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Functions of subintimal tissue:

Due to high elasticity of elastin fibres and their concentration in mobile parts of the synovial membrane, the subintimal tissue prevents the nipping of synovial membrane during movements.

FUNCTIONS OF THE SYNOVIAL MEMBRANE

1.It performs three functions:
2.It takes part in the production of synovial fluid.
3.It helps in absorption of the fluid.
4.It shows phygocytic activity.

SYNOVIAL FLUID:

It is an egg-ablumen like fluid present in the joint cavities of synovial joints, bursae and tendon sheaths.

PROPERTIES:

It is a dialysate of plasma with addition of hyaluronic acid. It shows following properties :-
1.It is slightly yellow, viscous, clear fluid.
2.It is slightly alkaline in pH. However, the pH changes with activity. In exercise it turns towards acidic side.
3.Its volume is very small,0.5 ml fluid can be aspirated from the knee joint.

It contains :-

i) Hyaluronic acid or synovial mucin (secreted by Type- A cells).
ii)The cell population is comprised of monocytes, macrophages, lymphocytes, neutrophils and synovial cells.
iii)A morphous metachromatic material which is formed as a result of wear and tear process.

FUNCTIONS:

1.It provides nutrition to the articular cartilages, discs and menisci. It lubricates the joints and in this manner increases their efficiency.
2.It prevents the erosion of articular cartilage during movements and weight bearing.
3.It exhibits phagocytic activity due to its cellular components.
4.The volume, viscosity and colour of the synovial fluid is variable in different joints and in different species. It contains no fibrinogen.

Synovial fluid and mechanisms of Joint Lubrication :

Various models have been proposed but none of them is satisfactory (Dowson et al. 1975). These models are touched briefly :-
Fluid film lubrication or hydrodynamic lubrication. Elastohydrodynamic lubrication.
1.Boundary lubrication.
2.Weeping lubrication.
3.Boosted lubrication.


Boosted lubrication is receiving greater attention in the recent years. It tells us that when the articular cartilages are compressed, the synovial fluid is trapped in the irregularities present on the surface of the articular cartilage.

As the compression increases, the small molecules of the fluid are passed into the joint cavity over the contact area. The fluids present in irregularities of the articular cartilage becomes very viscous due to increased synovial mucin content. Thus this fluid forms a more effective coat which protects and lubricate the joint surfaces.

ARTICULAR DISCS:

These are pads of white-fibrocartilage interposed between the two articular bony surfaces.
Examples:
i)Temporomandibular joint.
ii)Knee joint.
iii)Sternoclavicular joint.
iv)Acromioclavicular joint.
v)Radio-ulnar joint (i.e., inferior radio-ulnar joint).

Features:

i)They are composed of dense fibrous tissue with few cartilage cells.
ii)At the periphery they are continuous with the articular capsule through the medium of loosely arranged vascular connective tissue.
iii) They are covered by flattened cells which are continuous with the synovial cells of the synovial
membrane.
iv) They are a nervous.
v)They are avascular except at the periphery.
vi) They partly or completely divide the joint cavity into proximal and distal compartments.
vii)On the proximal surface of the disc one type of movement takes place (e.g., flexion and extension) while on the distal surface another type of movement occurs (e.g., rotation or gliding).

Functions:

i)They help to maintain perfect lubrication of the joints.
ii)They serve to minimize attrition of the articular surfaces.
iii)They increase the range of mobility by converting the single joint cavity into two, e.g.
iv)Temporomandibular joint
v)Sternoclavicular joint
iv)Inferior radio-ulnar joint

They act as shock absorbers. Actually the cartilage is non-weight bearing so it cannot act as shock absorber.

• They strengthen the joint by acting like an accessory ligament.
• They help to make a joint more harmonious.

FATTY PADS (HAVERSIAN GLANDS)

These are pads of fatty tissue occupying spaces where the articular surfaces are uneven. They are usually situated between :

(a) Synovial membrane and fibrous capsule.
(b)Synovial membrane and bone.

Examples:
i)Hip joint (Haversian pad).
ii)Knee joint (i.e., Alar and infra patellar folds.)

Functions:
Talo-calcaneo navicular joint (Mid-tarsal joint).
They act like swabs to spread the synovial fluid.

INTRA ARTICULAR STRUCTURES:

These are those joint elements which are seen inside the articular capsule and take part in joint-stability. They include the followings:-
a)Fatty pads:
Examples:
i)Hip joint.
ii) Knee joint.

