Anatomy of Bones and Joints: Structure, Functions, and Classification, Study Guides, Projects, Research of Physiology

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• (^) Bone is a highly specialized tissue designed to perform multiple vital

functions of the body. Some of them are as follows:

◼ It gives shape to the body in the form of the human skeleton

◼ It gives protection to the body’s vital organ systems by forming

compact cavities (e.g., chest cavity protecting the heart and the lungs)

◼ It forms a major organ of our life, i.e., movement through systematic

arrangement of the joints and muscular attachments

◼ It serves as a storehouse for important nutrients and minerals

(e.g.,99% of the body’s calcium is stored in the bones) required to

maintain the perfectly balanced metabolism of our body

THE BONES AND JOINTS

◼ The body’s defence in the form of WBCs and platelets are also

produced in the bone marrow.

◼ Although it appears to be an inactive solid mass, bone factually is a

very active tissue, e.g., immediately following a fracture, the activity of

bone formation begins at the fractured bony ends. Therefore, fracture

reduction and achieving a correct anatomical alignment of the

fractured bony ends must be done as early as possible.

• (^) The systematic and schematic arrangement of bones of various

shapes and sizes forming the basic structure of the body is known as

the skeleton

• (^) The bones and the joints in a human skeleton are so organized that

they provide the basic stability to the body to facilitate the required

functional mobility.

• (^) The stability to the body is provided by the girdles and the functional

mobility is provided through the limbs and their joints.

Human skeleton

The skeleton is composed of 206 bones and is categorized into two

basic types:

1. Axial skeleton – It is composed of 80 bones and forms the upright

axis of the whole body.

2. Appendicular skeleton – It is composed of 126 bones which mainly

include both the shoulder and the hip girdles and the long bones of

both the upper and lower extremities

• (^) The 206 bones of the human skeleton are shaped and sized according

to their functions.

• (^) 1. The long bones (e.g., femur, tibia): These serve as levers to facilitate

muscular action.

• (^) 2. Short bones: These are generally found where only limited

movements are required. Mainly, these bones provide strength and

stability to the strong muscular action by providing a stable base (e.g.,

carpal bones).

• (^) 3. Flat bones: These are composed of a thin layer of cancellous bone

covered on both the sides by a parallel layer of compact bones, (e.g.,

skull, scapula, pelvic bones). The scapula provides free but stable

mobility to the shoulder and arm, whereas the skull and the pelvis

protect vital soft organ systems.

Types of bones

•

4. Irregular bones: These are irregular in

shape with small bony appendages, e.g.,

vertebrae. These bones provide stable

flexibility to the trunk while protecting the

spinal cord.

5. Sesamoid bones: These are small

rounded or triangular bones which develop

in the substance of a tendon or fascia to

protect and facilitate muscular action, e.g.,

patella – a typical resemblance to the

‘sesame seed’.

• (^) A bone consists of two major portions: cortex and medulla.
• (^) Cortex Cortex is the outermost hard layer giving shape, strength and

protection from injury. It has a smoother covering layer (periosteal

layer) which also gives attachment to muscles, tendons and ligaments.

It allows remodelling of the bone throughout the life.

•

Medulla is the softer inner lining of the bone with a cavity within it

(medullary cavity). Medulla is a storehouse of important minerals,

calcium and a major seat where the RBCs and WBCs originate.

Structural and microscopic composition of a

bone

Microscopic composition of bone

• (^) The structural unit of a lamellar bone is Osteon, which consists of a

series of concentric laminations surrounding the central longitudinal

Haversian canal which contains a nutrient artery supplying blood to

the bone

• (^) Bone cells There are three types of bone cells:
• (^) 1. Osteoblasts – bone forming cells
• (^) 2. Osteoclasts – bone resorbing cells
• (^) 3. Osteocytes – resting cells which can act as both osteoblasts and

osteoclasts as per the need

Microscopically, bone is classified as follows:

◼ Woven – It is an immature bone where the cells and the collagen are

arranged in a random pattern. It is found during the initial stage of bone

formation after fracture – when the bone is in the process of formation.

◼ Lamellar – The bone is in a fully matured stage where the cellular

distribution is set in an orderly fashion and the collagen fibres are also

properly oriented. The dense arrangement of lamellae is present in the

cortical bone, whereas the lamellae are arranged loosely in a cancellous

bone.

• (^) The development and growth of a long bone (except clavicle) begins

during the intrauterine period and continues to the period of skeletal

maturity (around the age of 16–18 years).

•

Events in the development and growth of a bone

• (^) 1. Development of the cartilaginous model
• (^) 2. Ossification of the cartilaginous model to bone

Development and growth of a long bone

• (^) Development of a cartilaginous model: The bone begins to develop

during the embryonic life from cartilaginous primordia (enchondral

ossification) by the process of condensation of the mesenchyme – as

hyaline cartilage surrounded by pericondrium.

