Cementum, PDL & Alveolar Bone

Cementum types

Type	Location	Role
Acellular extrinsic fiber cementum	Cervical and middle root (coronal two-thirds)	Main anchorage for Sharpey fibers
Cellular intrinsic (or mixed stratified) cementum	Apical third and furcations	Adaptation to wear and eruption; repair

Cementum is about half mineral by weight, avascular and not innervated; cementocytes sit in lacunae with canaliculi pointing toward the PDL (their nutrient source).
CEJ patterns (percentages vary by source): cementum overlaps enamel in about 60%, meets it edge-to-edge in about 30%, and leaves a gap of exposed dentin in about 10% (a cause of cervical sensitivity).

Group	Direction	Resists
Alveolar crest	Cementum just below the CEJ down and out to the alveolar crest	Extrusion and tipping
Horizontal	Right angles to the root	Horizontal and tipping forces
Oblique (most numerous)	From cementum coronally up to bone	Vertical (intrusive) occlusal forces
Apical	Radiate from the apex	Tipping and extrusion; protect apical vessels and nerves
Interradicular	Furcation crest to cementum (multirooted teeth only)	Tipping, torquing, extrusion

Gingival fiber groups (not PDL proper) include the transseptal fibers, which run tooth to tooth over the crest and are a major cause of orthodontic relapse.
PDL width is roughly 0.15–0.4 mm and is narrowest near the middle third of the root (the fulcrum of movement).

Alveolar bone proper (bundle bone) lines the socket and anchors Sharpey fibers; it is perforated by vessels (cribriform plate) yet seen on radiographs as the lamina dura.
Supporting alveolar bone: cortical plates plus trabecular (cancellous) bone between them.
Bone remodels with function: osteoclasts resorb on the pressure side and osteoblasts deposit on the tension side, the basis of orthodontic tooth movement.
Main PDL cells: fibroblasts (most numerous), cementoblasts, osteoblasts, osteoclasts, epithelial rests of Malassez and undifferentiated mesenchymal cells.

Calling the horizontal group the most numerous: the oblique group is.
Listing transseptal fibers as PDL fibers: they belong to the gingival fiber groups.
Putting cellular cementum at the cervical root: it concentrates apically and in furcations.
Thinking the lamina dura is a solid plate: it is the radiographic image of perforated alveolar bone proper.

Pick an answer to see right away whether it is correct and why.

0 of 5 answered

Answer key: 1 B · 2 B · 3 A · 4 B · 5 D