In the mandible, so-called odontogenic lesions develop above the inferior dental canal, while non-odontogenic lesions develop above, within or below the canal. Thus some conditions have a predilection for certain areas whilst others develop in one site only. For example, radicular dental cysts develop at the apices of non-vital teeth, while so-called fissural bone cysts develop only in the midline. The site or anatomical position of a lesion may therefore provide the initial clue as to its identity.

Relative radiodensity and internal structure

The radiodensity of the lesion should be assessed relative to the surrounding tissues. Radiolucent lesions are only evident within otherwise hard (radiopaque) tissues as a result of a decrease in mineralization, resorption of mineralized tissue or a decrease in thickness. As stated in Chapter 2, the amount of photoelectric absorption and hence opaque/whiteness of a structure is ∝ Z³. The atomic number (Z) of bone is approximately 12 (Z³ = 1728), whereas the atomic number of soft tissue is approximately 7 (Z³ = 343). A soft-tissue lesion replacing bone will therefore appear radiolucent.

Radiopaque lesions are evident in bone because of an increase in mineralization, increase in thickness, or superimposition on some other structure. They are also evident following calcification in soft tissues (see Ch. 27), having replaced air in the maxillary antra (see Ch. 28) or as a result of an increase in thickness of either normal or abnormal soft tissue.

Thus, relative to the surrounding tissues, the radiodensity of the lesion could be:

• Uniformly radiolucent

• Radiolucent with patchy opacities within (mixed)

• Radiopaque.

The relative radiodensity also helps in the differentiating process by highlighting the internal structure of partially or completely radiopaque lesions. The alternatives include:

• Fine bone trabeculae, e.g. ground glass appearance

• Thick, coarse trabeculae with enlarged trabecular spaces, e.g. honeycomb appearance

• Haphazard sclerotic bone, e.g. cottonwool patches

• Homogeneous dense cortical bone

• Discrete bony septa, which could be:

– thin or coarse

– straight or curved

– prominent or faint

• Cementum – oval or round amorphous calcification

• Identifiable dental tissue – enamel and/or dentine

• No specific pattern.

Effects on adjacent surrounding structures

The following structures need to be checked (see Fig. 25.4):

Fig. 25.4 Diagrams showing the radiographic appearance of some of the typical effects that a lesion can have on adjacent structures. A Expansion, displacement of ID canal, tooth and bone resorption. B Tooth displacement.

The teeth

There may be evidence of:

• Resorption, which is a feature of long-standing, benign but locally aggressive lesions, chronic inflammatory lesions, and malignancy

• Displacement

• Delayed eruption

• Disrupted development, resulting in abnormal shape and/or density

• Loss of associated lamina dura

• Increase in the width of the periodontal ligament space

• Alteration in the size of the pulp chamber

• Hypercementosis.

Surrounding bone

There may be evidence of:

• Expansion:

– Buccal

– Lingual

– In other directions

• Displacement or involvement of surrounding structures, including the:

– Cortex of the inferior dental canal

– Mental foramen

– Lower border cortex of the mandible

– Floor of the antrum

– Floor of the nasal cavity

– Orbits

• Ragged destruction

• Increased density (sclerosis)

• Subperiosteal new bone formation

• An increase in the normal width of the inferior dental canal

• Irregular bone remodelling, resulting in an abnormal shape or unusual overall bone pattern.

Surrounding soft tissues

There may be evidence of invasion of the soft tissues producing a soft tissue mass.

Knowing what effects on adjacent surrounding structures a lesion is having provides information about the nature of the lesion and its mode of growth. For example, the more dangerous the lesion, the more damaging and destructive its effects, and the faster its growth. Some lesions grow and expand in particular ways, such as the odontogenic keratocyst (keratocystic odontogenic tumour) which tends to infiltrate the cancellous bone and grow along the body of the mandible and produces little buccal or lingual expansion, whilst an ameloblastoma in the same site tends to expand and infiltrate in all directions.

Time present

The length of time the lesion has been present – days, weeks or years – should be determined, if possible. Unfortunately, this information is not always available. The patient may not be aware of the lesion and there may be no records of when it first became evident. If sequential radiographs are available, they can be very useful in providing information on the speed of growth, or development, of a lesion (see Fig. 25.5). This in turn may provide a clue about the nature of the lesion because slow-growing lesions tend to be benign, whilst fast-growing, aggressive lesions are usually malignant.

Fig. 25.5 Examples of two similar radiographs taken 5 years apart showing the increase in size of the cystic lesion over the years.

Footnote

This methodical, systematic description of the features of a lesion forms the basis of the step-by-step differential diagnostic process and is used throughout Chapters 26 and 27. As is shown later, several of the same features are shared by different conditions. Thus, no one feature alone gives enough information to provide the diagnosis. All the features should be considered carefully to determine their relevance and importance.

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