57 Maxillary implant overdentures: four or two implants 4.1 INTRODUCTION Lack of retention or stability of a maxillary conventional denture can be improved with an implant overdenture (IOD)1-4. Four implant IODs (4-IOD) combined with bar attachment systems are a favourable treatment option due to their high implant survival rates, low marginal bone level changes (MBLC), high patient satisfaction, improved masticatory performance and low complication rates for up to ten years1-6. 4-IODs are therefore considered the gold standard. Treating patients with a maxillary IOD can be an invasive and costly procedure, especially if reconstructive surgery is needed prior to implant surgery7 and may therefore be inaccessible for many patients. Next to this, for the aged and/or medically compromised patient, there is a demand for less invasive treatment options. The need for reconstructive surgery could possibly be reduced by retaining the maxillary IOD with less than four implants. Current knowledge comes from a small number of studies which show varying results regarding MBLC, implant survival, and patient related outcomes (PROMs)814. Three of these studies reported on 3-implant maxillary overdentures (3-IOD) retained by bars and solitary attachments8,10,11, of which two reported favourable clinical outcomes after at least five years10,11 and one reported favourable outcomes regarding patient satisfaction8. Four studies reported on 2-implant maxillary overdentures (2-IOD) retained by bars or solitary attachments9,12-14. While two studies experienced high implant losses in the first two years of function9,13, the other two studies reported favourable results during a short and long follow-up period12,14. It must be mentioned, however, that the studies on 2-IODs were not conducted as a randomised controlled trial and had relatively small groups of participants. To our knowledge there are no randomised controlled trials (RCTs) comparing four with two implants and no studies focusing on the masticatory performance after such a treatment. Therefore, the aim of this randomized controlled trial was to compare marginal bone level change, implant and overdenture survival, clinical, masticatory and patient related outcomes of maxillary implant overdentures with either two or four implants and a bar attachment system. 4.2 MATERIALS AND METHODS Patients Between February 2018 and September 2020, all the eligible edentulous patients experiencing functional problems with their maxillary conventional denture, who were referred to the Department of Oral and Maxillofacial Surgery (University Medical Center Groningen, the Netherlands) were asked to participate in a RCT. The patients were deemed eligible for participation if they had been edentulous for more than one year and if they had sufficient bone volume for the placement of four implants in the maxilla. Bone sufficiency was assessed by using cone beam computed tomography (CBCT). Patients were excluded from the study if they had been formerly treated with pre-prosthetic or reconstructive surgery in the maxilla, had a medical contra-indication for a surgical intervention, were smoking or had undergone radiotherapy in the head- and neck region. Forty participants were included in the study (Fig 4.1). All the participants received 4
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