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AN2.1-6 | General features of bones & Joints — SDL Guide (Part 3)

Joints: Where Bones Meet

A joint (or articulation, from Latin articulatio = junction) is where two or more bones are connected. Joints are classified in two ways:

Structural classification (by the tissue connecting them):

1. Fibrous joints — bones joined by fibrous connective tissue, no joint cavity.
• Sutures — skull bones (coronal, sagittal, lambdoid sutures). Immovable in adults.
• Syndesmosis — distal tibiofibular joint (joined by interosseous membrane). Slightly movable.
• Gomphosis — tooth in its alveolar socket. Periodontal ligament = fibrous tissue.

2. Cartilaginous joints — bones joined by cartilage, no joint cavity.
• Primary (synchondrosis) — joined by hyaline cartilage. Temporary — converted to bone at maturity. Examples: epiphyseal plates, first costochondral joint.
• Secondary (symphysis) — joined by fibrocartilage disc. Slightly movable. Examples: pubic symphysis, intervertebral discs, manubriosternal joint.

1. Synovial joints — bones separated by a joint cavity filled with synovial fluid. Freely movable. This is the most common joint type in the limbs.

Components of a synovial joint:
• Articular cartilage (hyaline) — covers bone ends, provides smooth, low-friction surface
• Articular capsule — fibrous outer layer (holds bones together) + synovial membrane (inner layer, produces synovial fluid)
• Synovial fluid — viscous, pale yellow. Acts as lubricant and nourishes articular cartilage
• Ligaments — thickenings of capsule or accessory structures that limit movement
• Accessory structures — intra-articular discs (menisci), labrum, bursae

Types of Synovial Joints & Movements

Synovial joints are further classified by the shape of their articular surfaces (which determines the movements possible):

Type	Shape	Movements	Examples
Plane (gliding)	Flat surfaces	Gliding (limited)	Acromioclavicular, intercarpal, intertarsal
Hinge	Convex fits into concave in one axis	Flexion + extension only	Elbow (humeroulnar), knee (mainly), ankle, interphalangeal
Pivot	One bone rotates in a ring	Rotation only	Atlantoaxial (C1-C2), superior radioulnar
Condyloid (ellipsoid)	Oval head in oval socket	Flexion/extension + abduction/adduction (circumduction)	Wrist (radiocarpal), metacarpophalangeal
Saddle	Each surface concave in one plane, convex in another	All movements except axial rotation	1st carpometacarpal (thumb base — gives humans precision grip)
Ball and socket	Sphere in cup	All movements including rotation	Shoulder, hip

Movements at synovial joints:
• Flexion/Extension — decrease/increase the angle at a joint (e.g., bending/straightening the elbow)
• Abduction/Adduction — moving away from / toward the midline of the body
• Rotation — turning around the long axis (medial/internal vs lateral/external)
• Circumduction — sequential combination of above (cone of movement)
• Special movements: Pronation/supination (forearm), Inversion/eversion (foot), Protraction/retraction (mandible, scapula), Opposition (thumb)

Systematic joint examination follows Look → Feel → Move:
1. Look: Swelling, deformity, skin changes, wasting of adjacent muscles, posture
2. Feel: Temperature, tenderness (localise to anatomical structures), crepitus on movement
3. Move: Active movement first (patient's own effort), then passive (examiner moves), then special tests (ligament stability, impingement)

You will practice this sequence on actual patients in your clinical postings.

Hilton's Law: The Neural Logic of Joint Pain

In 1863, the British surgeon John Hilton published a law so clinically powerful that it remains in every anatomy textbook 160 years later:

> "The same trunk of nerves, whose branches supply the groups of muscles moving a joint, also furnishes a distribution of nerves to the skin over the insertions of the same muscles and the interior of the joint."

In plain English: the nerve supplying a joint = the nerve supplying the muscles that move it = the nerve supplying the skin over those muscles' insertions.

Why this matters — the hip-to-knee example:
The hip joint is supplied by the obturator nerve and femoral nerve. The obturator nerve also supplies the adductor muscles of the thigh and the skin of the medial knee. When the hip joint is inflamed (as in Perthes disease, hip fracture, or severe osteoarthritis), pain is referred along the obturator nerve pathway — and the patient feels pain at the medial knee.

This is called referred pain, and Hilton's law predicts it anatomically.

Practical applications of Hilton's law:
• Hip pathology → knee pain (classic exam question; missing this leads to missed hip diagnoses)
• Shoulder pathology → outer arm pain (axillary nerve supplies glenohumeral joint + deltoid + skin over deltoid)
• Cervical spine → arm pain (cervical nerve roots supply both the vertebral joints and the arm dermatomes)
• Knee pathology → upper shin pain

Mnemonic: HILTON = H-I-L-T-O-N → Hip Is Linked TO kNee