7.3: Synovial Joints

By the end of this section, you will be able to

• Describe the structural features of a synovial joint
• Discuss the function of additional structures associated with synovial joints
• List the six types of synovial joints and give an example of each

Synovial joints are the most common type of joint in the body. A key structural characteristic for a synovial joint that is not seen at fibrous or cartilaginous joints is the presence of a joint cavity. This fluid-filled space is the site at which the articulating surfaces of the bones contact each other. Also unlike fibrous or cartilaginous joints, the articulating bone surfaces at a synovial joint are not directly connected to each other with fibrous connective tissue or cartilage. This gives the bones of a synovial joint the ability to move smoothly against each other, allowing for increased joint mobility.

Structural Features of Synovial Joints

Synovial joints are characterized by the presence of a joint cavity. The walls of this space are formed by the articular capsule (Figure \(\PageIndex{1}\)). The outer layers of the capsule is the fibrous layer comprised of dense irregular connective tissue structure that is attached to each bone just outside the area of the bone’s articulating surface. This layer of the capsule restricts bone movements and protects the more delicate underlying structures. Lining the inner surface of the articular capsule is a thin synovial membrane. The cells of this membrane secrete synovial fluid (synovia = “a thick fluid”), a thick, slimy fluid that provides lubrication to further reduce friction between the bones of the joint. This fluid also provides nourishment to the articular cartilage, which does not contain blood vessels. The ability of the bones to move smoothly against each other within the joint cavity, and the freedom of joint movement this provides, means that each synovial joint is functionally classified as a diarthrosis.

Friction between the bones at a synovial joint is prevented by the presence of the articular cartilage, a thin layer of hyaline cartilage that covers the entire articulating surface of each bone. However, unlike at a cartilaginous joint, the articular cartilages of each bone are not continuous with each other. Instead, the articular cartilage acts like a coating over the bone surface, allowing the articulating bones to move smoothly against each other without damaging the underlying bone tissue.

Figure \(\PageIndex{1}\): Synovial Joints. Synovial joints allow for smooth movements between the adjacent bones. The joint is surrounded by an articular capsule that defines a joint cavity filled with synovial fluid. The articulating surfaces of the bones are covered by a thin layer of articular cartilage. (Image credit: "Synovial Joint Structure" by Justin Greene is licensed under CC BY-NC-SA 4.0.)

Stabilizing Structures for Synovial Joints

To add more stability to the joint by preventing movements in specific directions, the bones are connected together by ligaments. These thick bands of dense regular connective tissue allow for normal movements at a joint, but limit the range of these motions, thus preventing excessive or abnormal joint movements (Figure \(\PageIndex{2a}\)). Ligaments are classified based on their relationship to the fibrous layer of the articular capsule:

• an extracapsular ligament is located outside of the articular capsule,
• a capsular ligament attaches to one bone and then fuses with the fibrous layer of the articular capsule, and
• an intracapsular ligament is located inside of the articular capsule.

At many synovial joints, additional support is provided by the muscles and their tendons that act across the joint. A tendon is the dense regular connective tissue structure that attaches a muscle to bone. As forces acting on a joint increase, the body will automatically increase the overall strength of contraction of the muscles crossing that joint, thus allowing the muscle and its tendon to serve as a “dynamic ligament” to resist forces and support the joint. This type of indirect support by muscles is very important at the shoulder joint, for example, where the ligaments are relatively weak.

Figure \(\PageIndex{2}\): Ligaments restrict the movements of joints such as the shoulder by tightening and preventing further rotation along their fiber axis.  Menisci in the knee joint provide a cup-shaped cushion for the round femoral condyles to better fit the flat tibial condyles. (Image credit: "Ligaments of the Shoulder" and "Menisci of the Knee" by Jennifer Lange are licensed under CC BY-NC-SA 4.0, based on images by University of British Columbia Clinical Anatomy.)

A few synovial joints of the body have a fibrocartilage structure located between the articulating bones that improves the complementary fits of the two bone ends. This is called an articular disc, which is generally small and oval-shaped, or a meniscus, which is larger and C-shaped (Figure \(\PageIndex{2b}\)). These structures can serve several functions, depending on the specific joint. In some places, an articular disc may act to create a deeper socket, thus enlarging the articular surface. Examples of this include the labrum of the shoulder and the hip. At other synovial joints, the disc can provide shock absorption and cushioning between the bones, which is the function of each meniscus within the knee joint. Finally, an articular disc can serve to smooth the movements between the articulating bones, as seen at the temporomandibular joint. Some synovial joints also have a fat pad, which can serve as a cushion between the bones.

