The minipig intraoral dental implant model: A systematic review and meta-analysis

OBJECTIVES

The objective of this report was to provide a review of the minipig intraoral dental implant model including a meta-analysis to estimate osseointegration and crestal bone remodeling.

METHODS

A systematic review including PubMed and EMBASE databases through June 2021 was conducted. Two independent examiners screened titles/abstracts and selected full-text articles. Studies evaluating titanium dental implant osseointegration in native alveolar bone were included. A quality assessment of reporting was performed. Random-effects meta-analyses and meta-regressions were produced for bone-implant contact (BIC), first BIC, and crestal bone level.

RESULTS

125 out of 249 full-text articles were reviewed, 55 original studies were included. Quality of reporting was generally low, omissions included animal characteristics, examiner masking/calibration, and sample size calculation. The typical minipig model protocol included surgical extraction of the mandibular premolars and first molar, 12±4 wks post-extraction healing, placement of three narrow regular length dental implants per jaw quadrant, submerged implant healing and 8 wks of osseointegration. Approximately 90% of studies reported undecalcified incandescent light microscopy histometrics. Overall, mean BIC was 59.88% (95%CI: 57.43-62.33). BIC increased significantly over time (p<0.001): 40.93 (95%CI: 34.95-46.90) at 2 wks, 58.37% (95%CI: 54.38-62.36) at 4 wks, and 66.33% (95%CI: 63.45-69.21) beyond 4 wks. Variability among studies was mainly explained by differences in observation interval post-extraction and post-implant placement, and implant surface. Heterogeneity was high for all studies (I2 > 90%, p<0.001).

CONCLUSIONS

The minipig intraoral dental implant model appears to effectively demonstrate osseointegration and alveolar bone remodeling similar to that observed in humans and canine models.

Pre-Clinical Models in Implant Dentistry: Past, Present, Future

Biomedical research seeks to generate experimental results for translation to clinical settings. In order to improve the transition from bench to bedside, researchers must draw justifiable conclusions based on data from an appropriate model. Animal testing, as a prerequisite to human clinical exposure, is performed in a range of species, from laboratory mice to larger animals (such as dogs or non-human primates). Minipigs appear to be the animal of choice for studying bone surgery around intraoral dental implants. Dog models, well-known in the field of dental implant research, tend now to be used for studies conducted under compromised oral conditions (biofilm). Regarding small animal models, research studies mostly use rodents, with interest in rabbit models declining. Mouse models remain a reference for genetic studies. On the other hand, over the last decade, scientific advances and government guidelines have led to the replacement, reduction, and refinement of the use of all animal models in dental implant research. In new development strategies, some in vivo experiments are being progressively replaced by in vitro or biomaterial approaches. In this review, we summarize the key information on the animal models currently available for dental implant research and highlight (i) the pros and cons of each type, (ii) new levels of decisional procedures regarding study objectives, and (iii) the outlook for animal research, discussing possible non-animal options.

Porous Titanium Granules in comparison with Autogenous Bone Graft in Femoral Osseous Defects: A Histomorphometric Study of Bone Regeneration and Osseointegration in Rabbits

Background

The high resorption rate of autogenous bone is a well-documented phenomenon that can lead to insufficient bone quality and quantity in an augmented area. Nonresorbable bone substitutes might perform better than autogenous bone in certain applications if they are able to provide adequate bone formation and graft osseointegration.

Purpose

The aim of this study was to compare the osseous regeneration and graft integration in standardized defects in the rabbit femur treated either with porous titanium granules or autogenous osseous graft.

Materials and Methods

Standardized femoral osseous defects were surgically induced in 45 New Zealand rabbits. Fifteen were treated with porous titanium granules (TIGRAN™-PTG) and membrane (PTGM), 15 with autogenous graft and membrane (AGM), and 15 with membrane alone (CM, control). At six weeks, the defects were assessed histologically and histomorphometrically.

Results

PTGM as compared to AGM presented similar percentages of newly formed bone tissue, but a significantly higher fraction of the region of interest was filled with the bone substitute material. Accordingly, the composite of new bone plus bone substitute material showed significantly higher volumes for PTGM. Yet, the smaller amount of remaining autogenous bone was far better osseointegrated than the titanium granules, which in large regions showed no connection to newly formed bone. Both PTGM and AGM as compared to CM presented higher values of newly formed bone.

