**1. Introduction**

Lack of vertical bone height in the posterior maxilla limits standard dental implant placement. In order to increase the vertical dimension in the posterior maxilla, a maxillary sinus floor elevation (MSFE) with graft material can be performed. [1,2] MSFE is a predictable preimplant surgical procedure with a high survival rate of the dental implants, exceeding 93.8% [3]. Pjetursson systematically reviewed the success of dental implants placed in combination with MSFE, and reported an implant survival rate after 3 years up to 98.3%, using rough surface dental implants, related to non-augmented jawbone [4].

Due to its osteoinductive, osteoconductive and osteogenic properties autogenous bone is still considered the golden standard as graft material [5–12]. This osteogenic capacity of autogenous bone grafts may be attributed to the presence of bone morphogenic proteins, attracting osteogenic cells from the adjacent tissues, thus mobilizing other growth factors essential for bone regeneration [4].

Bone grafts can be obtained either intraorally or extraorally. Harvesting these bone grafts has drawbacks, such as an extended operating time, donor site morbidity, hospitalization, unpredictable resorption rate of the bone grafts [9,13–15] and sensory disturbances [5,16,17]. Different types of bone substitutes have been developed to overcome these drawbacks (e.g., allograft, xenograft, alloplast and mixtures of different materials) [18,19]. The comparison of bone grafts from different origins has been the subject of study extensively. Meta-analyses have confirmed the superiority of autogenous bone grafts over allografts, xenografts and synthetic bone grafts with respect to new bone formation [20–22]. Ideally, such a bone substitute should be biomechanically stable, capable of degradation within an appropriate time frame, exhibiting osteoconductive, osteogenic and osteoinductive properties, biologically safe, low patient morbidity, volume stable, easy available on the market with low production costs and providing a favorable environment for the entry of blood vessels and bone-forming cells [23–26]. For cranio-maxillofacial purposes, autografts (due to the drawbacks) play a minor role today. In terms of costs and benefits, allografts were the most commonly used bone graft in the United States and xenografts were the most commonly used grafts in Europe [27]. Allografts are tissue grafts from a donor of the same species as the recipient, but not genetically identical, with a risk of immune responses, infection transmission and are known to have high failure rates with long-term use. Additionally, many osteoinductive properties are lost during the manufacturing of allografts [21,28–30]. In Europe, the use of allografts is often abandoned in clinical practice, advised by the Medical Device Regulation [31].

Xenografts are usually of porcine or bovine origin. The use of xenografts involves a number of risks and complications, e.g., disease transmission (Creutzfeldt-Jakob disease), immune responses, foreign body response and chronic inflammation. The production process can lead to a lack of viable cells and reduced osteoinductive properties [32]. For cranio-maxillofacial applications, bovine xenografts are allowed for safe use without reports of transmissible spongiform encephalopathies (TSE) and bovine spongiform encephalopathy (BSE) risk [27,33,34].

Alloplastic (synthetic) grafts are currently most commonly used for their osteoconduction, hardness and acceptability by bone. Most alloplasts consist of hydroxyapatite, a naturally occurring ceramic that is also the primary mineral of bone, or other calcium phosphate compounds, such as β–tricalcium phosphate (β–TCP). Calcium phosphates, like hydroxyapatite (HA), β–tricalcium phosphate (β–TCP) or biphasic calcium phosphate (BCP), a mixture of HA and β-TCP, are osteoconductive, biocompatible and simulate the chemical composition of natural bone. Calcium phosphates do not induce a sustained foreign body response or toxic reaction [35–37]. Hydroxyapatite is, at a physiological pH, the least dissolvable of the naturally occurring calcium phosphates, making it relatively resistant to resorption and suitable for clinical use [9,38–40]. β-TCP does not have osteoinductive properties and resorbs rather quickly, but not necessarily at the same rate as the formation of new bone [11,12,41–44].

In previous studies a mixture of 60% HA and 40% β-TCP (BCP) as graft material in an MSFE procedure demonstrated sufficient bone (re)generation after 6 months for placement of dental implants, although remnants of BCP could still be observed, indicating that the process of bone substitution was not yet completed [12,45]. After 9- and 12-months healing time, a high bone formation was still observed and remnants of BCP particles could still be detected [46]. A significant lower total bone volume is found for each biomaterial or combination of different graft materials compared to autogenous bone [11,47].

This study was based on the use of an autogenous bone graft, harvested from the maxillary tuberosity, in an MSFE procedure, as the golden standard [10]. Though, if autogenous bone graft volume is insufficient, a bone substitute can be supplemented to achieve sufficient graft volume for

completion of the MFSE procedure, thereby avoiding a second surgical intervention and minimizing donor site morbidity. Referring to previous studies on the use of BCP's only [12,46], it would be interesting to further study the use of a mixture of autogenous bone and Straumann® Bone Ceramic (SBC), a BCP (Straumann Holding AG, Basel, Switzerland).

Therefore, the purpose of this study was to determine whether a mixture of autogenous bone and BCP in an MSFE procedure leads to an improved bone formation compared to an MSFE with pure BCP and ideally a total remission of BCP remnants in the entire augmented MSFE area, eventually leading to a sufficient bone structure, qualitatively and quantitatively, for dental implant placement. Five subsequent patients were evaluated clinically, radiologically, histologically and histomorphometrically after a 6-month healing period. The results were compared with the results of the previously reported studies with pure BCP (no autogenous bone added), after 6-, 9- and 12-month healing time, which were conducted according to the same study protocol [12,46].
