Synovial Joints

Structure of Synovial Joints

A synovial joint or diarthrosis occurs at articulating bones to allow movement. It is distinguished by a surrounding synovial capsule.

Learning Objectives

Identify the structures of the synovial joint that allow it to move freely

Key Takeaways

Key Points

  • The bones of a synovial joint are surrounded by a synovial capsule, which secretes synovial fluid to lubricate and nourish the joint while acting as a shock absorber.
  • The ends of the joint bones are covered with smooth, glass-like hyaline cartilage which reduces friction during movement.
  • A synovial joint contains a synovial cavity and dense, irregular connective tissue that forms the articular capsule normally associated with accessory ligaments.

Key Terms

  • articulation: A joint or the collection of joints at which something is articulated, or hinged, for bending.
  • synovial membrane: A thin membrane of joints comprised of smooth connective tissue and that secretes synovial fluid.
  • synovial fluid: A viscous, non-Newtonian fluid found in the cavities of synovial joints. With its yolk-like consistency, its principal role is to reduce friction between the articular cartilage of synovial joints during movement.
  • articular cartilage: A tough, elastic, fibrous connective tissue found in various parts of the body such as the joints, outer ear, and larynx. A major constituent of the embryonic and young vertebrate skeleton, converted largely to bone with maturation.
  • diarthrosis: A joint that can move freely in various planes.

A synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in a mammal’s body. Diarthroses are freely movable articulations. In these joints, the contiguous bony surfaces are covered with articular cartilage and connected by ligaments lined by synovial membrane. The joint may be divided, completely or incompletely, by an articular disk or meniscus, the periphery of which is continuous with the fibrous capsule while its free surfaces are covered by synovial membrane.

The articular capsule is fibrous and continuous with the periosteum of articulating bones, surrounding the diarthrosis and uniting the articulating bones. The articular capsule also consists of two layers: (1) the outer fibrous membrane that may contain ligaments and (2) the inner synovial membrane that secretes the lubricating, shock-absorbing, and joint-nourishing synovial fluid. The bones of a synovial joint are covered by a layer of hyaline cartilage that lines the epiphyses of joint ends of bone with a smooth, slippery surface that does not bind them together. This articular cartilage functions to absorb shock and reduce friction during movement.

Synovial Membrane and Components

This illustration of the synovial joint includes the muscle, synovial cavity, bursa, joint capsule and synovial lining, tendon, enthesis, ligament, articular cartilage, and epiphyseal bone.

Synovial Joint: An illustration of the structure of a synovial joint.

A synovial membrane (or synovium) is the soft tissue found between the articular capsule (joint capsule) and the joint cavity of synovial joints. Synovial fluid is the clear, viscid, lubricating fluid secreted by synovial membranes. The morphology of synovial membranes may vary, but it often consists of two layers. The outer layer, or subintima, can be fibrous, fatty, or loosely areolar. The inner layer, or intima, consists of a sheet of cells thinner than a piece of paper.

Where the underlying subintima is loose, the intima sits on a pliable membrane called the synovial membrane. This membrane, together with the cells of the intima, acts like an inner tube, sealing the synovial fluid from the surrounding tissue and effectively stopping the joints from being squeezed dry when subjected to impact (such as when running). As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones. The main structural differences between synovial and fibrous joints are the existence of capsules surrounding the articulating surfaces of a synovial joint and the presence of lubricating synovial fluid within those capsules (synovial cavities).

Synoviocytes

The intimal cells are termed synoviocytes and can be either fibroblastic (type B synoviocytes) and macrophagic (type A synoviocytes). Both types have differences from similar cells in other tissues. The type B synoviocytes manufacture a long-chain sugar polymer called hyaluronan, which combines with a molecule called lubricin to give the synovial fluid a stringy, egg-white consistency. The water component of synovial fluid is effectively trapped in the joint space by the hyaluronan due to its large, highly negatively charged moieties. The macrophages are responsible for the removal of undesirable substances from the synovial fluid.

Structure of Synovium

The surface of a synovium may be flat or covered with finger-like projections (villi) to allow the soft tissue to change shape as the joint surfaces move on one another. Just beneath the intima, most synovia have a dense net of small blood vessels that provide nutrients for the synovia and the avascular cartilage.

In any one position, much of the cartilage is close enough to get nutrition directly from the synovium. Some areas of cartilage have to obtain nutrients indirectly and may do so either from diffusion through cartilage or by the stirring of synovial fluid.

Synovial Bursa

The synovial bursa is a small, fluid-filled sac lined by synovial membrane containing synovial fluid. It provides a cushion between bones and tendons and/or muscles around a joint.

Nerve and Blood Supply

Synovial joints are highly innervated but vascularized indirectly by nearby tissues.

