9.4 Synovial Joints
3 min read•june 18, 2024
Synovial joints are the most common and flexible joints in the body. They allow for smooth, pain-free movement thanks to their unique structure, which includes , a , and .
These joints come in six types, each with its own shape and . From the simple sliding of to the wide-ranging movement of , synovial joints are essential for our daily activities and overall mobility.
Synovial Joint Structure and Function
Components of synovial joints
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- covers the articulating surfaces of bones
- provides smooth, low-friction surface for joint movement
- Lacks blood vessels and nerves, receiving nutrients from (diffusion)
- Joint capsule surrounds the joint, providing stability and support
- Consists of outer and inner
- lines the inner surface of the joint capsule
- Secretes synovial fluid
- Composed of (macrophage-like cells that remove debris) and (fibroblast-like cells that secrete synovial fluid components)
- Synovial fluid is a clear, viscous fluid within the joint cavity
- Lubricates and nourishes the articular cartilage
- Contains (viscosity), (lubrication), and nutrients
- is the layer of bone beneath the articular cartilage
- Provides structural support and shock absorption
Roles of accessory structures
- are bands of dense regular connective tissue
- Connect bones to bones, providing stability and limiting excessive joint movement ()
- are bands of dense regular connective tissue
- Connect muscles to bones, transmitting muscle forces to enable joint movement ()
- are crescent-shaped fibrocartilaginous structures found in some synovial joints
- Improve and congruency, distribute load, and absorb shock (medial and of the knee)
- are fluid-filled sacs lined with synovial membrane
- Reduce friction between moving structures such as bones, tendons, muscles, and skin ( in the shoulder, in the knee)
- are adipose tissue within or around some joints
- Provide cushioning and fill space ( in the knee joint)
Classification of Synovial Joints
Types of synovial joints (synovial joint classification)
- Plane (gliding) joints have flat or slightly curved articulating surfaces
- Allow sliding or gliding movements ( in the wrist, in the ankle, acromioclavicular joint in the shoulder)
- have a cylindrical surface of one bone fitting into a trough-shaped surface of another
- Allow and movements in one plane (, in fingers and toes, )
- have a rounded surface of one bone articulating with a ring formed by another bone and a ligament
- Allow rotational movements ( between C1 and C2 vertebrae, in the forearm)
- Condyloid (ellipsoidal) joints have an oval-shaped condyle of one bone fitting into an elliptical cavity of another
- Allow , extension, , , and movements ( in the hand, in the wrist)
- have articulating surfaces with both concave and convex regions, resembling a saddle
- Allow flexion, extension, , , and movements ( of the thumb)
- Ball-and-socket joints have a spherical head of one bone fitting into a cup-like cavity of another
- Allow flexion, extension, abduction, adduction, , and circumduction movements (, hip joint)
Joint Mechanics and Function
Range of motion and joint stability
- Range of motion refers to the extent of movement possible at a joint
- Influenced by joint type, ligament flexibility, and muscle strength
- Joint stability is maintained through a balance of static and dynamic stabilizers
- Static stabilizers include joint capsule, ligaments, and bony structures
- Dynamic stabilizers involve muscles and tendons
Joint lubrication and movement
- Synovial fluid provides lubrication through various mechanisms
- Boundary lubrication: Lubricin forms a thin layer between articulating surfaces
- Fluid film lubrication: Synovial fluid forms a thin film between surfaces during movement
- describes the precise movements of joint surfaces
- Includes rolling, sliding, and spinning motions between articulating surfaces
Key Terms to Review (86)
Abduction: Abduction in anatomy and physiology is the movement of a limb or other part away from the midline of the body or from another part. It is a key action in joints, enabling movements such as spreading the fingers or arms.
Abduction: Abduction is the movement of a limb or body part away from the midline of the body. It is one of the primary types of body movements that occur at synovial joints, allowing for a wide range of motion and flexibility in the limbs and extremities.
Adduction: Adduction is a movement that brings a limb closer to the midline of the body or decreases the angle between two bones at a joint. It is opposite to abduction, which moves limbs away from the body's midline.
