ILIUM

 

The Ilium or the flank forms the upper extended plate-like part of the hip bone. It has upper and lower parts and three surfaces. The upper end is called the iliac crest which forms the two-fifths of the acetabulum and the lower end which is smaller than upper end is fused with pubis and ischium at the acetabulum. The upper part is much expanded, and has gluteal, sacropelvic and iliac (internal) surfaces. The posterolateral gluteal surface is an extensive rough area; the anteromedial iliac fossa is smooth and concave; and the sacropelvic surface is medial and posteroinferior to the fossa, from which it is separated by the medial border.

Ilium

Ilium

 

Iliac crest

The iliac crest is the superior border of the ilium. It is broad and convex upwards but sinuous from side to side, being internally concave in front and convex behind. Its ends project as anterior and posterior superior iliac spines. The anterior superior iliac spine is palpable at the lateral end of the inguinal fold; the lateral end of the inguinal ligament is attached to the anterior superior iliac spine. The posterior superior iliac spine is not palpable but is often indicated by a dimple, approximately 4 cm lateral to the second sacral spinous process, above the medial gluteal region.

The iliac crest has ventral and dorsal segments. The ventral segment occupies slightly more than the anterior two-thirds of the iliac crest. It has internal and external lips and a rough intermediate zone that is narrowest centrally. The dorsal segment, which occupies approximately the posterior 1/3^rd in humans. It has two sloping surfaces separated by a longitudinal ridge ending at the posterior superior spine. The tubercle of the iliac crest projects outwards from the outer lip approximately 5 cm posterosuperior to the anterior superior spine. The summit of the iliac crest, a little behind its midpoint, is level with the 4^th lumbar vertebral body in adults and with the 5^th lumbar vertebral body in children aged 10 years or less.

Anterior border

The anterior border descends to the acetabulum from the anterior superior spine. Superiorly it is concave forwards. Inferiorly, immediately above the acetabulum, is a rough anterior inferior iliac spine, which is divided indistinctly into an upper area for the straight head of rectus femoris and a lower area extending laterally along the upper acetabular margin to form a triangular impression for the proximal end of the iliofemoral ligament.

Posterior border

The posterior border is irregularly curved and descends from the posterior superior spine, at first forwards, with a posterior concavity forming a small notch. At the lower end of the notch is a wide, low projection known as the posterior inferior iliac spine. Here the border turns almost horizontally forwards for approximately 3 cm, then down and back to join the posterior ischial border. Together these borders form a deep notch, the greater sciatic notch, which is bounded above by the ilium and below by the ilium and ischium. The upper fibres of the sacrotuberous ligament are attached to the upper part of the posterior border. The superior rim of the notch is related to the superior gluteal vessels and nerve. The lower margin of the greater sciatic notch is covered by piriformis and is related to the sciatic nerve.

Medial border

The medial border separates the iliac fossa and the sacropelvic surface. It is indistinct near the crest, rough in its upper part, then sharp where it bounds an articular surface for the sacrum and finally rounded. The latter part is the arcuate line, which inferiorly reaches the posterior part of the iliopubic ramus, marking the union of the ilium and pubis.

• Gluteal surface

Gluteal surface is the outer surface of the ilium, which is convex in front and concave behind, like the iliac crest. It is rough and curved, convex in front, concave behind, and marked by three gluteal lines which divides into four areas. The posterior gluteal line is shortest, descending from the external lip of the crest approximately 5 cm in front of its posterior limit and ending in front of the posterior inferior iliac spine. Above, it is usually distinct, but inferiorly it is poorly defined and frequently absent. The anterior gluteal line, the longest, begins near the midpoint of the superior margin of the greater sciatic notch and ascends forwards into the outer lip of the crest, a little anterior to its tubercle. The inferior gluteal line, seldom well marked, begins posterosuperior to the anterior inferior iliac spine, curving posteroinferiorly to end near the apex of the greater sciatic notch. Between the inferior gluteal line and the acetabular margin is a rough, shallow groove. Behind the acetabulum, the lower gluteal surface is continuous with the posterior ischial surface.

