Classification of joints

A joint or articulation is a junction of two or more bones connecting and articulating with each other. Joints are supported and stabilized by various connective tissue structures. They are classified both structurally and functionally by either the histological type of tissue that holds the joints together or the range of movements they provide.

The degree of movements the joints allow is related to their provided functions. The joints not only facilitate movements of the human body but also provide stability and flexibility to the skeleton. Muscles that cross the joints stabilize them and activate their motions.

Depending on the dominant type of connective tissue, joints can be classified into three subdivisions: fibrous, cartilaginous and synovial. Based on the amount of permitted movements, there are also three types of joints: synarthroses (immovable), amphiarthroses (move slightly) and diarthroses (move freely).

Subdivisions from both mentioned classifications correlate with one another. Synovial joints are freely movable junctions. Therefore, they are also classified as diarthroses. Synarthroses and amphiarthroses are restricted in motions and correspond to fibrous and cartilaginous joints.

Fibrous joints

As the name suggests, fibrous joints connect adjacent bones with the help of dense fibrous connective tissue. The main component of dense fibrous tissue is collagen. Therefore, this joint type allows little to almost no movements, and its primary function is to provide strength and stability. The range of permitted motions depends on the length of the connective tissue fibers that link adjacent bones. Most fibrous joints are immovable, however, some allow slight movements.

Fibrous joints are fixed and have no joint cavity. The gap between adjacent bones can be either wide or narrow. Fibrous joints can be further subdivided into three smaller groups - sutures, gomphoses and syndesmoses.

Sutures are immovable (synarthroses) narrow joints that connect flat and plate-like bones. They are only found within the skull. Sutures strongly link the adjacent bones, and they help to protect the brain and form the face. At different periods during an individual’s lifetime, the sutures ossify and are converted into bony tissues. Sutures allow little movements till the early twenties, but later they become immovable, and gaps between the adjacent bones close. This fusion of the bones is called synostosis.

Gomphoses are also synarthroses. They are specialized narrow fibrous junctions found between the roots of teeth and the bony sockets located within the maxillae and mandible of the skull. This type is also known as the peg-and-socket joint. Teeth are fixed into their corresponding sockets with the help of numerous small and short bands called the periodontal ligaments.

Syndesmoses can move but only slightly, and they are classified as amphiarthroses. Bones are more widely separated at this joint type, and they are held together with the help of a broad sheet of connective tissue called an interosseous membrane or by a narrower band called a ligament.

Syndesmoses are found between the distal parts of the tibia and fibula at the inferior tibiofibular joint, between the interosseous borders of the ulna and radius at the middle radioulnar joint), and also at the posterior aspect of the sacroiliac joint. Syndesmoses of the legs and arms unite parallel bones and prevent their separation.

Cartilaginous joints

Cartilaginous joints are articulations where cartilages unite adjacent bones. Depending on the type of cartilage (hyaline or fibrocartilage) involved, these articulations can be subdivided into primary and secondary cartilaginous joints. The cartilaginous joints can be immovable (synarthroses) or allow slight mobility (amphiarthroses).

At the primary cartilaginous joints (also called synchondroses), adjacent bones are joined by hyaline cartilages. Also, this joint type includes sites where a bone is united only to hyaline cartilage. A good example of this type would be the junctions of the anterior ends of the ribs with their respective cartilages (costochondral joints). Also, this type is found between the epiphyses (ends) and diaphyses (shafts) of long bones. An epiphysis is the end of a long bone that is related to the growth plate, which allows the bone to grow in length.

The primary cartilaginous joints can be either permanent or temporary as they also tend to ossify with aging in the process called synostosis. Permanent synchondroses do not ossify and are found mainly within the thoracic cage. They include the mentioned costochondral joints and the first sternocostal joints. Temporary synchondroses are found within the epiphyseal plates, and once the body growth stops, the cartilage disappears and gets replaced by the bone, forming synostoses. Synostoses are found between the sacral vertebrae that fuse and form a single bone called the sacrum. The primary cartilaginous joints are classified as synarthroses due to the lack of movements between the bones and cartilages.

The secondary cartilaginous joints (also called symphyses) include fibrocartilages instead of hyaline cartilages. However, histologically fibrocartilage is a transition tissue between the hyaline cartilage and dense fibrous tissue. Symphyses can be either narrow or wide junctions and they have limited mobility. They are classified as amphiarthroses. Nevertheless, they are very strong articulations, and they firmly unite the adjacent bones to endure stress due to tension and compression.

Some examples of symphyses within the human body include the pubic symphysis forming the anterior median aspect of the pelvis and the manubriosternal joint found at the sternal angle. Both are narrow junctions. Also, the intervertebral discs between adjacent vertebral bodies of the spine are classified as symphyses. Interestingly, all symphyses are located more or less in the midline of the body.

