Zygomatic implants placed in atrophic maxilla: an overview of current systematic reviews and meta-analysis

In patients with sufficient bone volume in the edentulous or semi-dentate maxilla, rehabilitation of masticatory function with dental implants can be achieved with predictable success and acceptable long-term results. However, due to mechanical and anatomical difficulties, rehabilitation of severely resorbed maxilla rehabilitation with endosseous implants remains a challenge [1, 2]. Several surgical procedures have been advocated to treat the atrophic maxilla including grafting techniques (block, composite, Le Fort I inter-positional and iliac crest, and maxillary sinus grafts), sinus floor elevation, and guided bone regeneration (GBR) [3‐5]. There are also less aggressive alternatives including short implants, tilted implants, and zygomatic implants [6, 7].

To ensure acceptable success rates for standard dental implants without any bone augmentation procedures, the minimal bone height in the posterior region of the maxilla needs to be at least 10 mm [8]. Although in the posterior atrophic maxilla where the height of residual bone is at least 6–7 mm, and where the width of any residual ridge permits placement of at least 5-mm-diameter implants, short implants can be a safe choice [9]. Nevertheless, there are reports that short implants, alone or in conjunction with sinus floor elevation, less than 6 mm significantly diminished implant survival rate [3].

In cases where residual bone height and width does not permit placement of conventional dental implants, surgical procedures as well as ZIs (abbreviation of zygomatic implants) can be considered.

Over the past decades, different bone grafting procedures have been advocated prior to, or simultaneously with, implant placement in routine implant treatments with the aim of increasing the volume of load-bearing bone [10]. Conventional grafting with autogenous bone has been considered the “gold standard” in the treatment of the extremely atrophic maxillae, but due to high failure rates of 10–30%, additional time and higher costs, the development and introduction of a new standard with superior clinical outcomes is warranted [11].

Based on experiences in human and animal studies, Branemark et al. [12] reported that the introduction of an implant in the maxillary sinus would not necessarily jeopardize sinus health. Likewise, by considering high success rates using the zygomatic bone as an anchorage point for prosthetic rehabilitation in patients with maxillary defects [13], they developed a new type of implant named the zygomaticus fixture, which could obtain implant anchorage and stability in the zygoma. ZIs as described by Malevez et al. [14] are machined surface, self-tapping screws in commercially pure titanium which present a 45° angulated prosthetic head to compensate for the angulation between the zygomatic bone and the alveolus. By increasing the implant length with ranges from 30 to 52.5 mm, ZI could be placed in the atrophic upper jaws even with poor bone quality.

When considering all the available surgical options, treatment choice will be dependent on the characteristics of the patient, the amount of residual bone, and the general risks and the patient’s wishes to undertake [5, 15]. In severe maxillary atrophy, ZI offers a viable alternative to the rather invasive procedures including bone grafting or sinus-lift procedures since they eliminate the necessity of onlay bone grafting or sinus augmentation (and thus a graft donor site); a smaller number of implants are necessary to support fixed prostheses. Furthermore, in many instances, formal grafting is not a viable alternative due to the residual morphology of the atrophic maxillae following resorption. ZIs may also decrease patient morbidity, treatment time, and costs [15, 16].

To date, several systematic reviews (SR) have tried to elucidate the weight of evidence of clinical outcomes when utilizing ZIs, including survival/failure rate and complications. However, to the best of our knowledge, this is the first overview evaluating all evidence from the existing SR on this topic using an umbrella approach.

The flow diagram of the study selection process is shown in Fig. 1. A total of 223 references were obtained through searching the databases and additional 4 studies were retrieved by manual search. One hundred seventy five studies were obtained after duplication removal. Of these, 138 were excluded by reading the abstracts and the reviewers agreed on 37 potentially pertinent articles, which were submitted to full-text analysis. Thirty were excluded. The reasons for exclusion are shown in Table 1. Finally, seven SR (with two meta-analyses) were assessed (Fig. 1).

