PERIAPICAL CYST / RADICULAR CYST

The periapical cyst (also termed radicular cyst, and to a lesser extent dental cyst) is the most common odontogenic cyst. It is caused by pulpal necrosis secondary to dental caries or trauma. The cyst lining is derived from the cell rests of Malassez. Usually, the periapical cyst is asymptomatic, but a secondary infection can cause pain. On radiographs, it appears a radiolucency (dark area) around the apex of a tooth's root.

Radicular cyst is the most common odontogenic cystic lesion of inflammatory origin. It is also known as periapical cyst, apical periodontal cyst, root end cyst or dental cyst. It arises from epithelial residues in periodontal ligament as a result of inflammation. The inflammation usually follows death of dental pulp. Radicular cysts are found at root apices of involved teeth. These cysts may persists even after extraction of offending tooth; such cysts are called residual cysts.

Synonyms Periapical cyst, apical periodontal cyst, or dental cyst, radicular cyst , inflammatory odontogenic cyst 

Classification

It is classified as follows:
1) Periapical Cyst: These are the radicular cysts which are present at root apex.
2) Lateral Radicular Cyst:- These are the radicular cysts which are present at the opening of lateral accessory root canals of offending tooth.
3) Residual Cyst:- These are the radicular cysts which remains even after extraction of offending tooth.

Causes :
Dental cysts are usually caused due to root infection involving the tooth affected greatly by carious decay . The resulting pulpal necrosis causes release of toxins at the apex of the tooth leading to periapical inflammation. This inflammation leads to the formation of reactive inflammatory (scar) tissue called periapical granuloma further necrosis and damage stimulates the Malassez epithelial rests, which are found in the periodontal ligament, resulting in the formation of a cyst that may be infected or sterile (The epithelium undergoes necrosis and the granuloma becomes a cyst). These lesions can grow into large lesions because they apply pressure over the bone causing resorption . The toxins released by the breakdown of granulation tissue is one of the common causes of bone resorption.

Clinical Features :

• Usually small Symptomless  
• discovered  during routine radiographic examination. 
• As they   enlarge   may cause expansion of alveolar bone. 
• Pain  is seldom a feature unless there is acute exacerbation 
• occurs at any age
• usually rare in deciduous teeth. 
• May be related to any tooth in the arch
• The radicular cyst develops within periapical granuloma
• The inflammatory process within the granuloma stimulate the epithelial rest of  Malassez to proliferate  forming the cyst lining

Proposed mechanisms :

1. Degeneration and death of centeral cells within proliferating mass of epithelium, since the epithelium is vascular.

2. Proliferation of the epithelium forming arcades( Trapping of granulation tissues causing their necrosis)

3. Epithelization of an abscess cavity

Cyst expansion and bone resorption:
Radicular and many other cysts expand in a balloon like fashion, it expands in all directions.This is governed by the rate of local bone resorbtion.

Cyst  expansion:

• The cyst lumen contains protein (mainly derived from the plasma) & immunoglobulin.Cyst wall acts as a semipermeable membrane, as a result osmotically – active molecules are retained in Cyst  lumen.
• This result in the movement of fluid from the tissues into the lumen, increasing the hydrostatic pressure within the cyst causing it to expand in a unicentric ballooning pattern.  

Histopathology:

• Radicular cysts are lined wholly or in a part by non-keratinized stratified squamous epithelium, supported by a fibrous tissue capsule.
• In newly formed cysts, the epithelial lining is irregular and may vary in thickness i.e. hyperplastic. The fibrous capsule is richly vascular and diffusely infiltrated by inflammatory cells.
• In established cysts the epithelial lining is more regular in appearance and of fairly uniform thickness and the capsule tends to become more fibrous and less vascular with less inflammatory cells.
• Rushton bodies:Eosinophilic bodies,curved in shape mineralization of these bodies isassosiated with basophilia. They maybe haematogenous or represent sometype of epithelialproduct.
• Deposits of cholesterol crystals are common in the capsule of many radicular cysts. In histological sections cholestrol clefts are seen associated with giant cells. Due to discontinuity in the epithelium the cholesterol crystals are found in the cyst lumen.

Cyst contents:

• Varies from watery, straw colored fluid to semi-sol i d brownish material of paste – like consistency .
• Break down products of degenerating epithelial and inflammatory cells, and connective tissue components.
• Serum proteins : produced by plasma cells due to inflammation. (immunoglobulines).
• Water and electrolytes.
• Cholesterol crystals

Residual Radicular cyst:

It is a radicular cyst which persists after extraction of the causative tooth.They are a common cause of swelling of edentulous jaw in older age.May interfere with the fit of dentures.The lining is of thinner epithelium and shows mild degree of inflammation.

Lateral inflammatory cyst:

Forms on the side of non vital tooth as a result of the opening of a lateral branch of the root canal. Must be distinguished from the lateral developmental periodontal cyst.

Paradental cyst:

Paradental cysts occasionally result from inflammation around partially erupted teeth, particularly mandibular 3 rd molars. Usually between the ages of 20-25 years. The affected tooth is vital, but shows pericoronitis. Histologicaliy, the cyst shows the same lining of a radicular cyst but more intense inflammatory infiltrate is seen in the wall. 

Treatment of cysts:

• Enculeation : Is usually very effective.The affected (dead) tooth may be extracted or root filled  and preserved.
• Masrsupialization : Is for exceptionally large cysts where fracture is a risk.