ossification centre whereas epiphysis is the location of secondary ossification. Although primary ossification starts during the early fetal development, secondary ossification usually appears one month before birth and continues after that to replace the cartilage by bone. However a layer of the original cartilage remains in the epiphyseal plate and on articular cartilage. Articular cartilage which covers the articular ends of the bone facilitates bones movement at a joint. Epiphyseal plates which separate the epiphysis from diaphysis are the location of bone formation during bone length growth. Between 12 and 25 years of age, epiphyseal plate is ossified and becomes epiphyseal line (also known as physeal scar) and therefore the growth in bone length ceases (Seeley et al., 2002). 2.1.6 Bone remodelling Bone remodelling renews and maintains bone tissue. The woven (immature) bone which is developed during the bone formation and fracture healing does not have an organized structure and is not as strong as a mature bone. Through the remodelling process, osteoclasts degrade woven bone and osteoblasts generate a new bone matrix and form structurally organized and mechanically strong lamellar bone. Bone remodelling also occurs throughout life and renews and maintains bone structure based on its biomechanical environment. Bone remodelling repairs micro-damages in bone and adapts bone structure to the mechanical stress. It also regulates calcium homeostasis and haematopoiesis of bone. Bone senses and responds to its mechanical stresses and thereby strengthens the area of increased stress. However lower stress values can lead to bone resorption, a situation which can be seen in osteoporotic patients, astronauts returning from space and patients confined to bed for prolonged periods. This remodelling process occurs in a coupled activity of osteoblasts and osteoclasts in bone modelling unit (Marsell and Einhorn, 2011). Osteocytes are believed to sense micro- stress within bone tissue and stimulate and direct bone remodelling (Rubin et al., 2006). 44    2.2 Bone fracture Bone fracture normally occurs when a load induces a mechanical stress that exceeds the bone strength. This type
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Answer To: 38 2 CHAPTER 2: LITERATURE REVIEW 2.1 Introduction to bone biology Bone is a specific type of...

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Chapter 2: Literature Review
Contents
2.1 Introduction to bone biology    4
2.1.1 Bone function    4
2.1.2 Bone tissue morphology    4
2.1.3 Bone matrix    6
2.1.4 Bone cells    7
2.1.4 Bone development    7
2.1.6 Bone remodelling    9
2.2 Bone fracture    9
2.2.1 Fracture geometry    10
2.3 Bone fracture healing    11
2.3.1 Indirect healing    11
Hematoma formation (Inflammation)    12
Callus formation    12
Callus ossification    12
Bone remodelling    13
2.3.2 Direct healing    14
Contact healing    14
Gap Healing    15
2.3.3 Delayed healing and non-union    16
2.3.4 Importance of early stage of bone healing    17
2.4 Factors influencing fracture healing    17
2.4.1 The origins and role of mesenchymal stem cells (MSCs) during bone healing    18
2.4.2 Influence of growth factors on fracture healing    19
2.4.3 Angiogenesis during fracture healing    20
2.4.4 Mechanical factors in fracture healing    21
2.1 Mechanoregulation theories of bone healing    21
2.5.1Computational modelling of bone healing    24
2.6 Biomechanics of fracture fixation    28
2.6.1 Cast splints    28
2.6.1 External fixation    28
2.6.3 Internal fixation    30
2.1 Introduction to bone biology
Bone of the specific connective tissue because its healing process is unique compared to other tissues and bone healing does not lead to scare formation. Bone healing is a complex process, involving various biophysical, biological and biochemical processes at macroscopic as well as microscopic levels (Harwood et al., 2010). For understanding the mechanisms of bone healing in a better manner, it is important to review bone biology.
2.1.1 Bone function
There can be different functions of bone because it is the major element of skeleton system. It supports different tissues by providing framework for the body. Bones are attached to the skeleton muscles with the help of tendons, which further facilitate body movements through muscle contraction. Bones protects the skull, chest, ribs and many other vital organs from any injury or damage. Further, bone is the major store for minerals and the marrow within cancellous bone forms red and white blood cells.
