GENERAL ANATOMY - BONE

[SURIYA.KARTHIKEYHAN]

GENERAL ANATOMY - BONE

• The skeleton is composed of cartilage and bones. Bone is a living tissue, It is a highly specialized, hard form of connective tissue that makes up most of the skeletal system in the adult.
• The skeletal system has two functional parts:- 
  • Axial Skeleton - eg: Skull, Hyoid bone, Vertebral column, Thoracic (rib) cage.
  • Appendicular Skeleton -  eg: Limbs, Girdles.

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FUNCTIONS OF BONE:

1. Support - for the body.
2. Protection - for vital structures.
3. Movement - the mechanical basis (leverage).
4. Storage - salts e.g. calcium.
5. Blood cell production – by the bone marrow.

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HISTOLOGY OF BONE:

1. Mineralized matrix.
2. Living cells Bone matrix includes:

Inorganic.

• Hydroxapatite [Ca10(PO4)6(OH)2]
• Calcium carbonate [CaCO3].
• Small amounts of magnesium, sodium, potassium, fluoride, sulfate, carbonate and hydroxide ions.

Organic:

• Collagen and proteoglycans

TYPES OF BONE:

1. Compact Bone.
2. Spongy (trabecular or cancellous) bone.

• Distinguished by the relative amount of solid matter and by the number and size of the spaces they contain.
• All bones have a superficial thin layer of compact bone around a central mass of spongy bone.

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COMPACT BONE:

• Provides strength for weight bearing.
• Consists of Haversian Systems or Osteons.
• In long bones the amount of compact bone is greatest near the middle of the shaft.
• At the ends of the bone there is a thin covering of compact bone.
• Haversian systems have layers of bone called lamellae surrounding central Haversian canal Perforating or Volkmann’s canals lie perpendicular to the long axis.

CANCELLOUS OR SPONGY BONE:

• Consists of a meshwork of trabeculae (trabecular bone) oriented along lines of stress.
• The spongy bone may be replaced by a medullary  (marrow) cavity filled with marrow.
• At birth all bones have red marrow with active haematopoesis.
• With age this is replaced by yellow fatty marrow with no power of haematopoeis.
• Ends of long bones, sternum, ribs, vertebrae retain red marrow throughout l

CLASSIFICATION OF BONES(According to shape):

Long bones:

• They are tubular.
• Have a shaft and 2 ends.
• Humerus,Femur radius, ulna etc.

Short bones:

• Cuboidal.
• Seen at wrist (carpals) and ankle (tarsals).

Miniature Long bones:

• Look like long bones.
• Have only 1 primary and 1 secondary ossification centre.
• Metacarpals, Metatarsal and phalanges.

Flat bones:

• Serve protective functions
• Bones of skull vault like Parietal bone
• Sternum

Irregular bones:

• Have various shape
• Vertebrae
• Hip bone

Sesamoid bones:

• Develop in certain tendons and are found where tendons cross the ends of long bones.
• Protect the tendons from excess wear.
• Often change the direction of pull of the tendon.
• Patella, Pisiform bone.

 Accessory bones or supernumerary bones:

• When additional ossification centres appear or a part of a bone fails to fuse with the main part.
• In skull along sutural lines: Sutural or Wormian bones.
• Also sometimes found in the foot FABELLA- Usually seen embedded in lateral head of gastrocnemius that articulates directly with lateral femoral condyle.

Pneumatic Bones:

• Contain air filled spaces lined with epithelium.
• Eg:–Maxilla, Ethmoid, Sphenoid, Frontal bone.

Heterotopic bones:

• Bones which form in soft tissue due to trauma.
• Eg: horse riders develop bones in their thighs (riders’ bones).

CLASSIFICATION OF BONE((Developmental):

• Membranous Ossification: Ossify from mesenchyme arranged as sheets or membranes.
  • E.g. Flat bones.

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• Cartilaginous or Endochondral ossification:
  • The mesenchyme forms cartilage models which later ossify.
  • E.g. Long bones , Bones of the base of skull.
• Short bones ossify from only a primary ossification centre, Exception- Calcaneum which also has a secondary centre.
• Most secondary centres appear after birth, Exception- the centre for the lower end of the femur which appears just before birth.

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LAW OF OSSIFICATION:

• The secondary centre which is the first to appear, unites last, Exception- Secondary centre for lower end of Fibula appears first and fuses first.

TYPES OF EPIPHYSIS:

1. Pressure Epiphysis: At sites of pressure.
   • eg: Ischial tuberosity, Femoral condyles.
2. Traction Epiphysis: Because of pull of muscles.
   • eg: Tubercles of humerus, Trochanters of femur.
3. Atavastic Epiphysis: Ancestrally was a separate bones but have fused with neighboring bone.
   • eg: Corocoid process of scapula.
4. Aberrant Epiphysis: The epiphysis of first metacarpal is normally at the base but may be aberrant at the head

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BLOOD SUPPLY OF BONE:

1. Nutrient arteries: pass obliquely through the compact bone of the shaft of a long bone via nutrient foramina supply the deeper parts of the bone, the nutrient artery is directed away from the growing end of the long bone.​​​​​
2. Epiphyseal arteries: From arteries supplying the joints
3. Metaphyseal arteries: From neighbouring vessels, Reinforce the nutrient artery.

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TAKE HOME MESSAGE

1. DEFINE HILTON’S LAW,GIVE EXAMPLES
2. TYPES OF OSSIFICATION WITH EXAMPLES
3. TYPES OF EPIPHYSIS WITH EXAMPLES
4. LAW OF OSSIFICATION