94 Chapter 6 6.2 MATERIALS & METHODS The data used for this sub-analysis originate from two randomized clinical trials of which short- and medium-term results were published before6,7,22,23. The original studies reported on peri-implant bone change in fully edentulous patients with implant retained maxillary overdentures supported by four or six implants. The research protocols will be described briefly. A complete description can be found in the corresponding publications22,23. Patient selection All consecutive fully edentulous patients referred to the Department of Oral and Maxillofacial Surgery (University Medical Center Groningen, the Netherlands) between January 2006 and December 2009, suffering from a lack of retention or stability of their maxillary denture, were informed about the study. Patients were included if they were ≥18 years of age and fully edentulous for at least one year. Patients were excluded if their American Society of Anesthesiologists (ASA) score was over II, were smoking, had a history of radiotherapy in the head and neck region or previous pre-prosthetic surgery and implant placement. The research protocols were approved by the Medical Ethical Committee of the University Medical Center Groningen (NL32503.042.11) and were registered in the Dutch Trial Register (NL2828). All the participants signed an informed consent form. Participants with sufficient maxillary bone volume to place six endosseous implants in the maxillary anterior region were included in the anterior group (n=50), while participants with insufficient maxillary bone volume were included in the posterior group (n=66). Randomly, four or six implants were placed in either group. Participant characteristics are listed in Table 6.1. Table 6.1 | Patient characteristics at study baseline. Anterior group (n=50) Posterior group (n=66) Total Mean age (SD) in years 59.8 (8.0) 59.3 (8.6) 59.5 (8.5) Gender (male/female) 23/27 33/33 56/60 Mean edentulous maxilla period (SD) in years 14.2(13.1) 22.4 (13.1) 18.9 (13.7) Maxillary sinus floor augmentation No Yes Implant type 2-stage TiO2-blasted F-modified surface 1- stage Sandblasted, large grit, acid-etched SD: standard deviation. Treatment procedure All surgical procedures were performed by one experienced oral and maxillofacial surgeon (GMR). In the anterior group, TiO2-blasted implants with a fluoride-modified surface (OsseoSpeedTM Ø 4.0 S, Astra Tech AB, Mölndal, Sweden) with a length of at least 11 mm were inserted
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