Orthopaedics Anatomy · Orthopaedics

Joint Classifications

Joints are classified by the tissue uniting the bones: fibrous, cartilaginous, or synovial. Synovial joints are further divided into six subtypes by their shape and movement.

✦ The Mnemonic

"Firmly Connected Structurally — Fibrous, Cartilaginous, Synovial"

Fibrous (synarthrosis) · Cartilaginous (amphiarthrosis) · Synovial (diarthrosis) + 6 synovial subtypes

Fi Fibrous Joints United by fibrous tissue; little or no movement; types: sutures (skull), syndesmoses (tibiofibular), gomphoses (teeth)

Ca Cartilaginous Joints United by cartilage; primary (hyaline — epiphyseal plates) or secondary (fibrocartilage — symphyses, IVDs)

Sy Synovial Joints Most common; synovial cavity + fluid; capsule + ligaments; 6 subtypes below

Pl Plane/Gliding Flat surfaces; gliding movement; acromioclavicular, intercarpal, intertarsal joints

Hi Hinge Uniaxial; flexion/extension only; elbow, knee, ankle, interphalangeal joints

Pi Pivot Uniaxial rotation; atlantoaxial, proximal radioulnar joints

Co Condyloid (Ellipsoid) Biaxial; 2 planes; radiocarpal (wrist), MCP joints

Sa Saddle Biaxial but greater mobility; first carpometacarpal (thumb) joint

Ba Ball-and-Socket Multiaxial; most movement; hip, shoulder (glenohumeral) joints

📚 Clinical Breakdown

The distinction between primary and secondary cartilaginous joints: primary = hyaline cartilage = epiphyseal plates (eventually ossify in adulthood) and the 1st sternocostal joint. Secondary = fibrocartilage = stronger, slightly moveable = pubic symphysis, intervertebral discs, manubriosternal joint.

Synovial joint components: articular hyaline cartilage (no periosteum, no nerve supply — hence degeneration is painless until bone exposed); joint capsule (fibrous outer layer + synovial inner layer); synovial fluid (ultrafiltrate of plasma + hyaluronic acid — nutrient for cartilage); reinforcing ligaments (intra- or extra-articular); sometimes intra-articular discs/menisci.

Close-packed vs open-packed position: close-packed = maximum congruence of articular surfaces + tight ligaments (most stable). Hip: full extension + internal rotation + abduction. Knee: full extension + slight external rotation. Open-packed = some laxity — injury most likely in intermediate positions (hip: 90° flexion + adduction + internal rotation = posterior dislocation position).

Most stable joint position Close-packed — maximum congruency

Most moveable synovial joint Ball-and-socket (hip, shoulder)

Only biaxial + opposable Saddle joint (1st CMC — thumb)

Fibrocartilage joints Pubic symphysis, IVDs, menisci

⭐ Clinical Pearl

Osteoarthritis vs rheumatoid arthritis: OA — Heberden's nodes (DIP), Bouchard's nodes (PIP), DIP joints involved, worse at end of day; RA — MCP/wrist/PIP involvement, spares DIP, morning stiffness >1 hour, systemic features. The joints spared in RA include DIP joints and the thoracic spine — contrast with ankylosing spondylitis.

← Bone Healing Stages