Hydrophilic Dental Implant Surfaces: A Critical Review of the Jinno et al. (2021) Study and the Current Scientific Evidence

The study “Impact of a Hydrophilic Dental Implant Surface on Osseointegration: Biomechanical Results in Rabbit” by Jinno et al. (2021) investigated whether a hydrophilic dental implant surface could improve osseointegration compared with a conventional hydrophobic surface using a rabbit tibia and femur model. The researchers primarily evaluated implant stability through biomechanical measurements, including Implant Stability Quotient (ISQ) and removal torque values at multiple healing intervals. This article reviews the study’s methodology, discusses its findings within the context of the broader scientific literature, and highlights important considerations for interpreting its conclusions. Researchers who wish to read the original publication before continuing can access the study here:

Evaluating the Study Design in Context

The study by Jinno et al. (2021) contributes valuable information regarding implant stability under the specific experimental conditions investigated. However, like any preclinical study, its conclusions should be interpreted within the context of its methodology. Several aspects of the experimental design may have limited its ability to detect the biological advantages that have been reported for hydrophilic implant surfaces, particularly during the early stages of osseointegration and in clinically challenging situations.

1. Use of a Dense Cortical Bone Model

One important consideration is the selection of the rabbit tibia and femur as the implantation sites. These bones are characterized by a high proportion of dense cortical bone, providing excellent primary stability regardless of implant surface characteristics.

Hydrophilic implant surfaces are primarily intended to enhance early biological healing, especially in situations where bone quality is compromised, such as low-density cancellous bone, reduced vascularity, or medically compromised patients.

Supporting this concept, studies using ovine tibial models have noted that excellent outcomes for different implant surfaces may largely reflect the dense cortical structure of the bone rather than the influence of surface chemistry itself.

Consequently, evaluating implants in an ideal bone environment may reduce the likelihood of observing differences that could become clinically relevant in more demanding conditions.

2. Choice of Outcome Measures

The study assessed implant performance using:

• Implant Stability Quotient (ISQ)
• Removal Torque (RTQ)

Both measurements are well-established biomechanical indicators of implant stability. However, they primarily evaluate the mechanical interaction between bone and implant rather than the early biological events occurring at the implant surface.

Several histological investigations have demonstrated that hydrophilic surfaces can produce higher bone-to-implant contact (BIC) during the first weeks of healing compared with conventional hydrophobic surfaces, although these differences often diminish as healing progresses.

This suggests that relying exclusively on biomechanical outcomes may underestimate biological differences occurring during the earliest phases of osseointegration.

3. Lack of Histological Assessment

Another limitation is the absence of histological or histomorphometric evaluation.

The study did not report parameters such as:

• Bone-to-implant contact (BIC)
• Bone area fraction occupancy (BAFO)
• Cellular activity
• Vascularization around the implant

These measurements are commonly used to investigate the biological mechanisms underlying osseointegration and frequently reveal differences that are not detectable through mechanical testing alone.

Previous studies have reported improved early BIC, greater bone formation, and enhanced early healing around hydrophilic implant surfaces, particularly during the first few weeks after placement.

Including histological analysis would therefore have provided a more comprehensive assessment of the healing process.

4. Timing of the Evaluation

Measurements were performed at baseline and after 2, 4, and 8 weeks.

A substantial body of research suggests that the greatest biological advantage of hydrophilic surfaces occurs during the earliest healing period—approximately 3 to 14 days—when blood clot organization, angiogenesis, and woven bone formation are most active.

Several investigations have reported significantly greater early bone formation around bioactivated ultra-hydrophilic surfaces during this period.

Because the study did not include observations within the first week of healing, it may not have captured the phase during which hydrophilic surface modifications exert their greatest biological influence.

5. Healthy Animal Model

The experimental model consisted of healthy mature rabbits without systemic disease or compromised bone conditions.

This provides excellent experimental control but differs from many clinical situations in which implant surface modifications are expected to provide the greatest benefit.

Published research has demonstrated improved performance of hydrophilic implants in models involving:

• diabetes and hyperglycemia,
• low-density bone,
• immediate loading,
• compromised healing conditions.

For example, one study found that hydrophilic surfaces accelerated osseointegration in hyperglycemic animals and reduced the negative effects of diabetes on bone healing.

Accordingly, while the Jinno study is informative for healthy bone conditions, its findings may not necessarily be generalizable to higher-risk patient populations.

6. Surface Preservation and Hydrophilicity

Hydrophilic implant surfaces are known to be sensitive to storage and handling conditions.

Previous research has shown that biological aging of implant surfaces, including carbon contamination over time, can reduce surface wettability and potentially influence cell attachment and early healing.

The Jinno study does not provide detailed information regarding storage conditions, transportation, or handling of the hydrophilic implants before implantation.

Although this does not indicate that surface degradation occurred, reporting these details would improve reproducibility and help confirm that the intended hydrophilic properties were maintained throughout the study.

Evidence Supporting Hydrophilic Implant Surfaces

While the Jinno study did not identify significant differences between the tested implant surfaces under its experimental conditions, numerous other studies have reported advantages for hydrophilic surfaces, particularly during early healing or under challenging clinical circumstances.

Improved Early Bone-to-Implant Contact

Human histological studies comparing SLActive and SLA surfaces have demonstrated greater osseointegration with hydrophilic implants after 2 and 4 weeks, despite similar healing patterns later in the observation period.

These findings support the concept that hydrophilicity primarily accelerates early biological integration rather than altering the final outcome after complete healing.

Lower Clinical Failure Rates

A six-year retrospective review involving 2,918 implants reported a significantly lower failure rate for hydrophilic INICELL implants (0.5%) compared with hydrophobic TST implants (1.5%).

Although retrospective studies cannot establish causality, these findings suggest that improved early healing may translate into meaningful long-term clinical outcomes.

Benefits in Low-Density Bone

Experimental studies performed in rabbit iliac bone, which better represents low-density bone, have demonstrated:

• higher bone-to-implant contact,
• greater bone area fraction,
• higher removal torque values

for hydrophilic implants compared with conventional surfaces.

Interestingly, these results differ from those reported by Jinno et al., highlighting how the choice of implantation site may substantially influence the observed outcomes.

Overall Perspective

The findings of Jinno et al. (2021) are valid within the specific experimental conditions employed. However, several methodological characteristics may have reduced the study’s sensitivity to detect the biological advantages reported for hydrophilic implant surfaces in other investigations.

These considerations include:

Taken together, the available evidence suggests that hydrophilic implant surfaces may offer their greatest advantages during the early phases of healing and in compromised clinical situations. Therefore, rather than contradicting the broader literature, the Jinno study may be best interpreted as demonstrating that under favorable healing conditions and in dense cortical bone, both implant surface types can achieve comparable mechanical stability. This interpretation aligns more closely with the growing body of evidence indicating that the benefits of hydrophilic surfaces are context-dependent rather than universal across all experimental models.