1 Classification of Joints

A. Structural Classification

1. Fibrous Joints
   • Sutures
   • Syndesmosis
   • Gomphosis
2. Cartilaginous Joints
   • Primary cartilaginous joints → synchondrosis
   • Secondary cartilaginous joints → symphysis / amphiarthrosis
3. Synovial Joints
   • Ball-and-socket / spheroidal joints
   • Sellar / saddle joints
   • Condylar / bicondylar joints
   • Ellipsoid joints
   • Hinge joints
   • Pivot / trochoid joints
   • Plane joints

B. Functional Classification

According to Degree of Mobility

1. Synarthroses → immovable
   • Example → sutures of fibrous joints.
2. Amphiarthroses → slightly movable
   • Example → secondary cartilaginous joints.
3. Diarthroses → freely movable
   • Example → synovial joints.

Synarthroses

• Fixed joints at which there is no movement.
• Articular surfaces are joined by tough fibrous tissue.
• Often, edges of bones are dovetailed into one another, as in sutures of the skull.

Amphiarthroses

• Joints at which slight movement is possible.
• Articulating bones are covered by hyaline cartilage.
• A pad of fibrocartilage lies between the bone surfaces.
• Fibrous ligaments hold the bones and cartilages in place.
• Cartilages also act as shock absorbers.
• Example → intervertebral disc between the bodies of vertebrae.

Diarthroses / Synovial Joints

• Known as freely movable joints.
• Movement may be restricted at some joints by:
  • Shape of articulating surfaces.
  • Ligaments holding the bones together.
• These ligaments are made of elastic connective tissue.

C. Regional Classification

1. Skull type → immovable.
2. Vertebral type → slightly movable.
3. Limb type → freely movable.

D. According to Number of Articulating Bones

2 Movements at Synovial Joints

Angular Movements

Flexion

• Decreasing the angle between two bones or two parts.

Extension

• Increasing the angle between two bones or two parts.

Abduction

• Moving a part away from the mid-line.

Adduction

• Bringing a part towards the mid-line.

Rotatory Movements

Rotation

• Rotating along the vertical axis.

Circumduction

• Moving the extremity or part round in a circle so that the whole part inscribes a cone.
• When flexion, abduction, extension and adduction occur in sequence → circumduction.

Gliding

• One part slides on another.

3 Fibrous Joints

1. Sutures

• Present only in the skull.
• Two bones are separated by connective tissue.
• Sutural side of each bone is covered by:
  • Layer of osteogenic cells / cambial layer.
  • Capsular layer continuous with the periosteum.
• Area between the bones decreases with age.
• Osteogenic surfaces become opposed.
• Sutures may synostose and become obliterated as age advances.
• Sutures are peculiar to skull and are immovable.

Types of Sutures According to Shape of Bony Margins

1. Plane
   • Example → internasal suture.
2. Serrate
   • Example → interparietal sagittal suture.
3. Squamous
   • Example → temporo-parietal suture.
4. Denticulate
   • Example → lambdoid suture between parietal and occipital bones.
5. Schindylesis
   • Example → between rostrum of sphenoid and upper border of vomer.

Fontanelles

• Neonatal skull shows fontanelles, which are widened sutures.
• They are temporary.
• They permit moulding (temporary overlapping of bones) during normal vaginal childbirth.
• At six specific points on the sutures of the newborn skull are membrane-filled gaps called fontanelles.
• They also allow the underlying brain to increase in size.
• Anterior fontanelle is used to judge hydration of the infant.
• All fontanelles become bone by 18 months.
• Fontanelles represent intramembranous ossification in progress.

2. Syndesmosis

• Fibrous union between bones.
• May be represented as:
  • Interosseous ligament → inferior tibiofibular joint.
  • Tense membrane → posterior part of sacroiliac joint.
  • Interosseous talocalcanean ligament.

3. Gomphosis

• Peg-and-socket junction between a tooth and its socket.
• Periodontal ligament connects the dental element to the alveolar bone.
• Actually, gomphosis is an articulation between two bones.

