University of Plymouth Healing in Bone PDF

Year 2 BDS and DTH Life Sciences 2024/25 Healing in Bone Dr Amr Elraggal Clinical Lecturer in Dental Education BDS, MSc, PhD [email protected] Learning Objectives Describe different types of bone fractures Describe bone healing in terms...

Year 2 BDS and DTH Life Sciences 2024/25 Healing in Bone Dr Amr Elraggal Clinical Lecturer in Dental Education BDS, MSc, PhD [email protected] Learning Objectives Describe different types of bone fractures Describe bone healing in terms of primary and secondary fracture repair Detail the process and timeline of healing of bone in the context of physiological processes Identify the main healing phases of bone To identify the main cells involved in healing Discuss the main themes of pathophysiology of bone fractures Be aware of different types of healing (1º vs. 2º) Bone composition Inorganic: 67% hydroxyapatite (calcium phosphate) Organic: 33% 28% collagen 5% non-collagenous protein Bone cells Osteoprogenitor (Osteogenic) cells: develop into osteoblast, found in periosteum, endosteum, and canals of vital teeth. Osteoblasts: formation of bone, role in calcification, synthesis of protein. Osteoclasts: bone resorption. Osteocytes: maintenance of bone, exchange of calcium between bone and extracellular fluid (ECF).  Types of Bone Compact bone Cortical/ compact bone High density/ low porosity (5-30%) Cortical Bone is formed of osteons. Each Osteon is formed of many lamellae (formed in concentric circles). The lamellae are formed of:  Organic part which is collagen.  Inorganic part which is calcium phosphate or hydroxyapatite. In the center of each Osteon there are Haversian canals that include blood supply and innervation for the bone. The center of the bone is called medullary canal that contains the bone marrow----- blood cell production. The center of the bone is the medullary canal that is lined by spongy bone. It includes the bone marrow that is responsible for cell production.  Types of Bone Compact bone Osteons/ Haversian System Cylindrical structures, consisting of concentric layers (lamellae), surrounding central Haversian canal Osteon boundary – cement line. Osteons connected to each other & periosteum by Volkmann’s canals Exchange nutrients/ metabolic waste. Bone formation, Ca2+ homeostasis. Between adjoining osteons occupied by interstitial lamellae. Types of Bone Spongy bone Trabecular/ cancellous/ spongy bone Lamellae run parallel to bone surface Low density/ high porosity (30-90%) Softer, weaker, more flexible Trabeculae surrounded by bone marrow spaces Blood & nerve supply Collagen fibres run in parallel to bone surface  Types of Bone Woven bone Immature bone. Fewer collagen fibres, arranged in haphazard manner. Low density/ mechanical strength. Produced when osteoblasts produce osteoid rapidly. Eg foetal bone, adult bone fractures. Evident as fibrous matrix. Later replaced by lamellar (cortical or cancellous) bone in foetal tissues. Origin of Bone Cells Osteoblasts, Osteocytes, Osteoclasts Osteoblasts Begin to secrete bone matrix proteins Most abundant is type I collagen, but non-collagenous proteins are also secreted Synthesis and secretion of osteoid is initially unmineralized, but osteoblast secretion of proteins stimulate calcification Initially mineralised bone matrix takes form of immature woven bone, but over time this is remodelled into stronger lamellar bone, by working in conjunction with osteoclasts Osteoclasts Clear away damaged and necrosed tissue at site of injury. Resorption of tissue facilitates release of growth factors bound within bone matrix eg TGF-β1, BMP. Initiate intracellular signalling cascades, which promote recruitment and proliferation of mesenchymal stem cells / osteoprogenitors to site of injury and stimulate osteoblast differentiation. Stimulates remodelling of newly formed bone via production of local signalling molecules eg M-CSF, RANKL. Osteoblast Regulation of Osteoclast Formation Osteoblasts express surface Receptor Activator of NF-κB Ligand (RANKL), eg high IL-1 or TNF-α RANKL binds to (RANK) on osteoclast precursor surfaces Haematopoetic stem cells differentiate & fuse to form multi-nucleated, mature osteoclasts = bone resorption Differentiation inhibited by osteoblast-derived, osteoprotegerin, OPG (RANKL decoy reception, eg high TGFβ Osteocytes Coordinate bone remodelling processes by acting as mechano-sensors that can respond to mechanical strain and microdamage to signal bone resorption/ formation Communication via gap junctions at end of cellular processes and through contact with bone marrow Develop meshwork of filamentous cellular processes that extend into surrounding bone tissue, running alone bone canaliculi and connect osteocyte to other cell types Allows for regulation of bone mass  Types of bone fracture Closed (simple) fracture – the broken bone has not pierced the skin. Open (compound) fracture – the broken bone juts out through the skin, or a wound leads to the fracture site. Infection and external bleeding are more likely. Greenstick fracture – a small, slender crack in the