Odontogenic cysts: Comparing the growth of radicular cysts and keratocysts

A problem presented at the UK MMSG Nottingham 2002.

Presented by:
Dr Gabriel Landini (The School of Dentistry, University of Birmingham)
S Franks, G Landini, V Magar, J Ward

Problem Description

A cyst is a pathological cavity with fluid or semi-fluid contents which is not created by the accumulation of pus. Cysts are usually lined by epithelium (avascular). Odontogenic cyst have an epithelial lining which derives from the epithelial residues of the tooth-forming organ (glands of Serres, rest of Malassez, or the reduced enamel epithelium).

The Study Group is asked to test two hypotheses that have been postulated to describe the growth of radicular cysts and to predict whether such cysts grow indefinitely or to a limiting size. The first hypothesis is that inflammation mediators released following the necrosis of the tooth's pulp induces the proliferation and the epithelial rests. The second theory is that proliferation of the epithelial tissue (which is avascular) may isolate areas of granulation tissue by enclavement of proliferating strands of epithelium which eventually degenerate (liquefaction necrosis) and create a cavity with increased osmotic potential.

The Study Group is also asked to investigate the additional impact that active mitosis of the epithelial cells lining keratocysts has on their rate of growth and to compare their growth characteristics with those of radicular cysts.

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Study Group Report

In this report we discuss the pathogenesis of odontogenic cysts based on current knowledge and formulate a simple mathematical model of cystic growth. The aim of the modelling is to establish the dynamics (i.e. the long term behaviour) of cyst enlargement based on osmotic pressure differences between the cyst contents and the stomal extracellular fluid. Such osmotic gradient results in water movement into the cyst across the epithelium lining which acts as a semi-permeable membrane.

The modelling assumes a (spherical) cyst with a semi-permeable shell of living cells and a core consisting of wa- ter and a generic osmotic material (fed by the continuous death of epithelial cells in the shell). The lining cells are assumed to behave like a Maxwell fluid, reflecting the action of physical stresses by the surrounding cyst capsule formed by fibroblasts and collagen fibres. The model couples the cyst radius and the osmotic pressure differences resulting in a system of two nonlinear ordinary differential equations.

Using the combination of asymptotic and numerical techniques it is shown that in all parameter regimes the long time behaviour of the cyst is the same and that linear radial expansion is predicted. Moreover, the model predicts that in the early and intermediate stages of cystic growth, osmotic pressure differences play an important role; however, for very large cysts, this role is negligible as cell birth dominates growth.

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Follow-Up Activities

The following publications have been written as a result of this problem:

A mathematical model of the dynamics of odontogenic cyst growth
JP Ward, V Magar, SJ Franks & G Landini (2004)
Analytical and Quantitative Cytology and Histology 26, 39–46.