Typical structure of a Synovial Joint

Most of the permanent joints in our body are synovial joints.
They are more complicated in structure and allow more free movement than any other type of joint, i.e. diarthroses.
Such free movement is possible because the ends of the articulating bones are covered with a smooth hyaline articular cartilage.
Synovial joints consist of a joint cavity that possesses a thick fluid called synovial fluid.
Synovial fluid cushions the ends of bones and reduces friction when we move our joints.
A flexible articular capsule encloses and protects synovial joints.

Parts of synovial joints:

A typical synovial joint consists of four essential structures, namely;
- Synovial cavity
- Articular cartilage
- Articular capsule
- Ligaments

1. Synovial cavity:

Also known as joint cavity, synovial cavity is the space between two articulating bones.
It contains folds of synovial membrane that sometimes contains pads of fat. These fatty pads help fill the spaces between articulating bones and also reduce friction.
The synovial membrane secretes a thick synovial fluid that lubricates the synovial cavity.
Synovial fluid gets its name because its thick consistency resembles the white of an egg.
Combined with the articular cartilage, synovial fluid provides an almost friction-free surface for the easy movement of joints.

2. Articular cartilage:

Articular cartilage is a hyaline cartilage which caps or covers the surface of bones facing the synovial cavity.
It has a silvery-blue luster and appears as polished as a pearl.
It acts like a shock absorber because of its thickness and elasticity.
In certain joint diseases, articular cartilage gets worn away and restricts the movement of joints making it more painful.
The cartilage itself is insensitive to feeling, since it has no nerve supply, but the other portions of the joint are supplied with pain-receptor nerve fibers.
Several (not all) synovial joints have articular disks or fibro-cartilaginous disks.
The role of such disks vary depending on the joints;
1. They may act as shock absorbers to reduce the effect of shearing (twisting) on a joint.
2. They prevent the jarring between the bones.
3. They adjust the unequal articulating surfaces of the bones so that the surfaces fit together more evenly.

In some joints, the fibro-cartilaginous disk forms a complete partition, dividing the joint into two cavities.
In the knee joint, the fibro-cartilages, called the medial and lateral menisci, are crescent-shaped wedges that form incomplete partitions.
These menisci serve to cushion as well as guide the articulating bones.
Many athletes, especially sprinters and football players, tear these menisci which is commonly referred to as torn cartilages.

3. Articular capsule:

Articular capsule lines the synovial cavity in the non-cartilaginous parts of the joint.
This capsule if reinforced with collagenous fibers is called fibrous capsule. Fibrous capsule is lax and pliable, permitting considerable movement.
The inner lining of the capsule is the synovial membrane, which extends from the margins of the articular cartilages.
The outer layer of the capsule is a fibrous membrane which extends from bone to bone across a joint and reinforces the capsule.
So called double-jointed people have loose articular capsules.

4. Ligaments:

The portion of fibrous capsule reinforced by a thick layer of collagenous fiber is called ligament.
Ligament joins or connects two articulating bones in a joint.
They vary in shape and even in strength, depending on their specific roles.
Most ligaments are considered inelastic, yet they are flexible enough to permit movement at the joints.
Under excessive stress, ligaments will tear rather than stretch.
Torn ligaments are extremely painful and are accompanied by immediate local swelling.
In general, ligaments are strong enough to prevent any excessive movement and strain. A rich supply of sensory nerves helps prevent a person from over-stretching ligaments.

Two other structures associated with joints, but not parts of them are;

1. Bursae:

Bursae (sing. bursa) are flattened sac like structures filled with synovial fluid.
Bursae are found wherever it is necessary to eliminate the friction that occurs when a muscle or tendon rubs against another muscle, tendon or bone.
They also cushion certain muscles and facilitate the movement of muscles over bony surfaces.
The inflammation of bursae is called bursitis.

2. Tendon sheaths:

Tendon sheath is a modification of bursa.
It is a long cylindrical sac like structure filled with synovial fluid.
It surrounds long tendons that are subjected to constant friction such as tendons of the wrist, palm and finger muscles.
Like bursae, tendon sheaths reduce friction and permit tendons to slide easily.

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