A joint or articulation (or articular surface) is the connection made between

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

s, ossicles, or other hard structures in the body which link an animal's

skeletal system A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure insid ...

into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw-Hill Connect. Web
p.274
/ref> They are constructed to allow for different degrees and types of movement. Some joints, such as the

knee In humans and other primates, the knee joins the thigh with the human leg, leg and consists of two joints: one between the femur and tibia (tibiofemoral joint), and one between the femur and patella (patellofemoral joint). It is the largest join ...

elbow The elbow is the region between the arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and the m ...

, and

shoulder The human shoulder is made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder m ...

, are self-lubricating, almost frictionless, and are able to withstand compression and maintain heavy loads while still executing smooth and precise movements. Other joints such as sutures between the bones of the

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, t ...

permit very little movement (only during birth) in order to protect the brain and the

sense organ A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system re ...

s. The connection between a tooth and the

jawbone In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable ...

is also called a joint, and is described as a fibrous joint known as a

gomphosis In anatomy, fibrous joints are joints connected by fibrous tissue, consisting mainly of collagen. These are fixed joints where bones are united by a layer of white fibrous tissue of varying thickness. In the skull the joints between the bones ...

. Joints are classified both structurally and functionally.

Classification

The number of joints depends on if sesamoids are included, age of the human and the definition of joints. However, the number of sesamoids is the same in most people with variations being rare. Joints are mainly classified structurally and functionally. Structural classification is determined by how the bones connect to each other, while functional classification is determined by the degree of movement between the articulating bones. In practice, there is significant overlap between the two types of classifications.

Clinical, numerical classification

*monoarticular – concerning one joint *oligoarticular or pauciarticular – concerning 2–4 joints *polyarticular – concerning 5 or more joints

Structural classification (binding tissue)

Structural classification names and divides joints according to the type of binding tissue that connects the bones to each other. There are four structural classifications of joints: *

fibrous joint In anatomy, fibrous joints are joints connected by fibrous tissue, consisting mainly of collagen. These are fixed joints where bones are united by a layer of white fibrous tissue of varying thickness. In the skull the joints between the bones ...

– joined by dense regular connective tissue that is rich in collagen fibersPrinciples of Anatomy & Physiology, 12th Edition, Tortora & Derrickson, Pub: Wiley & Sons *

cartilaginous joint Cartilaginous joints are connected entirely by cartilage (fibrocartilage or hyaline). Cartilaginous joints allow more movement between bones than a fibrous joint but less than the highly mobile synovial joint. Cartilaginous joints also forms th ...

– joined by cartilage. There are two types: primary cartilaginous joints composed of

hyaline cartilage Hyaline cartilage is the glass-like (hyaline) and translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-gray in color, with a firm consistency and h ...

, and secondary cartilaginous joints composed of hyaline cartilage covering the articular surfaces of the involved bones with

fibrocartilage Fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions. It owes its inflexibility and toughness to the former of these constituents, and its elasticity to the latter. It is the only type of ...

connecting them. *

synovial joint A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulat ...

– not directly joined – the bones have a synovial cavity and are united by the dense irregular connective tissue that forms the articular capsule that is normally associated with accessory ligaments. *

facet joint The facet joints (or zygapophysial joints, zygapophyseal, apophyseal, or Z-joints) are a set of synovial, plane joints between the articular processes of two adjacent vertebrae. There are two facet joints in each spinal motion segment and e ...

– joint between two articular

processes A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process, activities that produce a specific se ...

between two vertebrae.

Functional classification (movement)

Joints can also be classified functionally according to the type and degree of movement they allow: Joint movements are described with reference to the basic anatomical planes. *

synarthrosis A synarthrosis is a type of joint which allows no movement under normal conditions. Sutures and gomphoses are both synarthroses. Joints which allow more movement are called amphiarthroses or diarthroses. Syndesmoses joints are considered to be ...

– permits little or no mobility. Most synarthrosis joints are

s (e.g., skull sutures). *

amphiarthrosis Amphiarthrosis is a type of continuous, slightly movable joint. Types In amphiarthroses, the contiguous bony surfaces can be: * A symphysis: connected by broad flattened disks of fibrocartilage, of a more or less complex structure, which adhere ...

– permits slight mobility. Most amphiarthrosis joints are

s (e.g.,

intervertebral disc An intervertebral disc (or intervertebral fibrocartilage) lies between adjacent vertebrae in the vertebral column. Each disc forms a fibrocartilaginous joint (a symphysis), to allow slight movement of the vertebrae, to act as a ligament to hol ...

s). *

(also known as a ''diarthrosis'') – freely movable. Synovial joints can in turn be classified into six groups according to the type of movement they allow:

plane joint A plane joint (arthrodial joint, gliding joint, plane articulation) is a synovial joint A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones ...

ball and socket joint The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of ...

hinge joint A hinge joint (ginglymus or ginglymoid) is a bone joint in which the articular surfaces are molded to each other in such a manner as to permit motion only in one plane. According to one classification system they are said to be uniaxial (having ...

pivot joint In animal anatomy, a pivot joint (trochoid joint, rotary joint or lateral ginglymus) is a type of synovial joint whose movement axis is parallel to the long axis of the proximal bone, which typically has a convex articular surface. According t ...

condyloid joint A condyloid joint (also called condylar, ellipsoidal, or bicondylar) is an ovoid articular surface, or condyle that is received into an elliptical cavity. This permits movement in two planes, allowing flexion, extension, adduction, abduction, ...

and

saddle joint A saddle joint (sellar joint, articulation by reciprocal reception) is a type of synovial joint in which the opposing surfaces are reciprocally concave and convex. It is found in the thumb, the thorax, the middle ear, and the heel. Structure ...

. Joints can also be classified, according to the number of axes of movement they allow, into nonaxial (gliding, as between the proximal ends of the ulna and radius), monoaxial (uniaxial), biaxial and multiaxial. Another classification is according to the

degrees of freedom Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...

allowed, and distinguished between joints with one, two or three degrees of freedom. A further classification is according to the number and shapes of the articular surfaces: flat, concave and convex surfaces. Types of articular surfaces include

trochlear Trochlea (Latin for pulley) is a term in anatomy. It refers to a grooved structure reminiscent of a pulley's wheel. Related to joints Most commonly, trochleae bear the articular surface of saddle and other joints: * Trochlea of humerus (part of ...

surfaces.

Biomechanical classification

Joints can also be classified based on their anatomy or on their biomechanical properties. According to the anatomic classification, joints are subdivided into ''simple'' and ''compound'', depending on the number of bones involved, and into ''complex'' and ''combination'' joints: # Simple joint: two articulation surfaces (e.g.

shoulder joint The shoulder joint (or glenohumeral joint from Greek ''glene'', eyeball, + -''oid'', 'form of', + Latin ''humerus'', shoulder) is structurally classified as a synovial ball-and-socket joint and functionally as a diarthrosis and multiaxial joint. ...

hip joint In vertebrate anatomy, hip (or "coxa"Latin ''coxa'' was used by Celsus in the sense "hip", but by Pliny the Elder in the sense "hip bone" (Diab, p 77) in medical terminology) refers to either an anatomical region or a joint. The hip region is ...

) # Compound joint: three or more articulation surfaces (e.g.

radiocarpal joint In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carpal ...

) # Complex joint: two or