Additional Structures Associated with Synovial Joints

Additional structures located outside of a synovial joint serve to prevent friction between the bones of the joint and the overlying muscle tendons or skin. A bursa (plural = bursae) is a thin connective tissue sac filled with lubricating liquid. They are located in regions where skin, ligaments, muscles, or muscle tendons can rub against each other, usually near a synovial articulation. Bursae reduce friction by separating the adjacent structures, preventing them from rubbing directly against each other. Bursae are classified by their location. A subcutaneous bursa is located between the skin and an underlying bone. It allows skin to move smoothly over the bone. Examples include the prepatellar bursa located over the kneecap (Figure \(\PageIndex{3}\)) and the olecranon bursa at the tip of the elbow. A submuscular bursa is found between a muscle and an underlying bone, or between adjacent muscles. These prevent rubbing of the muscle during movements. A large submuscular bursa, the trochanteric bursa, is found at the lateral hip, between the greater trochanter of the femur and the overlying gluteus maximus muscle. A subtendinous bursa is found between a tendon and a bone. Examples include the subacromial bursa that protects the tendon of shoulder muscle as it passes under the acromion of the scapula, and the suprapatellar bursa that separates the tendon of the quadriceps femoris (thigh) muscle from the distal femur just above the knee (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{3}\): Subacromial Bursa. Bursae are fluid-filled sacs that serve to prevent friction between skin, muscle, or tendon and an overlying bone. The subacromial bursa of the shoulder joint protects the underlying muscle from wear and tear due to contact with the acromion process. (Image Credit: "Subacromial Bursa" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original "Slagter - Drawing Impingement of shoulder - no labels" by Ron Slagter.

A tendon sheath is similar in structure to a bursa, but smaller. It is a connective tissue sac that surrounds a muscle tendon at places where the tendon crosses a joint. It contains a lubricating fluid that allows for smooth motions of the tendon during muscle contraction and joint movements.

DISORDERS OF THE ...

Joints: Bursitis

Bursitis is the inflammation of a bursa near a joint. This will cause pain, swelling, or tenderness of the bursa and surrounding area, and may also result in joint stiffness. Bursitis is most commonly associated with the bursae found at or near the shoulder, hip, knee, or elbow joints. At the shoulder, subacromial bursitis may occur in the bursa that separates the acromion of the scapula from the tendon of a shoulder muscle as it passes deep to the acromion. In the hip region, trochanteric bursitis can occur in the bursa that overlies the greater trochanter of the femur, just below the lateral side of the hip. Ischial bursitis occurs in the bursa that separates the skin from the ischial tuberosity of the pelvis, the bony structure that is weight bearing when sitting. At the knee, inflammation and swelling of the bursa located between the skin and patella bone is prepatellar bursitis (“housemaid’s knee”), a condition more commonly seen today in roofers or floor and carpet installers who do not use knee pads. At the elbow, olecranon bursitis is inflammation of the bursa between the skin and olecranon process of the ulna. The olecranon forms the bony tip of the elbow, and bursitis here is also known as “student’s elbow” (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): Olecranon Bursitis. Normally this bursa provides a small fluid cushion for the tip of your elbow, but either hitting it hard or repeatedly resting on the elbow for long periods can cause inflammation. (Image source: "Olecranon Bursitis" by Harrygouvas, is in the public domain.)

Bursitis can be either acute (lasting only a few days) or chronic. It can arise from muscle overuse, trauma, excessive or prolonged pressure on the skin, rheumatoid arthritis, gout, or infection of the joint. Repeated acute episodes of bursitis can result in a chronic condition. Treatments for the disorder include antibiotics if the bursitis is caused by an infection, or anti-inflammatory agents, such as nonsteroidal anti-inflammatory drugs (NSAIDs) or corticosteroids if the bursitis is due to trauma or overuse. Chronic bursitis may require that fluid be drained, but additional surgery is usually not required.

Types of Synovial Joints

Synovial joints are subdivided based on the shapes of the articulating surfaces of the bones that form each joint. These shapes give rise to the type of movements they permit - if the bones run into each other, movement in that plane cannot occur. The six types of synovial joints are pivot, hinge, saddle, plane, condyloid, and ball-and socket-joints (Figure \(\PageIndex{5}\)) .

