Introduction

According to the UN report on world population ageing in 2020, globally there are 727 million persons aged 60 years and over. This number is expected to increase from the current 9.3% to 16% over the next three decades [1]. The incidence of caries among British adults has reduced but the incidence of severe periodontal disease has increased [2]. These primary causes of tooth removal result in a growing number of patients seeking solutions for full or partial tooth loss. Implant-supported prostheses are attractive due to their biological and functional benefits, offering excellent long-term results to patients [3]. Studies show that dental implants between 100,000-300,000 implants are placed per year [4]. The success rate depends mainly on the way the individual’s bone integrates with the implant surface called osseointegration. A retrospective cohort study to investigate causes of failure of implants, classified as early failure and late failure was conducted by Manor et al. The main cause of early failure was found to be the lack of osseointegration at 73.2%. Osseo-integration is the direct contact between living bone tissue and the implant surface by Branemark [5]. The implant’s primary stability depends on factors such cortical bone volume, implant size, and surface features. This stability weakens in the first few weeks due to bone changes. To compensate for this, the secondary stability, mainly from the newly formed bone, becomes crucial. This bond on the implant surface is biological, not mechanical [6]. Historically, several modifications to the implant surface properties such as structure, surface roughness, chemistry, surface charge, and wettability have been examined to improve the osseointegration [5]. Recently, microroughening methods such as sandblasting or electrochemical deposition and altering the surface activity through acid etching, anodising, or activation are used to boost osseointegration [7]. Specific growth factors such as Bone Morphogenetic Proteins (BMP) to stimulate the creation of bone tissue have been explored. This phenomenon involves the proliferation and differentiation of the undifferentiated stem cells, osteoprogenitor cells and preosteoblasts into osteoblasts enhancing implant osseointegration. Among the family of BMPs, rhBMP-2 and BMP-7 seem to possess the highest osteoinductive potential [8]. Studies involving local application of rhBMP at fracture sites and bone analysis for BMP expression showed elevated upregulation for 21 days during healing [9]. A review on in vivo studies of BMP coated titanium implant surface measured the bone formation outcome each week up to 3 weeks [10]. Another systematic review on the effects of bioactive drugs such as bisphosphonates, calcium phosphates and BMP by coating the implant surface or using carriers showed significantly improved osseointegration by BMPs and calcium phosphate, but only bone to implant contact was measured [11]. This review aimed to critically appraise the extent and strength of osseointegration achieved by the biofunctionalisation of the implant surface with rhBMP-2 by analysing the outcomes including new Bone Area (BA) or Bone Volume (BV), bone implant contact from histological evaluations and the Removal Torque (RT), the Resonance Frequency Analysis (RFA) or Implant Stability Quotient (ISQ) compared to that achieved by the Sand-blasted, Large grit Acid etched (SLA) titanium implants.

Materials and methods

To explore the research question, a systematic review of published Randomised Controlled Trials (RCT) was the selected research method. In vivo animal randomised controlled trials studies, with or without split mouth design, and assessing follow up outcomes from 3 weeks to 3 months with low risk of bias were included. Studies not included in the earlier reviews and not analysed for more outcomes according to this inclusion criteria were included in this review.

Studies that did not meet the inclusion criteria such as cohort and case control studies and in vitro studies which could not provide the outcome measures such as removal torque and Stability Quotient were excluded.

Inclusion and exclusion criteria: Healthy animals without systemic diseases or medication which may compromise osseous healing were included. Implants used must be SLA titanium implants. In vitro studies, animal studies without ethical approval, BMP placed as scaffolds at the implant site, BMP in the form of solution/gel/carrier dependent placed as a simultaneous guided bone regeneration procedure but not coated on the implant were excluded. Studies involving BMP-7, BMP-4, nanotube delivery and gene vector delivery of BMP were also excluded.

Intervention planned: Implant surface modification with rhBMP-2 adsorbed or as a covalent attachment or a carrier delivered on the surface of the implant.