The articular capsule is attached to an area adjoining the acetabular margin, most of which is covered by gluteus minimus. Posteroinferiorly, near the union of the ilium and ischium, the bone is related to piriformis.

• Iliac fossa

The iliac fossa, the internal concavity of the ilium, faces anterosuperiorly. It is limited above by the iliac crest, in front by the anterior border and behind by the medial border, separating it from the sacropelvic surface. It forms the smooth and gently concave posterolateral wall of the greater pelvis. Below it is continuous with a wide shallow groove, bounded laterally by the anterior inferior iliac spine and medially by the iliopubic ramus.

• Sacropelvic surface

The sacropelvic surface, the posteroinferior part of the medial iliac surface, is bounded posteroinferiorly by the posterior border, anterosuperiorly by the medial border, posterosuperiorly by the iliac crest and anteroinferiorly by the line of fusion of the ilium and ischium. It is divided into iliac tuberosity and auricular & pelvic surfaces. The iliac tuberosity, a large, rough area below the dorsal segment of the iliac crest, shows cranial and caudal areas separated by an oblique ridge and connected to the sacrum by the interosseous sacroiliac ligament. The sacropelvic surface gives attachment to the posterior sacroiliac ligaments and, behind the auricular surface, to the interosseous sacroiliac ligament. The iliolumbar ligament is attached to its anterior part. The auricular surface, immediately anteroinferior to the tuberosity, articulates with the lateral sacral mass. Shaped like an ear, its widest part is anterosuperior, and its ‘lobule’ posteroinferior and on the medial aspect of the posterior inferior spine. Its edges are well defined but the surface, though articular, is rough and irregular. It articulates with the sacrum and is reciprocally shaped. The anterior sacroiliac ligament is attached to its sharp anterior and inferior borders. The narrow part of the pelvic surface, between the auricular surface and the upper rim of the greater sciatic notch, often shows a rough pre-auricular sulcus (that is usually better defined in females) for the lower fibres of the anterior sacroiliac ligament. The pelvic surface is anteroinferior to the acutely curved part of the auricular surface, and contributes to the lateral wall of the lesser pelvis. Its upper part, facing down, is between the auricular surface and the upper limb of the greater sciatic notch. Its lower part faces medially and is separated from the iliac fossa by the arcuate line. Anteroinferiorly, it extends to the line of union between the ilium and ischium. Though usually obliterated, it passes from the depth of the acetabulum to approximately the middle of the inferior limb of the greater sciatic notch.

Muscle attachments

The attachment of sartorius extends down the anterior border below the anterior superior iliac spine.

The iliac crest gives attachment to the anterolateral and dorsal abdominal muscles, and to the fasciae and muscles of the lower limb.

The fascia lata and iliotibial tract are attached to the outer lip and tubercle of its ventral segment.

Tensor fasciae latae is attached anterior to the tubercle. The lower fibres of external oblique and, just behind the summit of the crest, the lowest fibres of latissimus dorsi are attached to its anterior two-thirds. A variable interval exists between the most posterior attachment of external oblique and the most anterior attachment of latissimus dorsi, and here the crest forms the base of the lumbar triangle through which herniation of abdominal contents may rarely occur.

Internal oblique is attached to the intermediate area of the crest.

Transversus abdominis is attached to the anterior two-thirds of the inner lip of the crest, and behind this to the thoracolumbar fascia and quadratus lumborum. The highest fibres of gluteus maximus are attached to the dorsal segment of the crest on its lateral slope.

Erector spinae arises from the medial slope of the dorsal segment.

The straight head of rectus femoris is attached to the upper area of the anterior inferior spine.

Some fibres of piriformis are attached in front of the posterior inferior spine on the upper border of the greater sciatic foramen.

The gluteal surface is divided by three gluteal lines into four areas. Behind the posterior line, the upper rough part gives attachment to the upper fibres of gluteus maximus and the lower, smooth region to part of the sacrotuberous ligament and iliac head of piriformis. Gluteus medius is attached between the posterior and anterior lines, below the iliac crest, and gluteus minimus is attached between the anterior and inferior lines.

