Joint if formed by the union of two or more bones or cartilages by other tissue.
Bone is the fundamental part of most joints; in some cases a bone and cartilage, or two cartilages, from a joint. The uniting medium is chiefly fibrous tissue or cartilage, or a mixture of these. Union of parts of the skeleton by muscles.
Joints vary in both structure and arrangement and are often specialized for particular functions. However, joints have certain common structural and functional features and may be classified into three types:
a. Fibrous Joints.
In this group the segments are united by fibrous tissue in such a manner as to practically preclude movement; hence, they are often termed fixed or immovable joints.
here is no joints cavity. Most of these joints are temporary, the uniting medium being invaded by the process of ossification. The chief classes in this group of joints are as follows:
There term suture is applied to those joints in the skull in which the adjacent bones are closely united by fibrous tissue – the sutural ligament.
Surrate: this have irregular interloking margins like the interfrontal suture.
Squamous the edge of bones are beveled and overlap like: the joint between the squamous part of temporal bone parietal bone.
Plane: the edge of bone are plane or slightly rough like the internasal suture and the horizontal part of palatine bone.
Foliate: in which the edges of one bone fit into fissure in adjacent bones.
In these the uniting medium is white fibrous or elastic tissue mixture. Examples are the union of the shift of the metacarpal bones and the attachments to each other of the coastal cartilages. In a syndesmosis, when the opposed bones are united by fibrous such as the fusion of the bodies of the radius and ulna and tibia and fibula.
This term sometimes applied to the implantation of the teeth in the alveoli, the gomphosis is not properly considered, a joint at all, since the teeth are not parts of the skeleton.
b. Cartilaginous joints.
The bones of cartilaginous joints are united by fibrocartilage or hyaline cartilage, or a combination of the two. The amount and kind of movement are determined by the shape of the join surfaces and the amount and pliability of the uniting medium. They are chiefly classified as:
Synchondrosis(Hyaline cartilage Joints)
Symphysis (Fibro-cartilaginous Joints)
1- Synchondrosis: (Hyaline Cartilage Joints)
This type of joint is a temporary one, for the cartilages is converted into bone before adult life. The hyaline cartilages that joins the bones is a persistent part of the embryonic cartilaginous skeleton. As joint between petrous part of the temporal bone and stylohyoid bone via the tympanicohyoid cartilage.
2- Symphysis (Fibro- cartilaginous joints)
This joint sometimes called a secondary cartilaginous joint. Represents an articulation in which the contiguous bone are united by fibro-cartilage during some phase of their existence. Fibro-cartilaginous joints include the pelvic symphysis and joints between the bodies of the vertebrae. A limit and variable amount of movement may exist.
c. Synovial joints:
This group of joints, also known as diarthrodial joints are characterized by the presence of a joint cavity with a synovial membrane in the joint capsule and by their mobility. They are often called movable or true joints. A simple joint is one formed by two articular surfaces; a composite joint one formed by several articular surfaces. The following structures enter into their formation:
The articular surface are in most cases smooth, and vary much in form. They are of specially dense bone, which differs histologically from ordinary compact substance. In certain cases of surface is interrupted by non- articular cavities known as synovial fossae.
The articular cartilages usually hyaline in type form a covering over the articular surfaces of the bones. They vary in thickness in different joints, they are thickest on those which are subject to the most pressure and friction.
The articular or joint capsule is in its simplest form. A tube the ends of which are attached around the articulating surfaces. It consist of two layers an external one, composed of fibrous tissue, and an internal one the synovial layer or membrane. The synovial membrane secrets a fluid the synovial membrane which lubricates the joint it resembles white of egg it also serves to transport nutrient martial to the hyaline articular cartilage.
Are strong bands or membranes usually composed of white fibrous tissue which bind the bones together. They are pliable but practically inelastic. In a few cases however the nuchal ligaments they are composed of elastic tissue they may be subdivided according to position, into extra an intra-capsular ligaments and collateral ligaments.
Articular Discs and Menisci.
These are of fibro-cartilage or dense fibrous tissue placed between the articular cartilages they divide the joint cavity partially or completely into two components.
A marginal cartilage is a ring of fibro-cartilage which encircles the rim of an articular cavity. It enlarges the cavity and tends to prevent fracture of the margin.
Classification of synovial joints:the synovial joints can be classified in three ways:-
Numerical classification: which depend on the number of the bones which made the joint:
Simple Joint: when there are two articular surfaces within the capsule.
Compound Joint: when there are more than two articular surfaces within the same capsule.
Anatomical classification:this depend on the shape of the articular surface which the made the joint:
Ball-and Socket Joint (Enarthrosis): when a convex hemispherical head fits into a cavity (shallow, e. g. shoulder joint; deep, e. g. hip joint). This type make all types of movements of joint.
Hinge Joint (Ginglymus):allows flexion and extension with limited rotation. The movable surface is usually concave (e. g. elbow joint).
Condylar Joint: has similar movement to hinge joint, but the articular surfaces are rounded condyles (e. g. knee joint, which is also complex).
Pivot Joint (Trochoid): has most movement around a longitudinal axis through the bones (e. g. atlanto- axial joint, rasioulnar joint).
Plane (Arthrodial Joint): allows a gliding motion, in which the two articular surfaces are plane (e. g. carpometacarpal joints, the joint of vertebrae and tarsal bone).
Axial movement classification:
Unaxial: in which the movement is one axis, this axis transverse (e. g. hing joint) and longitudinal (Pivot Joint).
Biaxil: in which the movement occurs around the horizontal axis at right angle to each other. (flexion , extension, addiction, abduction and circumduction).
Multiaxial: in which the movement occurs around the several axis the can make all movements in this type.
The synovial joints can do different types of movement depended on the shape of articular surface and ligaments of the joint; these movements are:
Gliding movement: this type on articular surface glide over the other articular surface, e. g. the movement between articular facet of vertebrae, carpal bone.
Angular movement: subdivided into followings:
Flexion: it means the angle between the articular bones is decrease.
Extension: it means the angle between the articular bones is increase.
Adduction: it means movement of limbs to ward the median plane of the body.
Abduction: it means movement of limbs far away from median plane.
Circumduction: in this type one articular surface circumscribed inside a conical shape surface of the bone. This type is rarely in animals.
Rotation: it mean one segment rotate above the longitudinal axis of other segment of articular bone e. g. atlanto-axial joint.