Introduction:
Bones meet each
other at joints or articulations. Some of these articulations unite the bones
firmly while others allow movement.
Bones are united by the following:
a) dense connective tissues forming fibrous joints e.g. radius and ulna;
b) a cartilage forming cartilaginous joints
e.g. between sternebrae, pelvic symphysis or the mandibles;
c) a fluid-filled cavity intervenes between
the bones forming synovial joints.
a +
b are relatively immovable and
are termed synarthroses while ( c )
is freely moveable and is termed diarthrosis.
1. Syndesmosis – the uniting material is
connective tissue and if it is extensive as between radius and ulna it is
called interosseous membrane.
2. Suture - special type of joint uniting
bones of the skull. The uniting medium
is either a fibrous seam or a narrow seam of cartilage from which the bone keep
growing. The edges of the bones may have
interlocking margins (sutura serrata) such as of the frontal bones; b) overlap
as the tiles of a roof e.g. parieto-temporal (sutura squamosa) c) the edges are
smooth or slightly roughened (sutura plana) and may completely ossify in old
age e.g. nasal bones.
3. Gomphosis – the implantation and
anchoring of the teeth in their alveoli by the periodontium (alveolar
periosteum).
1. Synchodrosis – the bones are united by a hyaline cartilage e.g. between
diaphysis and epiphysis of juvenile long bones, and between skull and hyoid
bone (between tympanohyoid and temporal bone) except in carnivores where it is
fibrous. Basisphenoid and presphenoid
bones of juvenile animals are also joined by a synchondrosis.
2. Symphysis – the two bones are joined by a
fibrocartilage or regular connective tissue.
Examples are the joints between the symmetrical halves of mandibles and
of the pelvic girdle, also joint between successive vertebral bodies. When the uniting medium becomes ossified,
these joints are converted into synostoses.
A
typical synovial joint consists of the following parts:
a) articular
extremities of at least two bones covered with a hyaline cartilage,
b) joint
capsule which surrounds the joint cavity and consisting of
i)
inner synovial layer, the synovial membrane (membrana
synovialis)
ii)
an outer tougher fibrous layer, the fibrous
membrane (membrana fibrosa).
c) joint ligaments – these
hold the bones together and restrict the movement of the joint. They are composed of dense connective tissue
which consists of regularly arranged collagen fibers.
Structure of a synovial joint
The articulating bones are separated by a fluid
filled space, the joint cavity.
The fluid is termed synovia or synovial fluid. The space is bound by a synovial membrane
which is attached around the periphery of the articulating bones. A synovial membrane is strengthened
externally by a fibrous capsule, while additional ligaments are placed to
restrict the movement to the required directions and extent only.
Articular surface is covered with articular hyaline cartilage. The cartilage is very thin in the dog, but
thicker in the larger joints of horse and ox, being thickest in the centre of
the convex surface and about the periphery in the concave ones. The cartilage is insensitive and avascular,
drawing its nutrients by diffusion from the synovial fluid, vessels in the
tissues periphery to the cartilage and from vessels close to the marrow spaces.
Synovial membrane
It completes the lining of the joint but may either
a)
be left entirely unsupported and thus pouch out
forming diverticular.
b)
rest directly on a tough outer fibrous capsule,
c)
may be separated from an outer fibrous capsule by
the interposition of pads of fats which are called synovial pads or cushions.
The inner surface of the membrane carries many projections
of various sizes (synovial villi or folds, (villi or plica synoviales) which increases the surface area of the
membrane. The membrane is vascular and
sensitive. The synovial membrane is
lined with a squamous epithelium (often incompletely) and secretes the
aminoglycans (lubricant component) of the
synovia. The nutritive component of
the synovia is derived from blood plasma.
The synovial membrane also have an absorptive function.
In the joint cavity of the same articulations are fibrocartilaginous discs. When they separate the cavity into two as
occurs in the temporo-mandibular joint, they are known as articular discs (disci articulares) while if they only
partially separate the cavity and are semilunar in shape as in the
femoro-tibial joint they are called menisci, menisci articulares. The
discs and menisci act as shock absorbers and render the articular surfaces
congruent. An articular labrus (labra articularia) or marginal cartilage is a
fibrocartilaginous rim placed around the circumference of certain concave
articular surfaces e.g. acetabulum.
Fibrous membrane
Is attached around the margins of the articular
surfaces. Local thickenings of this
fibrous layer form ligaments, some
of which may be intracapsular (e.g. cruciate ligaments of the stifle joint)
while others are extracapsular (e.g. the collateral ligaments). The ligaments hold together the bones
constituting the joint and guide them in their movements, or in some instances
to check the extent of the movement.
Movement in joints
There are two main types of movements in a joint:
1. Rotation – the moving bones turn about an
axis perpendicular to its articular surface.
2. Pendular movements in sagittal planes
(parallel to the median plane). These
predominate in the joints of limbs and include:
a)
flexion – reduces the angle between the two segment
of limbs,
b)
extension – opens the angle and brings the two
segments more closely into alignment,
c)
over – extension (dorsal flexion) – e.g. of the
fetlock at landing (metacarpal / tarsal – 1st phalanx),
d)
adduction – carries the moving part towards the
median plane,
e)
abduction – carries the moving part further from
the median plane,
f)
circumduction – combination of extension, flexion,
adduction and abduction allowing the extremity of the limb to describe a
circle, thus defining a cone.
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