Joints: Structure, Function, and Mobility

• Joints are where bones meet.
• Joints involve bones, teeth and cartilage.
• Joints are also called articulations.
• Joints are classified by function (amount of movement) and structure (what binds bones together).

Joint Classification by Structure

• Fibrous joints are joined by fibrous tissue.
• Cartilaginous joints are joined by cartilage.
• Synovial joints have bones separated by fluid-filled cavities.

Joint Classification by Movement

• Synarthroses are immovable joints.
• Amphiarthroses are slightly movable joints.
• Diarthroses are freely movable joints.

Joint Mobility Concepts

• Range of motion refers to the normal extent of mobility during joint movement.
• Range of motion is commonly used for freely movable synovial joints.
• Range of motion is measured in degrees with a protractor.
• Degrees of freedom represents the number of axes at which movement occurs in a joint.
• Degrees of freedom are best described in the context of synovial joints.

Factors Affecting Joint Stability

• Articular surface shape allows movement at the joint.
• Articular surface shape provides freedom of movement, which opposes joint stability.
• Number and position of ligaments prevent undesired movement.
• More ligaments generally result in greater strength and joint stability.
• Muscle tone is most important for stability.
• Muscle tone refers to the contractile activity of a relaxed muscle.
• Muscle tone keeps the tendons taught.

Fibrous Joints

• Gomphoses are peg-in-socket joints.
• Teeth in the jaw are the only example of gomphoses.
• Gomphoses are synarthrosis joints.
• Sutures lie between the bones of the skull and have interlocking irregular edges.
• Sutures are synarthrosis joints.
• Syndesmoses have bones connected by ligaments, allowing some give dependent on ligament length.
• The ulna and radius, along with the tibia and fibula, are examples of syndesmosis.
• Syndesmosis are amphiarthrosis joints.

Cartilaginous Joints: Synchondroses

• Synchondroses are articulations of bone with hyaline cartilage.
• Synchondroses are all immoble.
• Synchondroses are classified as synarthroses.
• Epiphyseal plates and costal cartilage are examples of synchondroses.

Cartilaginous Joints: Symphyses

• Symphyses feature a pad of fibrocartilage between articulating bones.
• Symphyses allow slight mobility.
• Symphyses are classified as amphiarthroses.
• The pubic symphysis and intervertebral discs are examples of symphyses.

Synovial Joints

• Synovial joints have an articular capsule which encloses the joint cavity.
• The articular capsule has an outer fibrous capsule and inner synovial membrane.
• The joint cavity is filled with fluid.
• Synovial fluid is secreted by the synovial membrane and acts as a weight-bearing lubricant.
• Synovial joints possess articular cartilage to protect the bone ends.
• Reinforcing ligaments reinforce and strengthen the joint.
• Extrinsic ligaments are outside and separate from the joint capsule.
• Intrinsic ligaments are part of the joint capsule.
• Synovial Joints are classified as diarthroses.

Bursae and Tendon Sheaths

• Bursae and tendon sheaths are not part of the joint, but are closely associated with them.
• Bursae are sac-like structures containing synovial fluid.
• Bursae alleviate friction where bone, muscle, tendons, and ligaments meet.
• Tendon sheaths are elongated bursae wrapped around a tendon.

Classification of Synovial Joints

• Synovial joints are classified based the articulating surfaces and type of movement allowed, ie degrees of freedom.
• Movement is described relative to a plane (axis).
• Uniaxial joints allow movement in one plane.
• Biaxial joints allow movement in two planes.
• Multiaxial joints allow movement in multiple planes.

Classification of Synovial Joints: Non-axial

• Plane joints are non-axial synovial joints.
• Plane joints are the least mobile synovial joints.
• Plane joints have flat articular surfaces.
• Plane joints allow short, gliding movements.