Outcomes: New bone formation measured by bone area, BV, bone height or density. Osseointegration measured by boneimplant contact which is the amount of bone in contact with the determined surface or length of the implant.

Stability measured by removal torque or Stability Quotient. Removal torque is measured by the torque required to remove an osseointegrated implant. Resonance frequency analysis is the resonance measured of the implant oscillating at the first flexural stress translated as Stability Quotient [12].

Search methods for selection of studies:The three databases Medline on Ebscohost platform, CINAHL Ultimate, Dentistry and Oral Sciences Sources were searched using the strategy planned according to the inclusion criteria and outcomes decided. MeSH terms like dental implants, oral implants, titanium implants, osseointegration, bone morphogenetic proteins, rhBMP-2, reversal torque, removal torque, bone implant contact, implant stability quotient and resonance frequency analysis were employed. The limiters used were the publication period 2013-2023 to gather up to date studies and publications in English. Systematic reviews and meta- analyses on the same subject were consulted to supplement the selection of papers. The PRISMA protocol was followed for selection of studies (Table 1). A total number of 94 papers were identified from all the databases. The rationale for excluding studies was in accordance with the exclusion criteria for study selection, participants, interventions, and the removal of duplicate papers. The final studies selected were 7 (Table 2).

Missing data: Pang et al. (2021) [13] were emailed to request missing data, but the information could not be obtained. As it was not very critical to the review, the study was still included in the selection.

Table 2: List of studies included.

Risk assessment of bias in the included studies: The risk of bias for the included studies was assessed based on the OHAT risk of bias tool for evaluating human and animal studies (Table 3). This tool was developed from guidance by the Agency for Healthcare research and Quality, the Cochrane Handbook, the Cochrane risk of bias tool for non-randomised trials, the SYRCLE tool for animal studies, comments from advisors and staff and other sources (OHAT Risk of bias tool, 2015).

The following domains were assessed across individual studies.

Selection bias.

Allocation concealment.

Internal Validity- appropriate comparison groups.

Confounding variables.

Identical experimental conditions.

Attrition/Exclusion bias.

Detection bias.

Outcome assessment.

Other bias.

The risk of bias was graded on a 4-point scale as definitely high, probably high, probably low, and definitely low with “not reported” data classed as probably high. The study by [20] Soo Yeon Yoo et al 2015 had an overall definitely low risk of bias. The study by [17] Kim et al. 2015 was a pilot study with only mean values assessed and no statistical analysis was performed, hence the risk of bias for internal validity was graded as probably high. In the study [14] Cardoso et al. 2017, the use of phosphate carriers masks the reliable outcome measurement of the effect of the addition of BMP, hence the confounding variables are graded as probably high risk of bias. No study was excluded as the overall risk of bias for all studies were low

Data extraction and management: The seven randomised controlled studies selected for review included over 252 implants. A comprehensive data extraction template inclusive of all characteristics, interventions, outcomes measured, results and statistical data was planned and tabulated for easy reference. The Gradepro GDT tool was used to create a summary of findings table and assess the quality of evidence. Further, the data was analysed and described as a narrative synthesis with inferences and implications.

The following information was extracted:

General information: Author, publication year, Title, Journal of publication, location, aims and study design.

Study eligibility: Inclusion and exclusion criteria

Participants: Animal used, Number of animals and implants, ethical approvals, site of implantation.

Interventions: Type and concentration of BMP used, carrier, method of implant surface modification, BMP release tests, details of surgical procedure, pre and post medications.

Comparators: Type of implant or carrier used as control.

Outcomes: Schedules, bone area, BV, bone-implant contact, Stability Quotient.

Others: Statistical analysis, significance, conclusions, conflict of interest, overall risk of bias.

Characteristics of included studies: In the randomised con randomised controlled trials by [13] Pang etal. 2021, carried out on rabbit tibia, 200 ng rhBMP-2 was added to a composite of collagen and nano-hydroxyapatite and adsorbed onto the implant surface.