The fourth area, below the inferior line, contains vascular foramina. The reflected head of rectus femoris attaches to a curved groove above the acetabulum.

Iliacus is attached to the upper two-thirds of the iliac fossa and is related to its lower one-third. The medial part of quadratus lumborum is attached to the anterior part of the sacropelvic surface, above the iliolumbar ligament.

Piriformis is sometimes partly attached lateral to the pre-auricular sulcus, and part of obturator internus is attached to the more extensive remainder of the pelvic surface.

Vascular supply Branches of the iliolumbar artery run between iliacus and the ilium; one or more enter large nutrient foramina lying posteroinferiorly in the iliac fossa. The superior gluteal, obturator and superficial circumflex iliac arteries contribute to the periosteal supply. The obturator artery may supply a nutrient branch. Vascular foramina on the ilium underlying the gluteal muscles may lead into large vascular canals in the bone. Innervation The periosteum is innervated by branches of nerves that supply muscles attached to the bone, the hip joint and the sacroiliac joint.

OSSIFICATION

Ossification is by three primary centers: one each for the ilium, ischium and pubis. The iliac centre appears above the greater sciatic notch prenatally at about the 9^th week and the pubic centre in its superior ramus between the 4^th and 5^th months. The pubis is often not recovered from prenatal remains due to its size and fragility and because it is the last of the hip bones to begin ossification (Scheuer and Black 2004). At birth the whole iliac crest, the acetabular floor and the inferior margin are cartilaginous. Gradual ossification of the three components of the acetabulum results in a triradiate cartilaginous stem extending medially to the pelvic surface as a Y-shaped epiphysial plate between the ilium, ischium and pubis, and including the anterior inferior iliac spine. Cartilage along the inferior margin also covers the ischial tuberosity, forms conjoined ischial and pubic rami and continues to the pubic symphysial surface and along the pubic crest to the pubic tubercle. The ossifying ischium and pubis fuse to form a continuous ischiopubic ramus at the 7^th or 8^th year. 

Secondary centres, other than for the acetabulum, appear at about puberty and fuse between the 15^th and 25^th years. There are usually two for the iliac crest (which fuse early), and single centres for anterior inferior iliac spine (although it may ossify from the triradiate cartilage) and symphysial surface of the pubis (the pubic tubercle and crest may have separate centres). Progression of ossification of the iliac crest in girls is an index of skeletal maturity and is useful in determining the optimal timing of surgery for spinal deformity. Between the ages of 8 and 9 years, three major centres of ossification appear in the acetabular cartilage. The largest appears in the anterior wall of the acetabulum and fuses with the pubis, the second in the iliac acetabular cartilage superiorly, fusing with the ilium, and the third in the ischial acetabular cartilage posteriorly, fusing with the ischium. At puberty, these epiphyses expand towards the periphery of the acetabulum and contribute to its depth. Fusion between the three bones within the acetabulum occurs between the sixteenth and eighteenth years. Delaere et al have suggested that ossification of the ilium is similar to that of a long bone, possessing three cartilaginous epiphyses and one cartilaginous process, although it tends to undergo osteoclastic resorption comparable with that of cranial bones. During development, the acetabulum increases in breadth at a faster rate than it does in depth. Avulsion fractures of pelvic apophyses may occur from excessive pull on tendons, usually in athletic adolescents. The most frequent examples of such injuries are those to the ischial tuberosity (hamstrings) and anterior inferior iliac spine (rectus femoris).

Complied & written by Dr. Palak Shah.

 

CARTILAGE

Cartilage is a connective tissue composed of cells and fibres embedded in a firm, gel-like matrix which is rich in a mucopolysaccharide. it is more elastic than bone.

• Cartilage doesn't have blood supply nor it has lymphatics.it's nutrition diffuses through the matrix.
• It doesn't have nerves hence it is insensitive.
• Cartilage is surrounded by a fibrous membrane known as perichondrium which is similar to the periosteum in both structure and function. The articular cartilage doesn't have perichondrium so it's regeneration after injury is inadequate. 
• When cartilage dies, it forms into a bone like tissue.

