When two or more bones or cartilages come in contact with each other an articulation or a joint is formed. The study of joints is known as arthrology. The previous term syndesmology is not correct because it means study of ligaments (G. Syndermos = ligament) which only form a component of a joint. The joints vary considerably in their structure and function.
The shape of articulating surfaces and arrangement of various structures which unite them, determine the range of mobility at a particular joint. Actually the structure of a joint is an index of its functional activity,movements.
CLASSIFICATION OF JOINTS:
The joints could be classified on the basis of structure, development and function. However, for undergraduate students a purely structural classification is highly beneficial.
All joints are mesodermal in origin. The mesodermal element intervening between the developing bones and cartilages is differentiated into various structural components of a joint like joint capsule, synovial membrane, ligaments, etc.
The joints show varied degree of freedom of movements, i.e. immovable, slightly moveable and freely moveable. This freedom of movements is according to the functional need of a particular joint.
In the light of forementioned discussion, the joints are classified, on the basis of structure, into following groups:
I.FIBROUS JOINTS:
In these joints the union is due to dense fibrous tissue. They are usually fixed, immovable and mainly limited to the skull. They are subdivided into three major groups depending upon the location.
SUTURES:
They show following features:-
1.These are the joints of skull bones.
2.They are immovable and fixed.
3.The union is affected by sutural ligament.
In the vault of the skull the veins of sutural ligaments communicate with:
i)Diploic veins of neighbouring bones.
ii)Intracranial venous sinuses.
iii)Sutures are the sites of active bone growth.
They show gradual ossification from within outwards, which commences at the age of twenty and finishes in old age. It is an interesting feature that the sutures are obliterated on the inner aspect only, in a middle-aged skull, and are clearly visible on the external surface of vault of skull (i.e., pericranial surfaces). The process of obliteration of a suture is called synostosis.
Varieties of Sutures:
There are following varieties of sutures depending upon :
i)The shape of articulating surfaces.
ii) The mode of fusion of articulating bones.
Serrate Sutures :
Sagittal suture between two parietal bones. Here the shape of articulating surfaces is saw like.
Denticulate suture:
Example:
Lambdoid suture present between parietal and occipital bones. The shape of articulating surfaces is tooth-like. Here the two bones are more firmly united as compared with the serrate sutures.
Squamous sutures:
Temporo-parietal suture present between temporal and parietal bones. Here the two articulating bones overlap each other and are bevelled reciprocally, i.e., one bone internally and the other externally.
Limbous suture:
It is a slight modification of squamous suture. Here the bevelled surfaces may be mutually serrated or ridged. Modified Temporo-parietal suture.
Plane Sutures:
Examples:
i)Suture between horizontal plates of two palatine bones (Interpalatine suture).
ii)Palato-maxillary suture.
iii)Zygomatico-palatine suture.
Here the interlocking surfaces are slightly rough and irregular. Pure plane surfaces are seldom found. They can resist extreme torsional forces. 6. Wedge and groove sutures (Schindylesis):
bone.
Vomero-sphenoid suture – the suture between vomer bone and rostrum of sphenoid bone.
Here a ridged-bone fits into the groove present on a neighbouring
GOMBHOSIS (Peg and Socket Joint) :
It is a specialized fibrous joint in which the teeth fit into their sockets situated in the maxilla and mandible (i.e., upper and lower jaws). The ligament which connects the tooth with the socket or alveolus is called periodontal ligament.
It provides an independent and firm suspension for each tooth. It is characteristically seen in mammals and crocodiles. In non- mammals the periodontal ligament is absent and the tooth is firmly cemented directly to the bone. This cementing provide a brittle attachment.
SYNDESMOSIS:
It is a type of fibrous joint in which the two bones are united with each other by interosseous ligaments. It is a rarity in mammals and the only true example in man is the inferior tibiofibular joint. In this joint a slight degree of movements could be observed which is due to twisting or stretching of an interosseous ligament. This ligament never shows signs of ossification.
Examples:
i)Inferior tibio-fibular joint.
ii)Old sacro-iliac joint (it is a synovial joint which is converted into syndesmosis in older age).
iii)Coracoclavicular joint.
iv)Some joints between the vertebrae (i.e., joints between vertebral arches).
Interosseous membranes of fore-arm and leg which connect:-
a. Radius with ulna, and
b. Tibia with fibula, respectively.
CARTILAGINOUS JOINTS:-
In these joints the bones are united by a cartilage. They are usually slightly movable and are found in those places where stability and strength is required instead of free movements. The opposed bony surfaces are firmly bound together in order to minimize the risk of dislocations, which may produce serious effects in these places. They are mainly limited to the axial skeleton, e.g., vertebral column.
They are subdivided into two major groups on the basis of morphology and function :-
I.PRIMARY CARTILAGINOUS JOINTS (SYNCHONDROSES) They show following features:
1) These are the joints seen mainly in developing appendicular bones.
2)The bony ends and the shaft remain united with each other by a plate of hyaline cartilage. This cartilaginous plate is known as epiphy seal plate.
3)These joints are temporary because at the age of 18-20 years the epiphyseal cartilage is ossified.
4)They are completely immovable.
Examples:
i)Joints between the ends and shaft of growing long bones.
ii)Neurocentral joints of vertebrae.
iii)Spheno-occipital
iv)synchondrosis
1st sternocostal joint. (It persists and here bending and twisting could occur because the costal cartilage is about 2.5 cm long).