They show following features :-
1.These joints are mainly seen in axial bones.
2.These joints are formed by white fibro-cartilage which connects the articular surfaces covered by hyaline cartilage.
3.They are slightly movable and mobility depends upon sufficient amount of white-fibro cartilage.
4.Frequently a cavity is seen which is not lined by synovial membrane.
5.This cavity contains tissue fluid.
Examples:
i) Public symphysis.
iii) Manubriosternal joint. In early life it is primary but in later life it becomes secondary due to conversion of hyaline cartilage into white-fibro cartilage.
ii) Joints between the bodies of vertebrae (Here a false joint cavity is present).
Typical vertebrae are united with each other by three joints:
a)Symphysis (between bodies of vertebrae).
b)Synovial (between articular processes of vertebrae).
The Joints
- Syndesmosis (between laminae of adjacent vertebrae).
SYNOVIAL JOINTS:
The synovial joints are highly evolved, uniquely constructed, specialized articulations, where all possible movements can occur. These joints are seen in all vertebrates and show modifications according to functional needs. All synovial joints possess a definite joint cavity lined by synovial membrane and filled with a lubricating-synovial fluid.
Synovial joints show following characteristics:
1)The bony articular surface are covered by articular cartilage and are not attached to each other.
2)The bony articular surfaces are connected with each other by tubular fibrous capsule (articular capsule).
3)The fibrous capsule is lined by mesothelial membrane called synovial membrane. This membrane does not cover the articular cartilage covering the articular bony ends.
4)Between the articular bones a specific joint cavity is present which is lined by synovial membrane.
5)The joint cavity is filled with synovial fluid secreted by the synovial membrane.
6)The joint capsule in strengthened by the presence of accessory ligament.
7) The joint cavity may be divided partially or completely by an articular disc composed of white-fibro cartilage.
8)Different types of movements are always possible.
To appreciate the morphology and classification of synovial joints, it is profitable to consider the following embryological points :-
1. Joint capsule migration:
During foetal life the capsule of a synovial joint is attached to the epiphyseal line. However, in adult life the attachment of the capsule is shifted from epiphyseal line. As a result of this the epiphyseal line may become intra or extra capsular, e.g., Epiphyseal line of the head of femur becomes intra-capsular whereas the epiphyseal line of the lower end of femur becomes extra capsular.
2.False ligaments formation:
Localized thickenings of the joint capsule may develop and are termed as false ligaments, e.g., Glenohumeral ligaments of shoulder joint.
3. True ligaments formation:
Thick fibrous bands appear independent of the joint capsule. They may be separated or fused with the capsule, e.g., coraco-humeral and transverse humeral ligaments of shoulder joint.
4.Conversion of phylogenetically degenerated portions of tendons into ligaments:
The distal parts of certain tendons of muscles, which were functional in lower vertebrates, have converted into ligaments of joints. These tendons were actually freely passing over the joints in lower forms, e.g.
i) Piso-hamate and pisometacarpal ligaments of wrist and carpometacarpal joints represent the degenerated distal portion of tendon of flexor capri ulnaris muscle.
ii)Tibial collateral ligament of knee joint is the lower degenerated portion of tendon of adductor magnus muscle.
iii) Oblique popliteal ligament of knee joint represents one of the degenerated distal expansion of tendon of semimembranosus muscle.
Fate of intra-articular mesenchyme :
It persists in the form of: Synovial membrane.
ii)Intra-articular discs and menisci.
iii)Ligaments (cruciate ligaments of knee joints).
SYNOVIAL JOINTS can be classified according to following schemes:-
Depending upon the shape of articulating surfaces :
i)Homomorphic: When the two articulating surfaces are plane the synovial joint is called homomorphic, e.g.
ii)Plane joint (Intermetatarsal joints).
iii)Saddle joint (Carpometacarpal joint of thumb).
iv)Hetromorphic: When there is varied appearance of two articulating surfaces, then the synovial joint is called hetromorphic, e.g.
v)Ball & Socket Joint (shoulder and hip joints).
vi)Condyloid joint (Temporomandibular joint and knee joint.
vii)Ellipsoid joint (Radiocarpal joint).