The BMP release tests were conducted as invitro tests. New bone area, bone-implant contact, and removal torque were assessed at 4 weeks [13]. In 2017, Cardoso et al. [14] studied the effects of 1 µg rhBMP-2 with Phosphorylated Pullulan (PPL) adsorbed onto the implant surface. These randomised controlled trials involving 120 implants, also tested the effects of phosphates on osseointegration by coating implants with 10% polyphosphoric acid, 1% PPL and 10% PPL against SLA titanium implants as controls. BV and bone-implant contact B were measured at 1 month and 3 months [14]. In the study by [15]. Pan et al. 2016, hyaluronic acid hydrogel with 10 µg/ml.

BMP was employed in dried, wet, and immediate dip coated on dried gel forms on 28 implants. BMP release tests were determined up to day 3. Bone area and bone-implant contact were measured at 1 week and 4 week [15]. (Pan et al. 2016) [16]. Yoo et al. 2015, conducted a randomised controlled trial on 8 rabbits with 32 implants and BMP at 50 µg/ml (PLGA) molecules electrosprayed on the implant surface. Bone area and bone-implant contact were measured at 3 weeks and 7 weeks [16]. In the pilot study by [17]. Kim et al. 2015, 24 implants were used and different concentrations of BMP 0.1 mg/ml, 0.5 mg/ ml and 1.0 mg/ml were adsorbed directly and dried. The BMP release test were measured from the first 6 hours to 4 days. BH, BV, bone-implant contact and changes in Stability Quotient were measured at 8 weeks [17].

Results

This review compared the effect of rhBMP-2 on the implant surface to the existing SLA surface on the indicators of the extent and quality of osseointegration. Of the seven selected invivo studies, six studies modified the titanium implants with rhBMP-2 added onto a carrier and one directly in wet and dried layered coatings. The analyses at the bone-implant surface were performed at [3,4,6-8] weeks measuring the bone area, BH, BV and bone-implant contact. The de novo bone formation increased in the initial weeks in all implants biofunctionalized with rhBMP-2. A significant increase was seen in [2] studies in bone area, in 1 study in BH at 3, 4 and 8 weeks respectively. Later follow up analysis in 3 studies showed no statistical difference except for the heparinized implant with BMP-2 coating. The bone-implant contact showed statistically significant difference at [6,8] weeks in the implants with BMP-2 on Hydroxyapatite and collagen and Heparin carriers and none in all others, though the mean values were high in [6] studies. Both the removal torque and the Stability Quotient at 8 weeks increased compared to the control but had no statistical difference. The most efficient carrier releasing BMP-2 was heparin which provided prolonged and constant dose delivery up to 28 days.

Question: Does rhBMP-2 biofunctionalized implant surface compared to SLA implant surface provide better osseointegration?.

Population: Animal studies-healthy animals without systemic diseases or medication affecting bone physiology

Intervention: rhBMP-2 functionalization of implant surface with carriers or dip coated.

Comparison: SLA titanium implant surface.

Discussion

The rhBMP-2 is a dosage dependent growth factor. Lower concentrations promote bone formation, but higher concentrations can trigger osteoclasts, resulting in bone loss. For significant new bone formation, rhBMP-2 must be released in an initial burst and have a sustained release over days or weeks [21]. To favour bone formation, the release of BMP should be at a low constant dose by incorporating it into a carrier [22].

At very high doses, BMP-2 can cause formation of haematomas as reported in an experimental study by Bouyer et al. [23]. Other side effects of BMP at large doses were swelling and pain. In 2021, [13] Pang et al. studied the effects of Hydroxyapatite and collagen as a carrier for rhBMP-2. The BMP release was sustained over 5 days in the col/Hydroxyapatite/ BMP group. The outcomes of bone area, bone-implant contact and removal torque measured at 4 weeks showed higher values but there was no statistically significant difference [13] (Pang et al. 2021). In the study by [14] Cardoso et al. a remarkable decrease in the bone formation was seen in all the outcomes compared between PPL-10/BMP and PPL10. In comparison with the negative control, the outcomes of PPL10/BMP group were similar at 1 month and lower at 3 months [14]. Hence, phosphates cannot be preferred as a carrier for BMP-2. The PPA and PPL themselves may promote osseointegration in unfavourable bone conditions.

