Osseous System PDF

OSSEOUS SYSTEM By Dr. Muktha Nair  DEFINITIONS A bone is an organ made up of several different tissues working together: bone (osseous) tissue, cartilage, dense connective tissue, epithelium, adipose tissue, and nervous tissue. The entire framework of bo...

OSSEOUS SYSTEM By Dr. Muktha Nair  DEFINITIONS A bone is an organ made up of several different tissues working together: bone (osseous) tissue, cartilage, dense connective tissue, epithelium, adipose tissue, and nervous tissue. The entire framework of bones and their cartilages constitute the skeletal system. The study of bone structure and the treatment of bone disorders is referred to as osteology.  FUNCTIONS OF BONE The skeletal system performs several basic functions: 1. Support. The skeleton serves as the structural framework for the body by supporting soft tissues and providing attachment points for the tendons of most skeletal muscles. 2. Protection. The skeleton protects the most important internal organs from injury. For example, cranial bones protect the brain, and the rib cage protects the heart and lungs. 3. Assistance in movement. Most skeletal muscles attach to bones; when they contract, they pull on bones to produce movement. 4. Mineral homeostasis (storage and release). Bone tissue makes up about 18% of the weight of the human body. It stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone. Bone tissue stores about 99% of the body’s calcium. On demand, bone releases minerals into the blood to maintain critical mineral balances (homeostasis) and to distribute the minerals to other parts of the body. 5. Blood cell production. Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis. o Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers. o It is present in developing bones of the foetus and in some adult bones, such as the hip (pelvic) bones, ribs, sternum (breastbone), vertebrae (backbones), skull, and ends of the bones of the humerus (arm bone) and femur (thigh bone). o In a new-born, all bone marrow is red and is involved in hemopoiesis. With increasing age, much of the bone marrow changes from red to yellow. 6. Triglyceride storage. Yellow bone marrow consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve.  TYPES OF BONES Long bones These consist of a shaft and two extremities. As the name suggests, these bones are longer than they are wide. Most long bones are found in the limbs; examples include the femur, tibia and fibula. Short, Irregular, flat and sesamoid bones These have no shafts or extremities and are diverse in shape and size. Examples include: Short bones - carpals (wrist), metacarpals (ankle) Irregular bones - vertebrae and some skull bones Flat bones- sternum, ribs and most skull bones Sesamoid (shaped like a sesame seed) bones- patella (kneecap).  BONE STRUCTURE A long bone is one that has greater length than width. A typical long bone consists of the following parts: 1. The diaphysis is the bone’s shaft or body—the long, cylindrical, main portion of the bone. 2. The epiphyses are the proximal and distal ends of the bone. 3. The metaphyses are the regions between the diaphysis and the epiphyses. In a growing bone, each metaphysis contains an epiphyseal (growth) plate, a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length (described later in the chapter). When a bone ceases to grow in length at about ages 14–24, the cartilage in the epiphyseal plate is replaced by bone; the resulting bony structure is known as the epiphyseal line. 4. The articular cartilage is a thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation ( joint) with another bone. Articular cartilage reduces friction and absorbs shock at freely movable joints. Because articular cartilage lacks a perichondrium and lacks blood vessels, repair of damage is limited. 5. The periosteum is a tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage. It is composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of cells. Some of the cells enable bone to grow in thickness, but not in length. The periosteum also protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons. The periosteum is attached to the underlying bone by perforating fibers or Sharpey’s fibers, thick bundles of collagen that extend from the periosteum into the bone extracellular matrix. 6. The medullary cavity or marrow cavity, is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels in adults. This cavity minimizes the weight of the bone by reducing the dense bony material where it is least needed. The long bones’ tubular design provides maximum strength with minimum weight. 7. The endosteum is a thin membrane that lines the medullary cavity. It contains a single layer of bone forming cells and a small amount of connective tissue.  