TYPES OF CARTILAGE

1. HYALINE CARTILAGE: It is the most common form of cartilage. Hyalos is the Greek word for glass which describes the appearance of the tissue which is translucent, blueish-white and shiny. The cartilage is usually only 2-4mm thick. It is the embryonic form of cartilage. It is found in ribs, joints, nose. layrnx and trachea. Hyaline cartilage collagen fibres are primarily type II, extremely thin, invisible to microscope due to similar refractory properties to the matrix itself.

2. FIBROCARTILAGE: It is found where tendons and ligaments meet the bone, at the Pubic Symphysis, the Sternoclavicular joint and Annulus Fibrosus. The fibrocartilage is a very strong and pliable connective tissue. It is reinforced with collagen fibre bundles that run parallel to each other, allowing a low level stretch. Because of the abundance of collagen fibres, fibrocartilage is white in colour. It lacks a perichondrium and is composed of type II and type I collagen fibres. 

3. ELASTIC CARTILAGE: It is found in the external ear (auricle or pinna), the Eustachian tube and Epiglottis. Elastic's cartilage main role is purely structural, offering flexibility and resilience due to mixture of elastic fibres and type II collagen fibres. It is yellow in appearance without the organized structure of fibrocartilage when viewed under microscope.

COMPOSITION OF CARTILAGE

Cartilage is made up of highly specialized cells called chondrocytes and chondroblasts (chondro refers to cartilage), and other extracellular material which forms the cartilage matrix.

All connective tissue types within the human body are derived from the embryonal mesoderm. Bone, the strongest of the connective tissues, is the last to form and can remain in cartilage form well after birth. Increased cartilage to bone ratio enables a flexible and pliable new-born to exit the birth canal. A new-born has 300 bones, as opposed to the 206 of the normal adult, and all of these originate from cartilage.

From the 7th week of embryonic life, the process of ossification or osteogenesis slowly replaces cartilage with bone. This process continues into early childhood. Cartilage grows in two ways. In interstitial growth, chondrocytes proliferate and divide, producing more matrix inside existing cartilage throughout childhood and adolescence. In appositional growth, fresh layers of matrix are added to existing matrix surface by chondroblasts in the perichondrium. The perichondrium is a dense layer of connective tissue which surrounds most cartilage sites. Its outer layer contains collagen-producing fibroblasts, while the inner layer houses large numbers of differentiated fibroblasts called chondroblasts.

Chondroblasts: As long as they are free to move, chondroblasts produce the elements of the extracellular matrix (ECM). This cell type first forms a matrix of hyaluronic acid, chondroitin sulphate, collagen fibers, and water during embryonal development. Chondroblasts eventually become immobile after becoming surrounded by the matrix, and are then referred to as chondrocytes.

Chondrocytes: They are the immobile form of chondroblasts. They are surrounded by the matrix and contained within allotted spaces called lacunae. A single lacuna can contain one or more chondrocytes. Chondrocytes have varying roles according to the type of cartilage they are found in. In articular cartilage, found in the joints, chondrocytes increase joint articulation. At growth plates, chondrocytes regulate epiphyseal plate growth. While chondroblasts are ECM manufacturers, chondrocytes maintain the existing ECM and are a less active form of the same cell.

Fibroblasts: It is found in all types of connective tissue. In cartilage, these cells produce type I collagen. In certain situations, fibroblasts transform into chondrocytes.

Extracellular Matrix: There is significantly more matrix than cells in cartilage structure, as the low oxygen environment and lack of vasculature do not allow for larger numbers. Because of this, there is little metabolic activity, and little to no new growth in cartilage tissue – one of the reasons the elderly commonly suffer from degenerative joint pain. Cartilage does continue to grow slowly, however. This can be seen in the larger ears and noses of older individuals.

