Periapical lesions are divided into the following categories.
1. Lesions of endodontic origin caused by infection from root canal. Bacteria and their by-products, acting as antigens, may elicit nonspecific inflammatory responses as well as specific immunological reactions in the peri-radicular tissues, and cause periapical lesion.
2. Lesions of non-endodontic origin can be benign or malignant.
In this chapter periapical lesions of endodontic origin are discussed in detail; and only general information is given on lesions of non-endodontic origin.
Periapical lesions of endodontic origin are very variable in their clinical manifestations. There is only a limited correlation between the clinical diagnosis and histopathological status of periapical tissues. That is why clinical manifestations and diagnostic terminology are discussed in this chapter separately from histopathological features of periapical lesions.
ETIOLOGY OF PERIAPICAL LESIONS OF ENDODONTIC ORIGIN
► Infection: bacteria cause caries which results in pulp inflammation. If no treatment is provided, pulp necrosis develops, and bacteria, bacterial toxins or products of inflammation extend down the root canal and cause inflammation of periradicular tissues, which is apical periodontitis. The microorganisms found in these cases are predominantly gram-positive microorganisms (cocci, bacilli and filaments, such as Actinomyces, En-terococcus and Propionibacterium) and fungi, mainly Candida albicans. The microbiological factors and the host defense mechanism interact, destroying large amounts of periapical tissue, giving rise to different types of periapical lesions.
► Trauma; direct physical trauma to a tooth results in immediate interruption of blood supply leading to necrosis of the pulp and inflammation in