2.1.2 Bone tissue morphology
Bones at the microscopic level are classified as cance
llous (trabecular) or cortical (compact). Cortical bone has a dense and sturdy structure with 5-10% porosity and forms the outer shell of a whole bone. However, In contrast, cancellous bone has a spongy structure with 50-95% porosity and it is usually bound within cortical bone covering. Cortical bone has a three dimensional interconnected plate or rod called trabeculae. Blood vessels and bone marrow are found to fill this porous network (Seeley et al., 2002, Wang et al., 2010). Figure 2-1 and Figure 2-2 illustrate the structure of cancellous and compact bone respectively.
Figure 2-1 Cancellous bone is formed by 3D interconnected rod or plate of bone called trabeculae. Figure is adapted from Seeley et al. (2002)
Figure 2-2 Compact bone is mainly composed of osteons. Osteon consists of concentric bone layers (lamellae) which surround a central canal containing blood vessels. Figure is adapted from Seeley et al. (2002)
Diaphysis is the shaft of the long bone, which consists primarily of cortical bone. The end of the long also known as epiphysis is mainly composed of cancellous bone (Figure 2-3). As reflected in Figure 2-3, ‘periosteum’ is the thin layer of connective tissue, which covers the outer surface of all the bones. The outer layer of periosteum is a fibrous tissue that contains blood vessels while the inner layer contains bone cells as well as undifferentiated stem cells. (Cowin, 2001, Seeley et al., 2002). ‘Endosteum’ covers internal surface of all the cavities inside the bones along with the medullary cavity of diaphysis. It does not contain the characteristic collagenous structure of periosteum rather; it is primarily a stem cell layer (Cowin and Doty, 2007).
Figure 2-3 Microscopic structure of a long bone. Figure is adapted from Seeley et al. (2002)
2.1.3 Bone matrix
Bone is composed of extracellular matrix (ECM), different types of cells and interstitial fluid. ECM of the bone consists of mineral (mainly hydroxyapatite crystals) as well as, organic (mainly collagenous and non- collagenous proteins) constituents. The unique stiffness and strength of bone stem from its composite structure. Bone’s mineral constituents occupy around 65% of the ECM and it mainly contributes to the compressive strengths of bone. On the other hand, its organic constituent provides tensile strength and flexibility for bone (Seeley et al., 2002).
2.1.4 Bone cells
Based upon the functions and origins, there are three different types of bone cells. The critical type of bone cell is known as Osteoblast, which is derived from mesenchymal stem cells (MSCs). They apply a process called ossification for synthesising the bone matrix and producing collagenous as well as different types of organic elements of bone tissue. Osteocytes are the mature cell type of osteoblasts, when they are confined in a bone matrix. Osteocytes occupy small cavities (lacuna) within the matrix and they are one of the most abundant bone tissue cells. Further, osteocytes mostly are inactive and do not divide, they can produce components required for bone matrix maintenance (Seeley et al., 2002)
The multinuclear huge cells are known as Osteoclasts, which participate in the breakdown of bone matrix. They do so by breaking the components of the bone matrix, which are organic and mineral in nature, with the help of a process called resorption (Seeley et al., 2002). Osteoclasts do not originate MSCs instead; they are derived from the hematopoietic stem cells within the bone marrow (Cowin, 2001). Bone also has a small number of committed osteoprogenitor cells and uncommitted undifferentiated MSCs, which have a critical role in bone healing. They are generally located in periosteum (Einhorn, 1998).
2.1.4 Bone development
Endochondral and intramembranous ossification are the major processes through which bone formation occur during foetal development. Bone tissue is synthesized directly by osteoblasts through intramembranous ossification. However, during the process of endochondral ossification, cartilage formation is followed by calcification and is ultimately replaced by bone. Woven bone with randomly oriented collagen fibres may be obtained from both of the ossification processes. Woven bone is degraded after bone formation and is replaced by a new matrix and lamellar bone generated from osteoblasts through the process of bone remodelling. Bone strength may also be enhanced due to organized matrix structure of lamellar bone (Doblaré et al., 2004).