The ECM of cartilage contains three characteristic elements:

• Collagen

A protein-based collagen matrix gives form and strength to cartilage tissue through a mesh-like structure of fibrils. Although there are many different forms of collagen in the human body, the collagen found in cartilage is primarily type II, with an attached FACIT (short for fibril-associated collagen with interrupted triple helix) XIV collagen which determines the diameter of these fibers.

• Proteoglycans

Proteoglycans are large molecules that bind with water, providing flexibility and cushioning qualities. Proteoglycan monomers bond to hyaluronic acid by way of link proteins, as is the case with the large proteoglycan Aggrecan (chondroitin sulphate proteoglycan 

• Collagen and aggrecan in hyaline cartilage

The high numbers of negative charges such constructions provide, together with a large surface area, make it possible for proteoglycans to bind to large amounts of water. This creates high osmotic pressure, increases load-bearing, and constitutes the gel-like consistency of the ECM.

• Noncollagenous Proteins

Noncollagenous elements of the ECM are small in number and supposed to play a role in maintenance and organization of the cartilage structure on a macromolecular level.

Complied and written by Dr. Palak Shah

Bones

Bone is a rigid organ that constitutes part of vertebrate skeleton in animals. Bone constitutes of 30% of flexible matrix and 70% of bound minerals.

The bone matrix is 90-95% composed of elastic collagen fibres also known as Ossein, rest of which is just ground substance. The collagen elasticity improves fracture resistance. 

Bone matrix

The hardness and rigidity of bone is due to the presence of mineral salt in the osteoid matrix, which is a crystalline complex of calcium and phosphate (hydroxyapatite). Calcified bone contains about 25% organic matrix (2-5% of which are cells), 5% water and 70% inorganic mineral (hydroxyapatite).

The matrix is hardened by binding of inorganic salts, calcium phosphate in an arrangement known as Calcium HydroxyLapatite. 

Bone is actively constructed & remodeled throughout the life by special bone cells known as Osteoblasts and Osteoclasts.

Osteoclasts are responsible for aged bone resorption and while Osteoblasts are responsible for new bone formation.

Role Of Osteoblast in Osteoclast formation:

1. Cell to cell contact: The process happens by direct contact between the two of them. It initiates with osteoclatogenesis mainly depends on interaction between two cells.

2. Ephrin2/ephB4

3. MSF/MCP-1

4. OPG/RANKL/RANK

5. LGR4/RANKL/RANK

6. Sema3A/Nrp

7. Osteoclast apoptosis induced by osteoblasts

8. Lysophosphatidic acid (LPA)

CLASSIFICATION OF BONES

Bones are classified in various types which are:

A. According to SHAPE 

   1. Long Bones: It has an elongated shaft known as diaphysis and two expanded ends known as epiphuses qhich are smooth and articular. Typical long bones have 3 surfaces in shaft separated by 3 borders like Humerus, Radius, Ulna, Femur, Tibia & Fibula. Miniature long bones have 1 epiphysis like Metacarpals, Metatarsals and Phalanges. Modified long bones have no medullary cavity like Clavicle.

   

   2. Short Bones: Its shape is generally cuboid, cuneiform, trapezoid or scapoid like Tarsals & Carpals.

   3. Flat Bones: It looks like shallow plates and it makes boundaries of alot of body cavities like bones of Skull, Ribs, Sternum and Scapula.

   4. Irregular Bones: It has irregular shape and structure like bones of Vertebrae, Hip and Base of Skull.

   5. Pneumatic Bones: Irregular bones containing large air spaces which make skull light in weight, resonance of voice and act as air conditioning chambers. Bones like that are Maxilla, Sphenoid, Ethmoid, etc.

   6. Sesamoid Bones: Bony nodules found embedded in tendons or joint capsules. They have no periosteum and ossify after birth, related to an articular surface, its surface of contact are covered in hyaline cartilage and lubricated by bursa or synovial membrane. Its function is to resist pressure, minimize pressure, maintains local circulation and alters direction of muscle pull. Example: Patella, Fabella, Pisiform, etc.