Blood and nerve supply One or more nutrient arteries supply the bone shaft; the epiphyses have their own blood supply, although in the mature bone the capillary networks arising from the two are heavily interconnected. The sensory nerve supply usually enters the bone at the same site as the nutrient artery, and branches extensively throughout the bone. Bone injury is, therefore, usually very painful. Short, irregular, flat and sesamoid bones These have a relatively thin outer layer of compact bone, with spongy bone inside containing red bone marrow. They are enclosed by periosteum except for the inner layer of the cranial bones, where it is replaced by dura mater.  HISTOLOGY OF BONE Bone is a strong and durable type of connective tissue. Its major constituent (65%) is a mixture of calcium salts, mainly calcium phosphate. This inorganic matrix gives bone great hardness but on its own would be brittle and prone to shattering. The remaining third is organic material, called osteoid, which is composed mainly of collagen fibres. Collagen is very strong and gives bone slight flexibility. It is used as the framework for building the calcium-rich inorganic matrix. The cellular component of bone contributes less than 2% of bone mass. The most abundant mineral salt is calcium phosphate [Ca3(PO4)2]. It combines with another mineral salt, calcium hydroxide [Ca(OH)2], to form crystals of hydroxyapatite [Ca10(PO4)6(OH)2]. As the crystals form, they combine with still other mineral salts, such as calcium carbonate (CaCO3), and ions such as magnesium, fluoride, potassium, and sulphate. As these mineral salts are deposited in the framework formed by the collagen fibers of the extracellular matrix, they crystallize and the tissue hardens. This process, called calcification, is initiated by bone-building cells called osteoblasts.  BONE CELLS 1. Osteoprogenitor cells are unspecialized bone stem cells derived from mesenchyme, the tissue from which almost all connective tissues are formed. They are the only bone cells to undergo cell division; the resulting cells develop into osteoblasts. Osteoprogenitor cells are found along the inner portion of the periosteum, in the endosteum, and in the canals within bone that contain blood vessels. 2. Osteoblasts are bone-building cells. They synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix of bone tissue, and they initiate calcification. As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes. 3. Osteocytes, mature bone cells, are the main cells in bone tissue and maintain its daily metabolism, such as the exchange of nutrients and wastes with the blood. Like osteoblasts, osteocytes do not undergo cell division. 4. Osteoclasts are huge cells derived from the fusion of as many as 50 monocytes (a type of white blood cell) and are concentrated in the endosteum. On the side of the cell that faces the bone surface, the osteoclast’s plasma membrane is deeply folded into a ruffled border. On the side of osteoclast cell that faces the bone surface Here the cell releases powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying extracellular bone matrix. This breakdown of bone extracellular matrix, termed bone resorption, is part of the normal development, maintenance, and repair of bone. In response to certain hormones, osteoclasts help regulate blood calcium level. They are also target cells for drug therapy used to treat osteoporosis.  COMPACT (CORTICAL) BONE Compact bone makes up about 80% of the body bone mass. It is made up of a large number of parallel tube-shaped units called osteons (Haversian systems), each of which is made up of a central canal surrounded by a series of expanding rings, similar to the growth rings of a tree. Osteons tend to be aligned the same way that force is applied to the bone; for example, in the femur (thigh bone), they run from one epiphysis to the other. This gives the bone great strength. The central canal contains nerves, lymphatics and blood vessels, and each central canal is linked with neighbouring canals by tunnels running at right angles between them, called perforating canals.  The series of cylindrical plates of bone arranged around each central canal are called lamellae. Between the adjacent lamellae of the osteon are strings of little cavities called lacunae, in each of which sits an osteocyte. Lacunae communicate with each other through a network of tiny channels called canaliculi, which allows the circulation of interstitial fluid through the bone, and direct contact between the osteocytes, which extend fine processes into them. Between the osteons are interstitial lamellae, the remnants of older systems partially broken down during remodelling or growth of bone.  