# Lecture 3: Joint Classification and Structure ## Unit 1.3: Human Gross Anatomy I - Musculoskeletal System --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Define a joint (articulation) and explain the functional classification of joints 2. Describe the structural classification of joints (fibrous, cartilaginous, synovial) 3. Identify the components and features of a synovial joint 4. Classify synovial joints by shape and movement type 5. Describe the types of movements possible at synovial joints 6. Apply knowledge of joint structure to understand common joint pathologies --- ## Introduction to Joints A joint, also called an articulation, is any site where two or more bones meet. The human body contains approximately 300 joints, ranging from immobile connections that bind bones firmly together to highly mobile articulations that allow free movement in multiple planes. Understanding joint anatomy is essential for physical examination, interpreting imaging studies, understanding mechanisms of injury, and treating musculoskeletal conditions. Joints are classified by two complementary systems: structural classification based on the type of tissue connecting the bones and whether a joint cavity is present, and functional classification based on the degree of movement permitted. The structural classification recognizes three categories. Fibrous joints connect bones with fibrous connective tissue and lack a joint cavity. Cartilaginous joints connect bones with cartilage and also lack a joint cavity. Synovial joints feature a fluid-filled joint cavity between the articulating bones and are lined by a synovial membrane. The functional classification also has three categories. Synarthroses are immobile joints that allow essentially no movement. Amphiarthroses are slightly movable joints that permit limited motion. Diarthroses are freely movable joints that allow movement in one or more planes. There is a general correspondence between structural and functional classifications: most fibrous joints are synarthroses, most cartilaginous joints are amphiarthroses, and all synovial joints are diarthroses, though exceptions exist. Panel A: Central human skeleton with representative examples of each joint type highlighted at their anatomical locations. Panel B: Structural classification flowchart showing Fibrous (no cavity, connected by CT), Cartilaginous (no cavity, connected by cartilage), and Synovial (cavity present, synovial fluid). Panel C: Functional classification showing Synarthrosis (immovable) with skull suture, Amphiarthrosis (slightly movable) with intervertebral disc, Diarthrosis (freely movable) with knee. Panel D: Matrix showing relationship between structural and functional types with arrows and color coding connecting to typical examples. --- ## Fibrous Joints Fibrous joints are characterized by bones connected by fibrous connective tissue with no joint cavity present. The amount of movement depends on the length of the connective tissue fibers: shorter fibers generally mean less movement. Most fibrous joints are synarthroses (immobile), though some are amphiarthroses (slightly movable). Sutures are fibrous joints found only in the skull. The bones are joined by a thin layer of dense fibrous connective tissue called the sutural ligament. In the mature skull, sutures permit essentially no movement and are classified as synarthroses. However, this immobility serves important purposes: the rigid connection protects the brain while allowing the skull to grow during childhood. In infants and children, the sutural ligament is wider and more flexible, accommodating brain growth. As aging progresses, many sutures undergo synostosis, in which the fibrous tissue is gradually replaced by bone, fusing the adjacent skull bones into a single unit. The pattern and timing of sutural closure vary among individuals and can be used in forensic age estimation. Sutures are described by their appearance. Serrate sutures have interlocking, saw-tooth-like edges, as seen in the sagittal suture between the parietal bones. Squamous sutures have overlapping beveled edges, as seen between the parietal and temporal bones. Plane sutures have relatively flat, abutting edges. Syndesmoses are fibrous joints in which bones are connected by a ligament or a sheet of fibrous tissue called an interosseous membrane. These joints permit more movement than sutures and are classified as amphiarthroses. The degree of movement depends on the length and flexibility of the connecting fibers. The interosseous membrane between the radius and ulna in the forearm and the similar membrane between the tibia and fibula in the leg are examples of syndesmoses. These membranes not only connect the bones but also provide attachment surfaces for muscles and allow limited movement while maintaining the relationship between the paired bones. The distal tibiofibular joint is a syndesmosis that is essential for the stability of the ankle mortise. Gomphoses are specialized fibrous joints found only between the teeth and the alveolar processes of the maxilla and mandible. The periodontal ligament anchors each tooth root in its bony socket. Although classified as synarthroses because of their minimal movement, gomphoses do allow slight movement that serves proprioceptive functions, providing sensory feedback about biting forces. Panel A: Sutures showing superior view of skull with sagittal