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Synovial Joint Anatomy and Physiology

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Last Updated on March 17, 2025

A synovial joint consists of two bony surfaces lined by cartilage and is surrounded by a fibrous capsule with a synovial lining. Synovial joints are synonymous with mobility. The synovial fluid within the joint provides lubrication and shock absorption, while hyaluronic acid contributes to its viscoelastic properties.

Synovial joint is the most common type of joint found in the human body.

The ends of the bones in the synovial joint are free to move about each other and there is no cartilaginous tissue connecting the adjacent bony surfaces.

Synovial joints are also called diarthroses or diarthrodial joint.

The bone ends are indirectly connected by a joint capsule that covers and encloses the joint. Ligaments play an important role in the stabilization of these joints.

Contents hide
1 Anatomy of Synovial joint
2 Different Types of Synovial Joints
3 Factors Responsible for Joint Stability
4 Clinical Significance
5 Other Joint Types
6 References

Anatomy of Synovial joint

A synovial joint has the following features

  • A joint capsule formed by fibrous tissue
  • A joint cavity enclosed by a joint capsule
  • Inner surface of the capsule is lined by synovial membrane
  • Synovial fluid which forms a thin film over the joint surfaces
  • Hyaline cartilage covering the joint surface

 

joints-synovial joint
Diagram of synovial joint

Articular Capsule

It surrounds the joint and is continuous with the periosteum of articulating bones.

It consists of two layers:

  • Outer Fibrous layer (outer) – consists of white fibrous tissue, also called capsular ligament. It keeps the articulating bones together and also supports the underlying synovium.
  • Inner Synovial layer (inner) or Synvium – It is formed of serous connective tissue. It absorbs and secretes synovial fluid. It also participates in nutritional exchange wit blood.

Articular Cartilage

It covers the articulating ends of both surfaces of the synovial joint. A thin layer of hyaline cartilage covers it. It acts as a shock absorber and also decreases the friction between surfaces.

Synovial Fluid

The synovial fluid is secreted by synovium and is present within the synovial joint. It acts as a lubricant and distributes the nutrition.

Accessory Structures

The accessory ligaments can be separate or part of the joint capsule and are adapted for resisting strain. This resists any extreme movements that may damage the joint.

The synovial joint may be divided, completely or incompletely, by an articular disc or meniscus as in the knee or labrum as in the hip. These help to prevent excessive compression of opposing joint surfaces. Ligaments and tendons assist in guiding motion and also have an important role in keeping joint surfaces together.

Blood and Nerve Supply

Articular arteries arise from the vessels around the synovial joint and supply the joint. These are mostly in the synovial membrane. The synovial joint per se is not a very vascular structure. A common feature of the Articular arteries form rich anastomoses for a better-enriched blood supply.

Articular nerves supply the joint.

Different Types of Synovial Joints

Uniaxial Synovial Joint

The visible motion occurs only in one plane of the body around a single axis which usually is located near or center of the joint. They are said to have 1 degree of freedom of motion.

Two types of uniaxial diarthrodial joints are found in the human body

  • Ginglymus or Hinge joint: This is called so because it resembles a door hinge. It permits motion only in one plane, forward and backward. The articular surfaces are connected together by strong collateral ligaments, which form their chief bond of union. Examples are the interphalangeal joints and the joint between the humerus and ulna.
  • Trochoid or Pivot-joint: The movement is limited to rotation. The joint is formed by a pivot-like process turning within a ring, or a ring on a pivot, the ring being formed partly of bone, partly of the ligament. The proximal radioulnar joint and articulation of the odontoid process of the axis are examples.

Biaxial Synovial Joint

A biaxial synovial joint permits the motion in two planes around two axes and has 2 degrees of freedom. Condyloid and saddle joints are two subtypes.

  • Condyloid Joints: An ovoid articular surface, or condyle, is received into an elliptical cavity in such a manner as to permit of flexion, extension, adduction, abduction, and circumduction, but no axial rotation. The joint surfaces are shaped in such a manner that the concave surface of one bony component is allowed to slide over the convex surface of another component in two directions. The wrist joint and metacarpophalangeal joint are examples of this form of articulation.
  • Saddle Joints: The opposing surfaces are reciprocally concavo-convex. The movements flexion, extension, adduction, abduction, and circumduction are allowed but no axial rotation. The best example of this form is the carpometacarpal joint of the thumb.

