Joints Classified according to manner in which adjacent bones are bound together 4 major classes: Bony, Fibrous, Cartilaginous and Synovial. Bony joints

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Classified according to manner in which adjacent bones are bound together

4 major classes: Bony, Fibrous, Cartilaginous and Synovial.

Bony joints

Immovable joint formed when gap between two bones ossifies forming a single bone

Eg: Attachment of first rib to sternum becomes a synostosis with age

Fibrous joints (Synarthrodial)

Joint at which adjacent bones are bound by collagen fibers that originate from one bone and penetrates the next. There are three types;

  • Sutures (short fibers allowing no movement)

A joint binding the bones of the skull to each other.

3 different structures (Serrate, Lap and Plane)

  • Gomphosis (short fibers allowing little-no movement)

Attachment of a tooth to its socket by fibrous periodontal ligament which is a collection of collagen fibers that extends from bone matrix of the jaw and insert into the tooth.

This short Ligament allows for small movement of tooth during mastication.

  • Syndesmosis (fibers are longer allowing more movement)

Attachment of two bones by long collagen fibers

Allows more mobility then previous fibrous joints.

Eg: joint binding distal end of tibia with fibula.

Joint binding shafts of radius and ulna (fibrous sheet connecting them called interosseous membrane)

Cartilaginous joints (Amphiarthrodial)

Bones are linked by cartilage. There are two types;

  • Synchondrosis (bones bound by hyaline cartilage)

Eg: attachment of first rib to sternum by hyaline coastal cartilage and

Temporary joint between the epiphysis and diaphysis of a long bone formed by cartilage of epiphyseal plate

  • Symphysis (bones joint by fibro-cartilage)

Eg: attachment of the right and left pubic bones by the cartilaginous interpubic disc called the pubic symphysis.

Synovial Joints

Joints whereby two bones are separated by a film of slippery synovial fluid.

Are the most structurally complex of all joints. The bones are separated by a space called the joint cavity that contains synovial fluid. The fluid is a lubricant that nourishes the joint cartilages and removes their wastes and contains phagocytes that clean up tissue debris resulting from cartilage wear and tear. The adjoining bones are each covered with articular cartilage (hyaline). A joint capsule encloses the cavity which contains the synovial fluid and this is made up of two structures; The inner synovial membrane and the outer fibrous capsule.

Accessory structures of s synovial joint include tendons, ligaments and bursae. A tendon is a strip of tough collagenous connective tissue that attaches muscle to bone. A ligament attaches one bone to another. A bursa refers to a fibrous sac that is filled with synovial fluid and located either between adjacent muscles or where a tendon passes over a bone to help cushion muscle and reduce friction between a tendon and bone.

A synovial joint is the body’s most moveable joint where the movements are almost friction-free.

There are 6 classes of synovial joints; ball-and-socket, condyloid, saddle, glidng, hinge and pivot joints.

Eg: Jaw, elbow, interphalangeal, hip and knee joints.

Understand and explain the trade-off between mobility and stability of a joint.

Whilst it is important for a joint to bear heavy loads and withstand compression while executing smooth and precise movement, it is equally important for the joint to be able to ensure stability of the joint and possess a mechanism that limits its range of motion. The type of joint along with the surrounding structures, are the key factors which contribute to the stability of the joint.

The factors responsible for stability include:

The joint type (if ball and socket type, is the socket quite deep or shallow). Each type is distinguished by the shape of their articular surface and their specific structure that allows for precise degree/s of freedom.

Ligaments which help to ensure stability at a joint by way of their strength and tautness exerted during a movement to limit the range of motion. Eg. Shoulder capsule and ligaments, cruciate ligaments of the knee

Muscles and tendons limit movement at a joint by adopting a state of tension called muscle tone to restrict movement. This is the first line of defense against dislocations that is for instance in the preventing shoulder dislocation by the tension in the biceps brachii muscle.

We could remove some ligaments from the collateral ligaments of the knee and it would certainly be able to function with increased mobility. However, it would constantly dislocate and this is why we have a tradeoff between mobility and stability.

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