SPONGY (CANCELLOUS, TRABECULAR) BONE About 20% of skeletal bone mass is spongy bone. To the naked eye, spongy bone looks like a honeycomb. This honeycomb arrangement has two main functions. The spaces contain red bone marrow, which produces blood cells; this means that spongy bone is much lighter than compact bone, reducing the weight of the skeleton. Microscopic examination reveals a bony framework formed from trabeculae (meaning 'little beams’). Like compact bone, the bone tissue is organised into osteons, with osteocytes living in lacunae and communicating with each other via canaliculi. The bone tissue here is not dense like compact bone, and so individual osteocytes are never very far away from an internal bone surface; the osteocytes are nourished by diffusion through the canaliculi and there is no need for a central canal in the middle of the osteon. The branching framework of pillars and plates in spongy bone is not arranged randomly. Rather, the bony honeycomb develops maximum strength along the directions from which maximum stress comes, so that even though it is not solid bone, it can absorb its share of applied force.  OSSIFICATION Ossification or osteogenesis is process of development of bones. It begins at birth and is not complete until 21st year of life. Bone formation occurs in four principal situations: (1) the initial formation of bones in an embryo and foetus, (2) the growth of bones during infancy, childhood, and adolescence until their adult sizes are reached, (3) the remodelling of bone (replacement of old bone by new bone tissue throughout life), and (4) the repair of fractures (breaks in bones) throughout life. Long, short and irregular bones develop in the foetus from rods of cartilage, called cartilage models. Flat bones develop from membrane models and sesamoid bones from tendon models. During ossification osteoblasts secrete osteoid, which gradually replaces the initial cartilage model; then the osteoblasts lay down calcium and phosphate salts through the osteoid, progressively calcifying it and converting it to the hard, rigid structure of mature bone. As the bone grows, the osteoblasts become trapped in the matrix of their own making and become osteocytes.  DEVELOPMENT OF LONG BONES In long bones the focal points from which ossification begins at small areas of osteogenic cells, at centres of ossification in the cartilage model. Ossification is accompanied by development of a bone collar at about 8 weeks of gestation. Later the blood supply develops and bone tissue replaces cartilage as osteoblasts secrete osteoid in the shaft. The bone lengthens as ossification continues and spreads to the epiphyses. Around birth, secondary centres of ossification develop in the epiphyses, and the medullary canal forms when osteoclasts break down the central bone tissue in the middle of the shaft.  During childhood, long bones continue to lengthen because the epiphyseal plate at each end of the bone, which is made of cartilage, continues to produce new cartilage on its diaphyseal surface (the surface facing the shaft of the bone). This cartilage is then turned to bone. As long as cartilage production matches the rate of ossification, the bone continues to lengthen. At puberty, under the influence of sex hormones, the epiphyseal plate growth slows down and is overtaken by bone deposition. Once the whole epiphyseal plate is turned to bone, no further lengthening of the bone is possible.  BONE REMODELING Bone is subject to constant mechanical stresses that damage, crack and weaken it over time. To combat this, there is a constant turnover of bone tissue, mediated by osteoblast and osteoclast activity. On average, about 10% of bone is replaced each year, but the rate at which individual bones are remodelled varies. Bones subject to high stress are remodelled faster than others. The distal part of the femur, for example, is generally renewed every 3-6 months. The process is not a simple replacement but reflects adjustment of bone to the stresses to which it is exposed. For example, remodelling of lower limb bones in regular runners strengthens the bones along the lines of regularly applied stress.  JOINTS A joint is the site at which any two or more bones articulate or come together, meaning the ends or edges of the bones are held together by connective tissues. Joints may allow flexibility and movement of the skeleton. In some joints, however, the participating bones are fastened together so firmly that no movement between them is possible. Fibrous joints: There is no synovial cavity, and the bones are held together by dense irregular connective tissue that is rich in collagen fibers. Cartilaginous joints : There is no synovial cavity, and the bones are held together by cartilage. Synovial joints : The bones forming the joint have a synovial cavity and are united by the dense irregular connective tissue of an articular capsule, and often by accessory ligaments.  