A capsule lined with a membrane makes a slippery fluid, the consistency of raw egg white, that lubricates cartilage-covered bone ends
Synovial joint — synovial fluid; the name synovia is attributed to Paracelsus.
Anatomy · Joints & cartilage
Synovial joint
Freely movable (diarthrosis)
Joints · classification
Synovial joints are the freely movable joints of the body. Their bone ends are covered in articular cartilage and enclosed in a fibrous capsule lined by a synovial membrane, which secretes the viscous synovial fluid that fills the joint cavity. They are classified by the shape of their surfaces into six types: plane (gliding), hinge, pivot, condyloid, saddle and ball-and-socket.
How to spot them
- ▸A joint cavity filled with slippery synovial fluid, between two cartilage-capped bones
- ▸A two-layered capsule: outer fibrous layer, inner synovial membrane
- ▸Ligaments reinforce the capsule; some joints have discs (menisci) or bursae
- ▸Shape of the surfaces decides the movement: uniaxial, biaxial or multiaxial
What to know
Six kinds exist: plane, hinge, pivot, condyloid, saddle and ball-and-socket
Synovial joint — the classification by shape.
The thumb's carpometacarpal joint is the classic example of the saddle type
Synovial joint — the saddle joint of the thumb allows opposition.
The atlantoaxial joint and the proximal radioulnar joint are examples of the pivot type
Synovial joint — the pivot joint: rotation about one axis.
A man cracked the knuckles of one hand for 60 years and not the other, and won an Ig Nobel Prize in 2009
Synovial joint — Donald Unger; no arthritis in either hand.
Key ideas
- Classification by shape: plane (intercarpal), hinge (elbow, ankle, interphalangeal), pivot (atlantoaxial, proximal radioulnar), condyloid (radiocarpal, metacarpophalangeal), saddle (first carpometacarpal, sternoclavicular) and ball-and-socket (shoulder, hip).
- Axes of movement: hinge and pivot are uniaxial, condyloid and saddle biaxial, ball-and-socket multiaxial; plane joints glide.
Quick-fire facts
- •Synovial joints are called diarthroses: freely movable joints.
- •Features: articular cartilage, joint capsule, synovial membrane, synovial cavity, synovial fluid, and reinforcing ligaments.
- •Synovial fluid contains hyaluronic acid and lubricin, and nourishes the avascular articular cartilage.
- •Some have extra structures: the menisci of the knee, the articular disc of the TMJ, the labrum of the shoulder and hip, bursae and tendon sheaths.
- •Hinge joints: elbow, ankle, interphalangeal; the knee is a modified hinge.
- •Pivot joints: atlantoaxial (turning the head 'no') and proximal radioulnar.
- •Saddle joints: the first carpometacarpal joint (thumb) and the sternoclavicular joint.
- •Donald Unger's 60-year knuckle cracking experiment won the Ig Nobel Prize in Medicine in 2009.
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