Anatomy of Synovial Joints

Learning Outcomes

• Students should be able to describe the features and function of different joint types and give examples.
• Students should be able to describe the structure of a synovial joint in detail.
• Students should be able to provide details about the properties of individual elements of a synovial joint, such as the knee joint.
• Students should be able to describe accessory structures of synovial joints, such as ligaments and bursae.

Joint Classification by Function

• Joints are formed when bones (or bone and cartilage) come together.
• Joint function balances movement and stability.
• Classification by function:
  • Synarthrosis: Immobile joint; firmly held together for protection and stability with no movement.
  • Amphiarthrosis: Joint providing protection and stability with some limited movement.
  • Diarthrosis: Freely moveable joint allowing extensive movement of the body (axial skeleton) and limbs (appendicular skeleton).

Joint Types by Structure

• Three joint types:
  • Fibrous: Joints held firmly together by fibrous connective tissue.
  • Cartilaginous: Bones held firmly together by hyaline cartilage or fibrocartilage.
  • Synovial: Bones not directly joined; fluid-filled joint cavity allows smooth movement between bones.
• Fibrous and cartilaginous joints may be synarthrotic or amphiarthrotic.
• All synovial joints are diarthrotic.

Fibrous Joints

• Bones are directly connected by fibrous tissue with no joint cavity.
• Gap between bones varies from narrow to wide depending on fibre length.
• Types of fibrous joints:
  • Sutures
  • Gomphoses
  • Syndesmoses

Fibrous Joints: Sutures

• Strongly bind skull bones.
• Narrow, synarthrotic joints for brain protection.
• Form the face.
• Begin as wider areas of connective tissue in newborns (fontanelles).
• Some sutures ossify, causing skull bones to fuse (synostosis).

Fibrous Joints: Gomphoses

• Narrow joint between a tooth root and its jaw socket (alveoli).
• Also known as a peg-and-socket joint.
• Anchored by short dense connective tissue bands (periodontal ligament).
• Immobile, therefore a synarthrosis.

Fibrous Joints: Syndesmoses

• Where two parallel bones are joined, such as the tibia and fibula.
• Narrower joints joined by ligaments (fibrous band).
• Wider joints joined by interosseous membrane (broad fibrous sheet).
• Prevents bone separation.
• Amphiarthrosis (limited movement), providing stability and weight-bearing support.

Cartilage

• Connective tissue made of cells and an extracellular matrix.
• ECM: Protein fibres + ground substance.
• Ground substance: Water + large organic molecules.
• Cartilage is a dense network of collagen or elastic fibres embedded in proteoglycans.
  • Chondroitin sulphate
  • Keratan sulphate
  • Hyaluronic acid (and water)
• Chondroblasts are precursor cells; mature chondrocytes are located in lacunae.
• Collagen provides strength; chondroitin provides resilience (rubbery).

Elastic Cartilage

• Surface of most cartilage is covered by perichondrium (collagen fibres).
• Chondrocytes are located within elastic fibres in the extracellular matrix.
• Provides strength and elasticity, supporting structures like the external ear.

Fibrocartilage

• Chondrocytes are scattered within bundles of collagen fibres in the extracellular matrix (no perichondrium).
• Strongest form, providing support and joining structures like intervertebral discs and menisci of the knee.

Hyaline Cartilage

• Many chondrocytes within fine collagen fibres in the extracellular matrix.
• Provides support, flexibility, and a smooth surface for joint movement.
• Weakest type; no perichondrium over articular cartilage or epiphyseal plate.
• Found on ends of long bones and epiphyseal plates.

Cartilaginous Joints

• Two types:
  • Synchondrosis: Bones joined by hyaline cartilage or a bone joined to hyaline cartilage.
  • Symphysis: Bones joined by fibrocartilage (but bone can have hyaline cartilage on its end).
    • Fibrocartilage contains thick collagen fibres, resisting pulling and bending forces better than hyaline cartilage.

Cartilaginous Joints: Synchondrosis

• Example: Epiphyseal growth plate.
• Region of hyaline cartilage joining diaphysis and epiphysis of a long bone.
• When growth ends, the plate becomes bone (synostosis).
• Immobile synarthrotic joint.
• Can be considered a temporary synchondrosis.

Cartilaginous Joints: Symphysis

• Example: Intervertebral symphysis.
• Intervertebral discs comprised of outer ring of fibrocartilage and soft, pulpy, highly elastic interior.
• Soft, pulpy, highly elastic interior provides cushioning.
• Allows small movements between adjacent vertebrae (amphiarthrotic).
• Allows large range of movement across the entire vertebral column.

Synovial Joints

• Most common joint type.
• Articular capsule joins articulating bones, creating a synovial cavity.
• Capsule has two layers:
  • Outer fibrous membrane (mostly collagen).
  • Inner synovial membrane (synovium).

Synovial Joints 2

• Surfaces of contacting bones are covered by a thin layer of hyaline cartilage.
• Filled with lubricating synovial fluid (reducing friction).
• Affords significant mobility (diarthrotic) but less stable than other joints.

Subchondral Bone

• Cartilage is avascular, aneural, and has no lymphatic system.
• If damaged, it cannot regenerate.
• Subchondral bone, connected to cartilage via a calcified layer, is vascularised.
• Exchange with chondrocytes depends on diffusion, which limits cartilage thickness.

The Synovial Membrane

• Synovial membrane (synovium) comprises connective tissue, collagen fibres, and adipose tissue (articular fat pads).
• Synoviocytes produce synovial fluid (synovia) - a thin film over structures within the joint cavity.
• Serves to lubricate and nourish articular cartilage.

Synovial Fluid

• Viscous, clear, or pale yellow, sterile fluid in healthy joints.
• High levels of hyaluronic acid, lubricin, and associated glycoproteins.
• Ultrafiltrate of plasma from capillaries in the synovium.
• Normal synovial fluid contains some leucocytes (mainly monocytes).

Menisci

• Some synovial joints have fibrocartilage pads (menisci) between the articulating bones; these are attached to the fibrous capsule.
• Two forms:
  • Articular disc (generally small and oval-shaped).
  • Meniscus (larger and C-shaped).
• In the knee, menisci provide cushioning & shock absorption between bones.

Ligaments

• Ligaments are strong bands of fibrous connective tissue.
• Accessory ligaments are located outside (medial and lateral collateral ligaments) and inside (anterior and posterior cruciate ligaments) the articular capsule.
• Strengthen and support the joint; anchor bones; preventing their separation.
• Allow normal movement but limit range.
• Additional support from muscles/tendons acting across the joint.

Bursae

• Bursa is a thin connective tissue sac filled with lubricating liquid outside a synovial joint.
• Prevent friction between bones and overlying muscle, tendons, or skin.
• Tendon sheaths are similar to bursae but surround tendons where they cross joints to permit smooth movement when the muscle contracts and the joint moves.

Some Further Reading

• Muscolino, J.E. (2017) Kinesiology: the skeletal system and muscle function
• Young, B., O'Dowd, G. & Woodford, P. (2014) Wheater's Functional Histology
• Tortora, G.J. & Derrickson, B.H. (2009) Principles of Anatomy and Physiology,

This quiz will test your knowledge on the features, functions, and classifications of different joint types. You'll need to describe the structure and accessory components of synovial joints, focusing on examples like the knee joint. Prepare to showcase your understanding of how joints balance movement and stability.