   7. Accessory (Supernumerary) Bones: They are not always present. They may be present as ununited epiphysis developed from extra centres of ossification. These are often bilateraland have smooth surfaceswithout any callus. Examples: Suture bones, lateral tubercle of talus (Os Trigonum), tuberosity of 5th metatarsal (Os Vesalianum), etc.

   8. Heterotropic Bones: Bones which develop in soft tissues. Rider's bone which develop in horse riders in adductor muscles.

B. According to Development

   1. Membrane (Dermal) Bones: The bones ossify in intramembrane or mesenchyme and thus derived from mesenchymal condensation. The skull and facial bones are its ideal example.

      Cartilaginous Bones: Ossification is happening in the cartilage (intracartilaginous or endochondral Ossification) like bones of limbs, vertebrate column and thorax.

      Membrano-cartilaginous: It ossifies partly in membrane and other in cartilage. Example: clavicle, mandible, occipital, temporal and sphenoid.

   2. Somatic bones: Most of the bones in human body are somatic bones.

       Visceral bones: These develop from pharyngeal arches like Hyoid bones, part of mandible and ear ossicles.

C. According to Region:

   1. Axial Skeleton 

   2. Appendicular Skeleton

D. According to Structure: 

   1. Macroscopically: It can be Compact or Cancellous:

      i. Compact Bone: It is also known as Cortical Bone. Compact bone is dense bone tissue found on the outside of a bone. Basically, in kindergarten when you drew skeletons, you were drawing compact bone. Compact bone is enclosed, except where it's covered by articular cartilage, and is covered by the periosteum. The periosteum is a thick fibrous membrane covering the entire surface of a bone and serving as an attachment for muscles and tendons. Vessels pass from the periosteum through pores into the compact bone and run through canals found throughout the tissue.

      ii. Spongy Bone (Cancellous Bone)

"Cancellous" makes it sound so negative, doesn't it? Spongy bone is on the interior of a bone and consists of slender fibers and lamellae—layers of bony tissue—that join to form a reticular structure. Spongy bone is supplied by fewer and larger vessels than compact bone. These vessels perforate the outer compact layer and are distributed into the spongy portion of bone, which is filled with marrow. Bone marrow is tissue found in long bones, like the femur, that contains stem cells.

   2. Microscopically: The bone is of 5 types, Lamellar, Woven, Fibrous, Dentine and Cement.

      i. Lamellar : Most of the human bones whether it be compact or cancellous are composed of thin plates of bony tissues also known as Lamellae. 

      ii. Woven bone : It is mainly seen in the fetal bone, fracture repair or in bone cancers.

      iii. Fibrous bone : It is found in the foetal bones.

      iv. Dentine : The teeths in humans.

      v. Cement : It occurs in the teeth.

STRUCTURE OF HUMAN BONE OF AN ADULT 

1. SHAFT: It is composed of periosteum, cortex and medullary cavity.

   a. Periosteum: It is a thick fibrous membrane covering the surface of the bone. It is made upof 2 layers, outer fibrous layer and inner cellular layer which are osteoblastic in nature. The outer fibrous layer is made up of elastic fibrous material like collagen. It consists of blood vessels and nerves which pass through dense and compact layer of bone known as bone cortex. The inner cellular layer also known as Cambium consists of Osteoblast cells. This layer thinner with age. 

   b. Cortex: This is made up of a compact bone which gives it the desired strength to withstand all possible strains.

   c. Medullary Cavity: The cavity is made up of red or yellow bone marrow. At birth the marrow is red everywhere with widespread active haemopoiesis, with advancement of age the red marrow atrophies at many places and is replaced by yellow, fatty marrow with no power of haemopoiesis. Red marrow is present in cancellous bones at the ends throughout the life.

2. TWO ENDS: The long bones are made up of cancellous bones which are covered with Hyaline cartilage.

PARTS OF THE BONE:

1. Epiphysis is ends and tips of the bone which ossify from secondary centres. it has following types:

   a. Pressure Epiphysis is articular and takes part in transmission of weight.

   b. Traction Epiphysis is nonatrticular and doesn't bear weight, it provides attachment to tendons which exert traction at the epiphysis. The traction epiphysis ossify later than the pressure epiphysis.

   c. Atavistic Epiphysis is phylogenetically an independent bone which in man becomes an independent bone.

   d. Aberrant Epiphysis isn't always present.