Learning Objectives

Identify the nerve and blood supply of synovial joints

Key Takeaways

Key Points

  • Although the articular capsule is innervated with the nerves necessary for movement, it lacks blood vessels because the arteries wrap around the joint in an anastomosis, bypassing direct capillary contact with the capsule.
  • The articular and epiphyseal branches given off by the neighboring arteries form a periarticular arterial plexus.
  • Exchange of gases (oxygen and carbon dioxide) and nutrients is achieved, albeit slowly, via diffusion or more efficiently during exercise via convection.

Key Terms

  • convection: The movement of groups of molecules within fluids such as liquids or gases.
  • osteomyelitis: An infection of the bone and bone marrow characterized by inflammation.
  • anastomosis: A cross-connection between two blood vessels.
This diagram depicts the elbow joint, including the anterior profunda brachii, superior ulnar collateral, brachial, inferior ulnar collateral, anterior and posterior ulnar recurrent, interosseus, dorsal interosseus, interosseus recurrent, radial recurrent, radial collateral branch of profunda, and anterior branch of profunda.

Elbow Joint: Diagram of the anastomosis around the elbow joint.

The blood supply of a synovial joint comes from the arteries sharing in anastomosis around the joint. The articular and epiphyseal branches of the neighboring arteries form a periarticular arterial plexus. The articular capsule is highly innervated but avascular (lacking blood and lymph vessels), and receives nutrition from the surrounding blood supply via either the slow process of diffusion or convection, a far more efficient process.

Numerous vessels from this plexus pierce the fibrous capsule and form a rich vascular plexus in the deeper part of the synovial membrane. The blood vessels of the synovial membrane terminate around the articular margins in a fringe of looped anastomoses termed the circulus vasculosus (circulus articularis vasculosus). It supplies the capsule, synovial membrane, and the epiphyses. After epiphyseal fusion in the growth of long bones, communication between the circulosus vasculosus and the end arteries of the metaphysis is established. This minimizes the chances of osteomyelitis in the metaphysis.

The synovial cartilage in the capsule acts somewhat like a sponge. A sponge will absorb fluid, but it will release little of that fluid unless it is squeezed. Exercising the joint, in effect, squeezes the synovial “sponge,” allowing gas exchange to occur and nutrients to flow into the cartilage. Flexing and extending the joint alternately squeezes the sponge and releases it to reabsorb more fluid.

Bursae and Tendon Sheaths

Joints are cushioned by small fluid-filled sacs called bursae and stabilized by tough bands of fibrous connective tissue called tendons.

Learning Objectives

List the components of a joint

Key Takeaways

Key Points

  • Synovial joints are made up of five classes of tissues. These include bone, cartilage, synovium, synovial fluid, and tensile tissues composed of tendons and ligaments.
  • Tendons are tough bands of fibrous connective tissue that connect muscles to bones.
  • Bursae are sacs filled with synovial fluid that provide cushioning around a joint between the bones and the muscles and tendons crossing the joint.

Key Terms

  • retinacula: A band around tendons that holds them in place for stabilization.
  • tendon: A tough band of inelastic fibrous tissue that connects a muscle with its bony attachment.
  • synovial fluid: A viscous, non-Newtonian fluid found in the cavities of synovial joints. With its yolk-like consistency, its principal role is to reduce friction between the articular cartilage of synovial joints during movement.
  • connective tissue: A type of tissue found in animals that functions in binding other tissue systems (such as muscle to skin) or organs. It consists of the cells, fibers, and a ground substance or extracellular matrix.

Synovial joints are made up of five classes of tissues: bone, cartilage, synovium, synovial fluid, and tensile tissues composed of tendons and ligaments. The synovial lining in the bursae and tendon sheaths, similar to that within joints, is a slippery, non-adherent surface allowing movement between planes of tissue. Synovial tendon sheaths line tendons only where they pass through narrow passages or retinacula, as in the palm, at the wrist, and around the ankle. Elsewhere, the tendon lies in a bed of loose fibrous tissue.

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Tendons: Illustration of the location of tendons in the hand

A tendon or sinew is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen, but a ligament joins one bone to another and fasciae connect muscles to other muscles. Tendons connect muscle to bone and move the bones or structures to which they are attached.

A bursa (plural bursae) is a small, fluid-filled sac lined by synovial membrane with an inner capillary layer of fluid (synovial fluid) with the consistency of raw egg white. It provides a cushion between bones and tendons or muscles around a joint. This helps to reduce friction between the bones and allows for free movement.

Bursae occur at sites of shearing in subcutaneous tissue or between deeper tissues such as muscle groups and fascia. Many bursae develop during growth, but new or adventitious bursae can occur at sites of occupational friction. Bursae are found around most major joints of the body, such as the shoulder and the knee. For example, to protect the knee and reduce friction from the various muscles, tendons, and ligaments that attach to and cross the knee joint, knees are cushioned by 14 different bursae: five in front, four laterally, and five medially.