4 Cartilaginous Joints

1. Primary Cartilaginous Joints

Synchondrosis / Hyaline Cartilage Joints

• Bones are united by a plate of hyaline cartilage.
• Joint is:
  • Immovable.
  • Strong.
• These joints are temporary.
• After a certain age, cartilaginous plate is replaced by bone → synostosis.
• Seen between epiphysis and diaphysis of long bones.
• Associated with:
  • Growth.
  • Epiphysial plates.
  • Length of the bone.
• Primary cartilaginous synchondroses become synostosed after some age and are no longer identifiable.

Examples

1. Joint between epiphysis and diaphysis of a growing long bone.
2. Spheno-occipital joint.
3. First chondrosternal joint.
4. Costochondral joints.
5. Xiphisternal joint.

2. Secondary Cartilaginous Joints

Symphyses / Fibrocartilaginous Joints

• Articular surfaces are covered by a thin layer of hyaline cartilage.
• Surfaces are united by a disc of fibrocartilage.
• These joints are permanent and persist throughout life.
• Symphysis menti is a misnomer because it is a synostosis.
• Typically occur in the median plane of the body.
• Permit limited movements due to the compressible pad of fibrocartilage.

Examples

1. Symphysis pubis.
2. Manubriosternal joint.
3. Intervertebral joints between the vertebral bodies.

Further Features

• Thickness of fibrocartilage is directly related to the range of movement.
• Secondary cartilaginous joints may represent an intermediate stage in the evolution of synovial joints.
• Intervertebral disc:
  • Varies from a few mm to 10 mm.
  • Contains dense connective tissue with a few chondrocytes.
• Collagen ligaments extend from the periostea of articulating bones across the symphysis and blend with perichondria of hyaline cartilage.
• Complete capsule is not formed.
• Plexuses of afferent nerve terminals are present.
• It can withstand stress of:
  • Compression.
  • Tension.
  • Shear, etc.
• Range of motion is limited.

Functions / Significance

• Primary and secondary cartilaginous joints are concerned with:
  • Strength.
  • Withstanding forces.
• They transmit weight/force.
• Growth occurs in both types.
• Both are concerned with movement.
• Rigidity of synchondrosis improves the efficiency of the nearby synovial joint.
• Movement of a vertebra is the summation of the effect of:
  • Synchondrosis (primary cartilaginous joint).
  • Symphysis.
  • Synovial joints associated with the vertebra.
• These joints act as shock absorbers and help in weight transmission.
• Synchondrosis allows growth of hyaline cartilaginous plates where endochondral ossification extends.

5 Synovial Joints

Characters of Synovial Joints

1. Articular Surfaces

• Covered with hyaline (articular) cartilage.
• Fibrocartilage is present in certain membrane bones, like:
  • Clavicle.
  • Mandible.
• Articular cartilage is:
  • Avascular.
  • Non-nervous.
  • Elastic.
  • Lubricated with synovial fluid.
• Provides slippery surfaces for free movements, like “ice on ice”.
• Surface of cartilage shows fine undulations filled with synovial fluid.

2. Joint Cavity

• Present between the articular surfaces.
• Filled with synovial fluid.
• May be:
  • Partially subdivided, or
  • Completely subdivided
  by an articular disc or meniscus.

3. Articular Capsule

• Joint is surrounded by an articular capsule.
• Made up of:
  • Fibrous capsule.
  • Synovial membrane lining the fibrous capsule.
• Fibrous capsule has rich nerve supply.
• It is sensitive to stretches produced by movements.
• This produces appropriate reflexes to protect the joint from sprain.
• This is called the “watchdog” action of the capsule.

Reinforcement of Fibrous Capsule

a. Capsular / true ligaments

• Represent thickenings of the fibrous capsule.

b. Accessory ligaments

• Distinct from the fibrous capsule.
• May be intra- or extracapsular.

Synovial Membrane

• Lines the whole interior of the joint except articular surfaces covered by articular cartilage.
• Secretes a slimy, viscous fluid called:
  • Synovia.
  • Synovial fluid.
• Synovial fluid:
  • Lubricates the joint.
  • Nourishes articular cartilage.
• Viscosity of fluid is due to hyaluronic acid secreted by cells of the synovial membrane.

4. Movement

• Varying degrees of movement are always permitted by synovial joints.

6 Classification of Synovial Joints and Their Movements

A. Plane / Gliding Type

• Movement → gliding movements.

B. Uniaxial Joints

1. Hinge Joint

• Movement → flexion and extension.