2. Diaphysis is an elongated shaft of the long bone which ossifies from primary centres.

3. Metaphysis are the epiphysial ends of the diaphysis. It contains growth plate which grows and ossifies near diaphysis and epiphysis. The metaphysis contains mesenchymal stem cells which gives rise to bone and fat cells, also it contains haematopoietic stem cells which gives rise to alot of blood vessels and osteoclasts. during childhood, the growth plate contains the connecting cartilage which helps in growth. The components of growth plate stop growing alltogether and completely and ossify in a complete bone. In an adult, the metaphysis functions to transfer loads of weight bearing joint surfaces to diaphysis.

DEVELOPMENT AND OSSIFICATION OF BONE

The development of the skeleton can be traced back to three derivatives[1]: cranial neural crest cells, somites, and the lateral plate mesoderm. Cranial neural crest cells form the flat bones of the skull, clavicle, and the cranial bones (excluding a portion of the temporal and occipital bones. Somites form the remainder of the axial skeleton. The lateral plate mesoderm forms the long bones

Bone formation requires a template for development. This template is mostly cartilage, derived from embryonic mesoderm, but also includes undifferentiated mesenchyme (fibrous membranes) in the case of intramembranous ossification. This framework determines where the bones will develop. By the time of birth, the majority of cartilage has undergone replacement by bone, but ossification will continue throughout growth and into the mid-twenties.   

Intramembranous Ossification

This process involves the direct conversion of mesenchyme to the bone. It begins when neural crest-derived mesenchymal cells differentiate into specialized, bone-forming cells called osteoblasts. Osteoblasts group into clusters and form an ossification center. Osteoblasts begin secreting osteoid, an unmineralized collagen-proteoglycan matrix that can bind calcium. The binding of calcium to osteoid results in the hardening of the matrix and entrapment of osteoblasts. This entrapment results in the transformation of osteoblasts to osteocytes. As osteoid continues to be secreted by osteoblasts, it surrounds blood vessels, forming trabecular/cancellous/spongy bone. These vessels will eventually form the red bone marrow. Mesenchymal cells on the surface of the bone form a membrane called the periosteum. Cells on the inner surface of the periosteum differentiate into osteoblasts and secrete osteoid parallel to that of the existing matrix, thus forming layers. These layers are collectively called the compact/cortical bone [2].

Five steps can summarize intramembranous ossification:

1. Mesenchymal cells differentiate into osteoblasts and group into ossification centers
2. Osteoblasts become entrapped by the osteoid they secrete, transforming them to osteocytes
3. Trabecular bone and periosteum form
4. Cortical bone forms superficially to the trabecular bone
5. Blood vessels form the red marrow

Endochondral Ossification

This process involves the replacement of hyaline cartilage with bone. It begins when mesoderm-derived mesenchymal cells differentiate into chondrocytes. Chondrocytes proliferate rapidly and secrete an extracellular matrix to form the cartilage model for bone. The cartilage model includes hyaline cartilage resembling the shape of the future bone as well as a surrounding membrane called the perichondrium. Chondrocytes near the center of the bony model begin to undergo hypertrophy and start adding collagen X and more fibronectin to the matrix that they produce; this altered matrix allows for calcification. The calcification of the extracellular matrix prevents nutrients from reaching the chondrocytes and causes them to undergo apoptosis. The resulting cell death creates voids in the cartilage template and allows blood vessels to invade. Blood vessels further enlarge the spaces, which eventually combine and become the medullary cavity; they also carry in osteogenic cells and trigger the transformation of perichondrium to the periosteum. Osteoblasts then create a thickened region of compact bone in the diaphyseal region of the periosteum, called the periosteal collar. It is here that the primary ossification center forms. While bone is replacing cartilage in the diaphysis, cartilage continues to proliferate at the ends of the bone, increasing bone length. These proliferative areas become the epiphyseal plates (physeal plates/growth plates), which provide longitudinal growth of bones after birth and into early adulthood. After birth, this entire process repeats itself in the epiphyseal region; this is where the secondary ossification center forms [3].

