This page is dedicated to understanding to a very high level the structure and function of the bones, joints, and spine. All of tis stuff is written by me from my studies on orthopedics
Note: All citations, references, links, sources, and used material will be labeled with a specific number i.e. Source 1 = (1)
Orthopaedics is the study of the bones, joints, and spine of the human body. Let’s first focus on the bones first. In a toddler’s body there is about 270 bones. There is 206 bones in the human physically mature adult body. (1)
Bones are classified by their shapes. There are 5 main types of bones,
- The long bones,
- The short bones,
- The flat bones,
- The irregular bones.
- The sesamoid bones
An example of a long bone is the femur or humerus. An example of a short bone is the bones of the wrist or ankles. For flat bone, skull bones. For irregular bones, that would be like the vertebrate.
The majority of the bones is a type of bone matrix, which is made of both living and non living tissue.
1. Living tissue in the bones comprise of bone cells, blood vessels, nerves. For the bone cells, there are 3 main types.
- Osteoblasts – bone cells that build up bone
- Osteoclasts – bone cells that tear down bone
- Osteocytes – mature bone cells that don’t form bones anymore
2. Non-living tissue is the bone matrix which is the collagen fibers and crystalline salts.
Crystalline Salts – The crystalline salt are of two elements, Calcium and Phosphates. The Calcium and Phosphates combine to form hydroxyapatite crystals.
Collagen Fibers – The Type I Collagen make up the organic part of of the non living bone matrix.
What gives the bone its strength?
Answer: Two main components, collegan and the calcium salts.
- Collagen Fibers – gives the bone high tensile strength – is almost as strong as reinforced concrete! (about 150 MPa)
- Calcium Salts – gives the bone high compression strength – stronger than even reinforced concrete! (about 200 MPa)
This suggest that at least for long bones, the bone is less resilient to torsion forces applied. It seems fractures in long bones occurs most when a torsion force is applied.
- The shear stress strength of long bones is low in comparison to its tensile and compressive load strength at around 50 MPa (2)
Let’s only look at the structure of the long bone, like a femur.
The bone is elongated in form. There are two expanded ends of the bone, which are the epiphysis, which forms a joint with another bone. At the very ends of the epiphysis is a layer of cartilage cover called the articular cartilage. For the epiphysis, they are not made of compact bone but of another type of bone called the cancellous or trabecular bone type which is spongy in appearance. The cancellous bone has a high surface area and high porosity, which the cavities being rod and plate like in form.
The shaft in the middle of the bone between the two epiphysis ends is called the diaphysis. Besides the area which the articular cartilage is covering, the rest of the long bone is covered in a layer called the periosteum. Inside the layer of the periosteum is another thicker layer of bone called the compact bone which is in the diaphysis middle region. This is also very strong, and tough, with a low porosity and make up around 80% of the bone weight of the body.
The cortical bone in the diaphysis forms a inner cavity called the inter medullar cavity in the middle of the bone. This cavity continues on into the spongy bone areas of the epiphysis and is filled with a tissue called marrow. There are two types of marrow, yellow and red. The yellow marrow is found in the inter medullary cavity in the diaphysis region while the red marrow is found in the epiphysis spongy bone area. The yellow marrow is used as fat storage while the red marrow is to create blood cells. The red bone marrow participates in the formation of red blood cells (RBCs) through the process of erythropoiesis
Bones, specifically long bones, can grow in three main ways, longitudinally, radially, or by density. This means that the long bone is either going to grow longer, thicker, or stronger.
The type we are trying to achieve is to get the long bones like our femur and tibia to increase longitudinally, especially after the stage of puberty and our epiphyseal growth plates have fused and even the epiphyseal line has disappeared.
First, the formation of bone is called Ossification. There is two types of ossification, Endochondral and Intramembranous
1. Endochondral Ossification: This type of ossification is the main way for most of the bones in the body to form and grow. There appears to be an initial piece of hyaline cartilage that grows over time. There are 5 main steps in this type of ossification (2)…
- Development of cartilage model
- Growth of cartilage model
- Development of the primary ossification center – It seems the actual endochondral ossification actually starts here.
