69 Maxillary implant overdentures: four or two implants Complications All the implants were placed without any complications. During the entire follow-up period technical complications were considerable (Table 4.5). Fractured or loosened abutment screws were replaced or retightened chairside. Prosthetic fracture problems could be solved within one day by the dental laboratory. 4.4 DISCUSSION Based on the results of this randomised controlled trial with a 1 year follow-up period a maxillary 4-implant overdenture is the more favourable treatment in terms of implant survival rate compared to a 2-implant overdenture and therefore remains the gold standard. However, 2 and 4-implant maxillary overdentures perform similarly in terms of MBLC, clinical outcomes, masticatory performance and PROMs. Marginal bone level change and implant survival The MBLC was low in both groups and comparable to other studies describing maxillary overdenture treatment retained by bars1,5,6. On studying 2-IODs, Zembic et al (2017) reported a high MBLC, with 70% of the implants showing 2mm bone loss or more during the first year of follow-up14. They attributed the higher MBLC to the compromised bone conditions of the maxilla, combined with the higher biomechanical stress when treating patients with an overdenture without palatal coverage. The study of Sanna et al (2009) assessed 44 participants with maxillary IODs during a mean follow-up period of 7 years12. Twelve of their participants’ IODs were retained by two implants (bars or solitary attachments) and similar MBLC patterns were reported. However, both studies showed favourable implant survival rates for 2-IODs and thus contradicting the present study in both aspects. In contrast, Bergendal et al (1998) observed high implant loss among the 2-IODs retained by bars or balls, after a mean follow-up period of 5.1 years9. In that study, the highest implant loss was among participants with low bone quantities and quality and short implants with a relatively small osseointegration area. The authors attributed the early implant losses due to implant overloads and insufficient or non-existent osseointegration. Although the implants used in the present study also had a relatively small osseointegration area (implant diameter: 3.5mm), the bone properties were not assessed and so the assumptions made by Bergendal could not be either confirmed or refuted. Yet, the implants that survived in the present study gave successful outcomes, which means the maxillary 2-IOD treatment cannot be entirely discarded. The higher implant load suggested by Bergendal et al (1998) was confirmed by Takahashi et al (2018) and Nishimura et al (2021) who reported higher biomechanical stress in the 2-IODs compared to the 4 and 6-IODs24,25. They used an edentulous maxilla model containing six implants connected to strain gauges to test several IOD setups for implant strain. The lower strains were attributed to the distribution of forces, especially when using splinted implants. Though not tested in vitro, higher strains may be expected in 2-IODs compared to 4-IODs since 4
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