Synovial joints
Juncturae synoviales
Also known as: diarthrosis, synovial joint, plane joint, hinge joint, pivot joint, condyloid joint, saddle joint, ball-and-socket joint
The freely movable joints of the body (diarthroses), characterised by a joint cavity containing synovial fluid, articular cartilage and a fibrous capsule lined by synovial membrane. They are classified into six types by the shape of their articular surfaces.
Overview
Every synovial joint has: articular cartilage (usually hyaline) covering the bone ends; a joint (articular) cavity; an articular capsule with an outer fibrous layer (continuous with periosteum; often thickened as intrinsic ligaments) and an inner synovial membrane that lines all internal non-articular surfaces; and synovial fluid. Many also have extrinsic (accessory) ligaments, articular discs or menisci (TMJ, sternoclavicular, knee, distal radioulnar), labra (shoulder, hip), fat pads, bursae and tendon sheaths. Synovial joints are richly innervated: by Hilton's law, the nerves supplying the muscles moving a joint also supply the joint and the overlying skin.
The six subtypes by shape and axes of movement are:
Plane (gliding) — flat surfaces, sliding movement (non-axial/multiaxial gliding): acromioclavicular, intercarpal, intertarsal, zygapophysial joints.
Hinge (ginglymus) — uniaxial flexion–extension: humero-ulnar (elbow), interphalangeal, talocrural (ankle).
Pivot (trochoid) — uniaxial rotation about a longitudinal axis: median atlanto-axial, proximal and distal radioulnar.
Condyloid (ellipsoid) — biaxial flexion–extension and abduction–adduction (plus circumduction): radiocarpal (wrist), metacarpophalangeal, atlanto-occipital.
Saddle (sellar) — biaxial, reciprocally concave–convex surfaces: carpometacarpal joint of the thumb, sternoclavicular (functionally), calcaneocuboid.
Ball-and-socket (spheroidal) — multiaxial (flexion–extension, abduction–adduction, rotation, circumduction): shoulder and hip.
The knee is often called a modified hinge (bicondylar) because it also rotates; the TMJ is a modified hinge with gliding (ginglymoarthrodial).
- Location
Most joints of the limbs; also the TMJ, facet joints, costovertebral and sacroiliac (anterior part) joints.
Parts & subdivisions
- Hyaline (fibrocartilage in TMJ and sternoclavicular joints).
- Outer layer of the articular capsule.
- Secretes synovial fluid; does not cover articular cartilage.
- Lubricates and nourishes.
- Synovial sacs reducing friction near joints.
- Fibrocartilage pads improving congruity.
- Gliding — acromioclavicular, intercarpal, facet joints.
- Uniaxial — elbow (humero-ulnar), interphalangeal, ankle.
- Uniaxial rotation — atlanto-axial, radioulnar.
- Biaxial — wrist, MCP, atlanto-occipital.
- Biaxial — thumb CMC, sternoclavicular.
- Multiaxial — shoulder, hip.
Anatomy
- Innervation
Articular branches of the nerves supplying the overlying muscles and skin (Hilton’s law); the capsule and ligaments have mechanoreceptors and nociceptors, the cartilage has none.
Function
- Permit free, controlled movement between bones
- Transmit and distribute loads with minimal friction
- Provide proprioceptive information for posture and coordination
- Physiology
Stability is a trade-off with mobility and depends on (1) shape and congruity of the articular surfaces, (2) ligaments, (3) the tone of surrounding muscles (most important in the shoulder) and (4) atmospheric pressure/synovial adhesion.
Clinical significance
Septic arthritis
Bacterial infection of a synovial joint (S. aureus commonest; N. gonorrhoeae in young sexually active adults): hot, swollen, painful joint with fever; emergency joint aspiration and washout — cartilage is destroyed within days.
Crystal arthropathies
Gout (monosodium urate, needle-shaped negatively birefringent crystals; 1st MTP) and pseudogout (calcium pyrophosphate, rhomboid positively birefringent crystals; knee, chondrocalcinosis).
Mnemonics
Six synovial joint types
"Please Help Paul Catch Some Balls": Plane, Hinge, Pivot, Condyloid, Saddle, Ball-and-socket.
Test yourself
Which joint is classified as a saddle (sellar) synovial joint?
- Carpometacarpal joint of the thumb
- Median atlanto-axial joint
- Radiocarpal joint
- Acromioclavicular joint
Answer: Carpometacarpal joint of the thumb
The first carpometacarpal joint (trapezium–1st metacarpal) is the classic saddle joint, allowing opposition. The median atlanto-axial is a pivot, the radiocarpal a condyloid (ellipsoid) and the acromioclavicular a plane joint.
Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.