Introduction
Periodontal Diseases
Plaque-induced periodontal diseases have traditionally been divided into two general
categories based on whether attachment loss has occurred: gingivitis and periodontitis.
Gingivitis is the presence of gingival inflammation without loss of connective tissue
attachment whereas in periodontitis there is or has been pathological detachment of collagen
fibres from cementum and apical migration of the junctional epithelium (Anon, 2003).
Gingival and periodontal inflammation represents the host response to bacterial plaque,
mediated by the environment in which the response occurs (Genco, 1990). The supporting
tissues of the teeth, including the periodontal ligament and alveolar bone, are destroyed in
periodontitis and the end point is invariably the same, tooth loss. In the local lesion of
periodontitis, as tissue damage progresses, gaps are formed between the gum and the root
surfaces of the teeth referred to as periodontal pockets. In severe disease these periodontal
pockets are several mm deep and are lined by ulcerated epithelium covering inflamed
connective tissue. In periodontitis gingival crevicular fluid (GCF) flows from the gingival
microcirculation into the periodontal pockets and the volume increases in proportion to the
severity of the local inflammatory process.Factors controlling the progression of gingivitis to
periodontitis remain to be fully elucidated and as a result key questions about the
pathogenesis of the disease remain unanswered. Identification of a biomarker to characterise
the transition between gingivitis and periodontitis would be a major discovery in terms of (1)
identifying gingivitis cases at risk of progressing to periodontitis, (2) monitoring periodontal
disease progression and (3) supporting appropriate therapeutic intervention.
Biomarkers
Biomarkers of disease should be objective measurements that act as indicators of normal
biological processes, pathogenic processes or pharmacological responses to therapeutic
intervention. There are at least three important technical attributes of a good biomarker: (1)
presence in a peripheral body fluid (e.g., blood, saliva, GCF); (2) ease of detection or
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quantification in robust and affordable assays; and (3) associated specifically with damage of
a particular tissue, preferably in a quantifiable manner. Properties of the diagnostic test in
which biomarkers are employed such as specificity and sensitivity are helpful in evaluating its
usefulness in clinical practice. Very simply the relationship between the result of the test and
the clinical diagnosis can be presented in a two by two table (Figure 1). The ideal test should
have a sensitivity and specificity of 1, however no such test is currently available in medicine
or dentistry. The positive predictive value is also important as it reflects the probability that a
positive test reflects the underlying condition, however its value depends on the prevalence of
the disease which may vary. The negative predictive value on the other hand is the portion of
patients with negative test results who are correctly diagnosed.
Improving on Current Diagnostic Markers
Novel biomarkers of disease are generally measured against a ‗gold standard‘. No such
gold standard exists for periodontal diseases as traditional biomarkers like bleeding on
probing (BOP) are associated with many false positives and are not readily quantifiable.
However, the absence of BOP is generally considered as an accurate negative predictor of
disease (Lang et al., 1990). In the search for novel biomarkers comparisons must be made
with current measures of disease. Conventional periodontal probes are used routinely to
obtain both probing depth (PD; distance from the gingival margin to the base of the probable
crevice) and clinical attachment loss (CAL; distance from the cementoenamel junction to the
base of the probable crevice). PDs can be recorded rapidly and provide an overall assessment
of the depth of periodontal pockets whereas CAL assessments are more difficult to measure
accurately, but give a better overall estimate of the amount of damage to the periodontium.
The crudeness of measurements to the nearest mm to assess attachment loss means that the
extent of disease is not determined by a reliable measure and thus disease progression may
occur, but not to a large enough extent to be measurable routinely. Electronic probeshave
better resolution but are not in routine clinical use (Jeffcoat et al., 1986).
Figure 1. Decision matrix for diagnostic and prognostic tests.
The threshold for attachment loss must therefore be considered carefully if it is to be used
as a gold standard for biomarker validation. Studies measuring 1 mm, 2 mm or 3 mm
attachment losses over a defined period of time have been reported and those studies with
large thresholds have a lower likelihood of false positives (and a higher likelihood of false
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negatives). Disagreement between the attachment loss measured and the biomarker may
therefore be a true reflection of better positive prediction by the latter. The majority of studies
choose a CAL of 2 mm, with radiographic bone loss employed as a complementary method to
diagnose disease progression. Despite inherent difficulties in defining the effectiveness of a
novel biomarker against crude clinical measurements of disease there are currently few
alternatives.
Biomarkers in GCF or Saliva?
