When two or more bones or cartilages come in contact with each other an articulation or a joint is formed. The study of joints is known as arthrology. (G. Arthron= joint). Previously this was known as syndesmology which is incorrect because it means study of ligaments. (G.syndermos= ligament) which is only a component of a joint.
Classification of joints:
The joint can be classified on the basis of structure, development and function.
- Fibrous joints (fibrous; Fixed , immovable. Skull type)
In this joints the union is due to dense fibrous tissue. They are usually fixed, immovable and mainly limited to the skull. They are subdivided into three main groups depending upon the location.
- Sutures: These are immovable and fixed. These are the joints of skull bones. There are following varieties of sutures.
- Serrate sutures: shape of the articulating surfaces is saw like. eg sagittal suture
- Denticulate suture: the shape of the articulating surfaces is tooth like. eg lambdoid suture
- Squamous sutures: Here the articulating bones overlap each other and are beveled reciprocally i.e one both internally and the other externally.eg temporo-parietal suture
- Limbous suture: it is slightly modification of squamous suture. Here the beveled surfaces may be mutually serrated or ridged. eg modified temporo-parietal suture.
- Plane sutures: here the interlocking surfaces are slightly rough and irregular. eg interpalatine suture
- Wedge and groove sutures (schindylesis): here a ridged-bone fits into the groove present on a neighbouring bone. Eg vomero-sphenoid suture
- Peg and socket joint (Gomphosis): it is a specialized fibrous joint in which teeth fit into their sockets situated in the maxilla and mandible.
- Syndesmosis : it is a type of fibrous joint in which the two bones are united with each other by interosseous ligaments. It is a rarity in mammals and the only true example in man is inferior tibiofibular joint.
- Cartilaginous joints (slightly movable, vertebral type): In these joints the bones are united by a cartilage. They are mainly limited to the axial skeleton. Eg vertebral column. These are divided into two main groups.
- Primary cartilaginous joints (synchondroses): the bony ends and the shaft remain united with each other by a plate of hyaline cartilage. This cartilaginous plate is known as epiphyseal plate. These are the joints mainly seen in developing appendicular bones. These joints are temporary because at the age of 18-20 years the epiphyseal cartilage is ossified. Eg joints between the ends and shaft of growing long bones.
- Secondary cartilaginous joints (Symphyses): these joints are formed by white fibro-cartilage which connects the articular surfaces covered by hyaline cartilage. These are permanent joint, slightly movable mainly seen in axial bones.eg Pubic symphysis, joints between bodies of vertebrae.
- Synovial joints (movable; limb type): these joints are highly evolved, uniquely constructed specialized articulation where all possible movements can occur. All synovial joints posses a definite joint cavity lined by synovial membrane and filled with a lubricating synovial fluid. It can be classified according to following schemes.
- Depending upon the shape of articulating surfaces;
- Homomorphic: when two articulating surfaces are plane the synovial joint is called homomorphic, eg plane joint (intermetatarsal joints), saddle joint (carpometacarpal joints)
- Hetromorphic joints: when there is varied appearance of two articulating surfaces, then the synovial joint is called hetromorphic eg Ball and socket joint (shoulder and hip joint), condyloid joint (temporomandibular joint and knee joint), ellipsoid joint (radiocarpal joint)
- Depending upon complexity of organization:
- Simple: when only two bones are participating, the joint is called simple joint. Eg cromioclavicular joint, shoulder joint
- Compound: these are formed by participation of more than two bones, eg elbow joint, knee joint.
- Complex joint: when intra articular discs or menisci are present the joint is called complex. Eg knee joint , temporomandibular joint.
- Depending upon axis of movement:
- Uniaxial: In these joints movement take place on one axis only i.e on transverse or longitudinal axis, e.g elbow joint
- Biaxial: In these joints movements take place on two axis. i.e transverse axis, longitudinal axis eg wrist joint.
- Polyaxial : In these joints movements take place on more than two axis. Eg shoulder joint and hip joint.
- Depending upon the types of movements:
- Gliding: eg plane joint (intermeta-tarsal joints)
- Angular: eg condyoid joint (knee joint)
- Rotatory: e.g pivot joint (superior radio-ulnar joint)
- Circumductory: eg Ball and socket joint (shoulder joint and hip joint).
- Architectural or structural classification:
- Plane joints: these are formed by the opposition of fairly flat articular surface. Eg intermetatarsal joint
- Hinge joints: they roughly resemble the hinges of a door, the articulating surface are so moulded that they restrict the to and fro movements to one plane, i.e they are uni-axial. Eg elbow joint and interphalangeal joints
- Pivot joints: these are uni-axial joints (in which movements take place around the longitudinal axis). Movement is restricted to a rotation around a longitudinal axis passing through the center of the pivot. Eg proximal radio-ulnar joint. Atlanto-axial joint.
- Condylar or condyloid joints (practically uniaxial):Here two condyles (male surfaces) are received into two concave (female surfaces) cavities.it allows the principal movement occurring in one plan, but a limited amount of rotation is possible about a second axis. Eg temporo-mandibular joint, knee joint.
- Ellipsoid joints: these are biaxial joints formed by reception of an oval, convex surface into an elliptical concavity. Primary movement are possible about two axes situated at right angles. Eg flexion and extension , radiocarpal joint and metacarpophalangeal joints.
- Saddle joints (seller joints): these are biaxial joints which are formed by reception of concavo-convex opposing surfaces. Primary movement occur in two planes at right angles to each other but because of the articular geometry, these are accompanied by a degree of axial rotation of moving bones, eg. Carpometacarpal joint of thumb
- Ball and socket joints (spheroidal joints): they are polyaxial joints and are formed by reception of globular head into a cup like concavity or socket. All types of movements are possible, eg shoulder joint, hip joint.