1 Introduction
Edentulism is considered a major health problem due to associated impairments and disabilities (WHO 2001). Prolonged edentulism has been associated with progressive alveolar resorption and enlarged tongue dimensions making the mandibular prosthesis wear more challenging for the patients when compared to the maxillary complete dentures.

To reduce alveolar bone resorption and to preserve the proprioception, Overdenture can be the treatment modality. Overdenture is any removable dental prosthesis that covers and rests on one or more remaining natural teeth, the roots of natural teeth, and/or dental implants (GPT-8). In addition overdentures also increase support, masticatory function and retention.

Retention of the denture can be increased by using attachments which can be extraradicular or intraradicular. Among these attachments, bar attachment which is extraradicular attachment helps in splinting of the abutment teeth and retention of the appliance.

But the diminished denture base dimensions required to accommodate bar attachment and tooth abutments render the prosthesis more susceptible to fracture.A review by Goodacre et al showed that fracture of acrylic resin overdentures occurred in 7% of the prosthesis, with a range from 3% to 24%. Mandibular overdentures are more vulnerable to fracture due to reduced cross sectional dimensions and compromised denture bearing foundation when compared to maxillary prosthesis.

This problem is overcome by incorporation of metal framework into the mandibular denture base which reinforces and strengthens the removable prosthesis.

This case report describes tooth supported overdenture with bar attachment over retained mandibular canines and the procedure for reinforcement of the denture base with metal framework.

2 Case report
A 52 year old woman reported to the Department Of Prosthodontics, Crown and Bridge, Subharti Dental College, Meerut with partially edentulous maxillary and mandibular arch. The patient’s detailed medical, dental and social history was not significant. Extraoral and intraoral examination was done. Intraoral examination showed number of teeth present 11, 12, 22, 23, 24, 33, 43. Clinical and radiographic examination of the patient showed root canal treatment done in all the teeth present.

Treatment plan was developed and discussed with the patient. In the maxillary arch fixed prosthesis was planned in relation to teeth 11, 12, 22, 23, 24, 33, 43 and rest of the arch was restored with Interim removable partial denture. In mandibular arch, tooth supported overdenture was planned with right and left canines as abutments (Figure 1).
 

Figure 1 Tooth supported overdenture was planned with right and left canines as abutment

2.1 Procedure
Preliminary mandibular impression was made with irreversible hydrocolloid material.

2.2 Preoperative view of mandibular arch with retained right and left canine
Canines to be used as abutments were reduced to 2mm above the gingival level and were given dome shape with chamfer as cervical finish margin (Figure 2).
 

Figure 2 Canines to be used as abutments were reduced to 2mm above the gingival level and were given dome shape with chamfer as cervical finish margin

Since coping with extension was planned for the abutment, post space was created with peeso reamer no.2 and then direct impression of the post space prepared was taken by Addition silicone impression material (Figure 3).
 

Figure 3 Addition silicone impression material

Impression was poured with die stone to obtain cast. Wax pattern of the prepared abutments was made with custom made post in acrylic resin on the cast obtained.

Tooth abutments were connected with the Castable bar attachment (OT Bar Mutiuse, Rhein83, Bologna, Italy) which was kept 2mm above the mucosa for hygiene maintenance.

Wax pattern was tried in patient’s mouth and checked for fit and orientation (Figure 4).
 

Figure 4 Wax pattern was tried in patient’s mouth and checked for fit and orientation

Casting of the wax pattern was done with Ni-Cr alloy, trimmed, sandblasted, finished and polished.

Metal coping with bar attachment was cemented in patient’s mouth (Figure 5) and alginate impression was taken again so that custom tray could be fabricated.
 

Figure 5 Metal coping with bar attachment was cemented in patient’s mouth

Full wax spacer was adapted on the mandibular cast and custom tray was fabricated. Border molding was done and final impression was made with medium body. Multiple pores were made in the tray before making impression for mechanical retention of the medium body.

Master cast was poured; spacer was adapted on the crest of the ridge. Spacer was removed from selected areas to provide tissue stops for the metal framework, bilaterally in the posterior region.

