Discussion

Many studies and theories have been developed in recent years in order to explain the cellular and molecular mechanism of cholesteatoma-related bone destruction in otitis media. However, none of these theories alone is sufficient to explain the mechanism of this destruction. It is very important to carry out these studies in order to both illuminate the destruction mechanism and to support the development of non-surgical treatments for cholesteatoma.^{Reference Hamed, Nakata, Sayed, Ueda, Badawy and Nishimura3} This study, to the best of our knowledge, is the first study in the literature that evaluates the local and systemic levels of tartrate-resistant acid phosphatase 5b, which is an enzyme specific for osteoclastic activity in cholesteatoma-related bone destruction.

Although there is no consensus in the literature, the theory that suggests cholesteatoma-related bone destruction is caused by increased osteoclastic activity is one of the most commonly discussed and widely acclaimed hypotheses. The study by Jeong et al. reported that the level of receptor activator of nuclear factor kappa B ligand, which is a molecule that stimulates osteoclastic activity in fibroblasts, was increased in cholesteatoma perimatrix.^{Reference Jeong, Park, Tae, Lee, Shin and Kim5} Similar to this finding, Imai et al. reported increased receptor activator of nuclear factor kappa B ligand protein in fibroblasts in the cholesteatoma perimatrix and increased number of osteoclasts in the bone tissue adjacent to the cholesteatoma tissue.^{Reference Imai, Sato, Iwamoto, Hanada, Terao and Ohta6} Additionally, Hamzei et al. reported an increase in the level of molecules that help activation of osteoclasts, such as receptor activator of nuclear factor kappa B ligand and osteoprotegerin; however, no increase in osteoclast number was observed.^{Reference Hamzei, Ventriglia, Hagnia, Antonopolous, Bernal-Sprekelsen and Dazert16} On the other hand, there are several studies that report no increased osteoclastic activity in cholesteatoma. For instance, Koizumi et al.^{Reference Koizumi, Suzuki, Ikezaki, Ohbuchi, Hashida and Sakai7} first reported that there was no increase in molecules that activate osteoclasts, such as receptor activator of nuclear factor kappa B ligand and osteoprotegerin in bone tissues with or without cholesteatoma and later showed that osteoclast density was decreased in cholesteatoma patients.^{Reference Koizumi, Suzuki, Kawaguchi, Hashida, Hohchi and Ohkubo8}

In a previous study we conducted, we found that the activity of both bone formation and destruction enzymes were decreased in cholesteatoma-related bone destruction and concluded that osteoclastic activity might have been decreased in cholesteatoma.^{Reference Yemis, Ozgur, Basbulut, Ozdemir, Akgul and Mehel9} However, in the aforementioned study, we had not studied any enzyme that can specifically demonstrate osteoclastic activity. In the current study, we show that the level of tartrate-resistant acid phosphatase 5b, which is an enzyme specific for osteoclastic activity, is decreased locally. This new finding provides supporting evidence to our previous study and suggests that osteoclastic activity decreases in cholesteatoma-related destruction.

Tartrate-resistant acid phosphatase 5b is a reliable marker that has been used for many years to show the presence of bone metastases in certain types of cancer, such as breast cancer and prostate cancer.^{Reference Lyubimova, Pashkov, Tyulyandin, Gol'dberg and Kushlinskii12,Reference Capeller, Caffier, Sutterlin and Dietl17,Reference Tauchert, di Liberto, Cordes, Thill, Salehin and Friedrich18} Since the enzyme is specific to osteoclastic activity, its level increases in many metabolic events that result in bone destruction. An experimental study by Rissanen et al. showed that the level of tartrate-resistant acid phosphatase 5b is a reliable marker for osteoclast number.^{Reference Rissanen, Suominen, Peng and Halleen11} Another study indicated that there is a direct correlation between the level of tartrate-resistant acid phosphatase 5b and the number of osteoclasts. The same study also showed that the level of tartrate-resistant acid phosphatase 5b, as well as the number and volume of osteoclasts, were increased in cell cultures stimulated with receptor activator of nuclear factor kappa B ligand.^{Reference Lv, Wang, Xu, Tao, Lv and Wang14} These findings suggest that tartrate-resistant acid phosphatase 5b, which we used in our study to demonstrate osteoclastic activity in cholesteatoma-related bone destruction, is a reliable marker. In the current study, no significant difference in systemic tartrate-resistant acid phosphatase 5b levels between the two groups were detected. However, local tartrate-resistant acid phosphatase 5b level was found to be decreased in the bone destruction group. We believe this might be due to the area of destruction being rather small.

The data obtained from our current and previous study suggest that osteoclastic activity is decreased locally in cholesteatoma-related bone destruction. In 1998, Chole showed that the number of osteoclasts decreased in cholesteatoma-related bone destruction and suggested that this reduction might be caused by the process of bone destruction being chronic.^{Reference Chole19} There are many studies in the literature with different results that support this idea. For this reason, we think the data that we gathered in the current study suggest a decrease in osteoclastic activity; however, this decrease might not be effective in bone destruction.

Although the data we obtained in our study provides important information about osteoclastic activity in cholesteatoma-related bone destruction, the lack of association between clinical parameters, such as the degree and duration of destruction, presence of ear discharge, the severity of hearing loss, and enzyme levels is a prominent limitation to the current study. Besides this, we measured the enzyme levels in the inflamed mucosa of chronic otitis patients without bone destruction as a control, which can be considered limiting for the effectiveness of our study. However, studies investigating the mechanism of cholesteatoma-related bone destruction generally compare cholesteatoma tissue with skin tissue taken from behind the ear because of their histological similarity.^{Reference Jeong, Park, Tae, Lee, Shin and Kim5,Reference Imai, Sato, Iwamoto, Hanada, Terao and Ohta6,Reference Xie, Pan, Yin, Ren and Liu20} In such studies, molecules that stimulate osteoclasts, such as receptor activator of nuclear factor kappa B ligand and osteoprotegerin, were evaluated, and the level of these molecules were shown to be higher in cholesteatoma compared with skin. However, it is known that these molecules that take an active role in bone turnover are secreted locally and then affect the neighbouring areas. Furthermore, the bone turnover cycle is affected by inflammation.^{Reference Walsh and Choi21} For this reason, we propose that the inflamed middle-ear mucosa is more suitable as a control tissue for comparison with cholesteatoma tissue, since this tissue is similar in its inflammatory properties to the cholesteatoma tissue and its closer proximity to the bone tissue than the retroauricular skin tissue is more relevant with regard to bone turnover enzymes.