suture between parietal bones, magnified inset revealing interlocking serrate edges and thin sutural ligament. Panel B: Squamous suture between parietal and temporal bones with overlapping edges, labeled as synarthrosis. Panel C: Syndesmosis showing anterior view of forearm with interosseous membrane connecting radius and ulna, similar view of leg between tibia and fibula with limited movement arrows. Panel D: Gomphosis cross-section through tooth in alveolar bone showing tooth root, periodontal ligament with radiating fibers connecting root to bone. --- ## Cartilaginous Joints Cartilaginous joints connect bones with cartilage and lack a joint cavity. They are divided into two types based on the type of cartilage involved. Synchondroses, also called primary cartilaginous joints, connect bones with hyaline cartilage. Most synchondroses are temporary joints that eventually undergo ossification, converting to bone. They are classified as synarthroses because they permit no movement while present. The most important examples are the epiphyseal plates of growing long bones, where hyaline cartilage connects the diaphysis to each epiphysis. Growth occurs at these plates until skeletal maturity, when they ossify and become epiphyseal lines. Once fused, no movement or further growth is possible. The first sternocostal joint, connecting the first rib to the sternum, is a permanent synchondrosis that remains as hyaline cartilage throughout life. The spheno-occipital synchondrosis at the base of the skull is particularly important for skull growth and typically fuses in early adulthood. Symphyses, also called secondary cartilaginous joints, connect bones with a disc of fibrocartilage, often with a thin layer of hyaline cartilage covering each bone surface. Unlike synchondroses, symphyses are permanent joints that persist throughout life without ossifying. They are located in the midline of the body and are classified as amphiarthroses because they allow limited movement. The intervertebral discs between adjacent vertebral bodies are symphyses; the cumulative effect of slight movement at each disc allows significant flexibility of the vertebral column as a whole. The pubic symphysis connects the two pubic bones anteriorly; during pregnancy, hormonal changes allow increased mobility at this joint to facilitate childbirth. The manubriosternal joint between the manubrium and body of the sternum is a symphysis that may partially ossify with age. Panel A: Synchondrosis showing longitudinal section through growing long bone with epiphyseal plate (hyaline cartilage in blue) connecting bony epiphysis to diaphysis with growth direction arrows. Panel B: First sternocostal joint with rib connected to sternum by hyaline cartilage, labeled as temporary or permanent synarthrosis. Panel C: Symphysis showing two vertebral bodies connected by intervertebral disc with central nucleus pulposus and outer annulus fibrosus, hyaline cartilage endplates visible. Panel D: Pubic symphysis with fibrocartilage disc between pubic bones, labeled as permanent midline amphiarthrosis joints. --- ## Synovial Joints Synovial joints are the most common type of joint in the body and the most structurally complex. They are characterized by a fluid-filled joint cavity between the articulating bones, which allows free movement. All synovial joints are classified functionally as diarthroses. The articular cartilage covers the ends of the bones where they articulate with each other. This cartilage is hyaline cartilage, typically 1-7 millimeters thick depending on the joint and the forces it experiences. The smooth surface of articular cartilage provides a low-friction interface for joint movement, with a coefficient of friction lower than ice on ice. Articular cartilage is avascular and aneural, receiving its nutrition from synovial fluid. Because it lacks blood supply, damaged articular cartilage heals poorly if at all, which has significant implications for arthritis and joint injuries. The joint capsule, or articular capsule, encloses the joint cavity. It consists of two layers. The outer fibrous layer is composed of dense irregular connective tissue continuous with the periosteum of the articulating bones. It provides strength and stability to the joint. The inner synovial membrane is a thin vascular layer that lines all internal surfaces of the joint cavity except the articular cartilage. The synovial membrane contains two cell types: fibroblast-like cells that synthesize hyaluronic acid and macrophage-like cells that remove debris from the joint fluid. Synovial fluid fills the joint cavity and is essential for joint function. It is a clear, viscous fluid derived from the ultrafiltration of blood plasma, with the addition of hyaluronic acid synthesized by the synovial membrane. This hyaluronic acid gives synovial fluid its characteristic viscosity. Synovial fluid serves three main functions. It lubricates the joint, reducing friction between the articular cartilage surfaces during movement. It nourishes the avascular articular cartilage by providing nutrients through diffusion. It also absorbs shock, helping to cushion the joint against impact. A phenomenon called weeping lubrication contributes to joint function: when cartilage is compressed, fluid within the cartilage matrix is squeezed to the surface, providing additional lubrication at the point of contact. The joint cavity