bone. This can occur in children, because their bones are more flexible that an adult’s bones. Hairline fracture – the most common form is a stress fracture, often occurring in the foot or lower leg as a result of repeated stress from activities such as jogging or running. Complicated fracture – structures surrounding the fracture are injured. There may be damage to the veins, arteries or nerves, and there may also be injury to the lining of the bone (the periosteum). Comminuted fracture – the bone is shattered into small pieces. This type of complicated fracture tends to heal more slowly. Avulsion fracture – muscles are anchored to bone with tendons. Powerful muscle contractions can wrench the tendon free and pull-out pieces of bone. This type of fracture is more common in the knee and shoulder joints. Compression (impacted) fracture – occurs when 2 bones are forced against each other. The bones of the spine, called vertebrae, can have this type of fracture. Older people, particularly those with osteoporosis, are at higher risk. Types of bone fracture Avulsion fracture: muscles are anchored to bone with tendons. Powerful muscle contractions can wrench the tendon free and pull-out pieces of bone. This type of fracture is more common in the knee and shoulder joints. Oblique/ spiral: A complete break at an angle across the bone. A spiral fracture or torsion happens diagonally and is longer than it is wide, resembling a corkscrew. Spiral fractures result from a twisting force or impact, such as a skiing or snowboarding accident. This fracture is most common in long bones such as the femur and tibia. Transverse: A straight and horizontal break completely across the bone. The ends of the broken bone can be displaced and pulled apart by the muscles pulling on the bone and angulated, usually due to a direct blow. Careful assessment of the bone angles should be considered for successful reduction. Mechanisms for Bone Healing Direct (primary) bone healing Indirect (secondary) bone healing Primary Bone Healing (Direct or Osteonal Bone Repair) Direct bone healing: Primary bone healing occurs when accurate reduction and rigid internal fixation has been performed. The central components of primary bone healing are “cutting cones” which consist of a spearhead of osteoclasts that cut a space in bone. Within this space, there is a central blood vessel and the walls of the cavity behind the osteoclasts are lined with osteoblasts, which lay down new bone. Multiple cutting cones traverse the fracture gap perceiving it to be a “giant microcrack.” Eventually, the fracture line is obliterated as it is removed and is replaced with new bone by the osteoblasts. Primary Bone Healing (Direct or Osteonal Bone Repair) Gap Healing: It is a modified form of primary bone repair. This occurs if a fracture has been rigidly fixed but the gap at the fracture site is in the order of a few hundred microns. In this situation, immature bone is initially laid down along the fracture surface of each bone end so that the gap now becomes small enough to allow cutting cones to traverse the fracture as in primary bone healing. Secondary (indirect) bone healing Secondary Bone Repair: The majority of fractures heal by secondary bone repair which is a form of wound healing. Bone ends are not in their position nor rigidly fixed. Involves response in periosteum and endosteum leading to formation of a callus. Callus is composed of immature bone. Fills intervening gap and unites bone ends. Some degree of mobility (mechanical stimulation) enhances secondary healing. It consists mainly of five stages: 1. Hematoma. 2. Inflammation. 3. Soft callus. 4. Hard callus. 5. Remodelling. Secondary (indirect) bone healing 1. Hematoma: Hematoma Immediately after a fracture, bleeding from the bone surfaces and from the surrounding soft tissue occurs. This blood clot is rich in a number of factors such as VEGF (vascular endothelial growth factor). These factors are important in initiating the inflammatory response and recruiting cells and blood vessels to the site of injury. It has been shown that removal of the hematoma results in delayed fracture healing and the absence of the hematoma in open fractures is likely to contribute to the impaired healing in these patients. Secondary (indirect) bone healing 2. Inflammatory phase: Sequential infiltration of neutrophils and macrophages. Macrophages engulf fibrin and red cells. Other inflammatory cells such as T-cells, B-cells and mast cells present. Inflammatory cytokines such as tumour necrotising factor-alpha (TNF-α), the interleukins (ILs), specifically IL-1, IL-6, IL-11, and IL- 18, recruit inflammatory cells and stimulate angiogenesis. IL-1 induces osteoblasts to produce IL-6 and stimulates angiogenesis at the fracture site. IL-6 also stimulates angiogenesis and in addition stimulates the differentiation of osteoblasts and osteoclasts. Secondary (indirect) bone healing 3. Soft callus Soft Callus In this phase, organization of the blood clot occurs with invasion of capillaries and fibroblasts. Fibroblasts and chondrocytes lay down a fibrocartilaginous