###### Abstract  
The influence of implant surface roughness on implant treatment outcome is still under debate. A rougher implant surface improves osseointegration but yields greater plaque accumulation and biofilm formation. Hybrid implants have a minimally rough component coronally and a moderately rough implant body. The aim of this split-mouth study is to evaluate the long-term outcome of treatment with hybrid and moderately rough implants after 6 years. As a secondary aim, Oral Health Quality of Life (OHQoL) was assessed after one and six years. Fully dentulous patients received an overdenture supported by two implants. One implant had a hybrid surface (MSC) and one implant had a moderately rough surface (DCC). Radiographic crestal bone loss (CBL), plaque score, bleeding on probing (BoP) and periodontal probing depth (PPD) were determined at one and six years. OHQoL was evaluated using the Oral Health Impact Profile-14 (OHIP-14) questionnaire and a Visual Analogue Scale (VAS). Twenty-one patients with 42 implants were evaluated after 6 years. No implants failed and a mean CBL of 0.26 mm (SD: 0.42) for the MSC group and 0.17 mm (SD: 0.29) for the DCC group was observed, which was not statistically significant. Periodontal parameters were comparable after 6 years and a significant improvement in OHQoL was observed. This randomized controlled trial concludes that hybrid implants are a predictable treatment alternative for moderately rough implants in patients with an overdenture supported by two implants. implant surface; moderately rough implants; hybrid implants; implant survival; crestal bone loss; oral health quality of life; implant-supported overdenture +
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rough [6]. Several studies comparing minimally and moderately rough implants have shown increased bone-to-implant contact for moderately rough implants [7; 8; 9]. On the other hand, a rougher surface facilitates greater plaque accumulation and formation of a biofilm, which could play an important role in peri-implant soft-tissue health [10; 11]. Initial crestal bone loss can represent normal remodeling in response to surgery or it can be a consequence of the re-establishment of the peri-implant biological width [12; 13]. Non-optimal implant components, poor clinical handling (e.g., traumatic surgery), restrictive prosthodontics (e.g., cement residues) or compromised patient characteristics (e.g., poor oral hygiene, smoking and drugs) could contribute to crestal bone loss occurring in the years thereafter [14]. Over time, this progressive crestal bone loss can lead to peri-implantitis, soft-tissue recession and, finally, implant failure. This occurs in approximately 5% of patients [13; 15]. [PERSON] et al., (2021) found that early crestal bone loss is a significant predictor of future peri-implantitis development. Crestal bone loss of 0.5 mm or more during the first year of function is associated with 5.43 times higher odds of developing peri-implantitis [16]. Hybrid implants could be a possible solution for the previously described issues. This type of implant has a minimally rough component coronally and a moderately rough implant body apically [17]. A systematic review by [PERSON] et al., (2017) suggests that implants with a minimally rough coronal part had statistically significantly less crestal bone loss in comparison to implants with a moderately rough and rough coronal part [13]. A hybrid design could possibly ensure better mucosal health and reduce the risk of peri-implant diseases, whilst the implant body could ensure osseometigration. The present split-mouth study aims to compare the long-term implant outcome between hybrid and moderately rough implants in patients with a bar-supported mandibular overdenture. The primary outcome is crestal bone loss (CBL). Peri-implant soft-tissue health, defined by plaque score, bleeding on probing (BoP) and periodontal probing depth (PPD), is the secondary outcome.