Ossification of a cartilaginous model to bone: Around the fifth week

of the intrauterine period, the ossification begins by the appearance

of the primary centre of ossification (PCO) at the middle of the

bone, when it is invaded by capillaries. The pericondrium is

converted to periosteum and the osteoid is produced around the

shaft. The shaft begins to grow longitudinally by the resorption of

the inner surface.

• (^) During the late fetal stage or early childhood, two secondary centres of

ossification (SCO) appear (the epiphyses). The first SCO appears at the upper

epiphysis followed by the second SCO at the lower epiphysis.

Both the SCOs are responsible for the radial growth of a bone.

The consolidation and thickening of the periosteum occurs by the process of

subperiosteal new bone deposition. The SCOs are also called apophysis (e.g.,

apophysis of the greater trochanter).

As the bone approaches adult size, the diaphysis and epiphyses are gradually

ossified fully, but are still separated by epiphyseal plates (cartilage). As the

epiphyseal plates cease to grow, the longitudinal growth of a bone stops.

At the end of the growth period of a bone, the epiphysis fuses with the

diaphysis leaving a definite area of hyaline layer at the articular surface of a

bone.

• (^) Throughout the lifespan, the bone has an ability for remodelling. A

variety of alterations in the size, shape and structure appear in a fully

matured healthy bone.

• (^) This remodelling occurs as a result of its hypertrophy as a response to

stress. Bone hypertrophy occurs in the plane of a stress. Note: The

time and the sequence of the appearance, and the fusion of epiphysis

have a great clinical relevance:

◼ In differentiating an epiphyseal plate from a fracture.

◼ In confirming a true bone-age of a person.

Re-modelling of bone

• (^) Epiphysis: In growing children, the long bone consists of two ends known

as epiphysis. It forms a support for the joint surface. It is susceptible to

developmental problems (epiphyseal dysplasia), degenerative changes,

injury and avascular necrosis due to ischaemia.

• (^) Diaphysis: A large part of the long bone or shaft constitutes diaphysis. It

is made up of strong cortical bone but due to mechanical disadvantage,

it always remains susceptible to fracture with angulation.



The process of healing is slow as compared to metaphysis. It may

develop dysplasias or infection. Bone has the ability for remodelling or

changing its shape in response to stress.

General structure of a long bone

• (^) Metaphysis: The portion of the bone adjacent to each epiphysis is

known as metaphysis. It is made up of cancellous bone where the

process of healing is fast. It is susceptible to bone infection, dysplasia

and tumours.

Growth plate :There is a thin plate of growth cartilage, one at each

end called ‘growth plate’. At the time of maturity, this growth plate

fuses with metaphysis.

The articular surfaces of the epiphyses are covered with an articular

cartilage. The rest of the bone is covered with periosteum which

provides attachment to tendons, ligaments and muscles. Although

mechanically weak, it helps in longitudinal growth of a bone. It is

susceptible to injury or slipping (slipped femoral epiphysis), tumour

and osteomyelitis. It is also susceptible to growth arrest as well as

deformed growth. Note:

Blood supply to a long bone

• (^) The blood supply to the long bone is derived from four vessels:
• (^) Metaphyseal vessels: Metaphyseal vessels are numerous small vessels

derived from the anastomosis at the joint. They enter the metaphysis

along the line of attachment of the joint capsule.

• (^) Nutrient artery: The major vessel providing nourishment to the long

bone enters the bone around the middle of the shaft and immediately

bifurcates running in opposite directions towards the proximal and

distal end of the bone as medullary vessels. Then each one further

divides into a number of parallel vessels towards the respective

metaphysis

• (^) Epiphyseal vessels: Epiphyseal vessels enter the epiphysis providing

independent nutrition to the epiphysis or epiphyseal site.

Periosteal vessels: The periosteum is richly supplied with blood

which it receives from a number of small vessels. These directly

enter the bone and supply mainly the cortical area of a bone. These

vessels play an important role in the process of the healing of the

bone following fracture.

• (^) EP: epiphyseal vessels. Directly enter

and supply epiphysis.

MP: metaphyseal vessels –

numerous small vessels derived

from the anastomosis around the

joint. Piercing metaphysis along the

line of joint capsule.

NV: major vessel entering the bone

around its middle-nutrient vessel.

The periosteum of a bone also has a

rich blood supply through the small

vessels supplying the bone cortex.

A: anastomosis.

• (^) Joints represent the sites where two bones come together.
• (^) These are designed on the basis of their function. Functions of the

joint are:

• (^) To protect vital organs from injury (e.g., brain)
• (^) To provide adequate mobility of the body part to facilitate functional

activity (e.g., limbs)

• (^) To provide only restricted mobility to facilitate the function of the

internal organs and protect them (e.g., lungs)

JOINTS

Broad classification of joints

•

Synovial joints are so called because the joint space has a special

lining membrane called the synovial membrane.

• (^) It secretes a highly thick oily lubricating fluid. Synovial fluid allows

freedom of movement to the joint and also provides nourishment to

the articular cartilage.

SYNOVIAL JOINTS

TYPICAL SYNOVIAL JOINT(KNEE JOINT)