Adduction: Adduction is a type of body movement that involves the action of moving a limb or body part towards the midline of the body. It is a key concept in understanding the mechanics of the pectoral girdle, synovial joints, and the muscles that control movement of the upper and lower limbs.
Ankle joint: The ankle joint, also known as the talocrural joint, is a synovial hinge joint that connects the distal ends of the tibia and fibula in the lower leg with the proximal end of the talus in the foot. It allows for dorsiflexion and plantarflexion of the foot, enabling movements such as walking, running, and jumping.
Ankle Joint: The ankle joint, also known as the talocrural joint, is a synovial hinge joint that connects the lower leg to the foot. It is responsible for the up-and-down movements of the foot, allowing for walking, running, and other locomotor activities.
Arthrokinematics: Arthrokinematics refers to the study of the motion and interaction between the articular surfaces of synovial joints. It examines the complex movements and relationships between the bones, cartilage, and other joint components during various types of joint motion.
Articular capsule: An articular capsule is a fibrous envelope that encloses the synovial cavity of a synovial joint, providing both stability and flexibility. It consists of two layers: an outer fibrous layer that strengthens the joint, and an inner synovial membrane that secretes synovial fluid for lubrication.
Articular cartilage: Articular cartilage is a smooth, white tissue that covers the ends of bones where they come together to form joints. Its main function is to reduce friction and absorb shock in synovial joints, facilitating smooth and pain-free movement.
Articular Cartilage: Articular cartilage is the smooth, slippery connective tissue that covers the ends of bones where they meet to form a joint. It plays a crucial role in the function and movement of synovial joints by providing a low-friction surface that allows for smooth, pain-free articulation between the bones.
Articular disc: An articular disc is a pad of fibrocartilage situated between the bones in some synovial joints, serving to absorb shock and improve the fit between bone ends. It ensures joint stability and allows for smoother movement.
Atlantoaxial joint: The atlantoaxial joint is a complex of joints located between the first and second cervical vertebrae (the atlas and axis) that allows for the rotation of the head. It is a pivotal type of synovial joint, characterized by its ability to support the head's rotation from side to side.
Atlantoaxial Joint: The atlantoaxial joint is a synovial joint located between the atlas (C1) and the axis (C2) vertebrae in the upper cervical spine. It is a pivotal joint that allows for the majority of the head's rotational movements.
Ball-and-socket joint: A ball-and-socket joint is a type of synovial joint in the human body where a rounded bone head (the ball) fits into a concave articulation (the socket), allowing multi-directional movement and rotation. This joint provides the greatest range of motion compared to other types of joints.
Ball-and-Socket Joints: A ball-and-socket joint is a type of synovial joint that allows for a wide range of motion. It is characterized by a rounded, ball-like projection on one bone that fits into a cup-like depression on another bone, enabling rotational and swinging movements in multiple directions.
Bursa: A bursa is a small fluid-filled sac lined by synovial membrane with an inner capillary layer of viscous fluid. It acts as a cushion to reduce friction between tissues of the body, such as between tendons and bones near synovial joints.
Bursae: Bursae are small, fluid-filled sacs located throughout the body that provide cushioning and reduce friction between tissues, particularly around joints. They play a key role in facilitating smooth movement by allowing muscles and tendons to glide over bones and other structures. Understanding bursae is essential when looking at joint classification, particularly synovial joints, where they are most commonly found.
Carpometacarpal joint: The carpometacarpal (CMC) joints are found at the base of the palm of the hand, where the carpal bones of the wrist meet the metacarpal bones of the hand. These joints allow for the thumb's wide range of motion, including opposition, which is crucial for grip and precision in hand movements.
Carpometacarpal Joint: The carpometacarpal joint is a type of synovial joint that connects the carpal bones of the wrist to the metacarpal bones of the hand. It plays a crucial role in the classification, structure, and movement capabilities of the hand and wrist.