- Development of the secondary ossification center
- Formation of articular cartilage and epiphyseal plate
2. Intramembranous Ossification: This type occurs during the formation of the flat bones of the skull but also in other bones in the skull and clavicle (2). This type of bone is formed from a type of tissue that is not cartilage, one of them called the mesenchyme tissue. There appear to be 4 steps in this type of ossification…
- Development of ossification center
- Formation of trabeculae
- Development of periosteum
Disorders & Pathologies
The most common types of disorders that affect the bone and skeletal system is osteoporosis. Osteoporosis is where the bones in the body lose their strength and density from increased bone porosity. This leads to an increase in the likelihood of fracture. It happens most often with women who have reached past menopause but it can also occur within men or women who are suffering from certain kinds of hormone pathologies
What exactly causes height increase?
Height increase is actually caused by at least 4 main processes, . Most people would tell you that height increase only comes about from the longitudinal growth of the long bones like the femur and tibia but they are not taking into consideration the growth of other bones in the body, specifically the short bones, the flat bones, and the irregular bones. The 32 vertebrate in our back are irregular bones and they do increase in size when we were still growing and that did contribute to our height. Our torso makes up about 1/3rd of our total height.
However, most people will only focus on the endochondral ossification in the long bones. It is important to note that the short bones also go through endochondral ossification just like the long bones.
Let’s look closer at the process of endochondral ossification. This type of ossification is the type of process in the first formation of long bone, the growth in length of the long bone, and the natural healing of bone fractures. Cartilage is used in this process.
The cartilage goes through a type of growth called interstitial growth which chondrocytes which are cartilage cells divide and multiple while are the same type excreting waste which is used to make up the cartilage matrix .
Another type of growth that happens is known as appositional growth in the endochondral ossification process. This is where the long bones get thicker so the width increases since the extracellular matrix on the edge of the chondrocyte stacked columns also increases from the excretion of the chondrocytes. This happens at the same time new chondroblasts are made in the perichondrium.
For the long bone, there are actually two main areas that experience endochondral ossification, not one. There is the middle part called the diaphysis but also the outer parts called the epiphysis. The middle part is the primary center while the outer parts is the secondary center.
For Primary Center In Middle Diaphysis Section (taken from Wikipedia)
The first site of ossification occurs in the primary center of ossification, which is in the middle of diaphysis (shaft). Then:
- Formation of periosteum: The perichondrium becomes the periosteum. The periosteum contains a layer of undifferentiated cells (osteoprogenitor cells) which later become osteoblasts.
- Formation of bone collar: The osteoblasts secrete osteoid against the shaft of the cartilage model (Appositional Growth). This serves as support for the new bone.
- Calcification of matrix: Chondrocytes in the primary center of ossification begin to grow (hypertrophy). They stop secretingcollagen and other proteoglycans and begin secreting alkaline phosphatase, an enzyme essential for mineral deposition. Then calcification of the matrix occurs and apoptosis of the hypertrophic chondrocytes occurs. This creates cavities within the bone. The exact mechanism of chondrocyte hypertrophy and apoptosis is currently unknown.
- Invasion of periosteal bud: The hypertrophic chondrocytes (before apoptosis) secrete Vascular Endothelial Cell Growth Factor that induces the sprouting of blood vessels from the perichondrium. Blood vessels forming the periosteal bud invade the cavity left by the chondrocytes and branch in opposite directions along the length of the shaft. The blood vessels carry hemopoietic cells, osteoprogenitor cells and other cells inside the cavity. The hemopoietic cells will later form the bone marrow.
- Formation of trabeculae: Osteoblasts, differentiated from the osteoprogenitor cells that entered the cavity via the periosteal bud, use the calcified matrix as a scaffold and begin to secrete osteoid, which forms the bone trabecula. Osteoclasts, formed from macrophages, break down spongy bone to form the medullary (bone marrow) cavity.
