Pore diameter of more than 100 microm is not requisite for bone ingrowth in rabbits

Ari I. Itälä, Heimo O. Ylänen, Clifford Ekholm, Kaj H. Karlsson, Hannu T. Aro

Full text link : https://pubmed.ncbi.nlm.nih.gov/11745521/

Pore Diameter of More Than 100 μm Is Not Requisite for Bone Ingrowth in Rabbits

1. Scientific Reference

• Study Title: Pore Diameter of More Than 100 μm Is Not Requisite for Bone Ingrowth in Rabbits
• Authors: Ari I. Itälä, Heimo O. Ylänen, Clifford Ekholm, Kaj H. Karlsson, Hannu T. Aro.
• Journal: Journal of Biomedical Materials Research (Applied Biomaterials)
• Publication Year: 2001
• DOI: Not specified in the provided material.

2. Scientific Background

The design of porous biomaterials plays a central role in achieving stable bone integration. Whether used in orthopedic surgery or implant dentistry, porous implant surfaces are intended to facilitate bone tissue penetration, creating a biological connection that contributes to long-term implant stability. Among the numerous structural parameters investigated over the years, pore diameter has received particular attention, with many experimental studies suggesting that pores larger than 100 μm provide the most favorable environment for mineralized bone formation.

Despite this widespread assumption, previous investigations have often relied on porous materials with heterogeneous architectures, making it difficult to isolate the specific influence of pore size. To overcome this limitation, the present study employed laser-perforated titanium implants with highly standardized pore dimensions. This experimental design allowed the investigators to evaluate the relationship between pore diameter and bone ingrowth under controlled biological conditions while minimizing structural variability. Although performed in an animal model, the findings contribute to the broader understanding of osseointegration and may inform future developments in implant surface engineering and bone-regenerative biomaterials.

3. Study Objective

The primary objective of this investigation was to determine whether a pore diameter exceeding 100 μm is truly required for successful bone ingrowth. Using a controlled rabbit model, the authors compared bone formation within laser-created titanium pores ranging from 50 to 125 μm under non-load-bearing conditions. The study specifically sought to evaluate whether smaller pore diameters could support mineralized bone formation comparable to that observed in larger pores.

4. Methodology

This research was designed as a preclinical in vivo experimental study using adult rabbits. Eight female rabbits received a total of 16 laser-perforated titanium implants placed within the cancellous bone of the distal femoral condyles. Two implant thicknesses (250 μm and 500 μm) were evaluated, with each implant containing rows of standardized pores measuring 50, 75, 100, and 125 μm in theoretical diameter.

Following a 12-week healing period, the specimens were harvested for histological assessment. Bone ingrowth was quantified using backscattered electron scanning electron microscopy (BEI-SEM), allowing precise measurement of the actual pore dimensions, the percentage of pore area occupied by newly formed bone, and the residual porosity of the regenerated tissue. Statistical comparisons were performed using two-way analysis of variance (ANOVA) followed by Tukey’s post hoc test.

5. Main Findings

After the 12-week healing period, microscopic evaluation demonstrated consistent bone formation within the titanium pores located in cancellous bone. Empty pores were observed only in implant regions that were not in direct contact with trabecular bone, indicating that local anatomical positioning rather than pore size influenced the absence of bone ingrowth in those areas.

Quantitative analysis revealed no statistically significant differences in the amount of newly formed bone among the four pore-size groups or between the two implant thicknesses. Depending on the experimental group, newly formed bone occupied approximately 64% to 76% of the available pore area. Likewise, the porosity of the regenerated bone remained low, ranging between 2% and 4%, with no measurable effect attributable to pore diameter or implant thickness.

One of the most noteworthy observations was the presence of well-organized lamellar osteonal structures throughout the investigated pore-size range. Secondary osteons, including central vascular canals and osteocyte lacunae, were identified even within the smallest pores. This finding suggests that, under the biological conditions of this model, mature bone architecture can develop without requiring pore diameters greater than 100 μm.

Overall, the experimental data indicate that, within the evaluated range of 50–125 μm, pore diameter alone did not determine the extent of bone ingrowth in unloaded titanium implants.

6. Clinical Interpretation

This study challenges one of the long-standing assumptions in biomaterial research—that pores larger than 100 μm are essential for successful bone ingrowth. Rather than demonstrating a clear biological threshold, the findings suggest that smaller pores can also support extensive mineralized bone formation when the implant is mechanically stable and surrounded by viable cancellous bone.

From a biomaterials perspective, these results highlight that pore diameter should not be considered in isolation when designing implant surfaces. Osseointegration is influenced by multiple interacting factors, including implant stability, the architecture and interconnectivity of the porous network, the biological environment, and local mechanical conditions. Consequently, optimizing implant performance is likely to require a comprehensive approach rather than focusing exclusively on increasing pore size.

For researchers involved in implant dentistry and oral reconstruction, the study provides valuable biological insight into bone ingrowth mechanisms. However, it should not be interpreted as evidence that smaller pores are clinically superior or that current implant design principles should be revised. The investigation was conducted under non-load-bearing experimental conditions, which differ substantially from the functional environment encountered by dental implants after prosthetic loading.