is a potential space containing only a small amount of synovial fluid under normal conditions, typically less than one milliliter in most joints. Negative intra-articular pressure helps maintain joint contact and stability. Panel A: Generic synovial joint longitudinal section showing two bones with articular cartilage covering opposing surfaces and joint cavity containing synovial fluid. Panel B: Joint capsule with outer fibrous layer continuous with periosteum and inner synovial membrane lining cavity except over cartilage. Panel C: Magnified inset of articular cartilage layers showing superficial, middle, deep zones over calcified cartilage and subchondral bone. Panel D: Synovial membrane inset with fibroblast-like and macrophage-like synoviocytes, blood vessels, fluid production, and weeping lubrication diagram showing fluid expressed under pressure. --- ## Accessory Structures of Synovial Joints Many synovial joints contain additional structures that enhance their function, stability, or durability. Ligaments are dense regular connective tissue structures that connect bone to bone. In synovial joints, ligaments reinforce and stabilize the joint capsule, limiting the range of motion to prevent abnormal movements that might damage the joint. Intrinsic or capsular ligaments are thickenings of the joint capsule itself. Extrinsic or accessory ligaments are separate from the capsule and may be located outside the capsule (extracapsular) or within the joint cavity (intracapsular). Ligaments are richly supplied with proprioceptors that provide information about joint position and movement, contributing to coordination and protective reflexes. Articular discs, commonly called menisci when wedge-shaped, are pads of fibrocartilage within certain synovial joints. They improve the fit between articulating surfaces that do not perfectly match, known as incongruent surfaces. They also absorb shock, distribute weight more evenly across the joint, and may direct the flow of synovial fluid. The menisci of the knee joint are the most familiar examples: the medial and lateral menisci are C-shaped wedges that deepen the tibial plateau and absorb the forces transmitted through the knee. The temporomandibular joint contains a complete disc that divides the joint into upper and lower compartments. The triangular fibrocartilage complex of the wrist is another important articular disc. Labra are rings of fibrocartilage that attach around the margin of certain joint sockets, deepening the socket and increasing stability. The glenoid labrum deepens the shallow glenoid cavity of the scapula, increasing the stability of the shoulder joint while still allowing extensive mobility. The acetabular labrum similarly deepens the acetabulum of the hip, contributing to hip stability. Labral tears are common injuries that can cause pain and mechanical symptoms. Bursae are closed sacs lined with synovial membrane and containing a thin layer of synovial fluid. They are located at sites where structures such as tendons, muscles, or skin would otherwise rub against bone, reducing friction and facilitating movement. Many bursae communicate with adjacent joint cavities, while others are entirely separate. Important bursae include the subacromial bursa between the rotator cuff tendons and the acromion, the prepatellar bursa superficial to the patella, the olecranon bursa at the elbow, and the trochanteric bursa at the lateral hip. Tendon sheaths are elongated bursae wrapped around tendons, particularly where tendons pass through confined spaces such as the carpal tunnel of the wrist or the fibrous tunnels of the digits. The synovial lining and fluid allow the tendon to glide smoothly within its sheath during movement. Panel A: Ligaments at knee joint showing collateral ligaments (extracapsular) and cruciate ligaments (intracapsular), labeled as intrinsic versus extrinsic. Panel B: Articular disc/Meniscus with superior view of tibial plateau showing C-shaped medial and lateral menisci, cross-section showing wedge shape deepening surface. Panel C: Labrum showing coronal section through shoulder with glenoid labrum deepening fossa, coronal section through hip with acetabular labrum deepening acetabulum. Panel D: Bursa showing sagittal section through knee with suprapatellar and prepatellar bursae, tendon sheath cross-section through finger showing flexor tendon within synovial sheath. --- ## Classification of Synovial Joints by Shape Synovial joints are further classified into six types based on the shape of their articular surfaces and the types of movement they allow. This classification helps predict the movements possible at each joint. Plane joints, also called gliding joints, have flat or slightly curved articular surfaces that allow sliding or gliding movements in various directions. They may be classified as nonaxial (movement without rotation around an axis) or multiaxial (able to move in many directions). Examples include the intercarpal and intertarsal joints between the small bones of the wrist and ankle, the acromioclavicular joint between the clavicle and scapula, and the facet (zygapophyseal) joints between vertebral articular processes. Hinge joints have a convex surface on one bone that fits into a concave surface on the other, like a door hinge. They are uniaxial, allowing movement in only one plane: flexion and extension. The elbow joint (specifically the