The study by Pan et al. employed Hyaluronic Acid (HA) as a carrier for BMP. At 4 weeks, the BA measured significantly higher values in the dip coated group, but the bone-implant contact showed no significant difference among the groups though higher values were seen in the dried coated group [15]. The randomised controlled trials by Yoo et al. 2015, at 3 weeks, there was seen a significant difference but none at 7 weeks in the BMP group in bone area. The bone-implant contact measures showed no significant difference at 3 and 7 weeks [16]. The pilot study by Kim et al. 2015, measured the outcomes for different concentrations of BMP-2 and found higher values in BH, BV and bone-implant contact at 8 weeks for 0.5 mg/ml and 1.0 mg/ml groups.

Table 4: Summary of findings using gradepro assessment tool.

Table 5: Results and significance.

The study by Lee et al. 2014, the new bone formation and bone-implant contact at 6 weeks showed significant increase in Hydroxyapatite/collagen compared to Hydroxyapatite/collagen/BMP group [19].

The review of these studies indicates a boost in new bone formation and bone-implant contact values when rhBMP-2 is present on the implant surface. However, the statistical analysis showed effectiveness in only 2 studies. As the study participants are animals, it is difficult to design a larger cohort which would have shown better statistically significant results. Some carriers like hydroxyapatite, collagen, hyaluronic acid, and phosphates, being osteoconductive themselves caused confounding outcomes. The studies should have measured removal torque and Stability Quotient and further follow-up. As different concentrations of BMP-2 were used in each of the studies, it was difficult to ensure homogeneity among the outcomes. The most effective dosage of BMP-2 being 20-100 µg/g of coating [24]. The effects of the carriers themselves should be considered and the inclusion criteria modified accordingly to the use of inert carriers.

Conclusion

Numerous investigations have revealed rhBMP-2 is a promising osseoinductive growth factor which can accelerate bone remodeling [25,26]. In the studies selected for this review, the rhBMP-2 was coated onto implants using carriers like Hydroxyapatite and collagen, Phosphorylated pullulan, Polylactide (L,D, co-gylcolide) PLGA, Heparin and Hyaluronic acid in 4 studies. The gradual liberation of rhBMP-2 from a depot, in these studies over a period of 3-5 days from the different carriers, aided the sustained de novo bone formation [22]. The study of the heparinized implant surface with BMP-2 bound as an electrostatic interaction and dried coated BMP measured the sustained release over 28 days and showed no statistically significant difference among them. Thus, the author concludes that the effectiveness of biofunctionalising an implant surface with rhBMP-2 as an osseoinductive agent is largely dependent on its sustained constant release into the peri-implant tissues. This can be achieved by dried coating or by carriers.

Implications for practice and future research: The review included SLA titanium implants as controls which are widely used nowadays. Establishing a successful implant surface which provides even better osseointegration will provide remarkable reduction in failures even in the presence of compromised bone physiology. Hence, further research involving carriers or techniques for sustained release of rhBMP-2 should be conducted to assess the true effectiveness of biofunctionalization of the implant surface with rhBMP-2.

Abbreviations: BMP-2: Bone Morphogenic Protein-2; CINAHL: Cumulative Index To Nursing and Allied Health Literature; DOPA: Dopamine Hydrochloride; ELISA: Enzyme-Like Immunosorbent Assay; Hap: Hydroxyapatite; Mesh: Medical Subject Headings; OHAT: Office of Health Assessment and Translation; PICO: Population, Intervention, Comparison and Outcome; PLGA: Poly (D,L Lactide-Co-Glycolide); PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analyses; RFA: Resonance Frequency Analysis; SLA: Sand-Blasted, Large Grit Acid Etched; SYRCLE: Systematic Review Centre for Laboratory Animal Experimentation.

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