Figure \(\PageIndex{5}\): Types of Synovial Joints. The six types of synovial joints allow the body to move in a variety of ways that are determined by the fit (shape) of the two bones. (Image credit: "Synovial Joints" by BlueLink is licensed under CC BY-NC 4.0 with notification of the original authors; modifications by Jennifer Lange.)

When describing anatomical motion, an action is performed within (parallel to) one of the three anatomical planes: sagittal, frontal, transverse. Each joint has an axis of rotation that is a straight line running parallel to the plane of movement around which an object rotates. Movement at the joint takes place in a plane about an axis. There are three axes of rotation:

• Anterior-Posterior Axis: Imagine a pin that runs through the shoulder joint from front to back (anteriorly and posteriorly). The pin becomes the axis of rotation. Because of the pin’s position, the only movement the shoulder could do about this axis is lateral/medial movement in the frontal plane.
• Medial-Lateral Axis: Mediolateral means the imaginary pin is inserted into the shoulder from a sideways (lateral) approach. The axis runs from the medial side of the joint to the lateral side. The position of the pin allows only movements toward the front or back (anterior and posterior) in the sagittal plane.
• Longitudinal Axis - In this case the pin passes vertically through the shoulder joint and into the length of the humerus. Because the pin runs through the joint from top to bottom, it would effectively prevent the arm from moving forward, backward, or side-to-side. The only movement it will allow is twisting in the transverse plane (i.e., rotation).

Ball-and-Socket Joint

The joint with the greatest range of motion is the ball-and-socket joint. At these joints, the rounded head of one bone (the ball) fits into the concave articulation (the socket) of the adjacent bone (Figure \(\PageIndex{5}\)) . The coxafemoral joint (hip) and the glenohumeral (shoulder) joint are the only ball-and-socket joints of the body. At the hip joint, the head of the femur articulates with the acetabulum of the hip bone, and at the shoulder joint, the head of the humerus articulates with the glenoid cavity of the scapula.Ball-and-socket joints are classified functionally as multiaxial joints. The femur and the humerus are able to move in both anterior-posterior and medial-lateral directions and they can also rotate around their long axis. The shallow socket formed by the glenoid cavity allows the shoulder joint an extensive range of motion. In contrast, the deep socket of the acetabulum and the strong supporting ligaments of the hip joint serve to constrain movements of the femur, reflecting the need for stability and weight-bearing ability at the hip.

Plane Joint

At a plane joint, also known as a gliding joint, the articulating surfaces of the bones are flat or slightly curved and of approximately the same size, which allows the bones to slide against each other (Figure \(\PageIndex{5}\)) . The motion at this type of joint is usually small and tightly constrained by surrounding ligaments. Based only on their shape, plane joints can allow multiple movements, including rotation. Thus plane joints can be functionally classified as a multiaxial joint. However, not all of these movements are available to every plane joint due to limitations placed on it by ligaments or neighboring bones. Thus, depending upon the specific joint of the body, a plane joint may exhibit only a single type of movement or several movements. Plane joints are found between the carpal bones (intercarpal joints) of the wrist or tarsal bones (intertarsal joints) of the foot, between the clavicle and acromion of the scapula (acromioclavicular joint), and between the superior and inferior articular processes of adjacent vertebrae (zygapophysial joints).

Pivot Joint

At a pivot joint, a rounded portion of a bone is enclosed within a ring formed partially by the articulation with another bone and partially by a ligament (Figure \(\PageIndex{5}\)) . The bone rotates within this ring. Since the rotation is around a single axis, pivot joints are functionally classified as a uniaxial type of joint. An example of a pivot joint is the atlantoaxial joint, found between the C1 (atlas) and C2 (axis) vertebrae. Here, the upward projecting dens of the axis articulates with the inner aspect of the atlas, where it is held in place by the transverse ligament. Rotation at this joint allows you to turn your head from side to side. A second pivot joint is found at the proximal radioulnar joint. Here, the head of the radius is largely encircled by a ligament that holds it in place as it articulates with the radial notch of the ulna. Rotation of the radius allows for forearm movements.

Hinge Joint

In a hinge joint, the bar-shaped convex end of one bone articulates with the concave end of the adjoining bone (Figure \(\PageIndex{5}\)) . This type of joint allows only for bending and straightening motions along a single axis, and thus hinge joints are functionally classified as uniaxial joints. the slightly-rounded end of one bone fits into the slightly-hollow end of the other bone. In this way, one bone moves while the other remains stationary, similar to the hinge of a door.  A good example is the elbow joint, with the articulation between the trochlea of the humerus and the trochlear notch of the ulna. Other hinge joints of the body include the knee, ankle, and interphalangeal joints between the phalanx bones of the fingers and toes.

