Q2
LONG ANSWER
10M
★ High-yield
Classify joints with examples. Describe the structure of a synovial joint and add a note on Hilton's law.
Definition
A joint (articulation) is the junction between two or more bones, or between bone and cartilage.
Structural Classification
| Type | Uniting medium | Mobility |
|---|---|---|
| Fibrous | Fibrous tissue | Immobile or slightly mobile |
| Cartilaginous | Cartilage | Slightly mobile |
| Synovial | Joint cavity with synovial fluid | Freely mobile |
Fibrous Joints
| Subtype | Description | Examples |
|---|---|---|
| Suture | Thin layer of fibrous tissue; skull only | Coronal, sagittal, lambdoid; types — serrate, squamous, plane, denticulate, schindylesis |
| Syndesmosis | Bones united by an interosseous ligament or membrane | Inferior tibiofibular joint, interosseous membranes |
| Gomphosis | Peg-in-socket | Tooth in its alveolus, by the periodontal ligament |
- Synostosis = complete bony fusion of a former joint, as when sutures close in adult life
Cartilaginous Joints
| Subtype | Cartilage | Features | Examples |
|---|---|---|---|
| Primary (synchondrosis) | Hyaline | Temporary; ossifies; immobile; always in the midline? No — both | Epiphyseal plate, first sternocostal joint, spheno-occipital |
| Secondary (symphysis) | Fibrocartilage between hyaline-covered surfaces | Permanent; slightly mobile; always in the midline | Symphysis pubis, intervertebral discs, manubriosternal joint |
Synovial Joints — Structure
- Articular cartilage — hyaline (fibrocartilage in the temporomandibular, sternoclavicular and acromioclavicular joints); avascular, aneural and alymphatic, nourished by synovial fluid
- Articular capsule — outer fibrous layer and inner synovial membrane
- Synovial membrane — lines everything except the articular cartilage; secretes synovial fluid
- Joint cavity — a potential space containing synovial fluid
- Ligaments — capsular, extracapsular, intracapsular
- Accessory structures — menisci, labra, fat pads, bursae, tendons
- A dialysate of plasma with hyaluronic acid added by synovial cells
- Functions — lubrication, nutrition of articular cartilage, shock absorption, removal of debris (by phagocytosis)
- Viscous and slightly alkaline; only 0.5–2 mL in a large joint
Types of Synovial Joint
| Type | Axes | Movements | Examples |
|---|---|---|---|
| Plane (gliding) | Non-axial | Gliding | Intercarpal, intertarsal, acromioclavicular |
| Hinge (ginglymus) | Uniaxial | Flexion, extension | Elbow, ankle, interphalangeal |
| Pivot (trochoid) | Uniaxial | Rotation | Superior radio-ulnar, atlanto-axial |
| Condylar (bicondylar) | Biaxial | Mainly flexion and extension | Knee, temporomandibular |
| Ellipsoid | Biaxial | Flexion, extension, abduction, adduction — no rotation | Wrist, metacarpophalangeal |
| Saddle (sellar) | Biaxial with some rotation | All, including opposition | First carpometacarpal (thumb), sternoclavicular |
| Ball and socket | Multiaxial | All movements including circumduction | Shoulder, hip |
Blood and Nerve Supply — Hilton’s Law
Hilton's law: the nerve supplying a joint also supplies the muscles that move it and the skin over their insertions.
- Explains referred pain — hip disease refers pain to the knee, since both are supplied by the obturator and femoral nerves
- The nerve supply is sensory (pain and proprioception) and vasomotor; joints have no motor supply
- Articular arteries form a periarticular anastomosis, ensuring supply in all positions of the joint
- The capsule and ligaments are richly innervated; the articular cartilage and synovial membrane are insensitive
Factors Determining Range and Stability
| Factor | Effect on range | Effect on stability |
|---|---|---|
| Shape of articular surfaces | Deep socket restricts | Deep socket stabilises (hip) |
| Ligaments | Restrict at extremes | Strong but inelastic |
| Muscle tone | Little | The chief factor in the shoulder |
| Apposition of soft parts | Limits elbow and knee flexion | — |
| Atmospheric pressure | — | Significant at the hip |
| Tension of opposing muscles | Hamstrings limit hip flexion with the knee extended | — |
Applied Aspects
- Articular cartilage has no blood supply and cannot regenerate; damage is repaired by fibrocartilage, which is mechanically inferior — the basis of osteoarthritis
- Aspiration of synovial fluid distinguishes septic arthritis (turbid, high neutrophils, low glucose) from gout (negatively birefringent needle crystals) and pseudogout (positively birefringent rhomboids)
- Haemarthrosis in haemophilia damages cartilage and leads to ankylosis
- Referred pain from the hip to the knee is a classic trap — always examine the hip in a child complaining of knee pain
- Dislocation is commonest where mobility is greatest — the shoulder, because stability has been traded for range
All questions in General Anatomy & Osteology
- Q1Classify bones with examples. Describe the parts of a long bone and its blood supply. Add a note on the types of epiphysis. ★
- Q2Classify joints with examples. Describe the structure of a synovial joint and add a note on Hilton's law. ★
- Q3Classify skeletal muscle according to shape and action. Describe its nerve supply and add a note on the functional groups of muscles. ★
- Q4Describe the types of blood vessels. Define anastomosis and end artery with examples, and add a note on portosystemic anastomoses. ★
- Q5Describe the structure of skin and fascia. Add a note on the lymphatic system and the thoracic duct. ★
- SN1Describe the epiphyseal plate with its zones. Add a note on the Salter–Harris classification. ★
- SN2Classify cartilage with its sites and features. Add a note on articular cartilage. ★
- SN3Describe the structure and types of a synovial joint. Add a note on synovial fluid. ★
- SN4Define end artery. Give its types with examples and the consequences of its occlusion. ★
- SN5Describe the types of ossification with examples. Add a note on the cells of bone. ★
- SN6Describe the deep fascia with its modifications and functions. Add a note on compartment syndrome. ★
- SN7Describe the anatomical position, the planes of the body and the terms of movement.