Yuk Kenali Tulangmu!
Channel: M. Rifki Al Ikhsan, MD
Date: 2020-11-28

The bones (Ossa) form the bony skeleton of the body. The adult
skeleton is made up of nearly 200 single bones, which are connected
by joints. Bone is not a dead, but a living tissue that is very well supplied with
blood. In 10 % of bones there is a constant remodelling of bone
even after completed growth. If osteogenesis and osteoclasis take
place at different sites, this will change the form (shape) of the bone
(modelling); if both processes take place at the same site, the form
does not change (remodelling). With a constant loading of bones,
osteogenesis and osteoclasis will be balanced. Only when one process
outweighs the other, does the bony structure begin to change:
if osteogenesis predominates, this leads to a greater density of
bones (osteosclerosis); if more bone is degenerated (by osteoclasis)
than newly formed, one speaks of osteolysis or osteoporosis. These
processes can be localised or generalised.
Bone consists of organic substances (mainly type I collagen, and
bone cells [osteocytes, osteoblasts, osteoclasts]) and inorganic matrix
(containing salts such as calcium phosphate, magnesium phosphate
and calcium carbonate as well as calcium, potassium and sodium
compounds with chlorine and fluorine). 99 % of the body's
calcium reserves are bound in the bone. This is equivalent to
1–1.5 kg calcium in the human body. Only 1 % is not bound in bone, but is found elsewhere, e. g. in the blood or muscles. Organic
substances and inorganic matrices form a composite, its mechanical
properties depend on the ratio of the individual components.
Osteogenesis (bone formation) and osteoclasis (bone resorption) are
influenced by various factors such as mechanical load, hormones,
growth factors, matrix molecules and cytokines.
Functions of bone are:
• Support (the entire skeletal system)
• Protection (skull and vertebral canal)
• Calcium reservoir (the entire skeletal system)
• Blood formation (the entire skeletal system, age-dependent) 
In the case of inactivity or immobilisation (e. g. by a plaster
cast), lack of gravity (astronauts), and diseases such as osteoporosis
or spreading of malignant tumour cells in bone (bone
metastases) the bone mass is reduced, and the stability of the
bone is at risk. The goal of treatment is to balance this disequilibrium
in favour of a regeneration (osteogenesis) or preservation
of bone mass by means of an early initiated therapy
with exercises or drugs. The functional early mobilisation after
injury and operations through physical therapy is therefore of
very great importance.
Long bones
(Ossa longa)
Femur, Humerus • Have a shaft (Corpus) and ends
(Extremitates)
• The medullary cavity (Cavitas
medullaris) is in the shaft
Flat bones
(Ossa plana)
Os parietale, Scapula Consist of 2 compact lamellae
with integrated cancellous bone
in between
Short bones
(Ossa brevia)
Carpal bones, tarsal
bone
Have no medullary cavity, but a
core of cancellous bone
Irregular bones
(Ossa irregularia)
Vertebrae Bones that do not fit into the
above categories
Air-filled bones
(Ossa pneumatica)
Maxilla, Os ethmoidale
Bones with one or more air-filled
spaces and lined with mucosa
Sesamoid bones
(Ossa sesamoidea)
Patella, Os pisiforme Bones embedded in tendons
Accessory bones
(Ossa accessoria)
Os trigonum Accessory bones that do not
occur regularly
Functional adaptation
Construction principle/architecture
The shape of bones is genetically determined; in contrast their
structure depends considerably on the type and amount of mechanical
stresses acting upon them. In this respect, bones are
formed according to an economic construction principle: with a
minimum of material, a maximum of strength is achieved (minimum-
maximum principle). The tube (of long bones) is the optimal
architectural element for a body that is subject to bending: being
hollow inside and flexible in all directions (high peripheral tensions,
inside = 0 = neutral fibre = no tension). The light weight
construction is particularly evident when one considers the weight
relations: the bones account for only 10 % of body weight, the muscles
over 40 %.
Bones have an outer layer of compact or cortical bone (Substantia
compacta or corticalis) consisting of osteons and lamellae, and an
inner light weight construction due to trabeculae (Substantia
spongiosa, cancellous or spongy bone trabeculae) with interposed
spaces, filled with bone marrow or medulla (› Fig. 1.14 ).
The plate-like structure of flat bones allows the distinction between
an outer and an inner cortical layer with spongy bone in between.
On the flat cranial bones, the outer layer of the Substantia compacta
is referred to as Lamina externa and the inner layer as Lamina
interna. The Substantia spongiosa is referred to as diploe. The hollow
spaces of pneumatised (air-filled) bones are lined with mucous
membrane. This construction, which is only found in the cranial
region, is a useful means of minimising the weight.