Structure of Cementum

Question 1

Where is the alveolar bone?
What is the purpose of alveolar bone?
What does it depend on?

Answer 1

Boundary at the level of root apices of teeth.

• Mineralised tooth support.
• Muscle attachment
• Bone marrow
• Reservoir for ions (calcium)
• Plasticity and remodelling

Depends on:

• Tooth eruption for development and tooth retention for maintenance
• Requires functional stimuli to maintain bone mass

Question 2

Are what fetal age does alveolar bone develop?

Explain how this occurs

Answer 2

Second month of life.

Developing tooth buds are enclosed by grooves that are open to the oral cavity.
Major portion of the alveolar process begins with root formation and eruption of the teeth.

Question 3

Give details about the inside of alveolar bone

Answer 3

Two layers of compact (cortical) bone.
Spongey bone inside - higher SA to mass ratio. This is to make the bone lighter and to allow blood vessels&nerves to innovate.
On the inside, compact bone gives attachment to the fibres of the periodontal ligament. This is sometimes called ‘bundle bone’ due to bundles of fibres attaching from the periodontal ligament.

Question 4

What are the 3 layers on the alveolar bone coming from the root outwards towards the bone?

What connects cementum to alveolar bone?

Answer 4

1. Bone lining the alveolus (compact bone)
2. Central spongiosa
3. Outer cortical plates (vary in thickness, thicker in mantle than maxilla, thicker in premolar and molar regions). This makes the mandible more heavy than the maxilla.

Collagen fibres to help withstand pressures.

Question 5

Explain the parts of bundle/compact bone

Answer 5

It is the part of the alveolar bone where periodontal ligament fibres are inserted.
There are bundles of fibres and Sharpey’s fibres (principal fibres of the periodontal ligament that are
embedded in the bone or cementum).
Sharpey’s fibres are seen perpendicular to the bundle bone.
Other fibrils are arranged parallel to the bundle bone surface.

Question 6

What is lamina dura called?

Answer 6

Radiographically is more radiodense due to presence of thick bone without trabeculations.

Question 7

What is the cribriform plate?

Answer 7

Alveolar bone proper has many openings for blood vessels and nerves – is perforated.

Question 8

What does interradicular septum and interdental septum separate?

Answer 8

Interradicular - separates roots of multi-root tooth

Interdental separates two adjacent teeth.

Question 9

Give the 5 features of the nerve that comes into the bottom of the tooth

Answer 9

• Mesencephalic sensory neurons of the trigeminal nerve
• Motor nucleus of the trigeminal
• Sensory nucleus of the trigeminal
• Spinal sensory trigeminal nucleus
• Fibres of the masticatory musculature

Question 10

What are the 3 sources of blood supply to the alveolar bone?

Answer 10

1. Peridontal
2. Alveolar
3. Supraperiostal/mucogingival

Question 11

What are features of the two types of spongey bone found in the alveolar bone?

Answer 11

Type 1 - trabecular are regular and horizontal like a ladder. Seen commonly in the mandible

Type 2 - irregularly arranges delicate and numerous trabecular. Commonly seen in maxilla.

Question 12

Where is the trabecular bone the most dense?

Between the teeth, where is the bone most dense?

Answer 12

Next to the teeth where it forms the cribiform plate.

Near the crest and least dense at the apex.

Question 13

Why are there canals in the alveolar bone?

Answer 13

Nutrient canals are anatomic structures of the alveolar bone through which neurovascular elements transit to supply teeth and supporting structures.

Question 14

What affect does tooth loss have on alveolar bone?

Answer 14

Degradation of the bone

Question 15

Explain what injury does to the alveolar bone

Answer 15

Injury can lead to triggers causing building up of bone.
Initially it causes little formation and lots of absorption which can be due to inflammation.
Then during repair, bone formation peaks and bone resorption goes down.

Question 16

What 5 cells regulate bone loss and deposition?

Answer 16

Osteoprogenitor cells: Naïve osteoblasts
Osteoblasts: bone forming
Osteocytes: encased osteoblasts
Osteoclasts: bone degrading
Bone-lining cells: regulate movement of calcium and phosphate into and out of bone

Question 17

Explain some features of osteoclasts

Answer 17

• Large, mulinucleate cells
• Dissolve both mineral and organic matrix
• Unusual polarised morphology
• Specialised membrane areas
• Secrete unique markers

Question 18

Explain resting lines and reversal lines

Answer 18

When bone deposition by osteoblasts is halted temporarily then resumed, a resting line is formed.

Resorption by osteoclasts results in reverse lines which are scalloped - these represent the limit of a phase of resorption on which new bone deposition has occurred.

Question 19

What is cementum?

Functions?

Answer 19

It is a bonelike tissue which covers teeth roots.

• Joins the enamel at the cervix of the teeth
• Anchors the teeth to the bony walls of the tooth socket
• Anchors gingival and periodontal fibres
• Protects dentine

Question 20

Where does cementum come from?

Answer 20

• Formed continuously, slowly
• Compensated for tooth wear
• Thickest at apex
• Not vascularised unlike bone

Question 21

What are 3 possible places for cementoblasts to come from?

Answer 21

1. Pulp cells (make odontoblasts which become cementoblasts)
2. HERS cells: epithelial-mesenchymal transformation
3. Dental follicle cells: fragmentation of HERS cells allows dental follicle to contact the dentine surface.

Question 22

What is the composition of cementum?

How about bone?

Answer 22

65% inorganic (hydroxyapatite)
23% organic (type 1 collagen, proteoglycans, glycoproteins)
12% water

60% inorganic (hydroxyapatite)
25% organic
15% water

Cementum is harder than bone.

Question 23

Give the mineral content of cementum

Answer 23

Enamel = 95%
Dentine = 70%
Cementum = 65%
Bone = 60%

Question 24

What are the 4 types of cementum?

What is this due to?

Answer 24

1. Acellular, afibillar
2. Acellular, extrinsic-fiber
3. Cellular, intinsic-fiber
4. Cellular mixed fibre

Due to the faster rate of matrix formation for cellular cementum.

Question 25

Compare acellular and cellular cementum

Answer 25

Acellular:
No cells slow rate of deposition, close incremental lines, thin precementum layer

Cellular:
Cementocyte cells, fast rate of deposition, far apart incremental lines, thick precementum layer.
Ground section through it shows lucunae (space that cementocytes occupy) and canaliculi (channel for their processes)

Question 26

Explain extrinsic and intrinsic cementum

types of fibrillar cementum

Answer 26

Extrinsic: fibres come from outside such as Sharpey’s fibres from the periodontal ligament perpendicular to cementum, major role in anchoring teeth

Intrinsic: fibres are produced by cementoblasts, parallel to cementum surface, found at sites undergoing repair (following resorption), does not anchor teeth

Mixed fibre contains both of these.

Question 27

What are clinical considerations of alveolar bone and cementum?

Answer 27

• Essential for anchorage
• Protection to root dentine (preventing sensitivity) • scaling/root planing removes cementum!
• new cementum is important for new attachments
• slow deposition at apical root
• Hypercementosis (oral surgery aspect)
• Applied forces-Tooth movement and Orthodontic treatment
• Importance of quality/quantity of alveolar bone (Creating a biological base for dental implants)