For Secondary Center In Outer Epiphysis Section (taken from Wikipedia)
About the time of birth, a secondary ossification center appears in each end (epiphysis) of long bones. Periosteal buds carry mesenchyme and blood vessels in and the process is similar to that occurring in a primary ossification center. The cartilage between the primary and secondary ossification centers is called the epiphyseal plate, and it continues to form new cartilage, which is replaced by bone, a process that results in an increase in length of the bone. Growth continues until the individual is about 26 years old or until the cartilage in the plate is replaced by bone. The point of union of the primary and secondary ossification centers is called the epiphyseal line.
Let’s now see how Apositional Bone Growth occurs. It seems that the long bones get thicker by forming more bones right underneath the periosteum. On the inside, the osteoclasts degrade the long bone to create the hollow cavity in the middle. Eventually once physical maturity is reached, the rate at which bones are formed underneath the periosteum and the rate which osteoclasts degrade the inner bone is about the same so the thickness of the bone stays constant.
Now let’s remember that the cartilage in the growth plates are the type called hyaline. Let’s look to see what exactly is Hyaline Cartilage
The Epiphyseal Growth Plate (from Wikipedia)
The epiphyseal growth plate has 5 main layers to it, the resting zone, the proliferate layer ,the hypertrophic layer ,the calcification layer, and the ossification layer.
- Zone of resting cartilage. This zone contains normal, resting hyaline cartilage.
- Zone of proliferation / cell columns. In this zone, chondrocytes undergo rapid mitosis, forming distinctive looking stacks.
- Zone of maturation / hypertrophy. It is during this zone that the chondrocytes undergo hypertrophy (become enlarged). Chondrocytes contain large amounts of glycogen and begin to secrete alkaline phosphatase.
- Zone of calcification. In this zone, chondrocytes are either dying or dead, leaving cavities that will later become invaded by bone-forming cells. Chondrocytes here die when they can no longer receive nutrients or eliminate wastes via diffusion. This is because the calcified matrix is much less hydrated than hyaline cartilage.
- Zone of ossification. Osteoprogenitor cells invade the area and differentiate into osteoblasts, which elaborate matrix that becomes calcified on the surface of calcified cartilage. This is followed by resorption of the calcified cartilage/calcified bone complex.
Hyaline Cartilage (taken from Wikipedia)
Is a type of cartilage found on many joint surfaces. It is pearly bluish in colour with firm consistency and considerable collagen. It contains no nerves or blood vessels, and its structure is relatively simple.
Hyaline cartilage is covered externally by a fibrous membrane, called the perichondrium, except at the articular ends of bones and also where it is found directly under the skin, i.e. ears and nose. This membrane contains vessels that provide the cartilage with nutrition.
If a thin slice is examined under the microscope, it will be found to consist of cells of a rounded or bluntly angular form, lying in groups of two or more in a granular or almost homogeneous matrix.
The cells, when arranged in groups of two or more, have generally straight outlines where they are in contact with each other, and in the rest of their circumference are rounded.
They consist of clear translucent protoplasm in which fine interlacing filaments and minute granules are sometimes present; embedded in this are one or two roundnuclei, having the usual intranuclear network.
The cells are contained in cavities in the matrix, called cartilage lacunae; these are actually artificial gaps formed by the shrinking of the cells during the staining and setting of the tissue for observation. The interterritorial space between the isogenous cell groups contains relatively more collagen fibers, causing it to maintain its shape while the actual cells shrink, creating the lacunae.
This constitutes the so-called capsule of the space. Each lacuna is generally occupied by a single cell, but during the division of the cells it may contain two, four, or eight cells. (see isogenous group)
Hyaline cartilage also contains chondrocytes which are cartilage cells that produce the matrix. Hyaline cartilage matrix is mostly made up of type II collagen and Chondroitin sulfate, both of which are also found in elastic cartilage.
Hyaline cartilage exists on the ventral ends of ribs; in the larynx, trachea, and bronchi; and on the articular surface of bones.
