Discussion - References.
The present sample of adults had a 7.6%prevalence of IO within the jaws with no sex predilection and a mean age of the patients with IO that did not differ from the rest of the sample. Owing to the small size of our patient sample, we should be cautious while interpreting the results and drawing conclusions. The prevalence of IO has, however, been reported in a number of studies (Farman et al. 1978, Geist & Katz 1990, Kawai et al.1992, Petrikowski & Peters 1997,Yonetsu et al. 1997,Williams & Brooks 1998, MacDonald-Jankowski 1999) and our findings are in agreement with their main observations. Investigations on adult patients have generally shown a prevalence in the range of 3-8%, no sex differences have been found, and the majority of the lesions have been observed in the mandibular premolar and molar areas.
Knowledge of the prevalence and characteristics of IO is mainly based on the cross-sectional studies. Obviously, longitudinal studies may provide valuable information concerning origin and development of lesions, and all our patients were followed for at least10 years. In addition, a subsample of 130 individuals were followed for 20 years or more; only two earlier longitudinal studies have been published (Petrikowski & Peters 1997, Williams & Brooks1998).
Petrikowski & Peters (1997) examined a population of 2991 orthodontic patients, both children and young adults (5-35 years). Few patients were 25 years or older, and the mean age of the group was 14.0 years. All had a pretreatment panoramic radiograph and a follow-up panoramic radiograph taken1-9 years later. The earliest age at which an IO was detected was 9.4 years. The proportions of patients with IO in the age groups above 10 years were similar. In total, re-examination of the patients over varying time periods showed that 40% of the IO lesions increased in size, whilst 45%remained static. In addition to the lesions diminishing in size, three cases demonstrated complete regression. The authors commented that this phenomenon had not been described earlier. Their conclusion was that IO develops during early adolescence and that the lesions are somewhat labile, especially during adolescence.
Williams & Brooks (1998) examined complete series of intraoral radiographs of 1585 adults; their mean age was 44 years and the mean follow-up period was 10.4 years. They found only small changes during the observation period and proposed that in cases where the diagnosis was certain, no action or further follow-up was indicated.
Moreover, the cross-sectional study of Yonetsu et al. (1997) on the prevalence of IO includes some CT examinations, providing detailed information about the location of the lesions. Eleven patients, all with lesions in the mandible, out of the 64 patients with IO had a CT examination performed for some other reason. Five lesions appeared as thickening of cortical bone and were classified as enostoses. Six lesions were located within medullary bone and were classified as central sclerosis. Four of these six presented as homogeneous, rather dense radiopacities within medullary bone, whilst two had a heterogeneous appearance with varying density. Because of the characteristics and the locations, the authors concluded that IO represented normal variants of bone, being developmental rather than reactive.
With in the jaws or close to them, there are other skeletal structures also with varying frequency of occurrence and varying features. Torus mandibularis and torus palatinus are relatively common structures and two studies on Scandinavian populations (Eggen & Natvig 1991, Haugen 1992) indicated that they present themselves in late adolescence or early adult hood. There after, they are relatively stable, even though cross-sectional studies have found somewhat varying prevalence in different age groups. Exostoses, i.e. protuberances of bone located in various areas of the jaws, represent a similar entity. Jainkittivong & Langlais (2000) performed a cross-sectional study on their prevalence and found that the occurrence was moderately related to increasing age and that the13-19-year group displayed a low prevalence. More or less extensive ossifications within the stylohyoid ligament are often observed in panoramic radiographs just posterior to the mandibular ramus. According to a longitudinal study by Omnell et al. (1998) on orthodontic patients, the majority of these ossification sites are established during adolescence. A few were recorded between the ages of 20 and 30 years.
Thus, it appears that several skeletal structures with varying frequency of occurrence are established during late adolescence and early adult hood. As for their period of development, it is similar to that of IO.T his might indicate a common causative factor, most probably genetic, and that the alterations might be considered anatomical variants.
It has been maintained (Gibilisco 1985) that with IO most frequently found located in the premolar and molar areas, they might represent residual roots from deciduous molars, resorbed and replaced by sclerotic bone. The longitudinal study on young patients by Petrikowski & Peters (1997) gave, however, no indication of root remnants being anaetiological factor. Histological examination might provide information about possible dentine remnants. But because there is no clinical indication for removal of these lesions, and because biopsy on merely scientific indications would be problematic for ethical reasons, such data are sparse. One investigation, however, correlated radiological and histological findings of different radiopaque conditions within the jaws and included seven cases of sclerotic bone, defined as a circumscribed bulk of compact bone situated where the bone is normally cancellous (Henrikson et al.1963). In one case, the histological examination clearly demonstrated sclerotic bone containing a retained root.
In our study, residual roots, whose periodontal structures disappeared and later presented themselves as IO, were observed in three patients. This indicates that though the majority of the cases diagnosed as IO cannot be explained by this factor, the phenomenon may be occasionally observed.
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