Implant stability at 6 weeks: the missing link in osseointegration

Clinical success in implantology relies on a seamless transition between mechanical primary stability and biological secondary stability. While the thread profile ensures initial anchorage, bone remodelling within the healing chambers determines the longevity of the integration. The 3D stabilisation thread design (TP 3DS) and the Oxion biocompatible coating were designed to optimise these processes. However, although their effects are documented at 3 and 12 weeks, the intermediate 6-week phase — a pivotal period for bone maturation — remains under-explored in current literature.

This experimental study, conducted on an ovine model, specifically aims to evaluate the synergistic impact of the TP 3DS macro-geometry and the Oxion surface on osseointegration at 6 weeks post-implantation. The objective was to quantify whether this combination improves biomechanical stability and bone occupancy compared to a conventional thread. The authors tested the hypothesis that a design limiting lateral displacement, coupled with a high-wettability surface, would accelerate the tissue response during this critical time window, thereby facilitating a more predictable secondary stabilisation.

Methodology: an ovine experimental protocol

This in vivo study evaluated the synergistic impact of implant macrogeometry and a bioactive coating on osseointegration at an intermediate stage. The protocol was conducted on a model of 5 female sheep, each receiving 4 implants per experimental group in the mandible.

The researchers compared four configurations of titanium alloy implants:

• TP Group: standard thread design (buttress), uncoated surface.
• TP + Oxion group: standard design combined with the Oxion biocompatible coating.
• TP 3DS Group: 3D stabilisation geometry (alternating curved and linear threads) with RBM (Resorbable Blast Media) surface, uncoated.
• TP 3DS + Oxion group: 3D geometry combined with Oxion coating.

The analytical protocol began with a surface characterisation via scanning electron microscopy (SEM) and contact angle quantification to evaluate wettability. After a 6-week healing period, the mandibles were harvested for histomorphometric and biomechanical analyses: measurement of bone-to-implant contact (BIC), bone area fraction occupancy (BAFO) and torque-out (removal torque) tests to quantify secondary stability.

Results: Homogeneous biomechanical and biological stability at 6 weeks

Data analysis at the 6-week intermediate stage reveals uniform performance across the different designs and surface treatments, despite distinct initial physicochemical properties.

Surface characterisation and wettability

Scanning electron microscopy (SEM) examination and goniometry confirmed the impact of the Oxion coating on the implant-fluid interface. The contact angle was significantly lower for Oxion-coated implants compared to uncoated surfaces (p < 0.001), indicating markedly superior hydrophilicity and wettability upon insertion.

Biomechanical stability and histomorphometry

Despite the initial wettability advantage, the torque-out (unscrewing torque) tests performed ex vivo did not show statistical superiority for the Oxion coating or the TP 3DS macrogeometry at this stage of the healing process. Histomorphometric measurements confirm this trend towards homogeneity.

Qualitative and histological observations

The qualitative evaluation of the histological sections (Kirkland red staining and calcified tissue) revealed:

• Effective and generalised osseointegration in the four experimental groups (TP, TP+Oxion, TP 3DS, TP 3DS+Oxion).
• A clear presence of calcified tissue (appearing red on the micrographs) in direct contact with the black implant surface.
• The absence of deleterious inflammatory reaction or interpositional fibrosis, confirming the biocompatibility of the Oxion coating.

In summary, while the synergy between the TP 3DS geometry and the Oxion coating does not accelerate osseointegration in a statistically detectable manner at the pivotal 6-week stage, the results demonstrate perfect tissue integration, ensuring the transition between initial primary stability and long-term secondary stability.

Analysis of results and clinical relevance

The data obtained at 6 weeks mark a crucial transitional stage in the osseointegration process. Although the Oxion coating significantly improves the wettability of the implant surface (p<0.001), this study demonstrates that this physical advantage does not translate into biomechanical or histomorphometric superiority at this intermediate stage. With statistically similar torque-out (p>0.112), BIC (p>0.130) and BAFO (p>0.105) values between the groups, the study suggests a healing plateau where differences in macrogeometry (TP 3DS vs buttress) and surface temporarily fade.

Limitations and perspectives

This study, although rigorous, relies on an ovine model (n=5) whose bone metabolism differs from that of humans. The absence of a significant difference at 6 weeks highlights a temporal limitation: the expected synergistic effect between the TP 3DS geometry and Oxion could manifest earlier (during the primary/secondary stability phase at 3 weeks) or later during the final remodelling. The study focuses exclusively on this intermediate measurement point, acting as a bridge between the already known data at 3 and 12 weeks.

Implications for practice

For the clinician, these results primarily confirm the safety and total biocompatibility of the Oxion coating, which maintains a stability equivalent to current standards without compromising osseointegration. Although the acceleration of healing is not statistically evident at 6 weeks in this model, the consistent performance observed across all histomorphometric parameters guarantees predictable reliability for conventional loading protocols.

Summary of results

At 6 weeks post-implantation in sheep, this study shows that despite a significantly increased wettability for the Oxion coating (p < 0.001), no statistical difference is observed for BIC (p > 0.130), BAFO (p > 0.105) or removal torque (p > 0.112). The TP and TP 3DS designs demonstrate equivalent histomorphometric performance, confirming stable osseointegration but without marked superiority of the surface or geometric innovations at this intermediate stage of healing.

In practical terms, for the practitioner:

• Loading protocol: The use of the TP 3DS macrogeometry or the Oxion coating does not, on its own, justify accelerating loading to 6 weeks compared to standard designs.
• Surface reliability: The Oxion coating is a safe and highly hydrophilic option, ensuring biocompatibility and secondary stability comparable to conventional RBM surfaces.
• Healing kinetics: Between 3 and 12 weeks, the benefits of innovative designs appear to diminish in favour of a uniform bone healing plateau, which warrants caution during the intermediate phases.

Technical glossary of the study

TP 3DS (Three-Dimensional Stabilization thread form): Specific implant macro-geometry incorporating curved and linear features along the thread crest. This design is intended to optimise retention and limit lateral displacement compared to conventional "buttress" type threads.

Oxion: Innovative biocompatible surface coating applied to titanium. In this study, it demonstrates significantly higher hydrophilicity (reduced contact angle, p<0.001) compared to uncoated surfaces, potentially promoting initial biological interactions.

BIC (Bone-to-Implant Contact): Histomorphometric parameter measuring the percentage of the implant surface in direct contact with the bone. It is the standard indicator of morphological osseointegration at the interface.

BAFO (Bone Area Fraction Occupancy): Histological measurement quantifying the area fraction occupied by bone within the threads and healing chambers. This parameter assesses the newly formed peri-implant bone density.

Torque-out: Ex vivo biomechanical evaluation measuring the torque required to remove the implant. This test quantifies the functional stability and the strength of the biological bond between the bone and the implant.

Healing chambers: Void spaces created by the geometry of the implant threads during insertion. These areas are the site of de novo bone formation and remodelling, critical processes for the transition from primary stability to secondary stability.

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