Circumduction: Circumduction is a circular movement of a body part, such as an arm or leg, that combines flexion, extension, abduction, and adduction in a continuous sequence. It allows the distal end of the limb to move in a circle while the proximal end remains stationary.
Circumduction: Circumduction is a type of complex joint movement that involves the circular motion of a body part around an axis. It is a combination of the four basic movements: flexion, extension, abduction, and adduction, allowing for a wide range of motion in a joint.
Collateral Ligaments: Collateral ligaments are strong bands of connective tissue that run along the sides of certain synovial joints, providing stability and limiting excessive movement. They are particularly important in the anatomy and function of selected synovial joints.
Condyloid joint: A condyloid joint (also known as an ellipsoid joint) is a type of synovial joint that allows movement in two planes, including flexion, extension, abduction, and adduction, but not rotation. It consists of an oval articular surface fitting into a complementary depression on the opposing surface.
Condyloid Joints: Condyloid joints, also known as ellipsoidal joints, are a type of synovial joint where an oval-shaped end of one bone fits into a similarly shaped cavity of another bone, allowing for a range of movement in two planes. These joints enable flexion, extension, abduction, and adduction, making them versatile in allowing various body movements. They play a critical role in the biomechanics of the body, facilitating smooth motions while maintaining stability.
Elbow joint: The elbow joint is a synovial joint that connects the upper arm bone (humerus) with the two bones of the forearm (radius and ulna), allowing for flexion and extension as well as limited rotation. It is a hinge type of joint supported by muscles, ligaments, and tendons to facilitate movement and stability.
Elbow Joint: The elbow joint is a hinge-type synovial joint that connects the upper arm (humerus) to the forearm (radius and ulna). It allows for flexion, extension, and a small degree of rotation of the forearm.
Extension: Extension refers to the movement of a joint or body part that increases the angle between the bones involved, straightening or lengthening the limb or structure. It is one of the fundamental types of body movements that occurs at synovial joints and is facilitated by the contraction of specific muscle groups.
Extrinsic ligament: Extrinsic ligaments are connective tissues located outside the joint capsule that connect two or more bones, providing stability to the synovial joints. They play a crucial role in limiting the range of motion to prevent excessive or abnormal movements.
Fat Pads: Fat pads are specialized areas of adipose tissue found in various parts of the body, particularly in synovial joints. These pads serve important functional roles in cushioning and protecting the joints from mechanical stresses and impacts.
Fibrous Capsule: The fibrous capsule is a dense connective tissue structure that encapsulates and stabilizes synovial joints, providing protection and support to the joint. It plays a crucial role in the anatomy and function of synovial joints.
Flexion: Flexion is the action of bending a joint or limb in the body which decreases the angle between the bones of the limb at the joint. It is a fundamental movement in daily activities and exercises, contrasting with extension which increases the angle.
Flexion: Flexion is a type of body movement that decreases the angle between two body parts, bringing them closer together. This term is closely related to the structure and function of muscles, the pectoral girdle, synovial joints, and various body movements and joint anatomy.
Hinge joint: A hinge joint is a type of synovial joint that allows movement in one plane, primarily permitting flexion and extension. It resembles the mechanical operation of a door hinge, enabling parts of the body to bend and straighten.
Hinge joints: Hinge joints are a type of synovial joint that allow movement primarily in one plane, enabling flexion and extension. These joints work like the hinge of a door, permitting movement back and forth, which is crucial for actions such as bending the elbow or knee. This specific movement pattern plays a significant role in various body functions and aligns with the classification of joints based on structure and function.
Hyaline cartilage: Hyaline cartilage is a type of connective tissue found in various parts of the body, characterized by its translucent, glassy appearance under a microscope. It provides support and flexibility to structures such as the nose, trachea, and ends of long bones where it reduces friction in joints.
Hyaline Cartilage: Hyaline cartilage is a type of smooth, glassy cartilage that serves as a protective and supportive connective tissue in various parts of the body, including joints, the respiratory system, and the skeletal system.