Synovial joints

Synovial joints represent the most common joint type in the human body and are also the main functional articulations. This type is defined by the presence of a fluid-filled cavity enclosed by a fibrous capsule. The articulating structures of the bones are not directly connected but contact each other within the joint cavity. This characteristic allows for increased joint mobility. Therefore, synovial joints are mainly located in the limbs. They provide the broadest range of motion and are classified as diarthroses. The main characteristics of these articulations are as follows:

• Synovial joints have a joint cavity that is encased in a capsule.
• Inside of the joint capsule is lined by a synovial membrane.
• The joint cavity contains a thick and slimy fluid called the synovial fluid. It is produced by the cells of the synovial membrane. Synovial fluid acts as a lubricant. It not only reduces friction but also nourishes articular cartilage.
• Ligaments inside (intracapsular ligaments) and outside (extracapsular ligaments) of the capsule stabilize and strengthen it. They prevent the separation of the articulating bones. And finally, they allow movements at the joint but limit their range, preventing excessive motions.
• The articular surfaces of bones within the synovial cavity are lined by a thin layer of hyaline cartilage. It allows the articulating bones to move smoothly against each other and prevents friction.
• Some synovial joints also contain fibrocartilages between their articulating surfaces. They are oval-shaped like discs, and therefore, called the articular discs, or they can be larger and more C-shaped, in which case they are called menisci. Menisci are found within the knee joint. The articular discs and menisci have various functions depending on the joint. They may firmly link the bones of the joint, smooth the movements, or they can function as cushions, providing shock absorption.
• A few synovial joints have some more additional structures called bursae (or bursas) that prevent friction between the bones and overlying connective tissue structures such as muscle tendons and skin. Bursae are located outside of the joint. They prevent the mentioned structures from rubbing directly against each other. In the human body, bursae are present in the knee and shoulder joints.

Types of synovial joints

Synovial joints can be further subdivided according to the type of movements they provide and the shape of their articulating surfaces. All synovial joints can be either uniaxial, meaning they provide movements in only one plane, biaxial, allowing bones to move in two planes, or multiaxial, permitting motions in more than two planes. Synovial joints can be grouped into the following six types depending on the shape of their articular surfaces:

• Plane (gliding) joints
• Hinge joints
• Pivot joints
• Ellipsoid (condyloid) joints
• Saddle joints
• Ball-and-socket joints

Plane (gliding) joints are multiaxial joints that allow movements more of a sliding nature. These articulations are formed between flat or slightly curved surfaces that are more or less similar in size. Therefore, the articulating bones slide against each other during motions. This joint type is found between the carpal and tarsal bones (intercarpal and intertarsal joints) and between the acromion of the scapula and the clavicle (acromioclavicular joint).

Hinge joints are uniaxial joints because they allow movements just in one plane, and they usually provide flexion and extension. The articulation is formed between a convex end of one bone and a concave end of another bone. An excellent example of this type is the humeroulnar joint between the trochlea of the humerus and the trochlear notch of the ulna. The humeroulnar joint is one of the junctions forming the elbow. Besides the humeroulnar joint, the knee and ankle joints are also hinge-type joints.

Pivot joints are also uniaxial junctions, allowing only rotational movements. In this case, the articulation occurs between a rounded end of one bone (central bony pivot) surrounded by an osteo-ligamentous ring. Two examples include the atlanto-axial joint between the upper two cervical vertebrae (C1 and C2) and the transverse atlantal ligament, and the proximal radioulnar joint between the head of the radius, radial notch of the ulna and annular ligament.

Ellipsoid (condyloid) joints move in two planes (they are biaxial), allowing flexion/extension (anterior-posterior movements) and abduction/adduction (side-to-side movements). Ellipsoid joints are formed between a rounded (convex) end of one bone and an elliptical, slightly concave end of the adjacent bone. For example, this type is found in the hand between the distal ends of the metacarpal bones and the proximal ends of the proximal phalanges (metacarpophalangeal joints). Also, the radiocarpal joint is an ellipsoid type articulation.

Saddle joints are biaxial articulations. Articulating surfaces of bones within this type have a saddle shape, and they are reciprocally concave and convex. The concavity of the surface of one bone is matched to the opposing convexity of another bone, resembling a rider sitting on a saddle on the back of the horse. For example, this type of joint is represented in the hand between the trapezium and the first metacarpal bone (first carpometacarpal joint).

Ball-and-socket joints are multiaxial junctions that allow the greatest range of movements. They ensure flexion/extension, abduction/adduction and rotation. Within this type of joint, a rounded head of one bone (the “ball”) articulates with a concave cup-shaped end of another bone (the “socket”). The only examples of ball-and-socket joints in the human body are the hip and shoulder (glenohumeral) joints.

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