Classification of Synovial Joints: Uniaxial

• Hinge joints are uniaxial.
• Hinge joints have a cylindrical projection that fits into a trough-shaped bone.
• Hinge joints permit flexion and extension.
• Hinge joints operate like a door hinge.
• Pivot joints are uniaxial.
• Pivot joints feature a round end of a long bone protruding into a sleeve of bone or ligament.
• Pivot joints rotate along the longitudinal axis.
• Pivot joints allow rotation.
• Pivot joints allow motion like shaking your head "no".

Classification of Synovial Joints: Biaxial

• Condyloid Joints are biaxial
• Condyloid joints feature an oval-shaped articular surface fitting into a complementary depression
• Condyloid joints permit all angular motions, including back and forth and side to side movement.
• Saddle joints are biaxial.
• Saddle joints have both concierge and context surfaces along with both bones.
• Saddle joints have a greater range of motion than condylar joints.

Classification of Synovial Joints: Triaxial

• Ball-and-socket joints are triaxial.
• Ball-and-socket joints are the most mobile.
• Ball-and-socket joints have a spherical hand of a bone that fits into a cup-like socket on another bone.
• Ball-and-socket joints allow complete freedom of movement.

Four General Movement Types

• Gliding (translation): One bone surface slips over another without appreciable angulation or rotation.
• Angular: Increases or decreases the angle between two bones.
• Rotation: Turning the bone around the long axis.
• Special movements: Only occur at specific joints.

Gliding Movements

• Gliding movements are simple.
• Gliding movements involve two opposing surfaces slide against one another.
• Gliding movements are back and fourth or side-to-side movements.
• The angle between bones doesn't change in gliding movements.
• Gliding movements are limited.
• Gliding movements occur at plane joints.
• Gliding movements occur at the intercarpal and intertarsal joints.

Angular Movements

• Angular movements increase or decrease the angle between two bones.
• Flexion is movement in the anterior-posterior plan, and decreases the angle between the bones.
• Extension is movement in the anterior-posterior plane, and increases the angle between the bones.
• Extension is the opposite of flexion.
• Hyperextension is the extension of a joint beyond 180°.
• Lateral flexion is laterally movement of the body in the coronal plane.
• Abduction moves away from the midline through lateral movement of the limb.
• Adduction "moves toward" lateral movement of the limb towards the midline.
• Circumduction is movement of the limb creating a cone in space.
• Circumduction involves flexion, extension, adduction, and abduction in rapid succession.

Rotational Movements

• Rotation involves pivoting motion.
• Rotation is the movement of the bone around its long axis towards/away from the midline.
• Lateral rotation means external rotation.
• Medial rotation represents internal Rotation.
• Pronation is the medial rotation of the forearm with the palm faces posteriorly, and ulna and radius forming crossed (x).
• Supination the Latin and ration to the forearm with the palm faces anteriorly, and ulna and radius forming parallel (=).

Special Movements

• Dorsiflexion is limited to the ankle joint where the superior surface of the foot moves toward the leg.
• Plantarflexion is limited to the ankle joint where the toes point inferiorly.
• Eversion occurs at the intertarsal joints where the sole of the foot turns laterally
• Inversion occurs at the intertarsal joints where the sole of the foot turns medially.
• Protraction is an anterior anti-angular movement in the transverse plan.
• Retraction is a posterior non-angular movement in the transverse plan.
• Depression is inferior movement of a part of the body.
• Superior Movement of a part of the body is Elevation.
• The thumb moves to the palmar tips as it crosses the palm during Opposition.
• Opposition enables the hand to grasp objects.
• Reposition is the opposing movement of Opposition.

Skeletal Muscles as Levers

• A lever is a straight, stiff object that moves along a fixed pivot point (fulcrum).
• The moment arm is the distance from the fulcrum to the force.
• Applied force is the force applied to the lever.
• Resistive force acts against the applied force.
• Mechanical advantage is the load near the fulcrum ,where effort is applied far from the fulcrum.
• Skeletal muscles and bones act as levers, decreasing force to move bones.
• Bones are the levers.
• Joints are fulcrums.
• Muscles apply the force.

