Root formation and eruption

Question 1

Describe root formation:

Answer 1

• Starts at CEJ
• Hertwig’s Epithelial Root sheath (HERS):
  1) OEE and IEE only - only two cell layer
  2) separates dental papilla and dental follicle
• Epithelial diaphragm = shelf-like extension of HERS at apex of root; determines apical foramen, size/shape
• Root formation: HERS fragments, cementoblasts lay down cementum, intrinsic fibres are incorporated

Question 2

List and describe the cementum types

Answer 2

• Acellular extrinsic fibre -> cervical 2/3 of root; major role in tooth anchorage
• Cellular intrinsic fibre -> Apical 1/3; sites of cementum repair
• Acellular afibrillar fibre -> coronal cementum (i.e. at enamel-cementum junction)
• Cellular mixed fibre -> apex of tooth and bifurcations

Question 3

List the anomalies in root formation:

Answer 3

• Hypercementosis -> common in Paget’s disease

- Cementicles -> globule masses of cementum

Question 4

Describe periodontal ligament

Answer 4

• Formed from dental follicle

- Collagen fibres grow from cementum and alveolar bone and join in middle

Question 5

Describe eruption movement:

Answer 5

• Pre-eruptive phase -> movement prior to root formation
• Eruptive phase ->Initiation of root formation to functional position of occlusion
• Post-eruptive phase -> After eruption

Question 6

Describe eruption mechanism:

Answer 6

• Dental epithelial and follicle (only theory with evidence):
  1) Reduced dental epithelium initiates osteoclast recruitment
  2) Dental follicle initiates alveolar bone remodelling
• Root formation
• Bone remodelling
• Periodontal ligament

Question 7

Define ‘ankylosis’

Answer 7

Early trauma to tooth causes tooth to erupt lower than occlusal plate

Question 8

Describe eruption pattern (“rule of sixes”)

Answer 8

• 6 weeks = initiation
• 6 months -> 30 months:

Every 6 months, 4 teeth erupt (i.e. one tooth in each quadrant).
- Central incisor -> lateral incisor -> 1st molar -> canine -> 2nd molar

Question 9

Describe primary teeth shedding

Answer 9

Replacement of deciduous teeth by permanent teeth

• Permanent teeth develop LINGUAL side of deciduous tooth
• Permanent tooth move under root apex
• Shedding mechanism:
  1) pressure form successional tooth
  2) Osteoclast/bone remodelling
  3) Odontoclast
  4) Resorption of soft tissues

Question 10

Describe permanent teeth pattern (“Rule of Fours”)

Answer 10

• 4 months = initiation of successional lamina
• Birth = 1st molar initial calcification
• 4 years = all crowns have initial calcification
• 8 years = all crowns complete
• 12 years = all crowns erupted
• 16 years = all roots complete