Question

list the functional and structural classification of the articulation. using the knee joint as an example. explain In detail the feature of the joint that classify it as synovial and freely moveable.

Answer 1

A joint, also called an articulation, is any place where adjacent bones or bone and cartilage come together to form a connection. Articulation provide the means for movement. The type and characteristics is determined by its degree and the type of movement. Articulation are classified both structurally and functionally.

STRUCTURAL

The structural classification of joints is based on whether the articulating surfaces of the adjacent bones are directly connected by fibrous connective tissue or cartilage, or whether the articulating surfaces contact each other within a fluid-filled joint cavity. Structural classification categorizes joints based on the type of tissue involved in their formations.

• Fibrous:- The bones of fibrous joints are held together by fibrous connective tissue.
• Cartilaginous:- Cartilaginous joints are those in which the bones are connected by cartilage.
• Synovial:- Synovial joints are the only joints that have a space between the adjoining bones.

FIBROUS JOINT

The bones of fibrous joints are held together by fibrous connective tissue. There is no cavity, or space, present between the bones, so most fibrous joints do not move at all.   There are three types of fibrous joints:

• Sutures: The types of joint found in the cranium. The bones are connected by Sharpey’s fibres.
• Syndesmoses: Found between long bones of the body, such as radio-ulnar and tibio fibular joints.
• Gomphoses: Joint found at the articulation between teeth and the sockets of maxilla or mandible

SYNOVIAL JOINTS

Synovial joints are the only joints that have a space between the adjoining bones. These joints have a synovial cavity. Synovial cavities are filled with synovial fluid, Synovial fluid lubricates the joint, reducing friction between the bones and allowing for greater movement. The ends of the bones are covered with articular cartilage, a hyaline cartilage. The entire joint is surrounded by an articular capsule composed of connective tissue. This allows movement of the joint as well as resistance to dislocation. The knees and elbows are examples of synovial joints.

Synovial joints can be sub-classified into several different types, depending on the shape of their articular surfaces and the movements permitted:

• Hinge – permits movement in one plane – usually flexion and extension.
  • E.g. elbow joint, ankle joint, knee joint.
• Saddle – named due to its resemblance to a saddle on a horse’s back. It is characterised by opposing articular surfaces with a reciprocal concave-convex shape.
  • E.g. carpometacarpal joints.
• Plane – the articular surfaces are relatively flat, allowing the bones to glide over one another.
  • E.g. acromioclavicular joint, subtalar joint.
• Pivot – allows for rotation only. It is formed by a central bony pivot, which is surrounded by a bony-ligamentous ring
  • E.g. proximal and distal radioulnar joints, atlantoaxial joint.
• Condyloid – contains a convex surface which articulates with a concave elliptical cavity. They are also known as ellipsoid joints.
  • E.g. wrist joint, metacarpophalangeal joint, metatarsophalangeal joint.
• Ball and Socket – where the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. It permits free movement in numerous axes.
  • E.g. hip joint, shoulder joint.

FUNCTIONAL

Functional classification of joints is based on the type and determined by the amount of mobility found between the adjacent bones.Joints are functionally classified as:

• Synarthrosis: joints are immobile or have limited mobility and include fibrous joints.
• Amphiarthrosis: joints allow a small amount of mobility and include cartilaginous joints.
• Diarthrosis. joints are the freely movable synovial joints.

KNEE JOINT

The knee joint is one of the strongest and most important joints in the human body. It allows the lower leg to move relative to the thigh while supporting the body’s weight. Movements at the knee joint are essential to many everyday activities, including walking, running, sitting and standing.The knee, is a synovial hinge joint formed between three bones: the femur, tibia, and patella.

Like with all the synovial joints, a joint capsule surrounds the bones of the knee to provide strength and lubrication. The outer layer of the capsule is made of fibrous connective tissue continuous with the ligaments of the knee to hold the joint in place. Oily synovial fluid is produced by the synovial membrane that lines the joint capsule and fills the hollow space between the bones, lubricating the knee to reduce friction and wear. Two internal ligaments, the anterior and posterior cruciate ligaments — also help to maintain the proper alignment of the knee.

As the knee is a synovial hinge joint, its function is to permit the flexion and extension of the lower leg relative to the thigh. The range of motion of the knee is limited by the anatomy of the bones and ligaments, but allows around 120 degrees of flexion. A special characteristic of the knee that differentiates it from other hinge joints is that it allows a small degree of medial and lateral rotation when it is moderately flexed.

Articular capsules may also possess ligaments that hold the bones together. Synovial joints are capable of the greatest movement of the three structural joint types; however, the more mobile a joint, the weaker the joint. Since they allow for free movement, synovial joints are classified as

Synovial joints allow for many types of movement including:

• Gliding: occur as relatively flat bone surfaces move past each other, but they produce very little movement of the bones.
• Angular:  movements are produced when the angle between the bones of a joint changes; they include flexion, extension, hyperextension, abduction, adduction, and circumduction.
• Rotational: involves moving the bone around its longitudinal axis; this can be movement toward the midline of the body or away from the midline of the body.
• Special movements: movements that cannot be classified as gliding, angular, or rotational; these movements include inversion, eversion, protraction, and retraction.