The term “articular cartilage” refers to the hyaline cartilage on the articular surfaces of bones.Articular Cartilage
Though it is often found in close contact with menisci and articular disks, articular cartilage is not considered a part of either of these structures, which are made entirely of fibrocartilage.
Chondrocytes (taken from Wikipedia)
Chondrocytes are the only cells found in healthy cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans. Although chondroblast is still commonly used to describe an immature chondrocyte, use of the term is discouraged, for it is technically inaccurate since the progenitor of chondrocytes (which are mesenchymal stem cells) can also differentiate into osteoblasts. The organization of chondrocytes within cartilage differs depending upon the type of cartilage and where in the tissue they are found.
From least- to terminally-differentiated, the chondrocytic lineage is:
- Colony-forming unit-fibroblast (CFU-F)
- Mesenchymal stem cell / marrow stromal cell (MSC)
- Hypertrophic chondrocyte
When referring to bone or cartilage, mesenchymal stem cell (mesoderm origin) are undifferentiated meaning they can differentiate into different variance of generative cells (MSC) are commonly known as osteochondrogenic (or osteogenic, chondrogenic, osteoprogenitor, etc.) cell. Undifferentiated mesenchymal stem cell lose their process, proliferate and crowd together in a dense aggregate of chondrogenic cells(cartilage) at the center of chondrification. These condrogenic cells will then differentiate to chondroblasts which will then to synthesize the cartilage ECM(extra cellular matrix). Which consists of ground substance(proteoglycans, glycosaminoglycans for low osmotic potential) and fibers. The chondroblasts then trap themselves in a small space that is no longer in contact with the newly created matrix called lacunae which contain extracellular fluid. The chondroblast is now a chondrocyte, which is usually inactive but can still secrete and degrade matrix depending on the conditions. The majority of the cartilage that has been built has been synthesized from the chondroblast which are much more inactive at a late age (adult hood) compared to earlier years (pre-pubesence)
Chondrocytes undergo terminal differentiation when they become hypertrophic during endochondral ossification. This last stage is characterized by major phenotypic changes in the cell.
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of vertebrates. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. Collagen, in the form of elongated fibrils, is mostly found in fibrous tissues such as tendon, ligament and skin, and is also abundant in cornea, cartilage, bone, blood vessels, the gut, and intervertebral disc. The fibroblast is the most common cell which creates collagen.
Collagen occurs in many places throughout the body. Over 90% of the collagen in the body, however, is of type one.
So far, 28 types of collagen have been identified and described. The five most common types are:
- Collagen I: skin, tendon, vascular ligature, organs, bone (main component of the organic part of bone)
- Collagen II: cartilage (main component of cartilage)
- Collagen III: reticulate (main component of reticular fibers), commonly found alongside type I.
- Collagen IV: forms bases of cell basement membrane
- Collagen V: cell surfaces, hair and placenta
Collagen has an unusual amino acid composition and sequence:
- Glycine is found at almost every third residue
- Proline (Pro) makes up about 17% of collagen
- Collagen contains two uncommon derivative amino acids not directly inserted during translation. These amino acids are found at specific locations relative to glycine and are modified post-translationally by different enzymes, both of which require vitamin C as a cofactor.
- Hydroxyproline (Hyp), derived from proline.
- Hydroxylysine (Hyl), derived from lysine (Lys). Depending on the type of collagen, varying numbers of hydroxylysines are glycosylated (mostly havingdisaccharides attached).
Cortisol stimulates degradation of (skin) collagen into amino acids.
Osteoid makes up about fifty percent of bone volume and forty percent of bone weight. It is composed of fibers and ground substance. The predominant fiber-type is Type I collagen and comprises ninety percent of the osteoid. The ground substance is mostly made up of chondroitin sulfate and osteocalcin.
Glycogen (from Wikie)
Glycogen is a multibranched polysaccharide that serves as a form of energy storage in animals and fungi. In humans, glycogen is made and stored primarily in the cells of the liver and the muscles, and functions as the secondary long-term energy storage (with the primary energy stores being fats held in adipose tissue).