Conclusions

This study demonstrated that PTG was similarly effective as autogenous osseous graft in achieving osseous regeneration while PTG performed markedly better in graft volume stability. The resulting higher total percentage of new bone combined with the bone substitute material in PTG could provide a superior foundation for implant placement.

Osteogenic Potential of Porous Titanium. An Experimental Study in Sheep

The search for osteoinductive as well as osteoconductive materials has led to the novel idea of using titanium in bone augmentations of the alveolar crest. Due to its excellent biocompatibility and favorable osteogenic properties, highly porous TiO₂ granules has been proposed as a promising material for non-resorbable synthetic bone grafts in the restoration of large bone defects, and for bone augmentation in dental applications.

OBJECTIVES

The aim of this study was to investigate the osteoconductive properties and biological performance of porous titanium granules used in osseous defects adjacent to the maxillary sinus in sheep. The experimental animal study involved 15 yearling sheep with a focus on the osteogenic potential of porous titanium used for subantral augmentation.

MATERIAL AND METHODS

Calibrated defects were prepared in the subantral region of sheep. The defects were randomized into tests and control group. The test defects were grafted with porous titanium granules (PTG), whereas control defects were left empty (sham). Defects were left for healing for 30, 60, and 90 days. After healing, the grafted areas were removed and finally osteoconductivity was analyzed by an orthopantograph (OPG} and histology.

RESULTS

Significantly more new bone formed in PTG grafted defects compared with sham. The control group showed significantly less expression of key inflammation cells, but with no significant difference in key inflammation cells compared with the experimental groups.

CONCLUSION

Porous titanium can offer as an effective alternative to calcium phosphate and bone collagen-based materials used for subantral augmentation of the maxillary bone in cases of dental implantation.

Vertical bone augmentation with titanium granule blocks in rabbit calvaria

To determine whether it is possible to vertically augment bone utilizing a block graft from compressed titanium granules mainly used previously for contained bone defects and to determine whether there exists a difference in osteoconductive properties between the white and the grey granules. In 11 rabbits, 4 titanium blocks were inserted on each rabbit's skull bone according to a randomized scheme. These blocks were made from standardized compressed titanium granules. Type A: PTG grey, small granules (Pourus Titanium Granules, Tigran, Malmö, Sweden); Type B: PTG grey, large granules; Type C: PTG white, small granules; Type D: PTG white large granules. After 12 weeks, the animals were sacrificed and specimens were collected for histology and μCT scanning. From both the μCT and histology, it can be said that bone formation was successfully achieved for all groups, and the granules maintained their volume. The histomorphometric BA (bone area) evaluation in the entire grafted area presented that there were no statistical differences between all groups tested. The lowest 1/4 BA in contact with the rabbit skull presented that groups A and C presented the highest mean BA, and group A presented significantly higher BA than that of group D (p = 0,049). No significant differences were noted between groups A, B and C. Within the limitation of this study, no differences were noted between small white or grey PTG blocks. The large granules presented less bone ingrowth area compared to the small granules and this trend was regardless of the different PTG types. The entire grafted area was not filled with new bone suggesting that bone migration occurred mostly from the existing cortical bone side suggesting contact osteogenesis.

Impact of particulate deproteinized bovine bone mineral and porous titanium granules on early stability and osseointegration of dental implants in narrow marginal circumferential bone defects

The use of two particulate bone graft substitute materials in experimental narrow marginal peri-implant bone defects was investigated with respect to early bone healing and implant stability. Porous titanium granules, oxidized white porous titanium granules (WPTG), and demineralized bovine bone mineral (DBBM) were characterized in vitro, after which the two latter materials were tested in experimental peri-implant bone defects in six minipigs, with empty defects as control. After mandibular premolar extraction, the top 5mm of the alveoli were widened to 6mm in diameter, followed by the placement of six implants, three on each side, in each pig. Six weeks of healing was allowed. The WPTG showed better mechanical properties. No significant differences in resonance frequency analysis were found directly after compacting or healing, and similar quantities of defect bone formation were observed on micro-computed tomography for all groups. Histomorphometric analysis demonstrated a more coronal bone-to-implant contact in the DBBM group, which also displayed more defect bone fill as compared to the WPTG group. The better mechanical properties observed for WPTG appear of negligible relevance for the early stability and osseointegration of implants.