The authors also acknowledge that the study evaluated only the quantity of bone formed within the pores, without assessing the mechanical strength of the bone–implant interface. Additional investigations involving loaded implants, different healing intervals, and mechanical testing would therefore be necessary before translating these observations into clinical recommendations.

Accordingly, this work should be viewed as a contribution to the fundamental understanding of osseointegration rather than as direct evidence supporting a change in implant surface design for routine clinical practice.

7. Clinical Applications

Although performed in a rabbit model, this investigation provides useful information for the development of porous biomaterials intended for orthopedic and dental implant applications. The findings suggest that successful bone ingrowth cannot be explained solely by pore diameter and that other structural and biological characteristics should also be considered during implant surface design.

For implant dentistry, these observations may support ongoing research into porous titanium implants, bone substitute materials, and advanced surface engineering strategies aimed at improving osseointegration. The study is particularly relevant to the design and evaluation of biomaterials intended for bone regeneration and implant-supported rehabilitation.

8. Level of Evidence, Limitations, and Transparency

This publication is a preclinical in vivo experimental animal study conducted in rabbits. As such, it provides valuable biological evidence regarding bone ingrowth into porous titanium structures but represents a lower level of evidence than controlled clinical trials or systematic reviews involving human subjects.

A major strength of the study lies in its highly standardized experimental design. Unlike many previous investigations that relied on irregular porous coatings or sintered bead structures, the authors used laser-perforated titanium implants with precisely controlled pore dimensions. This approach minimized geometric variability and enabled a more reliable comparison of the biological response across different pore diameters.

The authors also acknowledge several important limitations. First, the implants were evaluated exclusively under non-load-bearing conditions, meaning that the findings cannot be directly extrapolated to functional orthopedic or dental implants subjected to occlusal or mechanical forces. Second, the investigation quantified the extent of bone ingrowth but did not evaluate the mechanical strength of the bone–implant interface. Consequently, the biological presence of bone within the pores should not automatically be interpreted as equivalent to superior implant fixation. Finally, the authors suggest that additional studies incorporating earlier healing intervals, fluorochrome labeling, and mechanically loaded models would be necessary to better characterize the dynamics and functional significance of bone ingrowth.

Regarding transparency, no specific conflicts of interest are reported in the material provided. The study received financial support from the National Technology Agency of Finland (TEKES) and formed part of the activities of the Åbo Akademi Process Chemistry Group, but no industrial sponsorship influencing the reported findings is disclosed.

9. Key Study Highlights

• Study design: Preclinical in vivo animal experiment.
• Level of evidence: Experimental preclinical research.
• Animal model: Eight adult female rabbits.
• Number of implants: Sixteen laser-perforated titanium implants.
• Follow-up period: 12 weeks.
• Implant characteristics: Titanium plates measuring 250 μm or 500 μm in thickness with theoretical pore diameters of 50, 75, 100, and 125 μm.
• Primary outcome: Quantitative assessment of bone ingrowth using backscattered electron scanning electron microscopy (BEI-SEM).
• Principal finding: No statistically significant difference in bone ingrowth among pore diameters ranging from 50 to 125 μm.
• Additional observation: Mature osteonal bone structures developed even within the smallest pores.
• Scientific conclusion: Under unloaded experimental conditions, pore diameters greater than 100 μm were not required to achieve substantial bone ingrowth.
• Main limitations: Animal model, absence of mechanical loading, and no assessment of interfacial mechanical strength.

10. Scientific Impact

This study represents an important contribution to biomaterials research by re-examining one of the most widely accepted concepts regarding the relationship between pore size and bone ingrowth. Rather than confirming the traditional assumption that pores exceeding 100 μm are biologically necessary, the investigators demonstrate that extensive mineralized bone formation can occur across a broader range of pore diameters under carefully controlled experimental conditions.

Another significant contribution is the introduction of a highly standardized laser-perforated implant model. By reducing architectural variability commonly encountered in conventional porous materials, the study provides a more rigorous framework for investigating how individual structural parameters influence osseointegration.

For implant dentistry and orthopedic biomaterials, these findings encourage a more comprehensive view of implant surface optimization. They suggest that pore diameter should be considered alongside other design characteristics—such as pore architecture, interconnectivity, implant stability, and the biological environment—rather than as an isolated determinant of bone integration.

Although the results should not be interpreted as evidence for immediate changes in clinical implant design, they provide a valuable biological foundation for future research aimed at developing next-generation porous implant surfaces capable of enhancing osseointegration in both orthopedic and dental applications.

11. Editorial Conclusion

This experimental study provides compelling biological evidence that substantial bone ingrowth can occur within titanium pores smaller than the traditionally accepted 100 μm threshold when implants are placed under stable, non-load-bearing conditions. By combining a highly standardized implant design with quantitative microscopic analysis, the authors offer new insights into the relationship between pore geometry and osseous healing.

While these findings improve our understanding of bone–implant interactions, they should be interpreted within the limitations of a preclinical animal model. Further investigations incorporating functional loading, mechanical testing, and human clinical studies will be required before these observations can influence implant design or clinical decision-making in implant dentistry.