humeroulnar articulation), the interphalangeal joints of the fingers and toes, the knee joint (primarily), and the ankle joint (talocrural articulation) are hinge joints. Pivot joints allow rotation around a single axis. A rounded or pointed process of one bone fits into a ring formed by bone and ligament. Pivot joints are uniaxial, allowing only rotational movement. The atlantoaxial joint between the first and second cervical vertebrae allows rotation of the head. The proximal radioulnar joint allows the radius to rotate around the ulna during pronation and supination of the forearm. Condylar joints, also called ellipsoidal joints, have an oval convex surface on one bone fitting into a corresponding oval concave surface on the other. They are biaxial, allowing movement in two planes: flexion-extension and abduction-adduction. Circumduction, a combination of these movements, is also possible. The radiocarpal joint of the wrist, the metacarpophalangeal joints (knuckles), and the atlanto-occipital joint between the skull and first cervical vertebra are condylar joints. Saddle joints have articular surfaces that are both convex and concave, shaped like a saddle. Each bone surface is concave in one direction and convex in the perpendicular direction, and the two surfaces fit together with their curvatures oriented at right angles. Saddle joints are biaxial, allowing flexion-extension, abduction-adduction, and circumduction. The first carpometacarpal joint at the base of the thumb is the primary example; its saddle configuration allows the opposition movement essential for human grip. Ball-and-socket joints have a spherical head on one bone fitting into a cup-shaped socket on the other. They are multiaxial, allowing movement in all three planes: flexion-extension, abduction-adduction, and rotation. Circumduction is also possible. These are the most mobile joints in the body. The glenohumeral joint of the shoulder and the coxal (hip) joint are ball-and-socket joints. Panel A: Plane joint (intercarpal) with flat surfaces and sliding arrows labeled nonaxial/multiaxial gliding; Hinge joint (elbow) with convex trochlea in concave notch labeled uniaxial flexion/extension. Panel B: Pivot joint (atlantoaxial) showing dens of C2 in ring of C1 with rotation arrow labeled uniaxial; Condylar joint (radiocarpal) with oval surfaces and two axes labeled biaxial. Panel C: Saddle joint (1st CMC of thumb) with concave-convex surfaces at right angles labeled biaxial allowing opposition. Panel D: Ball-and-socket joint (shoulder) with spherical humeral head in glenoid socket, three axes with arrows in all directions labeled multiaxial most mobile. --- ## Movements at Synovial Joints The movements possible at synovial joints are described using specific terminology. Understanding these terms is essential for communicating about joint function in clinical settings. Joints are described by the number of axes around which movement occurs. Uniaxial joints allow movement around one axis, meaning in one plane. Hinge and pivot joints are uniaxial. Biaxial joints allow movement around two perpendicular axes, meaning in two planes. Condylar and saddle joints are biaxial. Multiaxial joints allow movement around three or more axes, meaning in three planes. Ball-and-socket and plane joints are multiaxial. Angular movements change the angle between articulating bones. Flexion typically decreases the angle between parts, bringing them closer together, as in bending the elbow. Extension increases the angle, straightening a joint or returning from flexion. When extension continues beyond the anatomical position, as when the neck bends backward, it is called hyperextension. Abduction moves a body part away from the midline, as when raising the arm laterally. Adduction moves a part toward the midline. Circumduction is a combination of flexion, abduction, extension, and adduction performed in sequence, causing the limb to describe a cone in space. Rotational movements turn a bone around its longitudinal axis without changing its position in space. Medial rotation, also called internal rotation, turns the anterior surface of a limb toward the midline. Lateral rotation, also called external rotation, turns the anterior surface away from the midline. Supination is a specialized rotation of the forearm that positions the palm anteriorly (as in the anatomical position), with the radius and ulna parallel. Pronation rotates the forearm to position the palm posteriorly, with the radius crossing over the ulna. Special movements are unique to particular joints. Dorsiflexion bends the foot toward the shin, decreasing the angle at the ankle anteriorly. Plantarflexion points the foot downward, as when standing on tiptoe. Inversion turns the sole of the foot medially, while eversion turns it laterally; these movements occur primarily at the subtalar joint. Protraction moves a structure anteriorly, as when jutting the jaw forward or reaching forward with the shoulder. Retraction moves it posteriorly, returning to the resting position. Elevation raises a structure, as when shrugging the shoulders or closing the jaw; depression lowers it. Opposition is the movement unique to the thumb that brings its pad to meet the pads of the other fingers, enabling the powerful precision grip that characterizes human hands. Reposition returns the thumb from opposition. Panel A: Angular movements showing flexion/extension at elbow with angle markers, abduction/adduction at shoulder with arm raising/returning, circumduction at shoulder tracing cone pattern. Panel B: Rotational movements showing medial/lateral rotation at hip with leg rotating, supination/pronation at forearm with radius-ulna relationship in both positions. Panel C: Special movements at ankle showing dorsiflexion foot up and plantarflexion foot down, subtalar showing inversion sole medial and eversion sole lateral. Panel D: Special movements at scapula/jaw showing protraction/retraction and elevation/depression, thumb showing opposition meeting fingers and reposition returning. --- ## Factors Affecting Joint Stability Joint stability is the ability of a joint to resist displacement of its articular surfaces. Different joints balance stability and mobility differently based on their functions. The shape and congruence of articular surfaces is a primary factor in joint stability. When articular surfaces fit together closely, like the hip joint where the spherical femoral head sits deeply within the cup-shaped acetabulum, the joint is inherently stable. When surfaces are less congruent, like the shoulder joint where the small, shallow glenoid fossa provides little bony support for the humeral head, stability must come from other sources. Deeper sockets provide more stability; labra can increase effective socket depth. Ligaments are critical for limiting the range of motion and preventing abnormal movements that might damage joint structures. Ligaments become taut at the limits of normal motion, stopping movement before damage occurs. They also provide proprioceptive information that helps coordinate muscle activity to protect the joint. Ligament laxity, whether constitutional or from injury, reduces joint stability. Muscle tone is often the most important factor in joint stability, particularly for joints with incongruent surfaces. The constant low-level contraction of muscles crossing a joint holds the articular surfaces together and resists displacement. The rotator cuff muscles are critical for shoulder stability; the hip abductors and external rotators are important for hip stability. Muscles act as "dynamic ligaments" that can adjust their tension to meet changing demands. Muscle weakness from any cause can predispose to joint instability. Additional factors contribute to joint stability. Negative intra-articular pressure, created by the sealed joint capsule, helps hold the articular surfaces together. Atmospheric pressure acts on this sealed space to resist distraction of the joint surfaces. The slight adhesion provided by synovial fluid, similar to the resistance felt when trying to separate two wet pieces of glass, contributes to surface contact. Panel A: Shoulder joint coronal section showing small shallow glenoid fossa, large spherical humeral head with surface area mismatch, thin labrum, labeled less congruent requiring muscular stability. Panel B: Rotator cuff muscles (supraspinatus, infraspinatus, subscapularis, teres minor) surrounding shoulder joint providing dynamic stability. Panel C: Hip joint coronal section showing deep hemispheric acetabulum, femoral head sitting deeply within, labrum further deepening socket, labeled highly congruent inherently stable. Panel D: Center diagram showing stability factors including bone shape, ligament strength, muscle tone, negative intra-articular pressure, synovial fluid adhesion with contribution arrows. --- ## Blood and Nerve Supply of Joints Understanding the blood and nerve supply to joints has important clinical implications for joint disease, surgery, and pain management. The blood supply to synovial joints comes from periarticular arterial plexuses, networks of vessels formed by branches of arteries that pass near the joint. Multiple arteries typically contribute to each plexus, providing collateral circulation that protects the joint from ischemia if one vessel is damaged. These vessels supply the joint capsule, ligaments, and synovial membrane, which are highly vascular structures. Articular cartilage, being avascular, receives nutrition only indirectly through diffusion from synovial fluid. This rich blood supply is important for joint metabolism, healing of capsular and ligamentous injuries, and the inflammatory responses seen in arthritis. The nerve supply to joints follows a principle described by Hilton's Law, formulated by the English surgeon John Hilton in the 19th century. According to this law, the nerves that cross a joint to supply the muscles acting on that joint also send branches to supply the joint itself and the skin over the insertion of those muscles. This pattern has practical significance: pain from joint pathology may be referred to the skin area supplied by the same nerves, and muscle spasm commonly accompanies joint pain because the same nerves supply both structures. Joint innervation includes several types of nerve fibers. Proprioceptors, including Ruffini endings, Pacinian corpuscles, and Golgi tendon organ-like receptors, provide information about joint position and movement, essential for coordinated movement and protective reflexes. Pain receptors, primarily free nerve endings, are abundant in the joint capsule and ligaments, explaining the pain associated with joint injuries and arthritis. Autonomic nerve fibers supply the blood vessels of the joint, regulating blood flow to the