Saddle Joint

Saddle joints are so named because both of the articulating surfaces for the bones have a saddle shape (U-shaped), which is concave in one direction and convex in the other (Figure \(\PageIndex{5}\)). Saddle joints are functionally classified as biaxial joints. The primary example is the first carpometacarpal joint, between the trapezium (a carpal bone) and the first metacarpal bone at the base of the thumb. This joint provides the thumb the ability to move away from the palm of the hand along two planes. Thus, the thumb can move within the same plane as the palm of the hand, or it can jut out anteriorly, perpendicular to the palm. This movement of the first carpometacarpal joint is what gives humans their distinctive “opposable” thumbs. The sternoclavicular joint is also classified as a saddle joint.

Condylar Joint

At a condylar joint, also known as an condyloid joint, the shallow depression at the end of one bone articulates with a rounded structure from an adjacent bone or bones (Figure \(\PageIndex{5}\)), similar to a ball and socket joint. However, the condylar joint surfaces are oval as opposed to round. The knuckle (metacarpophalangeal) joints of the hand between the distal end of a metacarpal bone and the proximal phalanx bone are condyloid joints. Another example is the radiocarpal joint of the wrist, between the shallow depression at the distal end of the radius bone and the rounded scaphoid, lunate, and triquetrum carpal bones. Functionally, condyloid joints are biaxial joints that allow for two planes of movement - sagittal and frontal. One movement involves the bending and straightening of the fingers or the anterior-posterior movements of the hand. The second movement is a side-to-side movement, which allows you to spread your fingers apart and bring them together, or to move your hand in a medial-going or lateral-going direction. Transverse plane movement, twisting, is prevented by oval shape of the articular surfaces.  An oval has two axes - a long axis and a short axis - on both the convex and concave surfaces.  When attempting to twist the long axis will run into the opposing bone's short axis, which stops the movement.

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Concept Review

Synovial joints are the most common type of joints in the body. They are characterized by the presence of a joint cavity, inside of which the bones of the joint articulate with each other. The articulating surfaces of the bones at a synovial joint are not directly connected to each other by connective tissue or cartilage, which allows the bones to move freely against each other. The walls of the joint cavity are formed by the articular capsule. Friction between the bones is reduced by a thin layer of articular cartilage covering the surfaces of the bones, and by a lubricating synovial fluid, which is secreted by the synovial membrane.

Synovial joints are strengthened by the presence of ligaments, which hold the bones together and resist excessive or abnormal movements of the joint. Ligaments are classified as extrinsic ligaments if they are located outside of the articular capsule, intrinsic ligaments if they are fused to the wall of the articular capsule, or intracapsular ligaments if they are located inside the articular capsule. Some synovial joints also have an articular disc (meniscus), which can provide padding between the bones, smooth their movements, or strongly join the bones together to strengthen the joint. Muscles and their tendons acting across a joint can also increase their contractile tension when needed, thus providing indirect support for the joint.

Bursae contain a lubricating fluid that serves to reduce friction between structures. Subcutaneous bursae prevent friction between the skin and an underlying bone, submuscular bursae protect muscles from rubbing against a bone or another muscle, and a subtendinous bursa prevents friction between bone and a muscle tendon. Tendon sheaths contain a lubricating fluid and surround tendons to allow for smooth movement of the tendon as it crosses a joint.

Based on the shape of the articulating bone surfaces and the types of movement allowed, synovial joints are classified into six types.

Synovial Joint Type	Number of Axes of Rotation	Planes of Movement	Examples
Plane	none - gliding only	varies	intercarpal, intertarsal, patellofemoral, acromioclavicular
Hinge	1	sagittal	elbow, knee, ankle, interphalangeal
Pivot	1	transverse	proximal radioulnar, atlantoaxial
Saddle	2	sagittal, frontal	metacarpophalangeal #1, sternoclavicular
Condylar	2	sagittal, frontal	wrist, metacarpophalangeal #2-5, atlanto-occipital
Ball & Socket	3	sagittal, frontal, transverse	shoulder (glenohumeral), hip

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Review Questions

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Critical Thinking Questions

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Glossary

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Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology