**5. Influence of hormone replacement therapy on periodontal parameters in patients with osteoporosis**

Hormone replacement therapy (HRT) represents an attractive method to counterbalance hormonal changes. The aim of HRT is not only to avoid climacteric signs and symptoms but also to protect the patients from cardiovascular disease and osteoporosis complications [29]. Having in mind that oestrogen deficit is an important risk factor for osteoporosis, it is of high importance to consider the role of oestrogen in the association between periodontal lesions and osteoporosis [30].

We proposed a comparative assessment of periodontal status in postmenopausal patients who were on hormone replacement therapy or not. The study was performed on a group of 23 female subjects, diagnosed with osteoporosis, aged between 50 and 62 years [30]. Subjects were divided into two groups. The first group, the study group, included patients undergoing hormone replacement therapy (n = 13); the control group included patients who did not follow this therapy (n = 10). The patients underwent periodontal clinical examination.

We noticed that the risk of tooth loss was similar for both groups but this risk shows a slight form of decrease with increasing treatment duration. Regarding the bleeding on probing, its value was approximately twice higher in patients without hormone replacement therapy, compared to the control group. The diagnosis of periodontal disease was higher in patients who did not receive replacement therapy than in those with HRT. In the group without hormone replacement therapy, we noticed more severe periodontal attachment losses than in the study group, with HRT. Moreover, the level of clinical attachment was proportional to the duration of hormone replacement therapy [30].

An important effect of low levels of oestrogen is the decrease in the inhibition of osteoclatogenesis, with a consequent increase in the activity of osteoclasts [31]. The result is a decrease in bone mass and bone resistance.

In vitro experiments indicate that neutrophil chemotaxis is reduced in the presence of low concentrations of estradiol. On the other hand, progesterone increases the chemotaxis of neutrophils, so any change in the balance of these hormones in plasma or gingival tissue can have a significant effect on neutrophil function in vivo [32].

Until recently, hormone replacement therapy was considered the only effective treatment recommended for the prevention of diseases associated with oestrogen deficiency [30]. After the publication of the Women's Health Initiative results in 2002 and 2004 [33], the use of HRT has become a complex debated issue. Women's Health Initiative and other data from dedicated studies suggested that the potential risks associated with HRT (increased risk of breast cancer and severe cardiovascular disease) are in direct relationship with the regimen administered, dose, mode of

administration, age of the patient, associated disease, and duration of treatment [34]. Therefore, based on current data, the purpose, dose and regimen of HRT should be individualized, with a complex evaluation of each case [30].

Oestrogen has two key roles in bone health. First of all, the hormone is essential for the normal maturation of the bone and for ensuring the acquisition of minerals so as to reach an optimal bone mass. Second, oestrogen maintains bone mass throughout adulthood during remodelling processes. Oestrogen deficiency leads to a reduction in bone mass and damage to bone microarchitecture, the two being the main aspects of osteoporosis.

Oestrogen deficiency can cause bone loss by acting directly on bone cells that are involved in bone turnover and by decreasing the influence that oestrogen has on the intestines and kidneys that regulate extra-skeletal calcium levels.

Oestrogen deficiency contributes to the deterioration of bone microarchitecture and to the reduction of bone strength which is determined by bone geometry, cortical thickness, porosity, trabecular morphology. Bone remodelling results in the modification of major determinants of bone strength [35].

Oestrogen has been shown to stimulate osteoprotegerin (OPG) expression in human osteoblasts. The hormone can thus have an effect on bone metabolism through the surrounding soft tissue cells. Oestrogen has been shown to inhibit the formation of osteoclastic cells in cultures of fibroblasts in the periodontal ligaments and mononuclear cells in the peripheral blood. This inhibitory effect was not found in cultures with gingival fibroblasts. This observation suggests that they are not as sensitive to oestrogen without a clear biological mechanism. It is known that oestrogen exerts its effects through intercellular receptors; oestrogen receptor concentration is thus an important determinant of the cellular response to oestrogen.

The inhibitory effect of oestrogen on osteoclastic cell formation corresponds to previous research obtained either by osteoblast-induced osteoclast production or by bone marrow cells used as precursors. In addition, in vivo studies using ovariectomized mice have shown that oestrogen deficiency has induced the presence of a large number of osteoclasts in the periodontium. The data thus suggest that oestrogen plays an important role in modulating osteoclast formation.

Another important finding was the increased number of osteoclast-like cells in cultures with periodontal ligament fibroblasts. These observations suggest a difference in the interaction between different osteoblast populations and mononuclear cells in peripheral blood.

Subsequent analysis of molecules that may be involved in the oestrogen inhibitory effect involved the evaluation of mRNA expression of RANKL, OPG, and oestrogen receptors. No effect of oestrogen on the expression of these genes was found in any population of fibroblasts. The data suggest that the oestrogen inhibitory effect on osteoclastic cells can be mediated independently of OPG and RANKL, being mediated by other compounds such as TGF β, TNFα, IL-1, IL-6, IL-7 [30].

There are studies that have shown that the risk of edentulous ridges is lower in patients following HRT [36, 37]; it was observed that HRT reduced the risk of edentulousness by 6% for each year of HRT [38]. The patients with osteoporosis present a significant decrease in alveolar ridge height in edentulous areas and loss of alveolar ridge height is associated with osteoporosis and osteopenia [6]. We demonstrated a higher number of teeth present in patients with hormone replacement therapy [30]. However, teeth loss does not represent an ideal surrogate assessment for periodontal disease, as it can also be caused by carious lesions or traumatic events.
