Implantoplasty facing the risk of mechanical fatigue

Peri-implantitis, which affects approximately 24% of implants after ten years of function, presents major therapeutic challenges. Given the complexity of regeneration protocols or explantation, implantoplasty — the intra-oral machining of the implant surface to eliminate the biofilm — has emerged as a common clinical alternative. However, this technique inevitably reduces the cross-section of the implant, raising a critical question for the practitioner: what is the real impact of this thinning on mechanical durability and the risk of fatigue fracture?

This experimental study aims to precisely evaluate the influence of machining depth on the cyclic mechanical behaviour of implants. The authors tested 250 grade 4 titanium implants, comparing wall reductions of 0.2 mm, 0.4 mm (nominal thread height) and 0.6 mm to an untreated control group, using the ISO 14801:2016 standard and finite element simulations.

The tested hypothesis is that the depth of reduction modifies the von Mises stress distribution. More specifically, the study seeks to determine whether machining to the base of the thread (0.4 mm) could optimise mechanical performance by eliminating stress concentrations at the thread-body junction, compared to a partial (0.2 mm) or excessive (0.6 mm) reduction.

Experimental protocol and biomechanical analysis

This in vitro study adopts a hybrid approach, combining numerical simulations and experimental validations to evaluate the impact of implantoplasty on the structural integrity of the devices. The sample consists of 250 grade 4 titanium implants (commercially pure), characterised by an internal connection and threads with a height of 0.4 mm.

The implants were divided into four groups according to the depth of wall reduction:

• Control group: untreated original implants.
• Group 0.2 mm: partial thread reduction.
• 0.4 mm group: machining corresponding to the exact height of the threads (complete removal).
• Group 0.6 mm: deep reduction cutting into the implant body.

The analysis methodology was based on two pillars: on the one hand, a finite element analysis (FEA) to map the von Mises stress distribution and simulate fatigue behaviour; on the other hand, physical fatigue tests performed on an MTS Bionix servo-hydraulic machine according to the parameters of the ISO 14801:2016 standard. The reliability of the model was confirmed by a high statistical correlation between the simulated and experimental results (correlation coefficients > 0.9).

Results: The critical impact of machining depth

The study validated the correlation between numerical finite element analysis (FEA) simulations and experimental fatigue tests, showing a correlation coefficient greater than 0.9. The analyses revealed that von Mises stresses are systematically concentrated at the junction between the implant thread and its body.

The fatigue tests, conducted according to the ISO 14801:2016 standard, demonstrate a significant reduction in mechanical strength proportional to the aggressiveness of the implantoplasty, with a notable exception for the 0.4 mm group. The obtained fatigue limits are as follows:

Milling group (Wall reduction)	Fatigue limit (Newton)	Performance reduction vs Control
Original (Untreated)	351 N	-
0.2 mm	255 N	-27.3 %
0.4 mm (Thread height)	301 N	-14.2%
0.6 mm	185 N	-47.3%

Major clinical finding: machining at 0.4 mm (corresponding exactly to the thread height of this system) offers better fatigue resistance (301 N) than superficial machining at 0.2 mm (255 N). The authors attribute this result to a reduction in stress concentration at the thread-body junction once the thread is completely removed, unlike partial machining which creates residual areas of weakness.

Qualitative observation of failure modes reveals a transition depending on the applied load:

• High loads: Fracture primarily occurs in the coronal region of the implant.
• Low loads (long cycles): The point of failure shifts towards the implant-abutment connection.

The study highlights that beyond the initial thread height (0.6 mm in this case), the loss of cross-sectional area leads to a drastic drop in mechanical reliability (185 N), compromising the long-term survival of the restoration under functional loading.

This experimental study, validated by a strong correlation (> 0.9) between numerical models and mechanical tests (ISO 14801:2016), reveals a counter-intuitive behaviour of implantoplasty on grade 4 titanium. While the fatigue limit logically drops from 351 N (intact implant) to 185 N for a 0.6 mm reduction, the results show that a depth of 0.4 mm — corresponding precisely to the thread height of the tested implant — offers better resistance (301 N) than a superficial reduction of 0.2 mm (255 N). Clinically, this suggests that incomplete thread removal creates stress concentration zones (von Mises) at the thread-body junction, acting as premature fracture initiators. Conversely, a perfectly smoothed surface down to the implant body better redistributes the loads, further preserving mechanical stability under cyclic loading. Although rigorous, the in vitro study on 250 implants presents inherent limitations: finite element simulations and ISO conditions do not replicate the biological complexity of the oral environment (acidic pH, residual biofilms) nor the variability of multidirectional masticatory forces. Furthermore, these data are specific to an internal connection implant system with 0.4 mm threads; extrapolation to other geometries must remain cautious. In practice, for the practitioner:
• Never exceed the thread depth during implantoplasty: a reduction beyond the original height (0.6 mm here) halves the fatigue resistance.
• Aim for complete smoothing of the threads rather than partial milling: removing the entire thread height (0.4 mm) paradoxically reduces mechanical resistance less than a superficial pass (0.2 mm).
• Be vigilant regarding the cervical zone: high-load fractures are concentrated in the coronal region, a critical point during functional loading.

Summary of results

This study on 250 grade 4 implants demonstrates that machining depth non-linearly influences mechanical strength. While the intact implant presents a fatigue limit of 351 N, a 0.4 mm reduction (complete removal of the thread) better preserves the structure (301 N) than a partial reduction of 0.2 mm (255 N), the latter creating deleterious stress concentrations. Excessive machining to 0.6 mm causes the strength to drop to 185 N, drastically increasing the risk of coronal fracture.

In practical terms, for the practitioner:

• Complete rather than partial machining: to minimise the notch effect and stress peaks, implantoplasty should ideally reach the base of the thread (thread root) rather than simply removing the crest.
• Respect the critical limit: never exceed the original thread depth (here 0.4 mm); any machining cutting into the implant body (0.6 mm) reduces its durability by nearly 50%.
• Caution regarding narrow diameters: on small-diameter implants or those subjected to high occlusal loads, assess the risk of connection fracture before initiating machining that will inevitably weaken the cross-section.

Technical glossary of the study

Implantoplasty: Mechanical decontamination procedure consisting of machining the implant surface intraorally using rotary instruments (diamond or tungsten carbide burs) to remove the bacterial biofilm in cases of peri-implantitis.

Finite element analysis (FEA): Numerical simulation method used to evaluate mechanical stress distribution and predict the fatigue behaviour of implants as a function of machining depth (0.2 mm to 0.6 mm).

von Mises stress: Scalar index used to predict the failure of a material under load; the study locates the maximum concentrations at the junction between the thread and the implant body.

Fatigue limit: Maximum cyclic load that an implant can withstand without fracture. In this study, it ranges from 351 N for untreated implants to 185 N for a 0.6 mm wall reduction.

ISO 14801:2016: International standard specifying the dynamic fatigue testing conditions for endosseous dental implants, used here to validate the experimental numerical models.

Commercially pure grade 4 titanium: Material constituting the 250 tested implants, characterised by its mechanical properties which are altered by the reduction of the cross-section during machining.

Peri-implantitis: Inflammatory process triggered by bacterial colonisation leading to bone loss, with a prevalence estimated at approximately 24% after 10 years of function according to the cited scientific societies.

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