125 General discussion and conclusions Technical complications Fractures of the denture base or teeth were reported most frequently. Specifically considering the attachment systems used, the replacement of nylon solitary attachments was a relatively frequently reported complication, while fractures or wear of the bar-clip interface were not reported. Other studies on 4-implant overdentures retained by bars and solitary attachments report a similar pattern1,6,13. Since studies on 2-IODs are scarce, no additional data are available considering technical complications. Therefore, conclusions are mostly based on the present thesis: the most frequent technical complications in maxillary implant overdenture therapy consist of fractures of teeth and the denture’s base. In solitary attached overdentures, the replacement of nylon inserts is relatively common. However, in general it can be concluded that the number of technical complications is low. The use of surgical templates In general, surgical templates are used to aid in fully guided placement of implants to enable immediate placement of the superstructure. However, since bone properties in patients with atrophic maxillae are compromised, immediate placement of a superstructure is not the primary objective of treatment. The primary objective of treating these patients is reliable and secure placement of the implants. Placement of implants in the native atrophic jaw, without any form of reconstructive surgery, can be reliably achieved using 3D virtual surgical planning. Using 3D VSP, the available bone volume and the surrounding structures can be virtually assessed, and the implants can be planned in a prosthetically preferred position17. In case the virtual planning is successful, a surgical template can aid in stabilised implant placement, thereby avoiding vital structures such as the maxillary sinus and the nasal cavity. Template stability and supporting surrounding structures are essential18. In case of low template stability a larger safety margin is needed surrounding the planned implant19. In case the low amount of bone volume prohibits a larger safety margin, additional (bony) support can be created via an open flap procedure20,21. The developed surgical template described in chapter 3 offers additional support that may be needed in atrophic edentulous maxillae, resulting in satisfying implant placement accuracy when using a semi-guided approach. Retreatment of failing implants in the rehabilitated maxilla Even though implant surgery has become a safe and predictable treatment for replacing teeth22, loss of implants does occur. Retreatment is associated with lower implant survival because the retreated sites are still subject to some, if not all, of the previous factors that led to the failure23. Maxillary retreatment24, as well as of sites with a lower bone quality and quantity25, have been shown to result in an even lower survival rate, though current research on retreatment is limited. Therefore, the present thesis (chapter 7) added valuable data on retreating patients with implants that have suffered from implant failure, specifically in a group of patients with multiple late maxillary implant failures after full arch rehabilitation. It was suggested that late failures, i.e. chronically infected sites, could result in lower bone quality and quantity25,26, which might be the reason for a lower implant survival rate after secondary treatment. The high survival rates reported in chapter 7 contradict this proposition, though the follow-up period
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