2. Pivot Joint

• Movement → rotation only.

C. Biaxial Joints

1. Condylar Joint

• Movements:
  • Flexion.
  • Extension.
  • Limited rotation.

2. Ellipsoid Joint

• Movements:
  • Flexion.
  • Extension.
  • Abduction.
  • Adduction.
  • Circumduction.

D. Multiaxial Joints

1. Saddle Joint

• Movements:
  • Flexion and extension.
  • Abduction and adduction.
  • Conjunct rotation.

2. Ball-and-Socket / Spheroidal Joint

• Movements:
  • Flexion and extension.
  • Abduction and adduction.
  • Circumduction.
  • Medial and lateral rotation.

7 Plane Synovial Joints

Examples

1. Intercarpal joints.
2. Intertarsal joints.
3. Joints between articular processes of vertebrae.
4. Cricothyroid joint.
5. Cricoarytenoid joint.
6. Superior tibiofibular joint.
7. Interchondral joint (5–9 ribs).
8. Costovertebral joint.
9. Costotransverse joint.
10. Acromioclavicular joint with intra-articular disc.
11. Carpometacarpal joints (except first).
12. Tarsometatarsal joints.
13. Intermetacarpal joints.
14. Intermetatarsal joints.
15. Chondrosternal joints (except first).
16. Sacroiliac joint.

8 Hinge Joints

• Articular surfaces are pulley-shaped.
• There are strong collateral ligaments.
• Movements are permitted in one plane around a transverse axis.

Examples

1. Elbow joint.
2. Ankle joint.
3. Interphalangeal joints.

9 Pivot / Trochoid Joints

• Articular surfaces comprise a central bony pivot (peg) surrounded by an osteoligamentous ring.
• Movements are permitted in one plane around a vertical axis.

Examples

1. Superior and inferior radioulnar joints.
2. Median atlantoaxial joint.

10 Condylar / Bicondylar Joints

• Articular surfaces include two distinct condyles.
• Condyles are convex male surfaces fitting into reciprocal female surfaces.
• These female surfaces are also sometimes known as condyles, such as in tibia.
• These joints permit movements:
  • Mainly in one plane around a transverse axis.
  • Partly in another plane → rotation around a vertical axis.

Examples

1. Knee joint.
2. Right and left jaw joints / temporomandibular joints.

Median Atlantoaxial vs Superior Radioulnar Joint

11 Ellipsoid Joints

• Articular surfaces include:
  • An oval, convex male surface.
  • An elliptical, concave female surface.
• Free movements are permitted around both axes:
  • Flexion and extension → around transverse axis.
  • Abduction and adduction → around anteroposterior axis.
• Combination of movements produces circumduction.
• Typical rotation around a third, vertical axis does not occur.

Examples

1. Atlanto-occipital joints.
2. Wrist joint.
3. Metacarpophalangeal joints.

12 Saddle / Sellar Joints

• Articular surfaces are reciprocally concavoconvex.
• Movements are similar to those permitted by an ellipsoid joint.
• There is addition of some rotation (conjunct rotation) around a third axis.
• This rotation cannot occur independently.

Examples

1. First carpometacarpal joint.
2. Sternoclavicular joint.
3. Calcaneocuboid joint.
4. Incudomalleolar joint.
5. Between femur and patella.

13 Ball-and-Socket / Spheroidal Joints

• Articular surfaces include:
  • Globular head → male surface.
  • Cup-shaped socket → female surface.
• Movements occur around an indefinite number of axes having one common centre.
• The following movements occur quite freely:
  • Flexion.
  • Extension.
  • Abduction.
  • Adduction.
  • Medial rotation.
  • Lateral rotation.
  • Circumduction.

Examples

1. Shoulder joint.
2. Hip joint.
3. Talocalcaneonavicular joint.
4. Incudostapedial joint.

14 Classification and Movements of Synovial Joints

Terminology and Definition

Male Surface

• An articulating surface which is:
  • Larger in surface area.
  • Always convex in all directions.

Female Surface

• An articulating surface which is smaller in all directions.

Simple Joints

• Joints with only two articulating surfaces:
  • Male.
  • Female.

Compound Joints

• Joint possessing more than one pair of articulating surfaces.
• Example → wrist joint.

Degrees of Freedom