Formation of the clavicles, mandibles and skull bones occurs through the process of intramembranous ossification. It begins at approximately the eighth week of foetal development and continues until two years of age. Further, it may also be involved in fracture healing process. Intramembranous ossification begins when MSCs within embryonic mesenchyme or medullary cavity of a fractured bone differentiate into osteoprogenitor cells, which are specialized to become osteoblasts. Osteocytes, which are trapped within the matrix, are formed from the osteoblasts from the bone matrix and this leads to the formation of numerous tiny trabeculae of the woven bone. The joining of trabeculae then forms Trabecular bone and differentiated MSCs lead to the development of periosteum around it. Outer surface of the bone is generated from osteoblasts of the periosteum. The compact bone is formed when the space between the trabeculae is filled by osteons, which are the unit of compact bone, and the final structure of bone is formed after the completion of remodelling process (Cowin, 2001, Seeley et al., 2002).
Bones at the base of skull along with remaining skeletal bones are formed by the process endochondral ossification. During the fourth week of foetal development, cartilage formation may begin and around eighth week of embryonic development some parts of this cartilage may start bone formation. However, endochondral ossification of some of the cartilages may continue until eighteen years of age.
Endochondral ossification starts by aggregation of MSCs. These cells differentiate into chondroblasts to form a hyaline cartilage model. As chondroblasts become trapped within the cartilage matrix, they differentiate into chondrocytes. Then, chondrocytes near the centre of cartilage increase in size and the cartilage matrix enlarges and becomes calcified. Due to lack of nutrients in chondrocytes in the calcified region, death of these cells may occur, which leads to the formation of enlarged lacunae. At the same time, the blood vessels invade the perichondrium around the cartilage and MSCs inside the perichondrium differentiate into osteoblasts. Perichondrium converts into periosteum as soon as osteoblasts initiate the formation of compact bone on the surface of the cartilage model. During next stage, increase in the blood vessels of periosteum occurs, which may further penetrate inside lacunae. The connective tissue of the blood vessels conveys osteoblasts and osteoclasts from the periosteum to form and develop bone matrix in the calcified region. This sector of bone development is known as primary ossification Centre and with the progress in bone development, medullary cavity may be created due to removal of bone by osteoclasts in the centre of diaphysis.
The diaphysis of long bones is the primary ossification centre whereas epiphysis is the location of secondary ossification. Early foetal development may mark the starting of primary ossification, secondary ossification usually appears one month before birth and continues after that to replace the cartilage by bone. A layer of original cartilage remains in the epiphyseal plate and on articular cartilage. Articular cartilage facilitates the bones movement at a joint.
Epiphyseal plates are the location of bone formation during the growth of bone length. Epiphyseal plate may be ossified into epiphyseal line (also known as physeal scar) between the developmental age of 12 and 25 years and this leads to the ceasing of bone length growth among adults (Seeley et al., 2002).
2.1.6 Bone remodelling
    Bone tissue is maintained by through bone remodelling. The woven (immature) bone, which is developed during the bone formation and fracture healing, does not have an organized structure and is not as strong as a mature bone. Through the remodelling process, osteoclasts degrade woven bone and osteoblasts generate a new bone matrix, form structurally organized, and mechanically strong lamellar bone. Bone remodelling renews as well as maintains bone structure based upon biochemical environment throughout life by repairing the micro damages and adapting bone structure according to the mechanical stress. Further, it also regulates haematopoiesis and calcium homeostasis of bone. Bone may sense and respond to the mechanical stress by strengthening the area of increased stress. Bone resorption may be observed due to lower stress values in osteoporotic patients, patients confined to bed for longer periods and osteoporotic patients. Osteoblasts and osteoclasts functions in collaborative action during remodelling process in bone modelling unit (Marsell and Einhorn, 2011). Osteocytes are believed to sense micro- stress within bone tissue, stimulate, and direct bone remodelling (Rubin et al., 2006).
2.2 Bone fracture
When the load exceeds the mechanical strength of bone, bone fracture may occur. An accident, which is an external impact can cause fracture by a muscular contraction, which is known as non- traumatic or pathological fracture. Muscle contraction applied on a weakened bone due to osteoporosis or some other diseases such as tumours may lead to non-traumatic fracture. . Osteoporosis is the main cause of pathological fractures in the elderly and is more common in women than men are. Bone tumours are the other main cause of pathological bone fracture as they can change the material properties of the bone and induces stress concentration. It would be essential to consider that osteoporosis not only increases the risk of fractures among elderly people but also the inability of their soft tissue to absorb the accidental load can increase the fracture risk (Doblaré et al., 2004).