The physeal growth plate is separated into various sections based on pathologic characteristics. 

• Reserve Zone
  • Storage site for lipids, glycogen, proteoglycan 
• Proliferative Zone
  • Proliferating chondrocytes leading to longitudinal growth
• Hypertrophic Zone
  • Site of chondrocyte maturation
  • Within the hypertrophic zone, the chondrocytes go through a transformation process. The chondrocyte mature and prepare a matrix for calcification; then they degenerate which allows calcium release for calcification of the matrix. 
• Primary Spongiosa
  • Site for mineralization to form woven bone
  • Vascular invasion occurs
• Secondary Spongiosa
  • Internal modeling with the replacement of fiber bone with lamellar bone
  • External modeling with funnelization

Five steps can summarize endochondral ossification:

1. Mesenchymal cells differentiate into chondrocytes and form the cartilage model for bone
2. Chondrocytes near the center of the cartilage model undergo hypertrophy and alter the contents of the matrix they secrete, enabling mineralization
3. Chondrocytes undergo apoptosis due to decreased nutrient availability; blood vessels invade and bring osteogenic cells
4. Primary ossification center forms in the diaphyseal region of the periosteum called the periosteal collar
5. Secondary ossification centers develop in the epiphyseal region after birth

BONE'S BLOOD SUPPLY

Introduction

Bone receives 5-10% of cardiac output Bones that receive tenuous blood supply scaphoid talus femoral head odontoid Blood supply to long bone comes from three sources  nutrient artery system metaphyseal-epiphyseal system periosteal system

Nutrient Artery System

High pressure system that branches from major systemic arteries Enter the cortex through the nutrient foramen and enter the medullary canal then branch into ascending and descending branches then branch into arterioles and supply the inner 2/3 of mature bone via the haversion system

Metaphyseal epiphyseal system

Arteries arise from periarticular vascular plexus e.g. geniculate arteries

Periosteal System

Low pressure system that supplies the outer 1/3 of bone connected by Volkman's artery (perpendicular to long axis) Haversion system (parallel to long axis)

Intracortical Vascularization

Intracortical vessels travel within canals Primary Haversian canals Secondary Volkmann canals

Direction of Arterial Flow

Normal intraosseous blood flow rate is 5-20ml/min/100g of bone  Mature bone flow is centrifugal (inside to outside) because of high pressure nutrient artery system and low pressure periosteal system Immature bone flow is centripetal (outside to inside) because low pressure periosteal system predominates Factors increasing blood flow hypoxia hypercapnia sympathectomy

Direction of Venous Flow

Mature bone  flow is centripetal (outside to inside) cortical capillaries drain to venous sinusoids, which drain to the emissary venous system

Growth Plate

Perichondrial artery is the major source of nutrition of the growth plate

Pathoanatomy

Fractures patterns of blood flow following fracture immediate phase initial decrease in blood flow after fracture flow is centripetal (outside to inside) because high pressure nutrient artery system is disrupted low pressure periosteal system predominates hours to days increase in blood flow (regional acceleratory phenomenon) peaks at 2 weeks and returns to normal in 3-5 months Intramedullary nails unreamed intramedullary nails preserve endosteal blood supply reaming devascularizes inner 50-80% of the cortex and delays revascularization of endosteal blood supply loose fitting nails spare cortical perfusion and allow more rapid reperfusion tight fitting nails compromise cortical perfusion and reperfusion is slow NERVE SUPPLY OF BONE  Nerves accompny the blood vessels. Most of them are sympathetic and vasomotor in function. A few of them are sensory which are distributed to the articular ends and periosteum of the long bones, to vertebra and to the large flat bones. COMPLIED AND WRITTEN BY: Dr. Palak Shah