Drawing of the knee joint showing bursae, ligaments, and tendons, as well as tendon of quadriceps femoris, suprapatella bursa, patella, prepatellar bursa, synovial cavity, infrapatellar fat pad, infrapatellar bursa, tibia, anterior cruciate ligament, posterior cruciate ligament, femur.

Knee joint: Diagram of the knee joint.

Stability and Range of Motion at Synovial Joints

Tendons provide stability at joints.

Learning Objectives

Explain the roles of tendons in movement and flexibility

Key Takeaways

Key Points

  • Although tendons have long been considered just a way to attach muscles to bones, research has shown that their springy properties also allow them to provide stability during locomotion with no active work.
  • The elasticity of tendons enables them to release stored energy during walking, allowing the muscles to generate greater force without changing length.
  • Many factors influence joint stability and range of motion.

Key Terms

  • pronation: The action of rotating the forearm so that the palm of the hand is turned down or back.
  • supination: The action of rotating the forearm so that the palm of the hand is turned up or forward
  • eversion: The condition of being turned outward.
  • plantarflexion: The movement that increases the approximate 90 degree angle between the front part of the foot and the shin.
  • dorsiflexion: The movement which decreases the angle between the dorsum (superior surface) of the foot and the leg, so that the toes are brought closer to the shin.

A tendon is a mechanism by which muscles connect to bone and that transmits force. However, over the past two decades, research has also characterized the elastic properties of tendons and their ability to function as springs. This characteristic allows tendons to passively modulate forces during locomotion, thus providing additional stability with no active work. It also allows tendons to store and recover energy with high efficiency.

Effect of Tendon Elasticity

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Achilles Tendon: The Achilles tendon, also called the calcaneus, provides stability and limits the range of motion at the ankle joint. It’s depicted in this diagram in relation to the tendo calcaneus.

During a human stride, the Achilles (calcaneal) tendon stretches as the ankle joint undergoes dorsiflexion. During the last portion of the stride, as the foot undergoes plantar flexion (pointing the toes downward), the stored elastic energy is released. Because the tendon stretches, the muscle is able to function with less or even no change in length, allowing it to generate greater force.

Joint Stability

Certain joints exhibit special movements including elevation, depression, protraction, retraction, inversion, eversion, dorsiflexion, plantar flexion, supination, pronation, and opposition. A number of factors influence joint stability. These include:

  • Shape of articular surfaces (how close they fit)
  • Strength and tension of capsule and ligaments (dependent on position)
  • Arrangement and tension of muscles
  • Contact with soft parts such as adipose tissue
  • Hormones
  • Disuse, causing decrease in synovial fluid, flexibility of ligaments and tendons, and muscle atrophy
  • Gravity and atmospheric pressure.

Typically, the more stable the joint is, the less is its range of motion and vice versa. Aging is another factor that influences motion due to decreased fluid, thinning of cartilage, shortening of ligaments, and loss of flexibility.

Synovial Joint Movements

Synovial joints allow an individual to achieve a wide range of movements.

Learning Objectives

Identify the different types of synovial joints

Key Takeaways

Key Points

  • Synovial joints achieve movement at the point of contact of the articulating bones.
  • Synovial joints allow bones to slide past each other or to rotate around each other. This produces movements called abduction (away), adduction (towards), extension (open), flexion (close), and rotation.
  • There are six types of synovial joints. Some are relatively immobile but more stable than mobile joints.

Key Terms

  • synovial joint: Also known as a diarthrosis, the most common and most movable type of joint in the body of a mammal.
  • abduction: The movement that separates a limb or other part from the axis, or middle line, of the body.
  • flexion: The act of bending a joint. The counteraction of extension.
  • adduction: The action by which the parts of the body are drawn toward its axis.

A synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in the body of a mammal. Synovial joints achieve movement at the point of contact of the articulating bones. Structural and functional differences distinguish synovial joints from cartilaginous joints (synchondroses and symphyses) and fibrous joints (sutures, gomphoses, and syndesmoses). The main structural differences between synovial and fibrous joints are the existence of capsules surrounding the articulating surfaces of a synovial joint and the presence of lubricating synovial fluid within those capsules (synovial cavities).

Several movements may be performed by synovial joints. Abduction is the movement away from the midline of the body. Adduction is the movement toward the middle line of the body. Extension is the straightening of limbs (increase in angle) at a joint. Flexion is bending the limbs (reduction of angle) at a joint. Rotation is a circular movement around a fixed point.

Image demonstrating the various joint movements, including (a) and (b) angular movements, flexion and extension at the shoulders and knees; (c) angular movements, flexion and extension of the neck; (d) angular movements, flexion and extension of the vertebral column; (e) angular movements, adduction, abduction, and circumduction at the upper limb at the shoulder; (f) rotation of the head, neck, and lower limb.