Fatigue may also induce bone fracture also (known as stress fracture). Micro-damages may be induced in the bone due to prolonged loading and if the micro-cracks grow faster than the process of bone remodelling, it may lead to formation of crack or bone fracture. This fracture type is usually seen among people who have extreme and continual physical activities including workers and athletes (Doblaré et al., 2004).
2.2.1 Fracture geometry
Bone fractures can be classified based on the location and morphology of the fracture. Simple, wedge and complex fractures (Figure 2-4) are the three types of diaphyseal fracture of long bone as classified by AO foundation. A simple fracture has two fragments with one fracture line, which a perfect cortical contact can be induced after reduction. Wedge and complex fractures both may have three or even more fragments. The main fragment of wedge fracture, unlike complex fracture, may have a contact after reduction. Based upon fracture pattern, each fracture type can be divided into 3 groups that is, transverse, oblique or spiral. Bending forces may produce oblique and transverse fracture while twisting or torsional forces may lead to spiral fractures. Similarly, a wedge or complex fracture can have a spiral geometry (resulted from torsional loads), bending/segmental geometry (resulted from bending loads) or multifragmentary/irregular geometry (resulted from large and complex loads Muller et al., 1987).    
Figure 2-4 Classification of fractures in diaphysis of long bones, Adapted from (AOTRAUMA)
2.3 Bone fracture healing
Following a fracture, bone can undergo two types of healing processes depending on the fracture gap size and mechanical stability of the fracture site.
2.3.1 Indirect healing
Indirect fracture healing also known as secondary healing is the natural process of bone healing which is the most common type of bone repair (Harwood et al., 2010, Marsell and Einhorn, 2011). A callus tissue, which act as biological splint could be considered as the characteristic of this healing type. Indirect healing occurs in fractures stabilized by relatively flexible fixation methods such as non-operative fracture treatment (e.g. casting technique) as well as some types of operative fixation strategies which allows a degree of movement in the fracture site (i.e. external fixations, intramedullary nailing, and locking plate) (Marsell and Einhorn, 2011, Claes et al., 2012a). As reflected in Figure 2-5, there may be four overlapping phases in indirect fracture healing: hematoma formation, callus formation, callus ossification and bone remodelling.
Hematoma formation (Inflammation)
Hematoma formation occurs at the fracture site due to damage of blood vessels and this leads to clot formation, which prevents further bleeding. Due to inadequate blood supply at the fracture site, bone cells may die leading to the swelling and inflammation. Hematoma is rich in platelets and macrophages, which release different signaling molecules such as cytokines and growth factors that initiate and direct the cellular events during fracture healing (Harwood et al., 2010). Hematoma may have osteogenic capacity during early stages of its formation (Mizuno et al., 1990). Multipotent MSCs in hematoma have been reported through experimental studies and these cells may originate primarily from endosteum, periosteum, and bone marrow. Some of them are derived from the surrounding soft tissues as well as the systemic circulation (Postacchini et al., 1995, Iwaki et al., 1997, Oe et al., 2007).
Further, blood vessels from the surrounding soft tissues penetrate the hematoma in order to deliver mononuclear phagocytes, which degrades dead bone tissues and cell debris during inflammation phase. During the first 24 hours, highest levels of acute inflammatory response may be observed and it completes by the end of first week due to dissolution of hematoma (Marsell and Einhorn, 2011, Griffon, 2005).
Callus formation
After a few days of fracture, early fracture callus may be formed, which is a granulated tissue formed from the synthesis of collagen fibers at the fracture site. Granulation tissue can elongate to twice its original size and can withstand a relatively large interfragmentary movement (IFM) which is present during the early stage of healing. Intramembranous ossification and endochondral ossification are the two simultaneous processes, which leads to differentiation of MSCs. This differentiation further helps to create a suitable environment for callus ossification (Harwood et al., 2010).
