Our systematic review and meta-analysis indicate that implants placed simultaneously with autogenous onlay grafts have a survival rate of 93.1% and 86% after a follow-up of <2.5 years and 2.5–5years respectively. Data on implant success is limited ranging from 84.6 to 100% with variable follow-up duration. Analysis of a limited number of studies indicated no significant difference in implant survival between the simultaneous and delayed placement of implants with onlay bone grafts.
Bone augmentation with autogenous onlay grafts has been used for decades in the field of oral implantology. Several systematic reviews have indicated that ridge augmentation using onlay bone grafts is a reliable surgical method for placing implants in ridges where it would otherwise not be possible [5, 33]. A staged treatment procedure consisting of initial bone grafting and implant placement following maturation of the graft is often used in the rehabilitation of deficient alveolar ridges. Simultaneous implant placement with onlay grafts has also been reported, but it has received limited attention in the literature. A 2017 systematic review and meta-analysis by Aghaloo et al. [12] has reported a high implant survival of 85.7 to 100% with delayed placement (8 studies) and a lower implant survival of 73.8 to 91% with simultaneous placement of implants (5 studies) in autogenous onlay bone grafts. However, the study was focussed only on completely edentulous maxillary patients, and it did not conduct a separate meta-analysis for implant survival with simultaneous and delayed implant placement.
Given such deficiency in literature, our meta-analysis presents important results for implantologists practicing onlay bone grafting. On a systematic search of literature with pre-defined inclusion/exclusion criteria, we could identify only 19 studies. The scarcity of literature is an indication that simultaneous implant placement is infrequently practiced with onlay grafts. Our results demonstrated a high pooled survival rate of 93.1% at a follow-up of <2.5years with simultaneous implant placement. However, with a longer follow-up of 2.5–5 years, it dropped to 86%. A high failure rate of >10% after 2.5 years in our meta-analysis is difficult to explain considering our study was a systematic review of prior published literature with different cohorts in different geographical regions. The difference could, however, partly be attributed to the different studies pooled in the two sub-groups based on follow-up duration. In our secondary analysis, we found no difference in implant survival between the simultaneous and delayed placement of implants. This, however, should be interpreted with caution as the 95% CI of the OR was quite wide, and only four studies were available for analysis.
At this stage, it is also important to consider the difference between implant survival and implant success. Implant survival is defined as the proportion of implants still in place at a given follow-up even if they are not in function while implant success takes into account other factors influencing implant function like patient symptoms, peri-implant bone loss, pocket depth, bleeding on probing, and implant mobility [34, 35]. Thus, even if the implant is surviving, it may not necessarily be successful. On descriptive analysis of studies, simultaneous implant placement was associated with a variable success rate of 84.6 to 100% but with a different follow-up duration. Only two comparative studies assessed implant success, and both reported no difference between simultaneous and delayed placement. For deriving strong conclusions, this data needs to be verified by future comparative studies.
Several different sites of autogenous grafts are available providing either membranous or endochondral bone. In our review, iliac and intra-oral grafts were the most commonly used bone grafts. It is known that iliac bone is endochondral in origin while intra-oral grafts are intra-membranous in origin which is similar to the recipient site. Zins et al. [36] have demonstrated that the difference in origin of bone grafts can influence graft resorption rates with faster resorption seen in endochondral grafts. In a large study involving 368 implants, Kang et al. [8] have demonstrated an earlier and higher vertical bone loss with iliac onlay grafts as compared to intra-oral grafts. However, no difference was seen in implant stability and implant survival in their study cohort. In our subgroup analysis based on the type of onlay graft, implant survival was higher with intra-oral grafts (95.7%) as compared to iliac grafts (85.8%). Important to note is that only four studies were available in the sub-group of intra-oral grafts, three of which had a follow-up of 6 months to 1 year.
Other than the type of grafts, several other factors can affect bone resorption with onlay grafts. Vertical augmentations often tend to have higher marginal bone loss as compared to horizontal augmentations [2, 12]. Since there was heterogeneity in the type of defects augmented in the included studies, we further analyzed implant survival based on this variable. Our results demonstrated a higher implant survival with single dimension defects (94.1% with vertical defects and 96.5% with horizontal defects) as compared to combined vertical and horizontal defects (83.3%). Due to the lack of assessment and variability of data presentation, we were unable to analyze the exact changes in marginal bone with simultaneous implant placement.
It has been suggested that simultaneous implant placement with onlay grafts can lead to better osteointegration of implants with limited marginal bone loss. The presence of implants during graft maturation can provide better fixation and stability thereby improving procedural success [25]. Simultaneous implant placement also leads to early loading of the graft. As most of the bone resorption occurs in the first year of grafting, the earlier functional stimulus with simultaneous implant placement may also reduce the crestal bone loss of onlay grafts. This theory was, however, not supported by the results of two comparative studies reporting data on marginal bone loss with both indicating a higher bone loss in the simultaneous implant group.
Our review needs to be interpreted with the following limitations. Foremost, the primary analysis of our study is from single-arm studies with their inherent bias. Only five non-randomized retrospective comparative studies were available with a limited sample size. The overall quality of the included studies was also not high. Secondly, we could only analyze only implant survival and not pool data for implant success and bone loss which are important outcome variables. Thirdly, there was heterogeneity in the included studies owing to differences in follow-up, type of grafts, recipient site, type of implants, etc. A sub-group analysis was attempted to assess the influence of these confounding factors but was restricted by the limited number of available studies. Furthermore, we could not assess the impact of type of implant placed (like butt-joint vs platform switch or smooth vs micro-roughened surfaces) on the outcomes due to lack of details from the included studies. Lastly, majority of the studies did not report the criteria or minimum alveolar dimensions required for simultaneous implant placement. The residual alveolar ridge is a factor of importance for the primary stability of any implant [9]. Difference in implant survival and success between the included studies could have been influenced by this factor.
Nevertheless, our review presents the largest pooled data (1368 implants) of implant survival following simultaneous implant placement with onlay bone grafts. Our study is also the first meta-analysis comparing outcomes of simultaneous and delayed implant placement with onlay grafts. Appropriate sensitivity and sub-group analysis were conducted to present comprehensive evidence to clinicians.
To conclude, data indicate that implants placed simultaneously with autogenous onlay grafts have a survival rate of 93.1% and 86% after a follow-up of <2.5 years and 2.5–5years respectively. Implant success has been assessed sparsely and ranges from 84.6 to 100%. A limited number of studies indicate no significant difference in implant survival between the simultaneous and delayed placement of implants with onlay bone grafts. There is a need for randomized controlled trials comparing simultaneous and delayed implant placement to provide robust evidence. Till then, clinicians should assess each case individually based on the quality of the native bone and its ability to provide primary stability for simultaneous implant placement.