← All explanationsPeriodontal ligament space

What is the narrow space around a tooth root?

The periodontal ligament space lies between the tooth root and the bony socket wall. It contains the periodontal ligament, part of the tooth-supporting tissues. On a radiograph, this area may appear as a narrow dark line. Its appearance is also influenced by the projection angle.

For training & practice ↓

Fictional enlarged sectional illustration of a single-rooted incisor and a schematic radiographic view. A narrow fibrous zone lies between root cementum and the tooth socket; it appears as a darker line in the radiographic view. — Fictional teaching illustration, not a patient case or treatment result.

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Between root and bone

The root is not directly fused to the surrounding socket wall. The periodontal ligament lies between them. This anatomical space is not a hole inside the tooth.

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A dark line on a radiograph

The periodontal ligament space is more radiolucent than adjacent hard tissues. It may therefore appear dark beside the brighter socket boundary known as the lamina dura.

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Interpret it in context

Width is not identical at every point. Projection and image quality also affect its appearance. An unusual finding is assessed alongside symptoms, examination and other findings.

Read the full explanation

Between root and bone
The root is not directly fused to the surrounding socket wall. The periodontal ligament lies between them. This anatomical space is not a hole inside the tooth.
A dark line on a radiograph
The periodontal ligament space is more radiolucent than adjacent hard tissues. It may therefore appear dark beside the brighter socket boundary known as the lamina dura.
Interpret it in context
Width is not identical at every point. Projection and image quality also affect its appearance. An unusual finding is assessed alongside symptoms, examination and other findings.

Notes on the teaching images

Fictional teaching illustration, not a patient case or treatment result.

Watch the short explanation ↓

At a glance

A normal structure?

Yes, it belongs to the tooth-supporting tissues.

Does dark mean empty?

No. Radiographic contrast reflects X-ray attenuation.

Diagnostic on its own?

No. Imaging and examination need to be combined.

Knowledge in motion

Explained in 36 seconds.

Three steps. No audio.
Each scene can also be selected manually.

Silent educational film with German on-screen text and simplified teaching images. Not a patient case or a promised treatment outcome. The full English explanation is available as text.

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Often confused

Space and pocket are different

The periodontal ligament space lies between root and bone. A periodontal pocket is examined from the gum margin and is not another name for this space around the root.

Quick answers

Is the space empty?

No. It contains the periodontal ligament between the root and socket. A dark line on a radiograph does not mean there is an empty cavity.

Does widening always mean disease?

Unusual widening may merit closer assessment, but it does not establish a diagnosis by itself. Projection, the rest of the image and clinical findings need to be considered together.

Is it the same as a gum pocket?

No. A periodontal pocket extends from the gum margin alongside the tooth. The periodontal ligament space surrounds the root within the socket. They are different structures.

Does it require a new X-ray?

Not automatically. Symptoms, examination and existing images are considered first. An additional image is taken only when justified for a specific clinical question.

At the practice

For the appointment

You can bring this question to your appointment:

Is the appearance at this tooth unusual, and how does it relate to the examination?

An examination can help clarify biting pain or questions about a radiographic finding.

Arrange an assessmentAppointment selection at Doctolib

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Learning layer for training & practiceUnderstand terminology and deepen knowledge

Periodontal ligament: The tissue occupying the space between a tooth root and its bony socket.
Lamina dura: The often radiopaque outline of the bony socket wall on a radiograph.

Make the distinction

The periodontal ligament space lies between root and bone. A periodontal pocket is examined from the gum margin and is not another name for this space around the root.

Knowledge questionWhere is the periodontal ligament space?

For learning, not for assessing personal symptoms. Your answer is not stored or sent.

Between the tooth root and the bony socket wall.
Between enamel and the tooth crown.
As a cavity inside every filling.

View answer

Between the tooth root and the bony socket wall.

It surrounds the root and contains the periodontal ligament.

Sources and context

Editorial research: 2026-11-11. General information, not an individual diagnosis or treatment recommendation. Teaching illustrations are simplified and do not depict a patient case.