Effect of Porous Titanium Granules on Bone Regeneration and Primary Stability in Maxillary Sinus: A Human Clinical, Histomorphometric, and Microcomputed Tomography Analyses

The aim of this randomized controlled study was to comparatively analyze the new bone (NB), residual bone, and graft-bone association in bone biopsies retrieved from augmented maxillary sinus sites by histomorphometry and microcomputed tomography (MicroCT) in a split-mouth model to test the efficacy of porous titanium granules (PTG) in maxillary sinus augmentation. Fifteen patients were included in the study and each patient was treated with bilateral sinus augmentation procedure using xenograft (equine origine, granule size 1000-2000 μm) and xenograft (1 g) + PTG (granule size 700-1000 μm, pore size >50 μm) (1 g), respectively. After a mean of 8.4 months, 30 bone biopsies were retrieved from the implant sites for three-dimensional MicroCT and two-dimensional histomorphometric analyses. Bone volume and vital NB percentages were calculated. Immediate after core biopsy, implants having standard dimensions were placed and implant stability quotient values were recorded at baseline and 3 months follow-up. There were no significant differences between groups according to residual bone height, residual bone width, implant dimensions, and implant stability quotient values (baseline and 3 months). According to MicroCT and two-dimensional histomorphometric analyses, the volume of newly formed bone was 57.05% and 52.67%, and 56.5% and 55.08% for xenograft + PTG and xenograft groups, respectively. No statistically significant differences found between groups according to NB percentages and higher Hounsfield unit values were found for xenograft + PTG group. The findings of the current study supports that PTG, which is a porous, permanent nonresorbable bone substitute, may have a beneficial osteoconductive effect on mechanical strength of NB in augmented maxillary sinus.

Microwave-assisted fabrication of titanium implants with controlled surface topography for rapid bone healing

Morphological surface modifications have been reported to enhance the performance of biomedical implants. However, current methods of introducing graded porosity involves postprocessing techniques that lead to formation of microcracks, delamination, loss of fatigue strength, and, overall, poor mechanical properties. To address these issues, we developed a microwave sintering procedure whereby pure titanium powder can be readily densified into implants with graded porosity in a single step. Using this approach, surface topography of implants can be closely controlled to have a distinctive combination of surface area, pore size, and surface roughness. In this study, the effect of various surface topographies on in vitro response of neonatal rat calvarial osteoblast in terms of attachment and proliferation is studied. Certain graded surfaces nearly double the chance of cell viability in early stages (∼one month) and are therefore expected to improve the rate of healing. On the other hand, while the osteoblast morphology significantly differs in each sample at different periods, there is no straightforward correlation between early proliferation and quantitative surface parameters such as average roughness or surface area. This indicates that the nature of cell-surface interactions likely depends on other factors, including spatial parameters.

Porous titanium granules in critical size defects of rabbit tibia with or without membranes

Recently, porous titanium granules (PTGs) have been indicated for the preservation of the dimensions of post-extraction sockets, as a filler in sinus lift procedures and for the treatment of peri-implant and periodontal defects, based on the osteoconductivity and dimensional stability of the titanium granules. However, there is a lack of information regarding the use of this material in larger defects and in conjunction with membranes. The objective of this study is to test the behavior of PTGs used to fill critical size defects in rabbit tibiae, with and without membranes. Critical defects were created in both tibiae of rabbits, divided randomly into three groups: Group A (defect filled with PTG), Group B (defect filled with PTG+collagen membrane) and a control group (empty defect). After six weeks, histomorphometric analysis was performed. The results showed more defect closures at the cortical area (87.37%±2.2%) and more bone formation at the marrow area (57.6%±1.3%) in Group B, in comparison with the other groups (P<0.05); the use of membranes improved the material stability expressed as more percentages of the original material when membranes were used (P<0.05). Finally, inflammatory reactions were observed when the granules were not protected by membranes. In spite of the limitations of this animal study, it may be concluded that PTG particles are osteoconductive and allow bone growth. The PTG particles must be covered by a membrane, especially when grafting larger defects, in order to control particle migration, promote clot stabilization and separate the PTG graft from undesired soft tissue cells.