Its not uncommon for practitioners to refer to specialists or secondary care units for treatment of a ‘periapical cyst’ or ‘radicular cyst’. The only surefire way of diagnosing the presence of a cyst is through histology, although such lesions do share some common features.

Periapical cysts are the most common form of dental cyst. They form as a result of pulpal necrosis and subsequent bacterial infection which causes inflammation. This triggers proliferation of Hertwig’s Epithelial Root sheath-a fancy name for the leftover cells at the apex from the tooth’s embryological development.  The original drawing of these cells is shown below.


Once the epithelial rests cells start to proliferate a cyst forms and expands in a balloon-like manner, the shape and extent of growth is dependent on  local anatomy and structures.


The cyst fluid is a collection of inflammatory cells and exudate and proteins which exert osmotic pressure resulting in continued expansion. The cyst itself can release molecules such as prostaglandins which are bone resorbing factors which further progress the cysts growth.

cysts-part-1-9-728There are, broadly, two types of periapical cyst. One is the ‘True’ cyst (A). This is a separate entity to the root canal system and is uninterrupted and independent from the root canal apex. The second type is the ‘Bay’ cyst (B). This has a cyst lining which is intimately associated with the root canal system (G).


A=True Cyst B=Bay Cyst C=Granuloma

The main radiographic features of a periapical cyst are as follows

  • The epicentre is localised to the apex of a tooth or in some occasions associated with the lateral surface of the root where lateral canals may reside
  • The periphery is usually well defined with a corticated border. This can become less well defined if the cyst become infected
  • If the cyst is large it may cause resorption of adjacent roots or result in pressure on vital structures.
  • Shape is generally round or ovoid
  • Internally consistently radiolucent and unilocular
  • Associated with a ‘dead’ tooth

Some examples are shown on the below slideshow. CREDIT


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One new and fairly powerful tool that has become more mainstream in managing these lesions is the use of Cone Beam CT. This allows the clinician to fully visualise the lesion, be it cystic or not, and interpret the relative effects on the tooth and adjacent structures prior to treatment. In the video found below we have a very well circumscribed area closely associated with the lateral incisor that had been root canal treated some 5 years previously and continued to give symptoms. The anterior teeth had been traumatised due to an accident. What is very interesting in the scan is that there seems to be a radiolucent fragment in the centre of the lesion. At the surgical stage the presence of a bay cyst was confirmed with globular mass of sealer puff at the centre. This may have been the catalyst for the inflammation and subsequent cystic lesion. AA