Hyaluronic Acid: Hyaluronic acid is a naturally occurring polysaccharide found in the extracellular matrix of various tissues throughout the body. It is a key component in the structure and function of connective tissues, particularly in synovial joints and the skin.
Hyperextension: Hyperextension is the movement of a joint beyond its normal range of motion, resulting in an excessive straightening or opening angle. This action can occur in various joints, such as the knee, elbow, and spine, potentially leading to injury if performed excessively.
Infrapatellar Fat Pad: The infrapatellar fat pad is a collection of adipose tissue located within the knee joint, situated between the patellar tendon and the joint capsule. It plays a crucial role in the biomechanics and function of the synovial joint of the knee.
Intercarpal Joints: The intercarpal joints are a series of small, gliding synovial joints located between the carpal bones in the wrist. These joints facilitate the complex movements and flexibility of the wrist, allowing for a wide range of motion and dexterity of the hand.
Interphalangeal Joints: The interphalangeal joints are the hinge-like synovial joints located between the phalanges, or finger and toe bones, that allow for flexion and extension of the digits. These joints are essential for the dexterity and range of motion in the hands and feet.
Intertarsal Joints: Intertarsal joints are a group of synovial joints located between the tarsal bones in the posterior region of the foot. These joints play a crucial role in facilitating various types of body movements, particularly those involved in the ankle and foot.
Intracapsular ligament: An intracapsular ligament is a type of ligament located inside the joint capsule of a synovial joint, serving the primary function of providing additional support and stabilization to the joint. These ligaments are crucial for maintaining the integrity and proper alignment of the joint components during movement.
Intrinsic ligament: An intrinsic ligament is a type of ligament located within and inherently part of the structure of a synovial joint, providing stability by connecting bones or cartilages within the joint itself. These ligaments are crucial for maintaining the integrity and proper function of the joint during movement.
Joint Capsule: The joint capsule is a fibrous structure that surrounds and encloses the articulating ends of bones in a synovial joint, providing stability and limiting the range of motion. It is an essential component that helps maintain the integrity of the joint and facilitate smooth, controlled movements.
Joint Lubrication: Joint lubrication is the process by which the surfaces of a synovial joint are kept moist and slippery, allowing for smooth and efficient movement. This is a crucial aspect of the proper functioning of synovial joints, which are the most common type of joint in the human body.
Joint Stability: Joint stability refers to the ability of a joint to maintain its proper alignment and function without excessive movement or dislocation. It is a critical factor in the overall health and function of the musculoskeletal system, as it ensures the efficient and safe transfer of forces between body segments during movement and activity.
Lateral menisci: Lateral menisci are C-shaped cartilaginous structures located in the knee joint that provide cushioning and stability to the joint. They play a critical role in load distribution, absorbing shock during movement, and enhancing the congruence between the femur and tibia. The lateral meniscus specifically helps to prevent excessive lateral movement of the knee and supports overall joint function.
Ligaments: Ligaments are flexible, fibrous connective tissue structures that connect bones to bones, stabilizing and supporting the skeletal system. They play a crucial role in the classification, anatomy, and development of joints throughout the body.
Lubricin: Lubricin is a glycoprotein that plays a vital role in the lubrication of synovial joints, helping to reduce friction between articular cartilage surfaces during movement. This molecule is secreted by synoviocytes and is crucial for maintaining the health of joint cartilage, promoting smooth motion and preventing wear and tear in the joints. It contributes to the overall function of synovial fluid, which is essential for joint mobility and integrity.
Medial Menisci: The medial menisci are crescent-shaped fibrocartilaginous structures located in the knee joint. They play a crucial role in the biomechanics and stability of the synovial joint, acting as shock absorbers and facilitating smooth movement between the femur and tibia.
Menisci: Menisci are crescent-shaped fibrocartilaginous structures found in certain synovial joints, such as the knee, that serve to stabilize the joint, distribute weight, and provide shock absorption during movement.
Meniscus: A meniscus is a C-shaped piece of tough, rubbery cartilage that acts as a shock absorber within certain synovial joints, such as the knee. It helps to cushion the joint and distribute loads evenly across the joint surfaces.
