Periodontium Anatomy
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Direct answer
Four tissues invest and support every tooth — the gingiva, the periodontal ligament, the root cementum and the alveolar bone — and together they form the periodontium, most of it differentiating from the dental follicle of the tooth germ. The gingiva is keratinised masticatory mucosa arranged as a free marginal collar, an attached zone fused to the underlying bone, and a junctional epithelium that adheres to the tooth; the healthy sulcus it lines measures 1–3 mm. Beneath it the periodontal ligament, 0.15–0.38 mm wide, suspends the tooth by Sharpey's fibres embedded in cementum on one side and the bundle bone of the alveolar bone proper on the other, the layer radiographs display as the lamina dura. Two measurements anchor everything that follows in periodontics: the junctional epithelium averages 0.97 mm and the connective tissue attachment 1.07 mm — Gargiulo's 1961 figures — which together create the biological width of roughly 2 mm that restorative margins must respect.
What you must remember
- Components and derivation: gingiva, periodontal ligament, cementum and alveolar bone; cementum, ligament and bone arise from the dental follicle, a point examiners like.
- Gingival divisions: marginal (free) gingiva, interdental papillae, attached gingiva and junctional epithelium; the free gingival groove marks the free–attached boundary in fewer than half of adults.
- Biological width per Gargiulo (1961): sulcus 0.69 mm, junctional epithelium 0.97 mm, connective tissue attachment 1.07 mm — a dentogingival unit of about 2 mm.
- Periodontal ligament width 0.15–0.38 mm, narrowest over the middle third of the root in a functioning tooth; it widens with reduced or traumatic function and disappears around ankylosed teeth.
- Oblique fibres are the most numerous ligament group and resist axial loading; alveolar crest, horizontal, interradicular and apical groups complete the set.
- Transseptal fibres run across the interdental crest independent of bone and persist even after crestal bone loss, keeping interdental lesions connected.
- At the cementoenamel junction, cementum overlaps enamel in about 60–65 per cent of teeth, meets it edge to edge in roughly 30 per cent and leaves a gap in 5–10 per cent.
- Alveolar bone proper is the perforated cribriform plate that lodges Sharpey's fibres; the lamina dura of the radiograph is this layer, not the ligament itself.
The biological width at the chairside
Picture a crown on a maxillary canine whose margin sits 3 mm below the gingival crest, with localised redness and bleeding at every recall although plaque control is excellent. The margin has invaded the attachment. With 0.97 mm of junctional epithelium and 1.07 mm of connective tissue attachment, roughly 2 mm of tooth coronal to the bone crest is already committed to the attachment apparatus, and no sulcus plus restoration can be packed inside it; the tissue answers with inflammation that home care will never settle. The remedy is surgical crown lengthening — osteotomy to leave at least 3 mm between the planned margin and the bone crest, that is 2 mm of attachment plus a 1 mm sulcus — or orthodontic forced eruption when cutting bone would threaten adjacent roots. That is the origin of the practical restorative rule "3 mm from margin to bone".
Where viva examiners catch students out
The commonest trap is arithmetic: asked for the width of attached gingiva, students quote the keratinised band — but attached gingiva equals keratinised tissue minus free gingiva and sulcus, so subtract probing depth from the margin-to-junction measurement. The second trap is calling every 4 mm reading a pocket; a pocket exists only when the junctional epithelium has migrated apically, while a deep reading in an oedematous but unmigrated gingiva is still health-plus-inflammation. Third, the lamina dura is alveolar bone proper seen radiographically — writing "periodontal ligament space margin" costs the mark. Finally, remember that transseptal fibres belong to the gingival group, not the alveolar crest group of the ligament, a distinction that decides a one-mark MCQ.
Frequently asked questions
What is the width of a healthy periodontal ligament, and where is it narrowest?
It ranges from 0.15 to 0.38 mm and is narrowest around the middle third of the root in a functional tooth; it widens with excessive or absent function and vanishes in ankylosis.
What are Gargiulo's dimensions of the dentogingival junction?
Sulcus 0.69 mm, junctional epithelium 0.97 mm and connective tissue attachment 1.07 mm, giving a biological width of about 2 mm.
Which periodontal ligament fibre group is most numerous and why?
The oblique group, because its obliquity converts axial occlusal loading into tension that the crest-to-apex fibre architecture can absorb.
How do cementum and enamel meet at the cementoenamel junction?
Cementum overlaps enamel in 60–65 per cent of teeth, meets edge to edge in about 30 per cent, and a gap exists in 5–10 per cent.
Which radiographic line represents alveolar bone proper?
The lamina dura, which is the mineralised bundle bone of the cribriform plate around the ligament space — not the ligament itself.