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Bony exostosis (BE) is described as an unknown aetiology peripheral localised benign bone overgrowth, with a base continuous to the original bone and which seems to have a nodular, flat or pedunculate protuberance1 located on the jawbone's alveolar surface.2 Different exostoses species can generally be found during the periodontal examination, eg mandibular tori, palatal tori, palatal alveolar exostoses and multiple exostoses. Mandibular tori were observed in more than a quarter of modern dry skulls and palatal alveolar exostoses in more than a half.3 Both BE species may be found more usually in young male dentate subjects, probably from alveolar bone origin.3 Multiple exostoses are found somewhat less usually than mandibular tori or palatal alveolar exostoses on the maxillary buccal surface below the mucobuccal fold in the molar region.4 Other types of BE have been found over the past years associated with unusual postoperative conditions. Buccal bony exostosis (BBE) development secondary to soft tissue graft procedures has been reported in a small number of cases, eg as a consequence of shallow vestibules increasing with the use of skin grafts5,6 or subsequent connective tissue graft.7 The periodontal literature describes BBE development also at sites where free gingival grafts (FGG) have been used to increase the amount of gingiva (Pack et al.8 two cases; Efeoglu and Demiel9 two cases; Czuszak et al.10 one case; Otero-Cogide et al.11 nine cases; and Echeverria et al.12 one case). (Table 1)

Table 1 Development of exostoses following free gingival grafts: case reports
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The main BBE causing agents are unclear, but all previous reports have been unanimous in suggesting that periosteal trauma seems to be associated with such exostoses development.5,6,7,8,9,10,11,12

The following case series describes the BBE development at nine sites (five cases) of previous FGG performed to increase the width of the attached gingiva. All surgeries were achieved by accidental or intentional periosteal fenestration of the receipt sites.

Case 1

In 1979, a healthy 50-year-old female patient had an FGG placed on the buccal level 44-45 area, to increase the attached gingiva width prior to a prosthetic treatment with a fixed cantilever partial denture. Five years later it was noticed that the graft was presenting a discreet progressive enlargement. In 1994, 15 years later, the patient requested correction of the area. The site was painless and very hard on palpation (Fig. 1) and had a dense radiographic appearance (Fig. 2).

Figure 1
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Pronounced exostosis at FGG site in case 1

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Figure 2
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Increased radiopacity at FGG site in case 1

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Full thickness flap elevation revealed the presence of a nodular osseous area. Collected bone was sent for a histological examination, revealing a very dense lamellar bone formation compatible to the exostosis diagnosis (Fig. 3). Healing occurred uneventfully. The patient was continuously observed until 2001.

Figure 3
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Histological examination from the FGG site in case 1

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During the surgical procedure, a small accidental incision occurred at the periosteum over the root surface (tooth 43) of the receipt site (Fig. 4). Re-entry procedures showed the root surface recovered by bone proceeding from the exostosis.

Figure 4
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Accidental periosteal fenestration at FGG site in case 1

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Case 2

In 1978 a 17-year-old male patient presented four aberrant frenums attached near to the gingival margin, buccally adjacent to teeth 15-14, 24-25, 34-43 and 44. Frenectomy technique was performed associated with free gingival grafts. Throughout the postoperative years a slow exostosis in the previously grafted sites was noticed (Figs 5,6,7,8). These areas had been presenting increased volume, hard density on palpation and also painless conditions; radiographic assessment revealed an increased radiopacity area related to teeth 24-25. The patient did not present toris mandibularis or toris palatinus. He was aware of these clinical findings but he did not wish to have them removed.

Figure 5
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Clinical appearance of bony exostosis at FGG site in case 2

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Figure 6
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Clinical appearance of bony exostosis at FGG site in case 2

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Figure 7
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Clinical appearance of bony exostosis at FGG site in case 2

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Figure 8
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Clinical appearance of bony exostosis at FGG site in case 2

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Case 3

In 1975 a 37-year-old female patient underwent two FGG involving the buccal surface of teeth 33-35 and 43 to increase the attached gingival width prior to prosthetic crowns fabrication and a partial removable denture with free-end denture bases. Throughout the following 26-year period a very slow and gradual increase in the area had been noticed, painless, hard on palpation — and radiograph assessment revealed increased radiopacity. The patient was satisfied with the results and did not wish to have them corrected.

Case 4

In 1986 a healthy 34-year-old female received an FGG at the buccal area of teeth 43 and 44 to increase the keratinised gingiva associated with frenectomy to remove a high frenum inserted close to the free gingival margin.

A linear fenestration was performed in the receipt site periosteum where the frenum was inserted. The clinical result was excellent, however the grafted area volume increased in the following years, presenting a larger hard consistent painless enlargement over the corresponding area. Since the patient was unconcerned, the BE was not removed.

Case 5

A 31-year-old woman was referred to our private dental office presenting with the lower lip frenum insertion close to the lower incisors' papillae, 31-41, and a narrow attached gingival zone. In 1987, frenectomy with linear periosteal fenestration was accomplished to remove the muscle insertions in association with an FGG. Healing followed uneventfully and a wide keratinised gingiva was obtained. The grafted area maintained a healthy aspect, but with a slow and progressive volume increased over the time. After 14 years, in 2001, in the exact area of the periosteum fenestration, an enlargement was clearly evident. Flap reflection showed quite a resistant compact bone formation which was partially removed. Histological examination confirmed the exostosis diagnosis.