Metacarpophalangeal joints: Metacarpophalangeal joints, commonly known as MCP joints, are the articulation points between the metacarpal bones of the hand and the proximal phalanges of the fingers. These joints play a crucial role in hand function, allowing for a wide range of movements essential for gripping and manipulating objects, and they are classified as synovial joints that facilitate various types of body movements.
Pivot joint: A pivot joint is a type of synovial joint that allows rotational movement around a single axis. It enables movements such as turning the head from side to side or rotating the forearm.
Pivot Joints: Pivot joints, also known as trochoid joints, are a type of synovial joint that allows for rotational movement around a single axis. They enable a limited range of motion, primarily rotation, and are essential for various body movements and functions.
Plane joint: A plane joint is a type of synovial joint where two flat surfaces slide over each other, allowing for limited movement in multiple directions. They are found in areas requiring flexibility and stability, such as the carpal bones in the wrist.
Plane Joints: Plane joints, also known as gliding joints, are a type of synovial joint that allow for limited movement in various directions due to their flat or slightly curved articular surfaces. These joints primarily facilitate sliding or gliding motions between the bones involved, making them crucial for certain body movements and overall flexibility. They play a key role in the classification of joints by representing one of the simplest forms of joint articulation, which is essential for activities requiring nuanced movement.
Prepatellar bursa: The prepatellar bursa is a small, fluid-filled sac located in front of the kneecap (patella), acting as a cushion to reduce friction between the patella and the skin during movement. This bursa is essential in facilitating smooth motion of the knee joint, especially during activities like kneeling or bending, where the knee experiences significant pressure.
Proximal radioulnar joint: The proximal radioulnar joint is a pivot type of synovial joint located between the radius and ulna bones near the elbow, allowing for the rotation of the forearm. It plays a crucial role in enabling movements such as turning the palm up or down.
Proximal Radioulnar Joint: The proximal radioulnar joint is the synovial joint located between the proximal end of the radius and the ulna, allowing for the rotational movements of the forearm. It is a crucial component of the upper limb's skeletal structure and a key joint in the overall function of the upper extremity.
Quadriceps Tendon: The quadriceps tendon is a thick, strong tendon that connects the quadriceps muscle group to the patella (kneecap) in the knee joint. It plays a crucial role in the functioning and stability of the synovial knee joint.
Radiocarpal joint: The radiocarpal joint is a synovial joint formed between the distal end of the radius and the proximal row of carpal bones in the wrist. It allows for movements such as flexion, extension, abduction, and adduction of the wrist.
Radiocarpal Joint: The radiocarpal joint, also known as the wrist joint, is a synovial joint located between the distal end of the radius bone in the forearm and the proximal row of carpal bones in the wrist. It is a pivotal joint that allows for a wide range of motion in the hand and wrist.
Range of Motion: Range of motion (ROM) refers to the measurement of the amount of movement a joint can achieve before it reaches its functional or anatomical limit. It is an important concept in the context of joint classification, synovial joint structure, types of body movements, and the anatomy of specific synovial joints.
Rotation: Rotation in the context of anatomy and physiology refers to the movement of a bone or body part around its own axis within a joint, allowing for circular motion. This type of movement can occur in different planes and is crucial for various bodily functions.
Rotation: Rotation is the circular movement of a body or object around its own axis or center of mass. It is a fundamental type of motion that is observed in various contexts, including the pectoral girdle, synovial joints, and body movements.
Saddle joint: A saddle joint is a type of synovial joint that allows movement in two planes, including flexion, extension, adduction, and abduction. It is characterized by the fitting together of complementary surfaces with concave and convex areas.
Saddle Joints: Saddle joints are a type of synovial joint where the opposing surfaces of the articulating bones are concave and convex, resembling the shape of a saddle. This unique configuration allows for a wide range of motion in two planes, making saddle joints highly versatile.