Triaxial or Multiaxial Synovial Joint

These joints permit movement in three planes around three axes and have 3 degrees of freedom of motion. Motion at these joints may also occur in oblique planes. The two types of joints in this category are plane joints and ball and socket joints.

  • Plane or Gliding: These joints permit gliding movement only it is formed by the apposition of plane surfaces, or one slightly concave, the other slightly convex, the amount of motion between them is limited by the ligaments or osseous processes surrounding the articulation. Joints between the articular processes of the vertebrae, the carpal joints [except that of the capitate with the scaphoid and lunate], and the tarsal joints [except the talus and the navicular] are examples
  • Ball and socket: These are the joints in which the distal bone is capable of motion around an indefinite number of axes, which have one common center. It is formed by the reception of a globular head into a cup-like cavity, hence the name ball-and-socket.Hip and shoulder joints are the examples

Factors Responsible for Joint Stability

Stability is maintained by the structure of the joint and soft tissue structures around it. The various factors maintaining stability are described below in the order of their importance.

Muscles

Muscles have an inherent tone. The tone of different groups of muscles acting on the joint is the most important and indispensable factor in maintaining stability. Without muscles, the knee and shoulder would be unstable, and the arches of the foot would collapse.

Ligaments

Ligaments are important in preventing any over movement, and in guarding against sudden accidental stresses. However, they do not help against a continuous strain, because once stretched, they tend to remain elongated. In this respect, the elastic ligaments are superior to the common type of white fibrous ligaments.

Bones

Bones help in maintaining joint stability only in the firm type of joints, like the hip and ankle. Otherwise, in most of the joints (shoulder, knee, sacroiliac, etc.), their role in joint stability is negligible.

Clinical Significance

Dislocation

Dislocation Elbow in Child
Dislocation Elbow in Child

This a condition in which the articular surfaces of the joint are abnormally displaced and the two articular surfaces have no contact in between. If partial contact is still retained, it is called subluxation.

Dislocation is commonly caused by trauma and is characterized by pain, deformity, and loss of function. It can be diagnosed by clinical examination and an X-ray is confirmatory.

Sprain

Sprain is the severe pain in a joint caused by a ligamentous tear but without any associated dislocation or fracture. The tear leads to effusion into the ligament and joint causing great pain.

Neuropathic joint

It is the result of its complete denervation to that all reflexes are eliminated and the joint is left unprotected and liable to mechanical damage. A neuropathic joint shows painless swelling, excessive mobility, and bony destruction.

It is commonly caused by leprosy, tubes dorsalis, and syringomyelia.

Joints are locations where bones make contact with each other.  Because their function is movement and support, the joints have been modified accordingly to serve the required function.

The joints are broadly classified into three categories – fibrous, cartilaginous, and synovial. Each category has further subcategories

Other Joint Types

Fibrous Joints

These are also called synarthroses or immovable articulations. In these joints, the bone surfaces are in almost direct contact. The bones are fastened together by intervening connective tissue. Often in such joints, there is no appreciable motion.

Sutura, gomphosis, and syndesmosis are further subtypes of these joints

  • Sutura: Sutura is the joint contiguous margins of the bones that are united by a thin layer of fibrous tissue. This kind of joint is found in the skull only. When the margins of the bones are connected by a series of processes, and indentations interlocked together, the articulation is termed a true suture or sutura vera. When the articulation is formed by roughened surfaces placed in apposition with one another, it is termed a false suture or sutura notha.
  • Gomphosis: Gomphosis is articulation in which the surfaces of bony components are adapted to each other like a peg in a hole. For example- the articulations of the roots of the teeth with the alveoli of the mandible and maxillae.
  • Syndesmosis: A type of joint in which two bony components are joined directly by a ligament, cord or aponeurotic membrane. For example- Tibiofibular syndesmosis

Cartilaginous Joints

These are also called amphiarthroses or slightly movable articulations. In these joints, the contiguous bony surfaces are either connected by broad flattened discs of fibrocartilage or hyaline growth cartilages. In this kind,  the cartilage directly unites one bony structure to another ( bone – cartilage – bone).

Cartilaginous joints are of two types

  • Symphyses: It is a type of joint where the intervening cartilage that connects the two bones is fibrocartilage in the form of disc or plate. for example symphysis pubis.
  • Synchondrosis:  The connecting material between two bones is hyaline cartilage. It is a temporary form of joint, for the cartilage is converted into bone before adult life. Such joints are found between the epiphyses and bodies of long bones and skull bones.

References

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