Bone

Published on 11/03/2015 by admin

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1 Bone

Chapter contents

Structure of bone

‘Normal’ radiographic bone appearances

Bone is a highly vascular connective tissue, in which bone cells are enclosed in a mineralised intercellular matrix interposed with a system of collagenous fibres.

The organic matrix forms one-third of the structure and gives resilience and a degree of flexibility; the mineral salts (mainly calcium and phosphorus) form the remaining two-thirds and provide the strength and weightbearing capabilities of the bone.

Structure of bone

There are two main types of bone: compact (dense) and cancellous (spongy). Compact bone forms the surface layers or cortex of mature bones and cancellous bone the interior aspect.

Compact bone

This type of bone is found mainly in the shafts of long bones where a strong, tubular structure is required. It consists of a number of cylindrical structures called haversian systems (Figs 1.1 and 1.2). Each system comprises:

Fig. 1.1 Transverse section of bone (microscopic).

A – Haversian canal

B – Canaliculi

C – Circumferential lamella

D – Interstitial lamella

Fig. 1.2 Longitudinal section of bone (microscopic).

1 – Haversian canal

2 – Volkmann’s canal

4 – Canaliculi.

A central haversian canal –

which contains blood, lymphatic vessels and nerves.

Lamellae –

rings of bone round the haversian canal.

Lacunae –

spaces between the lamellae, which contain osteocytes (mature bone cells).

Canaliculi –

channels carrying nutrient fluid, which connect the lacunae and communicate with the haversian canal.

Interstitial lamellae –

fill the spaces between adjacent haversian systems.

Circumferential lamellae –

rings of bone round the edge of the bone.

Volkmann’s canals –

join the various haversian canals.

Periosteum –

a membrane which surrounds the bone, except at the articular surfaces, where articular hyaline cartilage allows friction-free movement. The inner layer is vascular and cellular providing nutrition, growth and repair. The outer layer is fibrous and blends with tendons and ligaments.

Cancellous bone

This type of bone is found in the parts of bones where lightness, strength and area are required. It is similar in structure to compact bone but bone marrow is found between the trabeculae which form the internal support structure of the bone.

Elsewhere the red bone marrow becomes inactive yellow marrow.

Development

Some bones develop from rods of cartilage, e.g. the bones which form the limbs, the trunk and the base of the skull. Some bones develop from membranes, e.g. bones of the vault of the skull, the face and the clavicle. Some bones develop in tendons, e.g. sesamoid bones – the patella and fabella.

Ossification

Ossification is the formation of bone from connective tissue and requires:

adequate calcium and phosphate in the blood

a supply of vitamins A, C and D.

Growth of the bone is influenced by the following hormones:

Intramembranous ossification

This takes place in a membrane, e.g. in the vault of the skull in the developing fetus. At the point of ossification, osteogenic fibres and bone cells appear in the connective tissue and calcium salts are deposited to form osteoid tissue. Ossification spreads from the centre outwards.

Intracartilaginous ossification

The process by which bone formation takes place, e.g. in a long bone (Fig. 1.3).

Fig. 1.3 Section through a developing long bone.

A – Diaphysis

B – Epiphysis

C – Epiphyseal plate

D – Metaphysis

E – Medullary cavity

F – Periosteum

G – Articular hyaline cartilage.

Primary centre of ossification

This appears in the middle of the diaphysis. Osteoblasts (bone cells) appear and calcium is laid down.

Osteoblasts ‘build the bone’ by laying down fibres, matrix and calcium.

Osteoclasts are a type of cell which ‘destroy the bone’ and therefore mould the bone into the required shape – ‘remodelling’.

Osteoclasts are responsible for forming the medullary canals and sinuses within the bone.

At the same time as the diaphysis is being formed, bone is being built up on the outside of the shaft, which later forms the periosteum.

Secondary centres of ossification

These appear at the ends of the bone and form the epiphyses (singular epiphysis). The epiphysis is separated from the diaphysis by a thin layer of cartilage called the epiphyseal plate.

Growth

This occurs during childhood by the production of bone at the epiphyseal plate. It occurs at the side of the epiphyseal plate nearest the shaft, which is called the metaphysis.

Fusion

The fusion of the epiphysis with the diaphysis occurs when the bone reaches the desired size. A typical long bone can take up to 20 years to reach fusion.

Function of bone

supports soft tissue

supports the body weight

enables movement

protects organs, e.g. the brain

produces blood cells from the red bone marrow.

Blood supply

The blood to the bone supplies:

epiphyseal cartilage

There are several distinct points where the large blood vessels enter the bone, called the nutrient foramina. These usually point away from the dominant growing end of the bone. Numerous smaller vessels enter through the non-articular surfaces of the epiphyses.

The arterial blood feeds the bone and then drains into venous channels which leave the bone through the surfaces which are not covered with articular hyaline cartilage.

Nerve supply

Nerves are widely distributed in the periosteum and nerve fibres accompany the arteries into the bone via the nutrient foramen.

Types of bones

Short bones (cuboidal shape)

These are formed by cancellous bone with a thin covering of compact bone, giving strength but with limited movement.

Examples

Flat bones

These have a thin layer of cancellous bone enclosed in two thin layers of compact bone and are found where protection for underlying organs or extensive muscle attachment is required.

Examples

vault of the skull.

Irregular bones

These are composed of cancellous bone surrounded by a thin layer of compact bone.

Examples

Sesamoid bones

These develop in tendons, usually near a joint, and their main function is to protect the tendon from wear as it moves over the bony surface.

Examples

‘Normal’ radiographic bone appearances (Figs 1.4, 1.5 and 1.6)

Cortex –

is more dense than the medullary cavity and therefore absorbs more radiation, producing a reasonably solid ‘white line’ round the periphery of the bone.

Fig. 1.4 Normal bone appearances, left foot.

B – Medullary cavity (cancellous bone)

C – Joint cavity.

(Courtesy of Ernest Higginbottom.)

Fig. 1.5 Normal bone appearances, ankle.

A – Fused epiphysis

B – Epiphysis.

(Courtesy of Ernest Higginbottom.)

Fig. 1.6 Radiograph of the upper end of femur.

A – Compact bone

B – Spongy bone

C – Medullary cavity.

(From Bryan 1996.)

Medullary cavity –

is less dense than the cortex and therefore appears slightly darker.

Cancellous bone –

the trabeculae, which are the support structure of the cancellous bone, have the appearance of very fine ‘white lines’ throughout the bone.

Epiphyseal plate –

as this is formed by cartilage it is radiolucent and therefore care must be taken not to confuse it with a fracture. It has the appearance of a radiolucent area with 2 fairly regular margins, extending to the periphery of the bone and situated near the ends.

Fused epiphyses –

appear, usually, during the teenage years, and occur as a thin ‘white line’ along the site of the old epiphyseal plate. With age these lines are no longer visible.

Joint cavity –

this contains articular hyaline cartilage and synovial fluid and is radiolucent (and therefore not demonstrated).

Terminology

Learning these terms will enable the reader to work out the names of different parts of a bone.

The names of most aspects of bone can be built up logically from a combination of some of the following:

an adjective derived from the name of the bone

an adjective derived from the bone with which they articulate

an adjective derived from part of the bone with which they articulate

a descriptive term – see lists below (elevations, projections, holes and depressions).

Prefixes

Demi –

Epi –

Infra –

Inter –

Intra –

Sub –

Supra –

Related

Bones and Joints A Guide for Students