Joints: form and function

Question 1

What are synarthroses?

Answer 1

Fibrous/semi-rigid joints

• fibrous = ligamentous (sutures of the skull, syndesmous equine metacarpals, gomphosis (peridontal ligaments))
• cartilaginous = cartilage (synchondroses at growth plate, symphyses - mandible and pelvis midline between vertebral bodies)

Question 2

What are diarthroses?

Answer 2

Freely movable

- synovial (fluid filled cavity, shoulder, stifle, hip, etc)

Question 3

What are psuedarthroses?

Answer 3

False joint

- unhealed area of a bone has motion like a joint

Question 4

What is a suture?

Answer 4

site of growth - membrane between bone grows, bone grows into membrane at suture

Question 5

What are syndesmoses and gomphosis?

Answer 5

Bands of short ligaments

e. g. syndesmoses = equine metacarpals
e. g. gomphosis = peridontal ligament

Question 6

What is a synchondrosis?

Answer 6

Cartilaginous joint between the epiphysis and diphysis

Question 7

Where are symphasis located?

Answer 7

Mandibular
- some sp. mobility for mastication
- common site of fracture in cats
Pelvic
- becomes softer before parturition (relaxin)
Invertebral disc
- movement facilitated by ligament like fibrocartilage

Question 8

What is the general layout of a diarthroses?

Answer 8

Synovial joint - overall have a degree of joint congruity

• two bones ends are the epiphysis covered with the hyaline cartilage
• joint fluid is between the two
• surrounded by the synovial membrane

Question 9

Describe a hinge joint

Answer 9

Hinge - shaped to limit articulation to one plane e.g. the knee

Question 10

Describe a ball and socket joint?

Answer 10

Wide, almost infinite range of articulation e.g. the hip

Question 11

Describe the sternoclavicular joint

Answer 11

Starting with the clavicle and moving down to the acromion you move to the glenoid cavity which is caudal to the acromion
Then move to the coracoid process which attaches the scapula to the clavicle

Question 12

What is the synovium made up of?

Answer 12

Synovium = initmal layer + sub-intima

• sub-intima = loose fibrous/fatty/dense fibrous tissue
• no basement membrane
• initimal lining 100µm, 2-3 cells thick

Question 13

What cells are in the synovium?

Answer 13

Type (A) cells - macrophage like - phagocytosis
Type (B) cells - fibroblast like - produce synovial fluid
- produces plasma ultrafiltrate < 10KDa molecules get into synovial fluid from the blood

Question 14

What molecules act as a lubricant for the synovial fluid?

Answer 14

Water 
Hyaluronic acid 
Lubricin 
Surface active phospholipids 
Few mononuclear leukocytes 
Glucose/electrolytes at plasma concentrations/bathes cartilage

Question 15

What is lubricin?

Answer 15

Protection through lubrication 
- low friction: HA-lubricin complexes 
Mechanically - controlled lubrication 
- compression drives HA from cartilage 
- trapped in surface collagen network 
- HA-lubricin complex - bound network

Question 16

What is a synovial sheath?

Answer 16

Provide lubrication to tendons as they pass over joints

- poor healing after trauma can lead to adhesion and loss of function

Question 17

What are joint cavities?

Answer 17

In complex joints e.g. equine carpus, multiple cavities in a single joint
- may or may not communicate

Question 18

What are the functions of cartilage?

Answer 18

Resists compression 
Provides resillience 
Provides support 
Provides flexibility 
Aneural but avascular (sometimes canals)

Question 19

How are collagen II fibres organised through the superficial, mid and deep zones?

Answer 19

Run parallel to the surface (superfical zone)
As they move down and form Beninghoff Arcades they form a highly striated organisation and run perpendicular to the surface (mid + deep zones)

Question 20

What is special about proteoglycans?

Answer 20

Very large molecular weight 
- abundant in PG in cartilage
High -ve charge 
- high compressive modulus 
- repel self, attract H2O

Question 21

What is ACC thickness determined by?

Answer 21

Articular Calcified cartilage thickness determined by bone invasion

Question 22

What are the stages in synovial joint development?

Answer 22

Cellular condensations differentiate into cartilage anlagen
- one anlage per future bone
Interzone
- emerges between adjacent anlagen and filled with undifferentiated mesenchyme
Cavitation
- interzone cell populations distinct from remainder of growth cartilage of anlagen from outset

Question 23

What is elastic cartilage?

Answer 23

Greater toughness in bending, less plastic, more elastic
In larynx, ear, eustachian tube and epiglottis
- surrounded by perichondrium and often merged with stiffer connective tissue for support
- large quantities of elastin
- elastic cartilage of epiglottis contains large quantities of elastin and offers additional support
- elastic cartilage of epiglottis contains large quantities of elastin and offers additional flexibility

Question 24

What is fibrocartilage?

Answer 24

Additional resistance to tension
Type 1 collagen in bundles, less proteoglycan
Fibrochondrocytes between ECM bundles
In IV disc, ligament attachment sites menisci
No perichondrium