Glycogen is the analogue of starch, a glucose polymer in plants, and is sometimes referred to as animal starch, having a similar structure to amylopectinbut more extensively branched and compact than starch. Glycogen is found in the form of granules in the cytosol/cytoplasm in many cell types, and plays an important role in the glucose cycle. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of triglycerides (lipids).
Polysaccharide represents the main storage form of glucose in the body. Found in the liver and muscles, muscle glycogen is converted into glucose by muscle cells, and liver glycogen converts to glucose throughout the body including the Central Nervous System.
In the liver hepatocytes, glycogen can compose up to eight percent of the fresh weight (100–120 g in an adult) soon after a meal. Only the glycogen stored in the liver can be made accessible to other organs. In the muscles, glycogen is found in a low concentration (one to two percent of the muscle mass). The amount of glycogen stored in the body—especially within the muscles, liver, and red blood cells—mostly depends on physical training,basal metabolic rate, and eating habits such as intermittent fasting. Small amounts of glycogen are found in the kidneys, and even smaller amounts in certain glial cells in the brain and white blood cells.
White fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions. It owes its flexibility and toughness to the former of these constituents, and its elasticity to the latter. It is the only type of cartilage that contains type I collagen in addition to the normal type II.
Fibrocartilage is found in the pubic symphysis, the annulus fibrosus of intervertebral discs, meniscus, and the TMJ. During labor, relaxin loosens the pubic symphysis to aid in delivery, but this can lead to later joint problems.
Formation as a repair mechanism
If hyaline cartilage is torn all the way down to the bone, the blood supply from inside the bone is sometimes enough to start some healing inside the lesion. In cases like this, the body will form a scar in the area using a special type of cartilage called fibrocartilage. Fibrocartilage is a tough, dense, fibrous material that helps fill in the torn part of the cartilage. Yet it’s not an ideal replacement for the smooth, glassy articular cartilage that normally covers the surface of the knee joint.
Periosteum (taken from Wikipedia)
Periosteum is a membrane that lines the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of all bones.
Periosteum consists of dense irregular connective tissue. Periosteum is divided into an outer “fibrous layer” and inner “cambium layer” (or “osteogenic layer”). The fibrous layer contains fibroblasts, while the cambium layer contains progenitor cells that develop into osteoblasts. These osteoblasts are responsible for increasing the width of a long bone and the overall size of the other bone types. After a bone fracture the progenitor cells develop into osteoblasts and chondroblasts, which are essential to the healing process.
As opposed to osseous tissue, periosteum has nociceptive nerve endings, making it very sensitive to manipulation. It also provides nourishment by providing the blood supply. Periosteum is attached to bone by strong collagenous fibers called Sharpey’s fibres, which extend to the outer circumferential and interstitial lamellae. It also provides an attachment for muscles and tendons.
Cytokines (taken from Wikipedia)
Cytokines are small cell-signaling protein molecules that are secreted by numerous cells and are a category of signaling molecules used extensively in intercellular communication. Cytokines can be classified as proteins, peptides, or glycoproteins; the term “cytokine” encompasses a large and diverse family of regulators produced throughout the body by cells of diverse embryological origin.
Part of the difficulty with distinguishing cytokines from hormones is that some of the immunomodulating effects of cytokines are systemic rather than local. For instance, to use hormone terminology, the action of cytokines may be autocrine or paracrine in chemotaxis and endocrine as a pyrogen. Further, as molecules, cytokines are not limited to their immunomodulatory role. For instance, cytokines are also involved in several developmental processes during embryogenesis
Each cytokine has a matching cell-surface receptor. Subsequent cascades of intracellular signalling then alter cell functions. This may include the upregulation and/or downregulation of several genes and their transcription factors, resulting in the production of other cytokines, an increase in the number of surface receptors for other molecules, or the suppression of their own effect by feedback inhibition.
