Synovial Joint

Essential components

1. Articular surfaces
   The ends of articulating bones are usually covered by smooth hyaline cartilage. This reduces friction and spreads compressive load. Articular cartilage is avascular and has no perichondrium.
2. Joint cavity
   A potential space between the articular surfaces that contains a small amount of synovial fluid.
3. Articular capsule
   It encloses the joint cavity and has two layers:
   • Outer fibrous capsule: dense connective tissue that holds the bones together and stabilizes the joint.
   • Inner synovial membrane: vascular lining that covers the non-articular internal surfaces of the capsule. It secretes synovial fluid.
4. Synovial fluid
   A viscous lubricating fluid that reduces friction, nourishes the avascular articular cartilage, and assists shock absorption.
5. Ligaments
   Capsular, extracapsular, or intracapsular ligaments reinforce the joint and limit excessive movement.
6. Accessory structures
   Present in some joints: articular discs or menisci, labra, bursae, fat pads, and tendon sheaths. Discs and menisci improve congruity and distribute load.

Classification with examples

Clinical approach to a painful synovial joint

• Onset: acute or chronic
• Trauma, fever, recent infection, rash, urethral symptoms
• Pain pattern: inflammatory pain often causes morning stiffness and improves with activity
• Number and distribution of joints involved
• Previous attacks, gout, psoriasis, autoimmune disease, or family history

• Look: swelling, erythema, deformity, scars, muscle wasting.
• Feel: warmth, tenderness, effusion, synovial thickening, crepitus.
• Move: assess active and passive range of motion.
• Function: gait, grip, ability to bear weight or use the limb.
• Examine the corresponding joint on the other side and assess nearby joints.

• Osteoarthritis: mechanical pain, brief morning stiffness, bony enlargement, crepitus.
• Inflammatory arthritis: warmth, tenderness, prolonged morning stiffness, synovitis.
• Crystal arthritis: sudden severe pain and swelling, commonly gout or calcium pyrophosphate disease.
• Septic arthritis: acute severe pain, hot swollen joint, restricted movement, fever may occur. This is an emergency.
• Trauma/internal derangement: ligament, meniscal, cartilage, or fracture-related injury.

• X-ray for fracture, joint-space narrowing, osteophytes, erosions, or alignment.
• Ultrasound for effusion, synovial thickening, active synovitis, and needle guidance. It may identify effusion and synovial thickening more sensitively than clinical examination. Rheumatology, 2-Volume Set, 2022, pp. 2492-2525.
• MRI for cartilage, menisci, ligaments, marrow lesions, and early inflammatory disease.
• Blood tests as indicated: CBC, ESR/CRP, uric acid, rheumatoid factor, anti-CCP.
• Arthrocentesis for a new unexplained effusion or suspected infection/crystal arthritis: send fluid for cell count, Gram stain, culture, and polarized microscopy for crystals. The Merck clinical guidance stresses prompt aspiration to exclude septic arthritis.

Red flags