matrix between the fracture ends. A bridge of soft callus forms. The muscle and periosteum contribute to the external callus. The medullary tissue and the bone ends contribute to the internal callus. MSCs are attracted from periosteum, endosteum and bone marrow. TNF-α also has a key role in recruiting osteoprogenitor stem cells, stimulating apoptosis of hypertrophic chondrocytes and enhancing the recruitment of osteoclasts to the calcified cartilage callus. Secondary (indirect) bone healing 4. Hard callus In this phase, the matrix becomes mineralized; calcium-containing granules are accumulated in the cytoplasm and then transported to the extracellular matrix where they combine with phosphate and precipitate and become the nidus for apatite crystal formation. As the soft callus becomes stiffer, and osteoblasts are able to ossify the tissue. Provided there are regions where the callus is now under the breaking strain of bone (ie, approximately 2%), a bridge of hard callus is able to form. If there is excessive movement at this stage and there are no zones where the strain is under 2%, then a hypertrophic non-union will occur. The hard callus initially consists of a “repair” form of bone (ie, woven bone) which consists of type 3 as well as type 1 collagen. Secondary (indirect) bone healing 5. Remodelling Once the bone ends have been joined together by a solid bridge of woven bone, this tissue is gradually replaced with lamellar bone by a cutting cone process called remodelling. This new bone is laid down along the lines of stress in response to the mechanical requirements of the bone. The process of secondary bone healing is remarkable in that the bone regenerates without a scar and reforms healthy new bone. Fracture Repair Fracture Repair - Summary Time after fracture Event 12 hours Blood clot and fibrous exudate collects between bone fragments 24 hours Inflammation: Sequential infiltration of neutrophils and macrophages 48 hours Granulation tissue formation 5 days Earliest bone formation 3 weeks Fibrous union; some primary callus 6 weeks Periosteal shell of external callus with bone ends being joined by meshwork of woven bone 6 weeks – 6 months Progressive formation of secondary callus and subsequent remodelling  Types of bone graft Miron RJ. Optimized bone grafting. Periodontology 2000. 2024 Feb;94(1):143-60. Processes of Bone Healing - Grafting Osteogenesis Formation of new bone by surviving cells within graft (osteoblasts) Grafts must be well vascularised in order for bone forming cells within graft to survive Eg Cancellous autografts are only commonly used grafts that promote osteogenesis Processes of Bone Healing - Grafting Osteoconduction New bone generated from existing cells (physical graft) Creates proper architecture/ environment for bone formation; physical property of graft to serve as scaffold for viable bone healing Osteoconduction allows for in-growth of neovasculature and infiltration of osteogenenic precursor cells into graft site Osteoconduction is passive function of graft, but allows for bone formation through process of creeping substitution Collagen is prototype osteoconductive component eg Gel- foam, Bio-oss, hydroxyapatite Processes of Bone Healing - Grafting Osteoinduction Process of transformation of local undifferentiated cells into bone forming cells Protein or growth factors are released from resorbing bone matrix and stimulate mesenchymal stem cells (monocyte or fibroblast) to differentiate and form bone Production of bone in non-bony site Cytokines, hormones or force signals BMP, piezoelectric and mechanical loading Process of Bone Healing Migration/ proliferation of bone marrow-derived, MSCs (Osteoinduction) Enhanced by TGF-βs, BMPs, PDGF, VEGF MSC differentiation into mature osteoblasts (Osteoconduction) Induced by BMPs, TGF-β, VEGF. Inhibited by PDGF & bFGF Primary vs secondary bone healing Primary (direct) bone healing Secondary (indirect) bone healing No movement Movement present Absolute stability Relative stability Direct healing of bone Healing through intermediate stages Bone ends are in direct contact or small gap Big gap is present between bone ends present Less common Most common Application of BMP’s to Clinical Situations: Preclinical Studies Orthopaedics Long bone segmental defect repair Fracture repair Spinal fusion Implant fixation Osteonecrosis Oral and Maxillofacial Calvarial reconstruction Mandibular reconstruction Cleft palate repair Dental Periodontal repair Alveolar augmentation Subantral augmentation Endodontics Factors affecting bone healing 1. Injury variables: 2. Patient variables: Open fracture. Age Severity of injury. Nutrition Intra-articular fracture. Systemic hormones Segmental fracture. Soft tissue interposition. Damage of blood supply. 3. Tissue variables: 4. Treatment variables: Form of bone: cortical or cancellous. Decreasing fracture gap. Bone necrosis. Stabilizing fracture Bone disease. Restoring fracture segments. Infection.  Thank you Dr Amr Elraggal [email protected] This presentation is a modification of another one introduced by dr Yen Lin.