Discussion

BBE development subsequent to free gingival grafts has been reported in a small number of cases,8,9,10,11,12 since 1991 when the first two cases were described.8

The authors suggest that patients presenting toris or any kind of BE are highly susceptible to bony overgrowth responses. Another four reports have been adding hypotheses and clinical characteristics to this uncommon osseous proliferation. Efeoglu and Demirel9 state that 'it is also possible that other clinicians might have assumed the thick gingival grafts they saw during their patients' postoperative visits were not thick soft tissue grafts, but were, in reality, exostoses.' Czuszak et al.10 suggested that this exostoses development may be coincidental and not due to FGG. Otero-Cagide et al.11 speculated that the bone formation after an FGG may be the result of a periosteal trauma combination during site preparation and the activation of osteoprecursor cells contained in the connective tissue of the graft. Echeverria et al.12 previously noticed that the total exostoses that have been related after an autogenous FGG were located in the cuspid-premolar area. They suggested that the grafted areas may be influenced by factors acting at this level, eg excessive forces, surgical trauma and genetic factors.12

Among the related reports, all the authors suggest that the periosteal trauma seemed to be the main aetiological agent associated with the exostosis development.5,6,7,8,9,10,11,12 In cases of skin grafts, the occurrence of periosteum fenestration after the graft suture position has also been observed. This surgical trauma can be associated with the liberation of osteoprogenitor cells from the periosteum-bone interface inducing osteogenesis.6 We are in agreement with this because at our nine reported sites this osseous formation has been verified after surgical procedures in which periosteal trauma occurred, eg periosteal fenestration. At the first presented case the root was exposed and the exostosis covered the fenestration.

Frenectomies were achieved through linear receipt site periosteum fenestrations,13,16 with high muscle inserts removal in association with autogenous FGG placed over the fenestrated areas, when frenum attachments were toward the marginal gingiva interfering in oral hygiene. The majority of exostoses have been found in these areas. It is possible that when we were trying to leave the receipt site free from elastic fibres and muscle inserts (preparing the appropriate bed for the graft), micro-fenestrations may have occurred, and consequently, may have also stimulated the exostosis formation. It can be observed among the cases reported in the literature8,9,10,11,12 that the areas operated on have corresponded in the majority of cases, to muscle insert location areas (Table 1). Accidental lesions may probably stimulate the bony formation development. In our five cases, these osseous formation developments have always been related with accidental or intentional periosteal fenestrations. However, bony overgrowth can be seen in three of these cases covering tooth roots that have been supporting fixed and removable prostheses.

The concept that BE formation can occur in response to heavy occlusal forces with the purpose of reinforcing bone trabeculae, was initially described by Glickman and Smulow.17 This new bone formation, providing buttressing, was divided into exostosis and lippings. A previous study investigated the prevalence, characteristics and evidence for BBE or lippings formation in a sample of 416 selected teeth in 52 modern skeletal specimens.18 BBE and lippings were found at 7% and 17.6% respectively (adjacent to 25% of teeth).18 BBE were mainly observed around the upper premolar-molar area, especially in males. Lippings were seen in lower incisors, premolars and molars, with no gender distribution. However, the authors suggest that 'other factors may be of greater importance in the aetiology of buccal bone enlargements'.18

Case reports of benign osseous proliferation beneath posterior fixed partial denture pontics have been related in the dental literature.19,20,21,22,23,24,25 As aetiological agents we (the authors) suggest genetic factors, functional stresses and chronic irritation. Two patients (cases 3 and 5) had the graft areas associated with teeth that have been supporting removable partial dentures; the other one (case 1), to teeth that had been supporting a cantilever. It is possible that the combination of periosteal (fenestration) and occlusal function (as a low-grade irritation) is responsible for this osseous proliferation.

Clinically, a review of the published reports (Table 1) suggested that canines and premolars, 89.8%, are more susceptible to BBE development. This fact was also observed by Echeverria et al.12 On the other hand, reasons for BE formation are more speculative. As previously mentioned, there have been patients who have developed BBE in the presence of other extraoral exostoses, as well. However, it should be noticed that intraoral toris were only observed in 15% of all subjects (Table 1). Sonnier et al.3 observed that palatal alveolar exostoses and mandibular tori can be found more often in young male dentate subjects (suggestive of alveolar bone origin). In contrast, the collective results from published case reports and the findings of the present paper (Table 1) indicate that 90% of the 20 subjects, who presented BBE after FGG, are females. Despite these differences, the development of such osseous overgrowths may be associated with the presence of teeth and their surrounding periodontal structures.

Conclusion

In conclusion, based on previous reports, periosteal trauma, eg fenestration, seems to be the main aetiologic agent associated with BBE development in areas where an autogenous FGG was placed. However, other stimuli alone, or in combination acting at this level, eg functional stresses and genetic factors, particularly in autogenous grafts, are of special interest. Thus, clinical studies with larger samples are needed to establish whether periosteal fenestration is the main aetiologic agent. Another sample of patients who underwent FFG and periosteum linear fenestration has been showing discreet clinical signs of exostosis formation, which will be confirmed by future postoperative visits.