capsule and synovial membrane. --- ## Clinical Correlations Several common conditions affect joint structure and function. Osteoarthritis, often called "wear and tear" arthritis, is the most common form of joint disease. It involves progressive degeneration of articular cartilage, beginning with fibrillation and fissuring of the cartilage surface and eventually progressing to complete cartilage loss with exposure of underlying bone. As cartilage is lost, the subchondral bone becomes sclerotic (thickened) and bone spurs called osteophytes form at the joint margins. Bone-on-bone contact causes pain, stiffness, and progressive loss of function. Weight-bearing joints and frequently used joints are most commonly affected. Risk factors include age, obesity, previous joint injury, and genetic predisposition. Rheumatoid arthritis is a systemic autoimmune disease in which the body's immune system attacks the synovial membrane. The inflammation leads to synovial hypertrophy, with the thickened synovium (pannus) spreading over the articular cartilage. The pannus releases enzymes that destroy cartilage and erode bone. Unlike osteoarthritis, rheumatoid arthritis typically affects joints symmetrically on both sides of the body, commonly involving the small joints of the hands and feet. Systemic symptoms including fatigue and morning stiffness are common. Joint dislocation is the complete loss of contact between the articular surfaces of a joint. Subluxation refers to partial loss of contact. Dislocation typically results from trauma that forces the joint beyond its normal range of motion, often with associated ligament damage. The shoulder, a mobile but relatively unstable joint, is the most commonly dislocated major joint. Finger dislocations are also common. Reduction (restoration of normal position) must be accomplished, usually with closed manipulation, and the joint must be immobilized while damaged structures heal. Sprains are injuries to ligaments, ranging from stretching of fibers (grade I) through partial tearing (grade II) to complete rupture (grade III). Sprains result from forces that stress the joint beyond the limits of ligament strength. Pain, swelling, and instability are proportional to the severity of injury. Ankle sprains, particularly of the anterior talofibular ligament, are extremely common. Bursitis is inflammation of a bursa, typically from repetitive friction or direct trauma. The affected bursa becomes swollen, painful, and tender. Prepatellar bursitis ("housemaid's knee") results from prolonged kneeling. Olecranon bursitis affects the bursa over the elbow. Subacromial bursitis contributes to shoulder impingement syndrome. Panel A: Osteoarthritis coronal section through knee showing cartilage thinning with irregular surface, subchondral bone sclerosis, osteophytes at margins, joint space narrowing with normal comparison inset. Panel B: Rheumatoid arthritis sagittal section through MCP joint showing thickened inflamed synovium, pannus spreading over cartilage, bone erosions, symmetric hand involvement indicated. Panel C: Dislocation showing anterior shoulder with humeral head displaced anteriorly from glenoid fossa, stretched capsule and torn labrum indicated. Panel D: Bursitis sagittal section through knee showing inflamed distended prepatellar bursa as red swollen sac superficial to patella. --- ## Summary Joints are classified structurally as fibrous, cartilaginous, or synovial based on the type of tissue connecting the bones and the presence or absence of a joint cavity. They are classified functionally as synarthroses (immovable), amphiarthroses (slightly movable), or diarthroses (freely movable). Fibrous joints include sutures (skull only), syndesmoses (connected by ligament or membrane), and gomphoses (teeth in sockets). They allow minimal to no movement. Cartilaginous joints include synchondroses (connected by hyaline cartilage, often temporary) and symphyses (connected by fibrocartilage, permanent, midline). They allow limited movement. Synovial joints are freely movable joints with a joint cavity containing synovial fluid. Their essential components include articular cartilage, joint capsule with fibrous and synovial layers, and synovial fluid. Accessory structures include ligaments, articular discs (menisci), labra, bursae, and tendon sheaths. Synovial joints are classified by shape as plane, hinge, pivot, condylar, saddle, or ball-and-socket, with each type permitting characteristic movements. Joint stability depends on the shape of articular surfaces, ligaments, and muscle tone, with different joints emphasizing different factors based on their functional requirements. Hilton's Law describes the common nerve supply to joints, the muscles acting on them, and the overlying skin. --- ## Key Terms | Term | Definition | |------|------------| | Diarthrosis | Freely movable joint; all synovial joints are diarthroses | | Synovial membrane | Inner lining of joint capsule that produces synovial fluid | | Meniscus | Fibrocartilage disc within a joint that improves articular surface congruence | | Bursa | Synovial fluid-filled sac that reduces friction between structures near joints | | Hilton's Law | Principle that nerves crossing a joint supply the joint, muscles acting on it, and overlying skin | | Ligament | Dense regular connective tissue connecting bone to bone | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*