Key words: Dental Implant, Implant Abutment Screw loosening, pre load, splinted cement retained implant prosthesis

Introduction:

The replacement of missing teeth by implant supported prosthesis has become a popular option after the introduction of the concept of osseointegration with titanium fixtures^1,2. The use of root form endosseous implant has increased considerably and this restorative option has become more refined with the introduction of newer designs and concepts. Screw loosening, screw fracture, prosthetic component and implant fractures are the different prosthetic complications seen after loading the implant^3–5. Among these, screw loosening is the most common complication. This is seen more in single implant supported prosthesis with external connection and molars^6–9. Reasons for screw loosening include inadequate preload, inappropriate implant position and occlusal scheme, variations in hex dimension and abutment counterparts, differences in fit and accuracy, tension on abutment, improper screw design and excessive occlusal forces^10–12.

Case Report

A 45 year old female patient reported to The MAHE Dental College, Department of Prosthodontics with a chief complaint of dislodged implant crown and misfit implant crown in the right lower back tooth region. (Fig. 1)



On examination, implant crown in 47 was dislodged and the implant crown in 46 was rotated. RVG was made and the radiographical assessment revealed discrepancy between the transmucosal abutments and the fixture head which indicated implant abutment screw loosening in 46 and dislodged crown in 47 (Fig. 2a,2b,2c). Both the implant prosthesis were screw retained. (Fig. 3)



As the loose crown was causing discomfort to the patient, it was sectioned and the screw and abutment were retrieved. 46 and 47 were done under paltop implant system.

Suitable straight abutment were screwed and adequate preparation was done. After acquiring adequate clearance, the access holes were packed with putty impression material and an abutment level impression were made using addition silicone and light body for fabricating a splinted cement retained implant prosthesis in 46 and 47. (Fig.4, Fig.5)



Prior to cementation, the access holes were packed with putty impression material and GIC. The splinted PFM implant prosthesis was luted with GIC (Fig.6, Fig.7). Excess cement in the proximal aspects were flossed.

Discussion:

This case report illustrates one of the mechanical complication which can occur during prosthetic stage of implant surgery. The radiovisiography revealed discrepancy between the transmucosal abutments and the fixture head which indicated implant abutment screw loosening in 46 and dislodged crown in 47. The reason for screw loosening was due to inadequate application of tightening torque to generate the necessary preload. The screw loosening of implant abutment can cause issue for both the dentist and patient .It is essential to sacrifice the overlying restoration to give access to the screw. Screws have been studied considerably in the engineering literature and dental implant screws have been improved as a result.

Proper design of the occlusion and sound osseointegration, a reliable connection between implant and abutment is important for the appropriate functioning and stability of implant restoration. Several clinical studies have reported incidences of abutment loosening in various types of abutment connections. The external hex configurations is more prone to abutment screw loosening. Limited engagement of the external member and the presence of a short fulcrum point when tipping forces are acting, are the main reason for abutment screw loosening in external hex connections. In the external hex configuration, the axial preload of the abutment screw determines the stability of the connection. There is no lock form or positive locking by the external hex. The rotational position was determined by external hex but it does not absorb any lateral loading and the tensile force and so the stress is concentrated on the abutment screw. Different methods have been tried to reduce the screw loosening. The two important methods used to counteract screw loosening include the incorporation of an antirotational element and attaining optimum screw joint preload.

Several guidelines recommended for improving screw joint stability include centering the occlusal contact, flattening the cuspal inclination, proper tightening of the abutment screw, narrowing the buccolingual width of the restoration, and reducing cantilevers. Incorporation of these features into the restoration will help to decrease the incidence of abutment screw loosening. There are three methods for managing abutment screw loosening. The most conservative method is removal of the crown along with the abutment as described in the case report. The second method is to identify the access of the connecting screw head. The access may be labial, occlusal or lingual. This should be identified during the cementation of the prosthesis. The abutment connecting screw is then removed or re-tightened through the perforation made at the access in the crown. Depending on the location of the access the crown, it can be reused or discarded. The third method include vertical cuts made on the crown to separate the crown from the abutment.

CONCLUSION

A conventional mode of managing abutment screw loosening without consecrating the crown on various reasons for abutment screw loosening has been described. Inadequate preload is the most accepted reason for abutment screw loosening in two piece endosseous root form implants. Employing suitable tightening torque with wrench having torque gauge will deliver the optimum preload and splinted crowns can be utilized for such clinical situations.

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