Considering the growing number of SR on this clinically important topic, the important next step is to provide an overview of current SR with a high level weight of evidence to guide decision makers in healthcare. An umbrella review is an approach to analyze published SR in order to compare and contrast the findings of SR on a defined topic [52]. To the authors’ best knowledge, this is the first overview to summarize findings and conduct a critical appraisal and risk of bias assessment of SR evaluating clinical variables of ZIs placed in atrophic maxilla (survival/failure rate, surgical, and prosthetic complications).

Our results suggested that in the severely atrophic maxillae, ZIs can be a favorable choice as they showed good/excellent survival rates ranging from 95.2 to 100% even in long-term follow-ups (> 10 years), and this is comparable to conventional implants. Oral rehabilitation using implant placement has always been more sensitive in the maxilla than mandible. Furthermore, this gets more challenging when the available bone becomes insufficient and limited due to bone atrophy or maxillary sinus pneumatization, or a combination of both [35]. To solve this problem, in the severely atrophic maxilla, different protocols have been advocated; different grafting techniques [53], sinus floor elevation [54], GBR (guided bone regeneration), and ZIs [44]. The known gold standard procedure for oral rehabilitation was the reconstruction of the resorbed alveolae using autogenous bone grafts. They are supported by a notable body of scientific evidence [4]. Different types of grafts are available for ridge augmentation. In severe atrophy, large quantities of bone are needed to be able to place conventional endosseous dental implants, and for large regenerations, onlay grafts are the preferred method [55]. Bony defects are reconstructed either in conjunction with dental implant placement or as a separate surgery procedure prior to dental implant placement (staged approach). The latter is preferred in large defects where primary stability cannot be achieved properly [4]. Although being widely used, bone grafting techniques suffer a series of shortcomings, both clinical and biological, that prevents high success rates as of alternative treatment based on ZIs; they are associated with longer treatment times as it takes a minimum of 4–6 months for bone formation in regenerated sites to be able to place dental implants [56]. Moreover, research suggests that implants placed in pristine bone experience higher survival rates than in grafted bone [55, 57].

In 1998, Branemark et al. developed a technology for oral rehabilitation in severely atrophic maxilla named “zygomatic technique” in which zygoma bone was utilized for implant anchorage. ZIs were defined as machined surfaced self-tapping implants in commercially pure titanium that are available in different lengths ranging from 30 to 52.5 mm used for oral rehabilitation without the use of grafts [35]. They suffered various technical modifications over time. Compared with major bone grafting techniques, ZI placement is less invasive with long-term high survival rates and can be a good alternative to grafting techniques esp. in patients in whom bone grafts cannot be harvested [49]. Even when the crestal bone of the maxilla is not sufficient, ZIs have a minimum of 8–10 mm implant-bone contact. The engagement of as much cortical bone as possible has been used as it is a decisive factor for primary stability and therefore higher success rates [27]. ZIs have the anchorage in four cortical portions (the palatina alveolar crest and sinus floor are extra cortical portions used) compared to one or a maximum of two cortical portions with regular implant placement [15, 58]. However, this is not always the case as according to ZAGA classification suggested by Aparicio et al. [59] the extra-maxillary approach many times is only anchored in the zygoma. The critical success factors of ZI-based therapy are the primary stability as well as establishing a harmonious occlusion (book). Our findings are in line with Apricio et al. who established that in many cases, ZIs had shown better clinical results compared with bone grafting procedures representing a possible new “gold standard” for atrophic jaw treatment [11].

ZIs appear to be a consolidated therapeutic option for significantly atrophic maxilla offering a promising alternative to heavy bone grafting techniques with lower costs, fewer complications, shorter time for rehabilitation, less prosthodontic work needed, and significantly higher survival rates. They have been assessed in reviews with long-term follow-ups (more than 10 years) and showed reliable and predictable results. It does not lack in complications but they are mostly mild and facile to manage and rarely get catastrophic. To prevent serious complications, this treatment should be reserved only to professional clinicians with vast surgical experience and a good knowledge of the 3D anatomy. Nonetheless, this conclusion is based on a limited number of SRs and the majority of them presented moderate quality.