Joints and Skeletal System

• A joint connects two bones in the skeletal system.

Types of Joints

• The two main types of joints are solid and synovial.

Solid Joints

• Exhibits little to no mobility.
• Lack a joint cavity.
• Bones are joined by fibrocartilage or fibrous connective tissue.
• Solid joints are either fibrous or cartilaginous.

Fibrous Joints

• Bones are connected by fibrous tissue, and are sub-classified into sutures, gomphoses, and syndesmoses
• Sutures are only in the skull, and adjacent bones are linked by a sutural ligament.
• Gomphoses are located between the teeth and adjacent bone, with short collagen tissue fibres in the periodontal ligament.
• Syndesmoses are comprised of bones held together by an interosseous membrane, such as the middle radioulnar joint.

Cartilaginous Joints

• Bones are connected by cartilage.
• Synchondroses (primary cartilaginous)
• Bones are connected by hyaline cartilage.
• Two ossification centres in a developing bone remain separated by a layer of cartilage e.g growth plates.
• These joints allow bone growth and eventually become completely ossified.
• Symphyses (secondary cartilaginous)
• Occur where two separate bones are interconnected by fibrocartilage e.g., pubic symphysis and the joints between vertebral bodies.

Synovial Joints

• Characterized by a fluid-filled joint cavity contained within a fibrous capsule.
• Highly mobile.
• Bones are separated by an articular cavity.
• Includes: joint capsule, articular discs, fat pads, and tendons.

Major Features of Synovial Joints

• Hyaline cartilage covers articulating surfaces of bones.
• Joint capsule includes:
• Outer fibrous membrane - dense connective tissue, which may thicken to form ligaments.
• Inner synovial membrane - attaches at the interface between cartilage and bone, is highly vascularised, and produces synovial fluid for lubrication, and it is contained in the articular cavity.

Accessory Structures

• Bursae are closed sacs of synovial membrane outside the joint.
• Found between structures e.g., between tendon and bone/skin, reducing friction.
• Tendon sheaths are closed sacs of synovial membrane outside the joint that surround tendons, and reduce friction.
• Articular discs are usually fibrocartilage, and absorbs compression forces, adjusts to changes in shape of joint surfaces during movement, and increases range of movement.
• Fat pads are usually between the synovial membrane and fibrous capsule, and serve as a cushion between the bones.
• It is mobile, and changes location as the joint moves.

Subtypes of Synovial Joints

• Subtypes are described based on the shape of their articular surfaces and the movements they permit.
• Ball and socket (e.g., hip, glenohumeral) are multiaxial joints that allow movement around multiple axes (flexion and extension, abduction and adduction, rotation). This creates the most freely moving synovial joints.
• The spherical head of one bone articulates with the cuplike socket of another.
• Condylar (e.g., wrist) are biaxial joints that allow Movement around 2 axes at right angles to each other (flexion and extension, abduction and adduction).
• Bicondylar (e.g., knee) has two convex surfaces (condyles) articulating with a concave or flat surface.
• The movement occurs around one axis (flexion and extension), with limited rotation around a 2nd axis.
• Saddle (e.g., carpometacarpal joints of the thumb) are biaxial joints that allow movement around 2 axes at right angles to each other (flexion and extension, abduction and adduction).
  • Each articular surface has both concave and convex areas.
  • The articular surfaces fit together, concave to convex surfaces
• Hinge joints (e.g., elbow) are uniaxial joints that enable movement around one axis (flexion and extension).
• Pivot joints (e.g., atlanto-axial) allow uniaxial rotation of one bone around its own long axis.
• Plane joints (e.g., acromioclavicular) feature relatively flat articular surfaces, allowing sliding or gliding movements when one bone moves across the surface of another.