Shoulder Joint: The shoulder joint, also known as the glenohumeral joint, is a ball-and-socket synovial joint that connects the upper arm bone (humerus) to the shoulder blade (scapula). It is the most mobile joint in the human body, allowing for a wide range of motion and enabling various arm movements.
Subacromial Bursa: The subacromial bursa is a fluid-filled sac located between the acromion process of the scapula and the superior surface of the rotator cuff tendons. It serves to reduce friction and facilitate smooth movement of the shoulder joint during various arm motions.
Subchondral Bone: Subchondral bone is the layer of compact bone that lies immediately beneath the articular cartilage in synovial joints. It serves as a shock-absorbing cushion and provides structural support for the overlying cartilage.
Subcutaneous bursa: A subcutaneous bursa is a small fluid-filled sac located between the skin and bony prominences, primarily serving as a cushion to reduce friction during movement. It plays an essential role in the smooth motion of joints by facilitating the sliding of skin over underlying bone.
Submuscular bursa: A submuscular bursa is a fluid-filled sac located beneath a muscle, which serves to reduce friction between the muscle and underlying bone or other tissues. It facilitates smooth movement of muscles across bony structures or adjacent muscles within synovial joints.
Subtendinous bursa: A subtendinous bursa is a small fluid-filled sac located between a tendon and bone, serving to reduce friction and facilitate smooth movement of the tendon. It is commonly found in areas where tendons pass over bony prominences within synovial joints.
Suspensory ligaments: Suspensory ligaments are fibrous connective tissues that secure the ovaries to the lateral pelvic walls, providing support and maintaining their position in the female reproductive system. They contain blood vessels and nerves that supply the ovaries.
Synovial fluid: Synovial fluid is a viscous, non-Newtonian fluid found in the cavities of synovial joints, serving as a lubricant to reduce friction between the articular cartilages during movement. It also provides nutrients to the cartilages and acts as a shock absorber.
Synovial Fluid: Synovial fluid is a viscous, lubricating liquid found in the synovial joints of the body. It plays a crucial role in reducing friction, providing cushioning, and nourishing the articular cartilage within these joints. This fluid is closely connected to the topics of tissue types, joint classification, synovial joint anatomy, and body fluid compartments.
Synovial Joint Classification: Synovial joints are the most common and mobile type of joint in the human body. They are classified based on the shape and function of the articulating surfaces, allowing for a wide range of motion and stability.
Synovial membrane: The synovial membrane is a specialized connective tissue that lines the inner surface of capsules in synovial joints and secretes synovial fluid. This fluid lubricates the joint, reducing friction and wear between cartilages during movement.
Synovial Membrane: The synovial membrane is a thin, specialized connective tissue lining the inside of synovial joints, such as the knee, shoulder, and elbow. It plays a crucial role in the anatomy and function of these movable joints by producing synovial fluid and facilitating smooth, frictionless movement.
Tendon: A tendon is a tough, flexible band of fibrous connective tissue that connects muscles to bones, enabling the force from muscle contractions to be transferred to the skeleton for movement. Tendons play a critical role in joint mechanics by stabilizing and guiding joint movements.
Tendon sheath: A tendon sheath is a layer of synovial membrane that surrounds a tendon, providing a lubricated tunnel for the tendon to move through smoothly. This structure helps reduce friction between the tendon and surrounding tissues, facilitating ease of movement.
Tendons: Tendons are tough, fibrous connective tissues that connect muscle to bone, transmitting the force generated by muscle contraction to the skeleton and enabling body movements.
Type A Synoviocytes: Type A synoviocytes, also known as macrophage-like synoviocytes, are specialized cells found within the synovial membrane of synovial joints. These cells play a crucial role in maintaining the health and function of the synovial joint by performing various important tasks.
Type B synoviocytes: Type B synoviocytes are specialized cells found in the synovial membrane of synovial joints, responsible for the secretion of lubricating hyaluronic acid and other components of synovial fluid. These cells play a crucial role in maintaining joint health and function by producing the extracellular matrix and contributing to the overall homeostasis of the joint environment.