Periodontal ligament space

The periodontal ligament space is the thin radiolucent line between a tooth's root and the lamina dura, the space the periodontal ligament occupies. It isn't the same width along the whole root. When it looks widened, you read the pattern: localised or generalised, regular or irregular, and whether the lamina dura beside it is still intact.

Synonyms: PDL space; periodontal ligament space widening; periodontal membrane space

What is the periodontal ligament space?

Follow a root outline from the alveolar crest down to the apex. Just outside the root runs a fine dark line, and outside that a fine white one. The dark line is the periodontal ligament space, a radiolucent space between the root and the lamina dura [1]. The white line is the lamina dura itself.

You read the two together. Almost every useful call about the PDL space depends on what the lamina dura beside it is doing.

What does a normal periodontal ligament space look like on a radiograph?

It's a narrow, even-edged dark line that follows the root, but it isn't uniform. The ligament is thinnest around the middle of the root and slightly wider near the apex and near the alveolar crest [1]. A line that's a little wider at those two ends is the normal shape, not a finding.

Published normal widths are fractions of a millimetre, and they don't agree. A review gives 0.15 to 0.21 mm and notes that the width may decrease with age [1]. An ex vivo micro-CT study of premolars and molars in dry human mandibles measured 0.16 to 0.28 mm [2]. The two figures come from different methods, so treat them as a range rather than a single normal value.

Don't expect to measure it on a periapical radiograph. In that same ex vivo study, widths measured on periapical radiographs had no significant relationship to the micro-CT values, while CBCT measurements performed better [2].

No agreed numeric cut-off for "widened" turned up in the literature reviewed for this entry. Studies set their own. One study of systemic sclerosis, for example, counted a periapical PDL space greater than 0.19 mm [3].

What makes the PDL space look widened, and which causes matter?

One review sorts the causes of widening into four groups: traumatic, infectious, neoplastic and systemic [1]. The group rarely shows on the radiograph. What shows is where the widening sits, what shape it has, and whether the lamina dura survives it.

Shape and the lamina dura come first. Ask whether the widening is regular or irregular, and whether the lamina dura is still there [1]. Orthodontic movement widens the space but leaves the lamina dura intact. Malignant or locally aggressive lesions grow into the ligament space, giving irregular widening with destruction of the lamina dura [1, 5]. Irregular widening with loss of the lamina dura is an ominous sign [4].

Then look at where it is:

• Confined to the apex. Widening at the apex is usually the characteristic feature of inflammation of endodontic origin, and in one study of root-filled teeth it was the first radiographic sign of an apical lesion [1]. It often comes with a break in the lamina dura and an ill-defined periapical radiolucency [6].
• At the crest or between the roots. In periodontitis, an early vertical defect can first appear as abnormal widening, and widening at the apex or in the furcation is evidence of periodontal disease [1].
• Around a heavily loaded tooth. Occlusal trauma can widen the space, along with a thicker lamina dura, bone loss and more numerous, larger trabeculae. It's read together with clinical signs: mobility, wear facets, an unusual response to percussion [1].
• Across many teeth. Generalised widening is the pattern associated with systemic sclerosis, especially on posterior teeth that aren't mobile and have an intact lamina dura, in more than one quadrant [1, 8]. Reported frequency varies widely. Figures cited in one review range from 7% to 100% [1]. One panoramic study found widening in 79.6% of 49 patients [9]. In a larger study, 26% of patients with systemic sclerosis had a periapical PDL space over 0.19 mm, against 13% of controls [3].
• With malignancy. Early osteosarcoma of the jaws has been described as symmetrical widening at one or more teeth, apparently before any other radiographic feature, though it can also be asymmetrical [1, 10]. In one case series, widening was present in 28% of jaw osteosarcomas [1]. Chondrosarcoma involving teeth can widen the space [11], and non-Hodgkin lymphoma can show irregular, diffuse widening [1].
• After treatment of the jaws. Irregular widening around the involved teeth is the most common radiographic change in the alveolar process after radiotherapy, and in one study widening was the most common finding in patients with bisphosphonate-related osteonecrosis [1].

Evenness is not reassurance on its own. Symmetrical widening at one or a few teeth is one of the described early signs of osteosarcoma [1, 10].

What it's confused with

How do you tell true widening from an angulation or superimposition effect?

Start by distrusting the width itself. In a study on dry skulls, changing the horizontal beam angle across a 12° range in 3° steps, and the vertical angle by 10°, significantly changed the measured PDL width at incisor sites. Molar sites showed no significant change, possibly because the anatomy there makes the image less clear [14]. Those angle changes were within what you'd expect clinically. A wide-looking line on an incisor may be the projection, not the ligament.

Width measured on a periapical also didn't track the true width in the ex vivo micro-CT comparison [2]. That's why widening at the margin on its own is weak evidence. A 1987 review judged marginal widening of no diagnostic worth as a sign of early periodontitis. Of the criteria it assessed, only loss of crestal bone height held up, and only with at least two sequential radiographs [7].

So read what the projection can't easily fake:

1. The lamina dura beside it. Intact, broken, thickened or gone. That's the discriminator between orthodontic widening and an aggressive lesion [1].
2. The shape. Regular or irregular, apical or marginal, one tooth or many [1].
3. A second view. In their endodontic chapter, UK selection criteria advise that when one periapical doesn't give enough information, supplementary views should be considered, usually a second periapical with the horizontal beam angle shifted about 10–20° mesially or distally. The parallax separates roots and can remove overlapping structures [15].

When is a widened PDL space a reason for a second view or further investigation?

A radiograph alone doesn't give a definitive diagnosis of the pulp or the periradicular tissues. It's read with the clinical examination [15].

A second periapical comes first. In the endodontic chapter of UK selection criteria, it is the supplementary view when one periapical doesn't give enough information [15].

CBCT comes after conventional imaging, not instead of it. European endodontic guidance places CBCT after a clinical examination that includes conventional radiographs [16]. It is then considered case by case, as a small field of view under 5 cm, when conventional imaging is inconclusive. Examples are periapical signs with non-specific symptoms, or periradicular bone changes that suggest a root fracture [16].

Further work-up is for the patterns that don't fit a local cause. Asymmetrical widening together with a growing bony mass should prompt consideration of osteosarcoma and chondrosarcoma, and a more elaborate diagnostic work-up [1]. Case reports add follow-up radiographs and histopathology [6], and biopsy when malignancy is suspected [5]. The guidance cited here covers second views and CBCT. The malignancy work-up comes from reviews and case reports, not from a professional-body guideline.

Common errors

• Holding the line against one "normal" number. Reported normal widths differ between methods [1, 2], and studies set their own cut-offs for widening [3].
• Calling widening on an incisor from a single angled view. Beam angle alone changes measured width at incisor sites [14].
• Treating marginal widening alone as early periodontitis. It didn't hold up as a criterion. Crestal bone height on sequential radiographs did [7].
• Reading even widening as benign. Symmetrical widening at one or a few teeth is a described early sign of osteosarcoma [1, 10].
• Expecting a vertical root fracture to widen the PDL space. It isn't necessarily related to widening [12]. In one study of root-filled teeth at endodontic surgery, a J-shaped or "halo" radiolucency had the strongest association with fracture [13].
• Reaching for CBCT before a second periapical. Guidance puts conventional imaging first [15, 16].