Three Kinds of Levers

• First class lever: Muscle force and resistive force act on opposite sides of the fulcrum, yielding mechanical disadvantage and necessitating a large muscle force for external resistance; these are not common levers in the body.
• Second class lever: Muscle and resistive forces act on the same side of the fulcrum, creating a beneficial mechanical advantage where the muscle force has a longer moment arm than the resistive force; these are also not common levers in the body.
• Third class lever: Muscle and resistive forces on the same side of the fulcrum, with muscle force having a shorter moment arm than the resistive force, however these constitute the most plentiful levers in the body.

Temporomandibular Joint

• The temporomandibular joint articulates between the mandible (mandibular condyle) and the temporal bone (mandibular fossa).
• A loose articular capsule supported by ligaments allows a variety of motion.
• The temporomandibular can hinge during depression when the mandible sits in the mandibular fossa.
• Opening the mouth causes the mandible to articulate forward which is protraction and bracing against the articular tubercle for biting.
• Gliding motion allows for side-to-side motion (lateral excursion) during chewing.

Glenohumeral (Shoulder) Joint Characteristics

• It is a ball and socket joint, with a shallow glenoid cavity on the scapula paired with the large head of the humerus.
• The glenoid labrum compensates for the shallow fossa.
• Stability is sacrificed for free movement.
• There are few reinforcing ligaments, located primarily on the anterior aspect.
• Including the Coracoacromial, coracohumeral, and glenohumeral ligaments, muscle tendons that cross the joints are the primary stabilizers
• Several bursae reduce friction, especially in the shoulder joint.

Elbow Joint Characteristics

• There are three separate joints.
• The humeroulnar joint which is a true hinge-joint where the trochlea of the humerus articulates with the trochlear notch of the ulna which allows flexion and extension.
• The humeroradial joint where the Capitulum of the humerus articulates with the head of the radius.
• A pivot-joint, the radioulnar joint, is not functionally part of the elbow but provides a space for pronation and supination of the forearm.
• Side-to-side movements are restricted by strong radial and ulnar collateral ligaments, however side-to-side movement is restricted by the radial and ulnar collateral ligaments.
• The neck of the radius binds to the ulna via the annular ligament.

Hip Joint Characteristics

• The hip joint is ball and socket joint with limited movement.
• The hip joint has a deep socket formed by the acetabulum and the fibrocartilage acetabular labrum further deepens the socket.
• Ligaments reinforce its articular capsule.
• Including the iliofemoral (anterior), ischiofemoral (posterior), and pubofemoral (inferior) ligaments, muscles and tendons play a key role in reinforcing stability in the hip joint.
• The hip joint offers a large range of motion, but presents a restriction compared to the shoulder girdle.

Knee Joint Characteristics

• Primarily a hinge joint, it’s a bicondylar structure capable of slight ration and lateral gliding in a flexed position.
• The knee joint functions via three joints: 2x Tibiofemoral joints and the Patellafemoral Joint.
• Medial and lateral condyles of the femur function across the Condyles of the tibia.
• The patella articulates alongside the patellar surface of the femur.
• Two C-shaped fibrocartilage pads (lateral and medial) that stabilize the knee joint between the condyles of the femur and the tibia are know as Menisci.
• The articular capsule is incomplete as it only encloses the medial, lateral, and posterior aspects of the joint while the Anterior aspect is protected via the tendons on the Quadriceps Muscle.
• Movement (flexion) is severely restricted with a reliance on ligaments and key muscle groups.
• Collateral ligaments become taught during extension to reinforce the lateral and medial surfaces
• The fibular Collateral maintains lateral support protects against hyperadduction at the knee.
• The tibial Collateral which maintains medial support.
• Prevents the leg from moving too far laterally relative to the thigh.
• Cruciate ligaments run deep into the articular capsule and their names reference to their position.
• The anterior position inhibits hyperextention while the posterior position allows prevents hyperflexion.
• The paratellar ligaments connects the patella to the tibia in a static structure.