Body Movements I: Image demonstrating the various joint movements.

There are six types of synovial joints. Some are relatively immobile but more stable than mobile joints. Others have multiple degrees of freedom, but at the expense of greater risk of injury. The six types of joints include:

  • Gliding joints: only allow sliding movement
  • Hinge joints: allow flexion and extension in one plane
  • Pivot joints: allow bone rotation about another bone
  • Condyloid joints: perform flexion, extension, abduction, and adduction movements
  • Saddle joints: permit the same movement as condyloid joints and combine with them to form compound joints
  • Ball and socket joints: allow all movements except gliding

Image demonstrating the six different types of synovial joints, including pivot joint between C1 and C2 vertebrae, hinge joint (elbow), saddle joint (between first metacarpal bone and trapezius carpal bone), plane joint (between tarsal bones), condyloid joint (between radius and carpal bones of wrist), and ball and socket joint (hip).

Six Types of Synovial Joints: Image demonstrating the six different types of synovial joints.

Types of Synovial Joints

There are six different types of synovial joint based on their shapes, each allowing a different kind of movement.

Learning Objectives

Describe the different types of synovial joints

Key Takeaways

Key Points

  • Plane joints are flat and have slipping and gliding properties.
  • Hinge joints are formed between the cylindrical end of a bone and the trough-shaped surface of another bone, allowing flexion and extension in one plane.
  • Pivot joints are formed between the rounded end of the bone and a sleeve or ring of a bone, allowing up and down and side-to-side movement.
  • Condyloid joints occur where an egg-shaped surface of a bone fits into a concave space in another bone, allowing flexion, extension, abduction, and adduction movements ( circumduction ). The saddle joint resembles a saddle and permits the same movements as the condyloid joints.
  • Ball-and-socket joints occur where one bone ends in a spherical head and the other has a round socket, allowing all movements except gliding.
  • Saddle joints
    are biaxial, and movements are the same as those for condyloid
    joints; however, no axial
    rotation is possible.

Key Terms

  • ball-and-socket joint: A joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone.
  • acromioclavicular joint: A joint at the top of the shoulder that is the junction between the acromion (a bony process on the scapula) and the clavicle.
  • circumduction: A conical movement of a body part consisting of a combination of flexion, extension, adduction, and
    abduction.
  • condyle: A smooth prominence on a bone where it forms a joint with another bone.
  • Synovial joint: The most common and most movable type of joint in the body of a mammal.

There are six basic types of synovial joints. Anatomical joints may consist of a combination of two or more joint types. Some synovial joints are relatively immobile but stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury. The types of the synovial joints are based on their shapes and can be classified as plane, hinge, pivot, condyloid, saddle, and ball-and-socket. The following descriptions are in ascending order of mobility:

  • The articulating surfaces of the plane joint are usually flat to allow slipping and gliding properties. Examples include the carpals of the wrist and the acromioclavicular joint.
  • A hinge joint (ginglymus) is formed when the cylindrical end of a bone fits into a trough-shaped surface of another bone, like that of an elbow joint (between the humerus and the ulna). These joints act as a hinge, allowing flexion and extension in just one plane.
  • In a pivot joint, the rounded end of the bone fits into a sleeve or ring of bone. The atlanto-axial joint, proximal radioulnar joint, and distal radioulnar joint are examples of pivot joints.
  • The condyloid joint occurs where an egg-shaped surface of a bone fits into a concavity in another bone. Examples include the wrist joint (radiocarpal joint) and the temporomandibular joint. Some classifications make a distinction between condyloid and ellipsoid joints, but both allow flexion, extension, abduction, adduction, and circumduction movements.
  • The surface of a saddle joint has both convex and concave areas which resemble a saddle and permit the same movements as the condyloid joints. The carpometacarpal or trapeziometacarpal joint of the thumb (between the metacarpal and carpal, the trapezium) and the sternoclavicular joint are examples of saddle joints.
  • A ball-and-socket joint occurs where one bone ends in a spherical head and the other bone has a round socket. This joint creates the ball-and-socket movement found in such places as the hip and shoulder (glenohumeral). This type of joint allows for all movements except gliding.

The knee joint is an example of a compound joint/modified hinge joint where different types of joints combine. In this example, the condyles of the femur join with condyles of tibia and the saddle joint, where the lower end of the femur joins with the patella.

This example of a plane joint delineates the scapula, neck of scapula, clavicle, coracoid process, capsular ligament, and tendon of biceps.

Plane Joint: The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula.

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Ball and Socket Joint: Hip joint: the ball of the femur head fits in the socket of the acetabulum of the pelvis.

This anterior view of sternoclavical articulation includes the manubrium sterni, first rib cartilage, anterior disc, and sternal end of clavicle.

Saddle Joint: Sternoclavicular articulation. Anterior view.