The study of GCF samples, from defined sites of chronic periodontal inflammation,
allows non-invasive access to an inflammatory exudate that can be used to improve our
understanding of the inflammatory process. The protein concentration of GCF at
periodontally healthy sites (22 mg/ml) is similar to that of physiological extravascular fluids
(Curtis et al., 1988). By contrast, GCF from inflamed sites has a protein concentration in the
region of 70 mg/ml and is considered to be a classical inflammatory exudate (Bickel et al.,
1985). The cellular components of GCF are 70-80% granulocytes, 10-20% monocytes/
macrophages, 5% mast cells and 5% T lymphocytes. The fluid exudate contains proteins
synthesised and secreted in the inflamed gingiva and carried by the GCF to the gingival
crevice/pocket (Lamster, 1997). Here, they are augmented by proteins released from bacteria
and host cells, particularly polymorphonuclear leukocytes (PMNs), present in the periodontal
pocket. The constituents of GCF are therefore derived from a number of sources including
microbial plaque, host inflammatory cells, serum and tissue breakdown (Curtis et al., 1989).
The rationale behind the analysis of crevicular fluid is that inflammatory mediators present in
GCF will reflect events taking place both within the connective tissue and in the gingival
crevice/pocket. Despite the minute quantities of fluid that can be recovered from single
gingival crevices or periodontal pockets (Linden et al., 1997), the majority of investigations
on biomarkers of periodontitishave focused on GCF.
In periodontal disease individual sites undergo periods of active tissue destruction
followed by periods of quiescence or repair. The site specific nature of the disease therefore
points to the use of individual GCF samples for analysis since it is only by sampling at
individual sites that a site specific prognosis can be evaluated. Specific markers of disease
activity are more likely to be present at higher concentration in the micro-environment of the
gingival crevice. However the collection of GCF, although non-invasive, requires training and
expertise, and the technique should be perfected so as not to allow GCF sample
contamination with saliva or blood. As a result of some of the inherent difficulties with
sampling and analysis of minute quantities of GCF, researchers have looked to saliva as an
alternative oral fluid for biomarker discovery. In some ways this can be regarded as an
advantage as salivary analytes may offer a way of assessing subject-level (as opposed to site-
level) risk or status. Furthermore, collection of whole saliva is easy, non-invasive and rapid,
requiring no special equipment or expertise.There are, however, additional challenges in
using saliva for diagnostic purposes. Whole or mixed saliva is a complex fluid mixture
derived from the major and minor salivary glands, containing contributions from GCF, oral
bacteria, cells and other sources that make identification of the exact site of disease activity
difficult.A specific biomarker of periodontal disease is perhaps more likely to be present in
higher levels in GCF and may not be measurable robustly in saliva. Furthermore, heightened
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97
periodontal disease activity in a limited number of sites could be underestimated by a dilution
effect in saliva. In addition, salivary flow rate varies between individuals or as a result of
medications and could affect biomarker concentration in saliva.
Biomarkers Associated with Periodontal Disease Pathogenesis
Periodontal diseases reflect a spectrum of disease activity from mild gingivitis through to
chronic destructive periodontitis.Gingivitis is a reversible inflammatory condition and does
not result in the permanent destruction of the periodontal ligament or alveolar
bone.Inflammatory markers associated with gingivitis would therefore not predict reliably the
likelihood of a patient developing periodontal disease. Of the numerous constituents in GCF,
however, the vast majority constitute soft tissue inflammatory events, while only a few are
regarded as specific biomarkers of alveolar bone destruction. At present under-diagnosis of
periodontal disease results in low rates of appropriate therapy and a reliable biomarker would
help redress this balance in favour of improved prognosis. Novel biomarkers should correctly
identify current disease activity, predict sites vulnerable for future breakdown, and assess the
response to periodontal interventions, ultimately leading to improved clinical management of
periodontal patients. Since periodontitis is a multifactorial disease, measurement of multiple
biomarkers may be advantageous.
It has long been recognised that PMNs are the major cells of the acute inflammatory
response in mammalian tissue and the oral cavity is no exception (Wilton et al., 1976). Many
of the potential biomarkers studied in GCF are derived from PMNs and as such are early
markers of inflammation but may not be useful discriminators between gingival inflammation
and periodontal disease. Several biomarkers of disease are now discussed in more detail, in
order of the phase of the disease process in which they have a major role; initial
inflammation, extracellular matrix (ECM) breakdown and boneturnover/destruction (Figure
2). It must be pointed out that many of the putative biomarkers describedbelow are functional
in more than one phase of disease.
Figure 2. Simplified model of the progression of periodontal disease.
Biomarkers of Inflammation
During the initiation of an inflammatory response in the gingival and periodontal
connective tissue numerous inflammatory mediators, such as prostaglandin E2 (PGE
2
),
interleukin (IL)-1β, IL-6, or tumour necrosis factor (TNF)-αare released from cells of the
junctional epithelia, connective tissue fibroblasts, macrophages, and PMNs. Neuropeptides
are also released from nerve fibres innervating the gingival and periodontal tissues and
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represent an important component of the neurogenic inflammatory response (Lundy and
Linden, 2004). Subsequent to the initial inflammatory response T and B cells emerge at the
infection sites and secrete immunoglobulins (Kornman et al., 1997).
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