Joint Articulations: Structural and Functional

• An articulation is the point of contact between bones.
• Most joints are movable, some are immovable or allow limited motion.
• Movable joints enable coordinated and purposeful movements.

Classification of Joints

• Joints can be classified using structural or functional schemes.
• Structural classification names joints based on:
  • The type of connective tissue that joins bones (fibrous or cartilaginous joints).
  • The presence of a fluid-filled joint capsule (synovial joint).

Functional Classification of Joints

• Functional classification names joints depending on the degree of movement.
• Synarthroses are immovable joints.
• Amphiarthroses are slightly movable joints.
• Diarthroses are freely movable joints.

Fibrous Joints (Synarthroses)

• Bones fit together closely, allowing little to no movement.
• Sutures are found only in the skull, toothlike projections interlock with adjacent bones.
• Gomphoses exist between the root of a tooth and the alveolar process of the mandible or maxilla.
• Synchondroses have hyaline cartilage between articulating bones.
• Synostosis is a rigid, immovable joint as a result of when two bones fuse and the boundary disappears.

Cartilaginous Joints (Amphiarthroses)

• Bones are joined together by hyaline cartilage or fibrocartilage, allowing very little motion.
• Syndesmosis involves bones connected by a ligament.
• Symphyses is when a pad or disk of fibrocartilage connects two bones.

Synovial Joints (Diarthroses)

• These are freely movable joints.
• Structures of synovial joints include:
  • A joint capsule: sleeve-like casing around ends of bones, binding them together.
  • Synovial membrane: lines joint capsule and secretes synovial fluid.
  • Articular cartilage: hyaline cartilage covering articular surfaces of bones.
  • A joint cavity: small space between articulating surfaces of bones.
  • Menisci (articular disks): pads of fibrocartilage located between articulating bones.
  • Ligaments: strong cords of dense, white, fibrous tissue holding bones of synovial joint more firmly together.
  • Bursae: synovial membranes filled with synovial fluid to cushion joints and facilitate movement of tendons.

Types of Synovial Joints

• Uniaxial joints permit movement around one axis and in one plane.
  • Hinge joints: articulating ends of bones form a hinge-shaped unity allowing flexion and extension.
  • Pivot joints: a projection of one bone articulates with a ring or notch of another bone.
• Biaxial joints permit movements around two perpendicular axes in two perpendicular planes.
  • Saddle joints are synovial joints where articulating ends of the bones resemble reciprocally shaped miniature saddles; occurring only in thumbs.
  • Condyloid (ellipsoidal) joints is where a condyle fits into an elliptical socket.
• Multiaxial joints facilitates movements around three or more axes and in three or more planes.
  • Ball and socket (spheroid) joints are the most movable joints with a ball-shaped head of one bone fitting into a concave depression.
  • Gliding joints have relatively flat articulating surfaces that allow limited gliding movements along various axes.

