Synovial joints

 What are the characteristic features of a Typical Synovial Joint?

Following are the characteristic features of a typical synovial joint:

• Joint cavity is present between the bones, and the joint is surrounded by articular capsule.
• Most movable type of joint.
• Joint is surrounded by fibrous articular capsule.
• The fibrous capsule is lined by synovial membrane(it produces synovial fluid which percolates into the joint cavity and lubricates the articular surfaces).
• Ends of the articulating bones are covered by articular cartilage (mostly hyaline cartilage).The articular surfaces of bones which are formed by membranous ossification are covered by fibrocartilage. e.g. temporomandibular and sternoclavicular joint).
• Most are “simple” joint i.e they have just two articulating surfaces.
• Some are “compound” joint i.e having more than two articulating surfaces (e.g. elbow joint (humerus, radius and ulna) and knee joint (femur, tibia, patella).

What are the different types of Synovial Joint?

Following are the seven different types of synovial joints according to the axis of movement and shape of articular surfaces.

1. Plane
2. Hinge
3. Pivot
4. Condylar
5. Ellipsoidal
6. Ball and socket
7. Saddle

Plane Synovial Joints

Articular surfaces are flat (plane). They allow only gliding/sliding movement.  e.g. Intercarpal joints, intertarsal joints, acromioclavicular joint.

Uniaxial Synovial Joints

They allow movement around one axis.

1. Pivot joint

 One of the articular surfaces is like a central pivot(peg) and the other is an osseocartilagenous ring. It allows rotational movement. e.g. atlanto-axial and superior radioulnar joints.

2. Hinge joint

 The articular surfaces are pulley shaped.  It allows movement across transverse axis (flexion and extension). e.g. elbow, ankle and interphalangeal joints.

Biaxial synovial joints

They allow movements  around two axes.

1. Ellipsoid joint

One of the articular surface is oval-shaped and convex and the other is  reciprocally elliptical and concave. It allows movement around transverse axis (flexion and extension) and adduction and abduction around another axis. e.g. – wrist and metacarpophalangeal joints.

2. Condylar joint

One of the articular surface is a condyle (convex) and the other is reciprocally concave. They allow movement mainly around transverse axis ( flexion and extension) and slight rotation around vertical axis. e.g. knee joint, atlanto-occipital joint, temporomandibular joint (bicondylar).

Multiaxial synovial joints

They allow movement across multiple axes.

1. Ball & socket joint

One of the articular surface is a globular head and the other is like a socket. Movements occur across multiple axes (flexion, extension, adduction, abduction, medial and lateral rotation and  circumduction) e.g. shoulder and hip joint.

2. Saddle joint

Both the articular surfaces are reciprocally concavo-convex. Movements occur across multiple axes. e.g. 1st carpometacarpal and sternoclavicular joints.

What are simple, compound and complex synovial joints?

Simple synovial Joint: Only two bones articulate. e.g. shoulder joint, hip joint.

Compound synovial joint: More than two bones articulate. e.g. Elbow joint (humerus, radius and ulna) and knee joint ( femur, tibia and patella).

Complex synovial joint: Joint cavity is divided into two parts by an intra-articular disc. e.g. temporomandibular joint, sternoclavicular joint.

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What is Hilton’s law?

 Hilton’s law states that the nerve that supplies a muscles which acts on the joint also supplies the joint and the skin covering the joint.

Applied Aspects