FIBROUS JOINTS The bones forming these joints are linked with tough fibrous material. Such an arrangement often permits no movement. For example, the joints between the skull bones, the sutures, are completely immovable, and the healthy tooth is cemented into the mandible by the periodontal ligament. The tibia and fibula in the leg are held together along their shafts by a sheet of fibrous tissue called the interosseous membrane. This fibrous joint allows a limited amount of movement and stabilises the alignment of the bones.  CARTILAGINOUS JOINTS These joints are formed by a pad of tough fibrocartilage between the bones that acts as a shock absorber. The joint may be immovable, as in the cartilaginous epiphyseal plates, which in the growing child link the diaphysis of a long bone to the epiphysis. Some cartilaginous joints permit limited movement as between the vertebrae, which are separated by the intervertebral discs, or at the symphysis pubis, which is softened by circulating hormones during pregnancy to allow for expansion during childbirth.  SYNOVIAL JOINTS Synovial joints are characterised by the presence of a space or capsule between the articulating bones. The ends of the bones are held close together by a sleeve of fibrous tissue and lubricated with a small amount of fluid. Synovial joints are the most movable of the body.  CHARACTERISTIC OF SYNOVIAL JOINTS Articular or hyaline cartilage The parts of the bones in contact with each other are coated with hyaline cartilage. This provides a smooth articular surface, reduces friction. distributes weight and prevents damaging bone-to-bone contact. (articular cartilage) The cartilage lining, which is up to 7 mm thick in young people, becomes thinner and less compressible with age. This leads to increasing stress on other structures in the joint. Cartilage has no blood supply and receives its nourishment from synovial fluid. Capsule or capsular ligament The joint is wrapped in a sleeve of fibrous tissue that holds the bones together. It is sufficiently loose to allow freedom of movement but strong enough to protect it from injury. The capsule is formed by an extension of the periosteum covering the participating bones. Synovial membrane This delicate epithelial layer lines the capsule and covers all non-weight-bearing surfaces inside the joint. It secretes synovial fluid. Synovial fluid This is a thick. sticky fluid of egg-white consistency, which fills the synovial cavity. It : - nourishes the structures within the joint cavity - contains phagocytes, which remove microbes and cellular debris - coats and lubricates the moving parts of the joint - maintains joint stability - prevents the ends of the bones from being separated, as does a little water between two glass surfaces. Little sacs of synovial fluid (bursae) are present in some joints, e.g. the knee. They act as cushions to prevent friction between a bone and a ligament or tendon or skin where a bone in a joint is near the surface. Other intracapsular structures Some joints have structures within the capsule to pad and stabilise the joint, e.g. fat pads and menisci in the knee joint. If these structures do not bear weight, they are covered by synovial membrane. Extracapsular structures Ligaments that blend with the capsule stabilise the joint Muscles or their tendons also provide stability and stretch across the joints they move. When the muscle contracts, it shortens, pulling one bone towards the other. Nerve and blood supply Nerves and blood vessels crossing a joint usually supply the capsule and the muscles that move it. TYPES OF SYNOVIAL JOINTS  BALL AND SOCKET JOINT The head of one bone is ball-shaped and articulates with a cup-shaped socket of another. These joints allow a wide range of movement including flexion, extension, adduction, abduction, rotation and circumduction. Examples include the shoulder and hip.  HINGE JOINT The articulating ends of the bones fit together like a hinge on a door, and movement is therefore restricted to flexion and extension. The elbow joint is one example, permitting only flexion and extension of the forearm. Other hinge joints include the knee, ankle and the joints between the phalanges of the fingers and toes (interphalangeal joints).  GLIDING JOINT The articular surfaces are flat or very slightly curved and glide over one another but the amount of movement possible is very restricted; this group of joints is the least movable of all the synovial joints. Examples include the joints between the carpal bones in the wrist, the tarsal bones in the foot, and the processes of the spinal vertebrae (note that the joints between the vertebral bodies are the cartilaginous discs).  