The effect of a particular cytokine on a given cell depends on the cytokine, its extracellular abundance, the presence and abundance of the complementary receptor on the cell surface, and downstream signals activated by receptor binding; these last two factors can vary by cell type. Cytokines are characterized by considerable “redundancy”, in that many cytokines appear to share similar functions.
Mesenchymal Stem Cells
Mesenchymal stem cells are a type of connective tissue cell that can differentiate into a few different type of cells, specifically bone cells (osteoblasts), cartilage cells (chondrocytes), and fat cells (adipocytes). It is characterized as small cell body,long and thin, with a large nucleus. It seems that they have a high capacity for self renewal while still being able to keep their ability to differentiate into other types of cells. The actual differentiation can be induced mechanically or chemically. It seems that the ability to multiply and change into other cells seem to decrease as the person grows older. They secrete cytokines.
How Exactly do Bones Heal from Fracture (dislocation, etc) ?
Bone heal first by having a knowledgeable person put the fractured bone area back into the right position or relocation. The position is stabilized and then the natural bone healing process occurs. A fracture ultimately heals through physiological processes.
The healing process is mainly determined by the periosteum (the connective tissuemembrane covering the bone). The periosteum is one source of precursor cells which develop into chondroblasts and osteoblasts that are essential to the healing of bone. The bone marrow (when present),endosteum, small blood vessels, and fibroblasts are other sources of precursor cells. (from Wikipedia)
Phases of fracture healing (from Wikipedia)
There are three major phases of fracture healing, two of which can be further sub-divided to make a total of five phases;
- 1. Reactive Phase
- i. Fracture and inflammatory phase
- ii. Granulation tissue formation
- 2. Reparative Phase
- iii. Cartilage Callus formation
- iv. Lamellar bone deposition
- 3. Remodeling Phase
- v. Remodeling to original bone contour
After fracture, the first change seen by light and electron microscope is the presence of blood cells within the tissues adjacent to the injury site. Soon after fracture, the blood vessels constrict, stopping any further bleeding. Within a few hours after fracture, the extravascular blood cells form a blood clot, known as a hematoma. All of the cells within the blood clot degenerate and die. Some of the cells outside of the blood clot, but adjacent to the injury site, also degenerate and die. Within this same area, the fibroblasts survive and replicate. They form a loose aggregate of cells, interspersed with small blood vessels, known as granulation tissue.
Days after fracture, the cells of the periosteum replicate and transform. The periosteal cells proximal (closest) to the fracture gap develop into chondroblasts which form hyaline cartilage. The periosteal cellsdistal to (further from) the fracture gap develop into osteoblasts which form woven bone. The fibroblasts within the granulation tissue develop into chondroblasts which also form hyaline cartilage. These two new tissues grow in size until they unite with their counterparts from other parts of the fracture. These processes culminate in a new mass of heterogeneous tissue which is known as the fracture callus. Eventually, the fracture gap is bridged by the hyaline cartilage and woven bone, restoring some of its original strength.
The next phase is the replacement of the hyaline cartilage and woven bone with lamellar bone. The replacement process is known as endochondral ossification with respect to the hyaline cartilage and bony substitution with respect to the woven bone. Substitution of the woven bone with lamellar bone precedes the substitution of the hyaline cartilage with lamellar bone. The lamellar bone begins forming soon after the collagen matrix of either tissue becomes mineralized. At this point, the mineralized matrix is penetrated by channels, each containing a microvessel and numerous osteoblasts. The osteoblasts form new lamellar bone upon the recently exposed surface of the mineralized matrix. This new lamellar bone is in the form of trabecular bone. Eventually, all of the woven bone and cartilage of the original fracture callus is replaced by trabecular bone, restoring most of the bone’s original strength.
Complications of Fracture Healing
The main complications include:
- Delayed Union: Poor blood supply or infection.
- Non-Union: Bone loss or wound contamination.
- Fibrous Union: Improper immobilization
Various studies have found that pulsed electromagnetic fields (PEMF) have increased the rate of bone healing.
Low intensity pulsed ultrasound, applied twenty minutes per day, increases the rate of bone healing.