Articular Menisci :

Examples:

i)Temporomandibular joint
ii) Knee joint

Muscle-tendons :

i)Tendon of popliteus at the knee joint.
ii)Long Head of Biceps brachii at the shoulder joint

Ligaments:

Examples:
i)Ligamentum teres of the hip joint.
ii) Cruciate ligaments of the knee joint.
iii)Intra-articular ligaments of heads of 2nd to 9th ribs. These ligaments are absent in joints of Head of the first, tenth, eleventh and twelfth ribs.
iv)Intra-articular ligament of 2nd chondrosternal joint (sternocostal joint).

Fibrocartilagenous rims or labra:

Examples:
i) The glenoidal labrum of the shoulder joint.
ii)Acetabular labrum of the Hip joint.
iii)They help to deepen the joint cavity.

MOVEMENTS OF JOINTS

The movements permitted at a joint (e.g., synovial) are of following:

• Gliding Angular
• Rotation
• Circumduction
• Miscellaneous

Frequently these movements are combined in a complex manner in order to show an infinite variety. It is seldom true that one kind of movement is found in any particular joint.

Where the movements are limited, the reciprocal articular surfaces are approximately of equal size, but where the movements are free, the more movable bone usually possesses the larger articular surface, e.g., the head of the humerus is extremely large as compared with the genoid cavity of the scapula-bone.

Various movements are discussed below:

CLIDING MOVEMENTS:

It is the simplest kind of movements in which one surface crawls over the other without any angular or rotatory movements.

Examples:
1)Intercarpal joints
2)Inter tarsal joints
3) synovial joints

ANGULAR MOVEMENTS:

It implies diminution or increase in angle between the adjoining bones.

The angular movements are of four types:

A) FLEXION:

It occurs along a transverse or obliqueiy transverse axis. It usually results in approximation of two morphologically ventral surfaces. There are following exceptions:

Carpometacarpal joint of the thumb:

Here the axis of movement is antero-posterior and not transverse.

Hip Joints:

Here the approximation of morphologically dorsal surface of the thigh and ventral surface of the trunk, takes place during flexion. This reversal is due to rotation of lower limb during development.

EXTENSION:

It also occurs along a transverse axis and is characterized by approximation of two morphologically dorsal surfaces. The exceptions are the same as in Flexion.

ADDUCTION:

It occurs around an antero-posterior axis. It implies the approximation towards the median plane of the body. The exception to this definition is Carpo-meta carpal joint of the thumb where the axis of movement is ransverse instead of antero-posterior.

ABDUCTION:

It also occurs along the anteroposterior axis. It is characterized by de-approximation (i.e., away from) from the median plane, the exception is the same as in adduction.

ROTATION OF ROTATORY MOVEMENTS:

This term is often used to denote a form of movement in which a bone moves around some longitudinal axis. The axis of the rotation may lie in:

i) A separate bone, e.g., the pivot of dens (2nd Cervical vertebra), around which the arch of the atlas moves.the same bone, e.g., during the rotation of humerus at the shoulder joint, the axis of movements lie in the humerus.
ii)Two bones oblique axis e.g., in superior radioulnar joint the axis of movements passes from radius to ulna.

The best examples of rotatory movements are:

i)Supination and pronation of the fore-arm. (It occurs at the superior radio-ulnar joint).
ii)Inversion and eversion of the Foot. (It occurs at the mid- tarsal joint).

CIRCUMDUCTION:

It is a derived movement in which the elements of flexion, extension, adduction and abduction are compounded. This movement occurs at the following joints:
1.Shoulder joint
2.Hip joint

Carpometacarpal joint of the thumb MISCELLANEOUS MOVEMENTS:

1.The axial line of the hand is passing through the middle finger and the axial line of the foot is passing through the second toe.
2.The movements of adduction and abduction of the fingers and toes are actually the movements towards and away from these axial lines, i.e., middle finger and second toe.
3.Pronation means a rotatory movement in which the palm of the hand faces backwards.
4.Supination means a rotatory movement in which the palm of the hand faces forwards.
5.Inversion means the rotatory movement of the foot in which the sole faces inwards.
6.Eversion means the rotatory movement of the foot in which the sole faces outwards.
7.Opposition in this movement the palmar surface of fingers are brought in contact with the palmar surface of the thumb. It is the characteristic movement of the thumb of the hand and forms integral part of grasping, holding and many other skilled movements.
8.Protraction means to move forward, e.g., movement of lower jaw and shoulder girdle.
9.Retraction means to move backwards. Examples are the same as of Protraction.