PIVOT JOINTS These joints allow a bone or a limb to rotate. One bone fits into a hoop-shaped ligament that holds it close to another bone and allows it to rotate in the ring thus formed. For example, the head rotates on the pivot joint formed by the dens of the axis held within the ring formed by the transverse ligament and the dens (odontoid process) of the atlas.  CONDYLOID JOINT A condyle is a smooth, rounded projection on a bone and in a condyloid joint it sits within a cup-shaped depression on the other bone. Examples include the joint between the condylar process of the mandible and the temporal bone, those between the metacarpal and phalangeal bones of the hand, and those between the metatarsal and phalangeal bones of the foot. These joints permit flexion, extension, abduction, adduction and circumduction.  SADDLE JOINT The articulating bones fit together like a person sitting on a saddle. The most important saddle joint is at the base of the thumb, between the trapezium of the wrist and the first metacarpal bone. The range of movement is similar to that at a condyloid joint but with additional flexibility; opposition of the thumb, the ability to touch each of the finger tips on the same hand, is due to the nature of the thumb joint.  DISORDERS OF JOINTS Rheumatoid arthritis (RA): It is a chronic progressive inflammatory autoimmune disease mainly affecting peripheral synovial joints. It is a systemic disorder in which inflammatory changes affect not only joints but also many other sites, including the heart, blood vessels and skin. Up to 90% of affected individuals have rheumatoid factor (RF-autoantibodies) in their body fluids. High levels of RF, especially early in the disease, are strongly associated with accelerated and more severe disease. Symptoms include joint pain and stiffness, particularly in the morning and after rest Joints can be visibly swollen, hot and tender. The joints most commonly affected are those of the hands and feet, but in severe cases most synovial joints may be involved. With each exacerbation there is additional and cumulative damage to the joints, leading to increasing deformity, pain and loss of function.  Osteoarthritis (OA) It is a degenerative non-inflammatory disease that results in pain and restricted movement of affected joints. In its early stages, OA is often asymptomatic. It is very common. with the majority of people over 65 showing some degree of osteoarthritic changes. Articular cartilage gradually becomes thinner because its renewal does not keep pace with its breakdown. Eventually, the bony articular surfaces come in contact and the bones begin to degenerate. Bone repair is abnormal and the articular surfaces become misshapen, reducing mobility of the joint. Chronic inflammation develops with effusion (collection of fluid) into the joint, possibly due to irritation caused by tissue debris not removed by phagocytes. Sometimes there is abnormal outgrowth of cartilage at the edges of bones that becomes ossified, forming osteophytes. In most cases, the cause of OA is unknown (primary OA), but risk factors include excessive repetitive use of the affected joints, female gender, increasing age, obesity and heredity.  Sprains, strains and dislocations These damage the soft tissues, tendons and ligaments round the joint without penetrating the joint capsule. In dislocations there may be additional damage to intracapsular structures by stretching, e.g. to the long head of biceps muscle in the shoulder joint, the cruciate ligaments in the knee joint, or the ligament of head of femur in the hip joint. If repair is incomplete, there may be some loss of stability, which increases the risk of repeated injury. Penetrating injuries These may be caused by a compound fracture of one of the articulating bones or by trauma. Healing may be uneventful or delayed by the presence of fragments of damaged or torn joint tissue (bone, cartilage or ligaments), which cannot be removed or repaired by normal body mechanisms and prevent full joint recovery. Infection is another risk. Chronic inflammation can lead to permanent degenerative changes in the joint.  Gout This condition is caused by the deposition of sodium urate crystals in joints and tendons, provoking an acute inflammatory response. Risk factors include male gender, obesity, heredity, hyperuricaemia. and high alcohol intake. Primary gout, the most common form, occurs almost always in men and is associated with reduced ability to excrete urate or increased urate production. Secondary gout usually occurs as a consequence of diuretic treatment or kidney failure, both of which reduce urate excretion. The sites most commonly affected are the metatarsophalangeal joint of the big toe and the ankle, knee, wrist and elbow joints.