Anatomy Chapter 9: Joints and Articulations

• Chapter 9 covers joints in human anatomy

Joints and Articulations

• Rigid skeletal elements meet at joints or articulations
• "Arthro" is a Greek root meaning joint
• Articulations can be bone to bone, bone to cartilage, or teeth in bony sockets
• Joint structure enables resistance to crushing, tearing, and other forces

Classifications of Joints

• Joints are classified by function or structure
• Functional classification is based on the amount of movement allowed

Functional Classifications

• Synarthroses are immovable and common in the axial skeleton
• Amphiarthroses are slightly movable and common in the axial skeleton
• Diarthroses are freely movable and common in the appendicular skeleton

Structural Classifications

• Structural classification is based on material binding bones and the presence/absence of a joint cavity
• Structural classifications include fibrous, cartilaginous, and synovial joints

Fibrous Joints

• Bones are connected by fibrous connective tissue in these joints
• Fibrous joints lack a joint cavity
• Fibrous joints are typically immovable or slightly movable

Fibrous Joint Types

• Sutures are joints held together with short, interconnecting fibers and interlocking bone edges and found only in the skull
• Syndesmosis are joints held together by a ligament; fibrous tissue length varies but is longer than in sutures
• Gomphosis are peg-in-socket fibrous joints held by the periodontal ligament

Cartilaginous Joints

• Cartilage unites the bones
• There is no joint cavity
• Two types exist: synchondroses and symphyses

Synchondroses

• Hyaline cartilage unites bones via epiphyseal plates and joints between the first rib and manubrium

Symphyses

• Fibrocartilage unites bones, resisting tension and compression
• Slightly movable joints provide strength with flexibility
• Intervertebral discs consist in these joints

Synovial Joints

• Synovial joints are the most movable type
• They are all diarthroses
• They have a fluid-filled joint cavity

General Structure of Synovial Joints

• Articular cartilage covers the ends of opposing bones with hyaline cartilage and absorbs compression
• Joint cavity (synovial cavity) is unique to synovial joints and holds a small amount of fluid
• Articular capsule is a two-layered capsule enclosing the cavity
• Fibrous capsule is made of dense irregular connective tissue , strengthens the joint
• Synovial membrane consists of loose connective tissue, lines joint capsule and covers internal joint surfaces, and makes synovial fluid

Synovial Fluid

• Synovial fluid is a viscous fluid similar to raw egg white
• It’s a filtrate of blood arising from capillaries in the synovial membrane
• It contains glycoprotein molecules secreted by fibroblasts

Reinforcing Ligaments

• They are thickened parts of the fibrous capsule
• Extracapsular ligaments are outside the capsule
• Intracapsular ligaments are inside the capsule

General Structure of Synovial Joints

• Richly supplied with sensory nerves that detect pain and monitor stretch of capsule
• Synovial joints have a rich blood supply
• Most of this blood supply is to the synovial membrane
• The supply constitutes the basis of synovial fluid
• Synovial joints have branches of major nerves and blood vessels

Synovial Joints with Articular Discs

• Only some examples contain articular discs
• The knee contains a meniscus

Synovial Joint Functions

• Synovial fluid is a lubricant
• Friction can overheat and destroy tissue
• Bursae and tendon sheaths are closed bags of lubricant and reduce friction between body elements

Factors Influencing Joint Stability

• Include articular surfaces like the elbow, knee, and hip joints
• Ligaments' strength increases with the number of ligaments in a joint
• Muscle tone is the most important factor
• Muscle tone keeps tension on muscle tendons

Movements Allowed by Synovial Joints

• Three basic types of movement: gliding, angular movement, rotation

Synovial Movement Types

• Gliding occurs when one bone slips across another's surface
• Angular movement changes the angle between bones
• Rotation refers to movement around a bone's long axis

Gliding Joints

• Flat surfaces of two bones slip across each other
• Gliding occurs between carpals, articular processes of vertebrae, and tarsals

Angular Movements

• They increase or decrease the angle between bones

Special Movements

• Supination occurs when the forearm rotates laterally and the palm faces anteriorly
• Pronation occurs when the forearm rotates medially and the palm faces posteriorly

Special Foot Movements

• Dorsiflexion lifts the foot so the superior surface approaches the shin
• Plantar flexion depresses the foot, elevating the heel
• Inversion turns the sole of the foot medially
• Eversion turns the sole of the foot laterally

Protraction and Retraction

• Protraction refers to nonangular movement anteriorly
• Retraction refers to nonangular movement posteriorly

Elevation and Depression

• Elevation lifts a body part superiorly
• Depression moves the elevated part inferiorly

Opposition

• Opposition is when the thumb moves across the palm to touch the tips of other fingers

Synovial Joints Classified by Shape

• Plane joints have flat articular surfaces and allow short gliding movements
• Intertarsal and intercarpal joints are plane joints
• Movements are nonaxial, and gliding does not involve rotation

Hinge Joints

• The cylindrical end of one bone fits into a trough on another bone
• Angular movement is allowed
• Elbow, ankle and joints between phalanges are hinge joints
• Movement around one axis only, which is known as uniaxial

Pivot Joints

• Pivot joints are uniaxial, rotating bones turn around the long axis only
• Examples include the proximal radioulnar joint and the joint between the atlas and axis

Condyloid Joints

• They allow the moving bone to travel side to side (abduction-adduction) and back and forth (flexion-extension)
• This form of movement is biaxial, moving around 2 axes

Saddle Joints

• Each articular surface has concave and convex surfaces
• They are biaxial

Ball-and-Socket Joints

• Spherical head of one bone fits into a round socket of another
• This movement is multiaxial
• The joints allow movement in all axes
• Shoulder and hip joints are ball-and-socket

Glenohumeral Joint

• This shoulder joint is freely movable but lacks stability
• Its articular capsule is thin and loose
• Muscle tendons, especially from the “rotator cuff,” provide joint stability

Elbow Joint

• Allows flexion and extension
• The joint between the humerus and ulna has hinge properties
• Tendons of the biceps and triceps brachii provide further stability

Hip Joint

• The hip joint is ball-and-socket
• Movement is limited by ligaments and the acetabulum, which provide significant stability
• Muscle tendons contribute somewhat to stability

Knee Joint

• The knee is the largest and most complex joint
• Primarily a hinge joint, with slight rotation when flexed
• Structurally bicondyloid and contains two fibrocartilage menisci

Knee Joint Capsule

• It covers posterior and lateral aspects of the knee
• The capsule covers with tibial and femoral condyles
• It does not cover the anterior aspect of the knee
• Anteriorly covered by patellar, medial, and lateral retinacula

Ligaments of the Knee Joint

• They become taut when the knee extends
• Extracapsular ligaments include the fibular and tibial collateral, oblique popliteal, and arcuate popliteal ligaments

Intracapsular Ligaments

• Cruciate ligaments cross each other like an “X”
• they originate from the proximal tibia to the distal femur
• Anterior and posterior cruciate ligaments exist

Ankle Joints

• A hinge joint between the united inferior ends of the tibia and fibula and the talus of the foot
• Allows dorsiflexion/plantar flexion only