Undercuts were blocked and Positioning clip A was placed on the bar (Figure 6) The position of the sleeve in the final metal framework was determined by the position of the Positioning clip A on the bar.
 

Figure 6 Undercuts were blocked and Positioning clip A was placed on the bar

Definitive cast was duplicated with reversible hydrocolloid material and refractory cast was poured.

Castable box was placed over the Positioning clip area duplicated in the refractory cast and the model was completed with inlay wax (Figure 7).
 

Figure 7 Castable box was placed over the Positioning clip area duplicated in the refractory cast and the model was completed with inlay wax

Spruing was done and casting was obtained (Figure 8).
 

Figure 8 Spruing was done and casting was obtained

Retentive clip was placed in its position in the metal framework.

Denture was finished with cast reinforcement (Figure 9) and retentive clip in place.
 

Figure 9 Denture was finished with cast reinforcement

3 Discussion
Overdentures have become a common treatment alternative for partially edentulous individuals with few remaining teeth. The preservation of remaining teeth at the same time preserves the residual alveolar bone around remaining roots (Polyzois et al., 1996). Thus overdenture serves several purposes like preserves natural teeth, preserves alveolar bone, increases masticatory efficiency, and preserves proprioception, increases support and retention.

The retention in overdentures is increased with the help of attachments which are attached to the abutment copings. Rodrigues (2000) presented clinical cases of mandibular implant overdenture using cast metal reinforcement. These overdentures had a bar attachment, and its reinforcement was an over-the- clip-of-the-bar attachment.

The coping becomes the fulcrum of overdenture movement and occupies a prosthetic space instead of the denture base. As a result, the overdenture around the coping is thin, and the overdenture is susceptible to deformation or fracture (Ohtani et al., 1991; Rodrigues, 2000).

Ohtani et al. (1991) examined denture repair cases (n=237) and found that the most frequent fractures of resin denture bases occurred in areas adjacent to abutment teeth in overdentures. Fracturing of the denture base around a coping is a longstanding problem and common clinical occurrence in prosthodontic practice (Polyzois et al., 1996; Rodrigues, 2000).

Several overdenture fractures have been reported in regions where copings exist (Ohtani et al., 1991).

Thus reinforcement of the denture base with metal framework adjacent to the top of the coping would be effective in reducing overdenture fracture. Dong et al. (2006) investigated the effect of reinforcement onoverdenture strain around the copings and at a midline. As according to them incidence of fracture of overdenture is high in these areas. They concluded that reinforcement over the top of the copings more effectively reduced strain in the midline of the overdenture and around the opings.

Thus the purpose of reinforcement of denture base is not only to prevent fracture, but also to improve functional rigidity for occlusal stability, and to distribute occlusal stress to the underlying denturebearing areas as uniformly as possible.

4 Conclusion
This paper describes not only the reinforcement of the denture base with a metal framework but also the inclusion of the attachment in the framework design to prevent fractures that could occur at the sites close to the tooth abutments as fracture of structures occurs because of crack initiation and propagation from areas of greatest strain, and since the greatest strain in this study was observed on the overdenture surface close to the top of each coping, an overdenture should be reinforced around the coping.

References
Dong J., Ikebe K., Gonda T., and Nokubi T., 2006, Influence of abutment height on strain in a mandibular overdenture, J. Oral. Rehabil., 33: 594-599

Ohtani T., Maeda Y., Enomoto K., Sogo M., Okada M., and Nokubi T., 1991, Study on denture repair case: Survey on resin denture base fractures, Part 1, Nihon Hotetsu Shika Gakkai Zasshi, 35: 977-982

Polyzois G.L., Andreopoulos A.G., and Lagouvardos P.E., 1996, Acrylic resin denture repair with adhesive resin and metal wires: Effects on strength parameters, J. Prosthet. Dent., 75: 381-387

Rodrigues A.H., 2000, Metal reinforcement for implant-supported mandibular overdentures, J. Prosthet. Dent., 83: 511-513