Shear's Cysts of the Oral and Maxillofacial Regions. Paul M. Speight

features and is now recognised as an entity – the orthokeratinised odontogenic cyst. Other variants, however, such as the bay or pocket variant of the radicular cyst, have little bearing on clinical management and are of academic interest only. There is a tendency for classifications to be overcomplicated by pathologists, who often describe subtle histological variations as new ‘entities’. The most useful classifications should be simple, should use globally recognised terminology, and should be easy to use and relevant for clinicians who treat the lesions. A straightforward uniform nomenclature facilitates communication between clinical specialties and enables precise reporting of lesions for statistical and research purposes.

International standards for classifications were first developed by the World Health Organization (WHO), which set up a global project for the histological classification of tumours in 1952 (reviewed by Sobin 1971 ). The project involved collecting samples of lesions that were reviewed by groups of international experts, who agreed a uniform nomenclature and established practical and clinically relevant diagnostic criteria. The first classification of cysts of the jaws was published in 1971 in Histological Typing of Odontogenic Tumours, Jaw Cysts and Allied Lesions (Pindborg and Kramer 1971 ). This first edition was deliberately inclusive and provided a comprehensive classification of jaw lesions so that all neoplasms and cysts of the odontogenic tissues could be considered in context, allowing pathologists and clinicians to make an informed diagnosis. The second edition, published in 1992, also included cysts of the jaws (Kramer et al. 1992 ), but the third edition (Barnes et al. 2005 ) omitted cysts and restricted the classification to tumours and a range of ‘tumour‐like’ lesions. Subsequently, the fourth (El‐Naggar et al. 2017 ) and fifth editions (WHO 2022a , b) have included the odontogenic cysts and have restored the status of the book as a complete classification of lesions of the odontogenic tissues.

      It is important to note that the WHO classifications still maintain the original principles of simplicity, relevance, and a uniform and well‐recognised terminology. Thus the WHO books should be regarded as a guide to terminology, definitions, and diagnostic criteria. They are not comprehensive and do not include detailed considerations of variants, unusual features, or differential diagnosis. In this book we have adopted a simple working classification of the odontogenic cysts using the WHO terminology. We do not include detailed subdivisions or variants in the classifications, but in each chapter we discuss terminology and classification, and include details of variants of each lesion where this might have a bearing on diagnosis or management.

      A further consideration is the use of the term ‘cyst’, which still causes disagreement and some controversy. In pathology dictionaries and general pathology textbooks, a cyst is usually defined as a closed capsule, cavity, or sac‐like structure that may be empty or have fluid or semi‐fluid contents. There is no requirement for a defined lining. Most contemporary oral pathology textbooks, however, choose to define a cyst as a pathological cavity lined by epithelium. Cyst‐like spaces not lined by epithelium have been described as pseudocysts and diagnostic terms such as ‘cavity’ have been used. Kramer (1974 ) defined a cyst as ‘a pathological cavity having fluid, semifluid or gaseous contents and which is not created by the accumulation of pus’. He discussed the terminology and was bemused by the requirement for an epithelial lining, stating that he was puzzled by ‘the definition that demands an epithelial lining’ and said: ‘I am not sure how, or why, the presence of an epithelial lining became included in the definition: clearly it creates difficulties, because so many lesions that have been accepted for generations as “legitimate” cysts must now be termed pseudocysts, or false cysts, or “cysts” in quotation marks’.

      The requirement for an epithelial lining probably became enshrined in oral pathology because the vast majority of cysts of the oral and maxillofacial tissues are odontogenic in origin and are lined by epithelium. Thus, the use of alternative terms such as ‘pseudocyst’ or ‘cavity’ clearly distinguishes cystic lesions without epithelium from odontogenic cysts. Although we recognise the term pseudocyst for lesions that are not lined by epithelium (see Chapter 17), we also agree with Kramer and suggest that ‘cyst’ can be used as a diagnostic term for lesions that are clinically or radiologically cystic. This is in keeping with common usage and understanding among clinicians and radiologists. For example, most clinicians, especially paediatric dentists, use the term cyst to describe mucoceles, whether they have an epithelial lining or not, and we therefore retain the diagnostic term mucous extravasation cyst for the non‐epithelial lined cystic lesion caused by spillage of mucus into the tissues (see Chapter 15). Similarly, we are content to use the well‐recognised diagnostic term simple bone cyst for the non‐epithelial lined bone cavity that clinically and radiologically presents as a cystic lesion (Chapter 17). It should be noted that the definition of cyst does not include a cavity caused by an accumulation of pus, which is defined as an abscess.

      In this book we do not attempt to suggest a definitive classification of cysts of the oral and maxillofacial regions, but we divide them into cysts of the jaws, cysts of the salivary and minor mucous glands, and developmental cysts of the head and neck. We discuss cysts that are specific to the maxillofacial regions, and cysts that are not peculiar to these regions are not included unless they have distinctive features that must be considered in the differential diagnosis. Cystic neoplasms such as unicystic ameloblastoma are not included either, although the possibility of a neoplastic origin of some of the odontogenic cysts is considered.

      Our classification is intended to be simple and to be particularly useful to pathologists and clinicians who must share and understand definitions and terminology. Many other classifications have been published and may well be perfectly satisfactory. Although readers may use any classification they find valuable as an aid to memory and understanding, they are encouraged to facilitate communication by using the WHO terminology and definitions (WHO 2022a , b).

Classifications

      In this edition of the book, the cysts are classified under three broad categories:

      1 Cysts of the jawsOdontogenic cysts (Chapters 3–12)Non‐odontogenic cysts (Chapters 13, 14, 16, and 17)

      2 Cysts of the salivary and minor mucous glands (Chapter 15)

      3 Developmental cysts of the head and neck (Chapter 18)

Cysts of the Jaws

      Odontogenic Cysts

The odontogenic cysts have been divided into cysts of inflammatory origin and cysts of developmental origin. These are convenient categories, since it is clearly understood that the inflammatory odontogenic cysts arise as a result of proliferation of odontogenic epithelium driven by chronic inflammation, resulting from either pulpitis or pericoronitis. The pathogenesis of the developmental cysts is less well understood, however, and in some cases there is evidence for a neoplastic origin. The pathogenic mechanisms involved in cyst development are discussed in Chapter 2, and details for each cyst type are presented in each chapter. Although these categories are widely used, they are not definitive, since some cysts classified as developmental may have an inflammatory origin. In particular, a variant of dentigerous cyst may be inflammatory in nature (see Chapter 5). In each category the cysts are listed in order of their approximate frequency (