Depending upon complexity of organization :
1.Simple: When only two bones are participating, the joint is called simple joint, e.g.
i)A cromio-clavicular joint.
ii)Shoulder joint.
Compound: These are formed by participation of more than two bones.
i)Elbow joint
ii)Knee joint.
Complex: When intra articular discs or menisci are present, the joint is called complex, e.g.,
i)Knee joint.
ii)Temporomandibular join.
Depending upon axis of movement:
Unioxial: In these joints movements take place on one axis only, i.e., on transverse or longitudinal axis, e.g., Elbow joint. Biaxial: In these joints movements take place on two axes.
ii)Transverse axis,
ii)Longitudinal axis, e.g., Wrist Joint.
Polyaxial: In these joints movements take place on more than two axes, e.g.
i)Shoulder joint
ii)Hip joint
Depending upon the types of movements :
1.Gliding: e.g., plane joint (Intermeta – tarsal joints).
2.Angular: e.g., Condyloid joint (Temporo-mandibular joint; knee joint).
3.Rotatory: e.g., Pivot joint (Superior radio-ulnar joint).
4.Circumductory: e.g., Ball and socket joint (shoulder joint and hip joint).
Architectural or structural classification :
According to this about 7 varities have been recognized:
1. Plane joints: These are formed by the apposition of fairly flat articular surface, e.g.
i)The Intermetatarsal joints.
ii)Some intercarpal joints.
Hinge joints:
They roughly resemble the hinges of a door, where the articulating surface are so moulded that they restrict the to-and-fro movements to one plane, i.e., they are uni-axial. The sides of the joint are typically provided with strong collateral ligaments, e.g.
i) Elbow joint
ii)Interphalangeal joints
Pivot joints: These are uni-axial joints (in which movements take place around the longitudinal axis). They are composed of pivot surrounded by an osteo-ligamentous ring. Movement is restricted to a rotation around a longitudinal axis passing. through the centre of the pivot, e.g.
i)Proximal radio ulnar joint.
ii)Atlanto-axial joint.
The pivot may habitually rotate within the ring as in case of ‘Proxial radioulnar joint’ in which the head of the radius rotates within a ring formed by the annular ligament and radial notch of ulna.
Conversely the ring may rotate around the pivot as in the articulation between the dens of the axis and ring formed by anterior arch of the atlas and its transverse ligament, i.e., Atlantoaxial joint, the ring rotates around the pivot, i.e., odontoid process of axis or 2nd cervical vertebra.
Condylar or condyloid joints:
Here two condyles (male surfaces) are received into two concave (female surfaces) cavities. It allows the principal movement occurring in one plane, but a limited amount of rotation is possible about a second axis set at 90° to the first one, e.g.
i)Temporo-mandibular joint
ii) Knee joint
Ellipsoid joints: These are biaxial joints formed by reception of an oval, convex male surface into an elliptical female concavity.
Primary movements are possible about two axes situated at right angles, e.g., Flexion and extension and abduction & adduction. These movements may be combined as a movement of circumduction. There is little appreciable rotation around the third axis, as this is prevented by overall shape of articular surfaces.
Saddle joints:
These are biaxial joints which are formed by reception of concavo-convex opposing surfaces. Primary movements occur in two planes at right angles to each other but because of the articular geometry, these are accompanied by a degree of axial rotation of moving bones.
Best known:
(i) Corpometacarpal joint of thumb.
Less important:
(ii) Ankle joint.
(iii) Calcaneocuboid joint.
Ball and socket joints:
They are polyaxial joints and are formed by reception of globular Head (male surface) into a cup like concavity or socket (female surface). All types of movements are possible.
i)Shoulder joint
ii)Hip joint