Representative Synovial Joints

• Humeroscapular joint:
  • Also known as the shoulder joint.
  • This is the most mobile joint because of the shallowness of the glenoid cavity.
  • The Glenoid labrum is a narrow rim of fibrocartilage around the glenoid cavity that lends depth to the cavity.
  • Is strengthened by ligaments, muscles, tendons, and bursae.
• Elbow joint:
  • The humeroradial joint is the lateral articulation of the capitulum of the humerus with the head of the radius.
  • The humeroulnar joint is the medial articulation of the trochlea of the humerus with the trochlear notch of the ulna.
  • Both components surrounded by a single joint capsule and stabilized by collateral ligaments.
  • A classic hinge joint, the medial and lateral epicondyles are externally palpable bony landmarks.
  • Olecranon bursa is inflammation of the elbow joint space that is called olecranon bursitis.
  • Trauma to the nerve results to unpleasant sensations supplied by nerve while severe injury may cause "wrist drop."
• Proximal radioulnar joint:
  • Located between head of radius and medial notch of ulna.
  • Stabilized by annular ligament.
  • Permits rotation of forearm.
  • Dislocation of the radial head is called a "pulled elbow."
• Distal radioulnar joint:
  • Point of articulation lies between the ulnar notch of the radius and the head of the ulna.
  • Permits pronation and supination of the forearm.
• Radiocarpal (wrist) joints:
  • Only the radius articulates directly with carpal bones distally (scaphoid and lunate).
  • They are synovial joints.
  • The Scaphoid bone is fractured frequently.
  • The portion of fractured scaphoid may become avascular.
• Intercarpal joints:
  • Located between 8 carpal bones.
  • Stabilized by numerous ligaments.
  • Joint spaces usually communicate.
  • Movements are generally gliding, with some abduction and flexion.
• Carpometacarpal joints:
  • Carpometacarpal joints are total of three joints.
  • One joint for thumb-wide range of movements.
  • Two joints for fingers-movements largely gliding type.
  • Thumb carpometacarpal joint is unique and important functionally
    • Has a loose-fitting joint capsule.
    • Has a Saddle-shaped articular surface.
    • Allows Movements of extension, adduction, abduction, circumduction, and opposition.
    • Opposition is the ability to touch the tip of thumb to tip of other fingers.
• Metacarpophalangeal joints:
  • Rounded heads of metacarpals articulate with concave bases of proximal phalanges.
  • The Capsule surrounding joints are strengthened by collateral ligaments.
  • Primary movements are flexion and extension.
• Interphalangeal joints:
  • Typical diarthrotic, hinge-type, synovial joints.
  • Occur between heads of phalanges and bases of more distal phalanges.
  • There are two categories:
    • PIP joints: proximal interphalangeal joints between proximal and middle phalanges.
    • DIP joints: distal interphalangeal joints which are located between middle and distal phalanges
• Hip joint:
  • The head of the femur and acetabulum contribute to a stable joint.
  • A joint capsule and ligaments contribute to the joint’s stability.
• Knee joint:
  • One of most complex, largest, and most frequently injured joints.
  • The tibiofemoral joint is supported by a joint capsule, cartilage, and numerous ligaments and muscle tendons.
  • Permits movements of flexion and extension, with limited internal and external rotation when the knee is flexed.
• Ankle joint:
  • Hinge type of synovial joint.
  • Articulation occurs between lower ends of tibia and fibula and upper part of talus.
  • The joint is "mortise” or wedge-shaped" where the lateral malleolus is lower than medial.
  • Internal rotation injury results in common "sprained ankle" involving the anterior talofibular ligament.
  • Other ankle ligaments can be involved in sprain injuries like the deltoid ligament.
  • External ankle rotation injuries is generally involved in bone fractures rather than ligament tears
    • First-degree ankle injury: lateral malleolus fractured
    • Second-degree ankle injury: both malleoli fractured
    • Third-degree ankle injury: fracture of both malleoli and articular surface of tibia
• Vertebral joints:
  • Vertebrae are connected to one another by several joints to form a strong, flexible column.
  • Bodies of adjacent vertebrae are connected by intervertebral disks and ligaments.
  • Intervertebral disks are made up of two parts: - The outer rim is the annulus fibrosus: made of fibrous tissue and fibrocartilage. - The central core is the nucleus pulposus made of a pulpy and elastic substance

Types and Range of Movement at Synovial Joints

• Range of Motion (ROM) is used to determine extent of joint injury.
• ROM can be measured actively/passively where the results of both methods are generally equal.
• It is measured using an instrument called a goniometer.
• Angular movements change the size of the angle between articulating bones.
  • Flexion: decreases the angle between bones by bending or folding one part on another.
  • Extension: increases the angle between bones, returning a part from its flexed position to its anatomical position.
    • Hyperextension is stretching or extending a part beyond its anatomical position
  • Plantar flexion: increases the angle between the top of the foot and the front of the leg.
  • Dorsiflexion: decreases the angle between the top of the foot and the front of the leg.
  • Abduction: moves a part away from the median plane of the body.
  • Adduction: moves a part toward the median plane of the body.
• Circular movements:
  • Rotation: pivot of the bone on its own axis.
  • Circumduction: movement of a part so that its distal end moves in a circle.
  • Supination: turns the hand palm faces up.
  • Pronation: turns the hand palm faces down.
• Gliding movements:
  • This the simplest movement where articular surface of one bone moves over articular surface of another without any angular or circular movement
• Special movements:
  • Inversion: turning sole of foot inward.
  • Eversion: turning sole of foot outward.
  • Protraction: moves a part forward.
  • Retraction: moves a part backward.
  • Elevation: moves a part up. - Depression: lowers a part.

Cycle of Life: Articulations

• Bone development and the sequence of ossification/skeletal maturity affect joints.
• Fontanels between cranial bones disappear and epiphyseal plates ossify at maturity.
• Older adults: ROM decreases and changes occur in gait.
• Skeletal diseases manifest as joint problems such as
  • Abnormal bone growth (lipping) influences the motion of joints.
  • Disease conditions can be associated with specific developmental periods.
• Hand is "reason for the upper extremity," and the thumb is the "reason for the hand".*
• The mobility of the upper extremity is extensive because of the arrangement of bones in a shoulder girdle, arms, forearm, hand and where location and method of attachment of muscles to bones enables proper functioning of the joints.
• Extensive ROM and mobility need to be able to position upper extremity and hand which then permits grasping and manipulation of objects to allow for effective external interaction.