Callus ossification
Intramembranous ossification may start at the periosteum in order to form a hard callus during first week of healing (Einhorn, 1998, Claes et al., 2012a). Direct bone formation starts at the area where the tissue strain is small enough to allow vascularization and differentiation of periosteal MSCs to osteoblasts (Claes et al., 2012a). During the same time period, soft callus formation occurs through the MSCs recruited from external soft tissue, periosteum, cortex and bone marrow. These differentiate into chondroblast in order to form a cartilage tissue around and within the fracture site called soft callus (Einhorn, 1998, Marsell and Einhorn, 2011). Moreover, granulation tissue and fibrous connective tissue may be formed between the cartilage wedges inside the fracture gap (Claes et al., 2012a). The cartilage may undergo calcification approximately 10 days after healing, when the cartilage tissue becomes abundant in the fracture site. Two weeks after the healing, hypertrophic chondrocytes become the dominant cell type in the cartilaginous callus (Einhorn, 1998). Hypertrophic chondrocytes may further release calcium and undergo the process of apoptosis in order to form calcified cartilage. This process is similar to the process of endochondral ossification, which occurs in growth plates (Einhorn, 1998, Claes et al., 2012a).
The callus is mainly composed of calcified cartilage after 4-5 weeks of fracture (Einhorn, 2005). Tissue strain gets reduced due to bridging of fracture gap by the calcified callus and this allows invasion of blood vessels within calcified cartilage (Claes et al., 2012a). These blood vessels also carry monocytes and MSCs and the MSCs differentiate into osteoblasts in order to form bone tissue. The monocytes differentiate to osteoclasts-like cells to absorb the calcified cartilage.
The callus after 6-7 weeks of fracture is usually composed of a combination of calcified cartilage and woven bone (Einhorn, 2005). During this stage, the strain is low, which allow replacement of the fibrous and granulation tissue within the fracture gap through intramembranous ossification (Claes et al., 2012a). These processes ay continue until woven bone replaces all the calluses. The indication of successful healing, also known as bony bridging many occur usually between week 8-17 and is highly dependent upon depending on the fracture geometry, fixation type and the patient conditions (Claes et al., 2012a).
Bone remodelling
Einhorn, (1998) suggested that the woven bone may undergo remodelling after the bridging of the fracture gap in order to form a structurally organized lamellar bone with a competent mechanical strength. The peripheral and medullary callus is resorbed during the remodelling process leading to the restoration of original form of the diaphysis. It may take years for the remodelling process to occur in humans (Claes et al., 2012a).
Figure 2-5 Phases of bone healing, adapted from Seeley et. al (Seeley et al., 2002)
2.3.2 Direct healing
Primary healing or the process of direct healing is a rare process and it mainly occurs under extremely rigid stabilization of fracture, which may be achieved through internal rigid fixations such as lag screws and compression plate. During this process. Bone healing occurs directly between bone fragments by intracortical remodelling. However, the process of osseointegration of the bone fragments may take months or years in order to complete. There is no callus formation during this type of fracture healing compared to the process of indirect healing (Cowin, 2001, Wang et al., 2010, Marsell and Einhorn, 2011). Direct bone healing may either occur through gap healing or contact healing.
Contact healing
Fracture may be healed by the process named contact healing, when the interfragmentary strain (i.e. interfragmentary movement/fracture gap size) of the fracture is less than 2% and the fracture gap size is less than 0.01 mm. Further, such conditions could be achieved through extremely rigid internal fixations associated with interfragmentary compression (Marsell and Einhorn, 2011).Contact healing is started through cutting cones formed at the end of the osteon adjacent to the fractures (Figure 2-6). In order to create longitudinal cavity, osteoclasts may work at the tip of the cutting cone. The blood vessels within these cavities bring with them perivascular osteoblastic precursors that differentiate to osteoblasts. Osteoblasts, which may be present at the rear end of cone from bone to fill cavities, may result in simultaneous Haversian systems remodelling and bony union. Since the fragments are in direct contact, these systems cross from one fragment to the other to generate “spot welds” which unite the fragments. The...
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