Joints
Classification of joints, synovial joint structure, movements and injuries
By the end of this unit you can…
- ✓Classify joints structurally (fibrous, cartilaginous, synovial) and functionally (synarthroses, amphiarthroses, diarthroses)
- ✓Describe the parts of a synovial joint and the factors that stabilize it
- ✓Name and demonstrate the movements allowed by synovial joints
- ✓Compare the six types of synovial joints and give examples
- ✓Describe the shoulder, elbow, hip and knee and common joint injuries and diseases
Key terms
Practice →1Classifying joints
A joint (articulation) is where two or more bones meet. Joints give the skeleton mobility and hold it together. They are classified two ways:
| Functional class | Movement | Typical structure |
|---|---|---|
| Synarthrosis | Immovable | Fibrous (sutures) or cartilaginous (epiphyseal plates) |
| Amphiarthrosis | Slightly movable | Cartilaginous (pubic symphysis, discs) or syndesmosis |
| Diarthrosis | Freely movable | Synovial — mostly in the limbs |
| Structural class | Joined by | Examples |
|---|---|---|
| Fibrous — suture | Short dense CT fibers; fuse in adults into synostoses | Skull bones |
| Fibrous — syndesmosis | Ligament or interosseous membrane; amount of movement depends on fiber length | Distal tibiofibular joint; radius–ulna interosseous membrane |
| Fibrous — gomphosis | Periodontal ligament | Tooth in its socket |
| Cartilaginous — synchondrosis | Hyaline cartilage plate | Epiphyseal plates; first rib–manubrium |
| Cartilaginous — symphysis | Fibrocartilage (with hyaline on the bone surfaces) | Intervertebral discs, pubic symphysis |
| Synovial | A fluid-filled joint cavity | Shoulder, elbow, hip, knee, most limb joints |
2Synovial joints
- Articular cartilage
- Hyaline cartilage covering the bone ends; absorbs compression.
- Joint (synovial) cavity
- A space filled with fluid — unique to synovial joints.
- Articular capsule
- Two layers: outer fibrous layer (dense irregular CT, strengthens the joint) and inner synovial membrane (loose CT that makes synovial fluid).
- Synovial fluid
- Viscous, egg-white-like filtrate of blood plasma plus hyaluronic acid; lubricates and nourishes the cartilage. Weeping lubrication: compression squeezes fluid out of the cartilage and it soaks back in when pressure is released.
- Reinforcing ligaments
- Capsular, extracapsular or intracapsular bands of dense regular CT.
- Nerves & blood vessels
- Nerves detect pain and monitor joint position and stretch; capillaries supply the synovial membrane.
Some synovial joints have articular discs (menisci) of fibrocartilage that improve the fit between bones (knee, jaw). Bursae are flattened fibrous sacs lined with synovial membrane that reduce friction where ligaments, muscles, skin or tendons rub against bone; a tendon sheath is an elongated bursa wrapped around a tendon.
Stability depends on three factors: the shape of the articular surfaces (minor role, except in the hip), ligaments (the more ligaments, the stronger — but they stretch only ~6% before snapping) and muscle tone (the most important factor in most joints, e.g. rotator cuff tendons at the shoulder).
3Movements at synovial joints
| Movement | Definition |
|---|---|
| Flexion / extension | Decreases / increases the angle between bones in the sagittal plane (bending vs straightening). Hyperextension goes beyond anatomical position |
| Abduction / adduction | Moving a limb away from / toward the midline (fingers and toes: relative to the middle digit) |
| Circumduction | Moving a limb so it describes a cone (flexion → abduction → extension → adduction) |
| Rotation | Turning a bone around its own long axis (medial / lateral) |
| Supination / pronation | Forearm rotation turning the palm anteriorly/up (supination) or posteriorly/down (pronation). In pronation the radius crosses over the ulna |
| Dorsiflexion / plantar flexion | Lifting the foot so its superior surface approaches the shin / pointing the toes |
| Inversion / eversion | Turning the sole medially / laterally |
| Protraction / retraction | Moving a part anteriorly / posteriorly in the transverse plane (jutting the jaw) |
| Elevation / depression | Lifting a part superiorly / moving it inferiorly (shrugging, opening the mouth) |
| Opposition | Touching the thumb to the tips of the other fingers — what makes the hand a grasping tool |
4Types of synovial joints
| Type | Shape | Movement | Example |
|---|---|---|---|
| Plane | Flat surfaces | Nonaxial gliding | Intercarpal, intertarsal joints |
| Hinge | Cylinder in a trough | Uniaxial — flexion/extension | Elbow, interphalangeal joints |
| Pivot | Rounded end in a ring | Uniaxial — rotation | Atlas–axis (C1–C2), proximal radioulnar |
| Condylar (condyloid) | Oval surface in an oval depression | Biaxial — flex/extend, abduct/adduct, circumduct | Wrist (radiocarpal), knuckles (metacarpophalangeal) |
| Saddle | Both surfaces concave and convex | Biaxial — more freedom than condylar | Carpometacarpal joint of the thumb |
| Ball-and-socket | Sphere in a cup | Multiaxial — the most freely moving | Shoulder, hip |
5Selected joints
Shoulder (glenohumeral) — ball-and-socket; head of the humerus in a shallow glenoid cavity deepened slightly by the glenoid labrum. Stability comes mainly from the rotator cuff tendons (supraspinatus, infraspinatus, teres minor, subscapularis) — mobility is bought at the price of stability.
Elbow — a hinge: the trochlea of the humerus in the trochlear notch of the ulna; very stable thanks to tight bony fit and the ulnar and radial collateral ligaments.
Hip (coxal) — ball-and-socket; the femoral head sits in the deep acetabulum (deepened by the acetabular labrum). Very stable: strong ligaments (iliofemoral, pubofemoral, ischiofemoral) and deep socket — mobility is sacrificed for weight-bearing stability.
Knee — the largest and most complex joint; primarily a hinge but also permits some rotation. The femoral condyles sit on the lateral and medial menisci of the tibia. Key ligaments: anterior cruciate (ACL) — prevents the tibia sliding forward and hyperextension; posterior cruciate (PCL) — prevents backward sliding; tibial (medial) and fibular (lateral) collateral ligaments — prevent sideways bending. The patella glides in front, held by the quadriceps tendon and patellar ligament.
6Joint injuries & disorders
| Condition | What happens |
|---|---|
| Sprain | Ligaments are stretched or torn; poorly vascularized ligaments heal slowly |
| Dislocation (luxation) | Bones forced out of alignment; must be reduced; subluxation = partial |
| Cartilage tear | Usually a meniscus (compression + rotation); cartilage is avascular so it rarely repairs — removed or repaired arthroscopically |
| Bursitis / tendonitis | Inflammation of a bursa / tendon sheath, often from overuse or a blow |
| Osteoarthritis (OA) | Most common chronic arthritis; “wear-and-tear” degeneration of articular cartilage; bone spurs; affects weight-bearing joints in older adults |
| Rheumatoid arthritis (RA) | Autoimmune — immune system attacks the synovial membrane; symmetrical; inflamed synovium forms a pannus that erodes cartilage, then fibrous tissue may ossify (ankylosis) |
| Gouty arthritis (gout) | Uric acid crystals deposit in soft tissues of joints — often the big toe; very painful |
| Lyme disease | Bacterial infection spread by ticks; can cause joint pain and arthritis |