Optimising immediate loading: the potential of porous implants

While immediate loading is appealing due to the reduction in treatment time, it exposes peri-implant tissues to a critical mechanical environment, particularly in high-stress posterior sectors. The practitioner faces a major challenge: limiting "stress shielding", the phenomenon where the excessive rigidity of the implant deprives the alveolar bone of the mechanical stimuli necessary for its maintenance and remodelling.

This finite element method (FEM) study aims to evaluate and compare the biomechanical and mechanobiological response of two Ti-6Al-4V implant designs: a conventional threaded model (Model A) and a porous implant (Model B) presenting a porosity of 64.26%. The objective is to determine how these architectures influence the maturation of the 0.2 mm peri-implant bone callus under two distinct protocols: immediate loading (simulated by a frictional interface) and delayed loading (bonded interface after mineralisation).

The authors test the hypothesis that a porous structure, by adjusting its elastic modulus to that of native bone, allows for a more physiological load transfer. This design would aim to promote tissue differentiation towards mature bone while reducing deleterious cortical stress concentrations during the early phases of healing, thus offering a promising alternative to traditional dense implants.

Methodology of numerical simulation

This study is based on a three-dimensional finite element analysis (FEA) aimed at modelling the biomechanical interaction between the implant and the bone. The protocol uses a virtual bone block incorporating a 0.2 mm thick peri-implant callus to simulate the initial healing phase.

• Model design: Two Ti-6Al-4V alloy implants were compared. Model A represents a conventional threaded implant. Model B is an implant structured as a porous scaffold displaying a porosity of 64.26%.
• Material properties: Bone tissues were modelled as poroelastic media to capture the dynamic response of fluids and solids under load.
• Loading protocols:
  • Immediate loading (IL): Simulated by a frictional bone-implant interface to reproduce the post-extraction mechanical environment.
  • Delayed loading (DL): Simulated by a tied interface, corresponding to complete healing and total tissue mineralisation.
• Mechanobiological analysis: The simulations were performed on ABAQUS/Standard. The Prendergast-Huiskes stimulus was applied to predict the cell fate of the callus, enabling the quantification of the formed tissue fractions (mature bone, immature bone, cartilage and fibrous tissue).

Results of the biomechanical and mechanobiological analysis

The comparative finite element study revealed significant disparities in stress distribution and biological response between the conventional threaded implant (Model A) and the porous implant (Model B), particularly under immediate loading (IL).

Under immediate loading, the porous implant allowed a drastic reduction in cortical stress (32.5 MPa versus 88 MPa for the conventional model). Notably: the mechanical stimulation within the 0.2 mm callus is multiplied by almost ten with Model B, reaching values of 20.5 to 31.6 MPa, whereas the threaded model generates only about 2.5 MPa.

Mechanobiological analysis, based on the Prendergast-Huiskes stimulus, shows contrasting results on tissue differentiation:

• Model B (Porous): Promotes significantly higher fractions of immature and mature bone within the callus.
• Model A (Threaded): Induces greater fractions of cartilage and fibrous tissue, suggesting less stable healing under immediate loading.
• Structural integrity: For both models, the internal stresses sustained by the implant (Ti-6Al-4V) remained below the yield strength of the material, ensuring the absence of permanent deformation.

These results indicate that the porous architecture of Model B optimises load transfer to the surrounding bone, reducing stress shielding and creating a mechanical environment conducive to early bone maturation.

A superior biomechanical response for immediate loading

The results of this numerical simulation highlight a major advantage of the porous design (Model B): a drastic reduction in stress in the cortical bone, decreasing from 88 MPa with a conventional threaded implant to only 32.5 MPa. For the implantologist, this data is crucial as it suggests a significant decrease in the risks of marginal resorption linked to mechanical overload, which is particularly feared during immediate loading.

The study also highlights a significantly more favourable stimulation of the peri-implant callus with the porous structure (up to 31.6 MPa compared to approximately 2.5 MPa for the conventional thread). Clinically, this stimulus intensity promotes cellular differentiation directed towards the formation of mature bone rather than fibrous or cartilaginous tissue. By adjusting the implant's stiffness (64.26% porosity) to that of the native bone, the porous design reduces the "stress shielding" effect, ensuring a more physiological load transfer from the early phases of healing.

However, these findings present limitations inherent to the finite element methodology (FEM). Although accurate for stress analysis, this static approach on a bone block with a fixed 0.2 mm callus cannot overlook the in vivo biological complexity, such as angiogenesis or the patient's metabolic variations. The use of TPMS structures derived from additive manufacturing (SLM) nevertheless appears to be a promising strategy to secure early loading protocols in high-stress areas.

Summary of the study

This finite element analysis demonstrates that the porous implant (64.26% porosity) reduces stress on the cortical bone by more than 60% compared to a conventional threaded implant (32.5 MPa versus 88 MPa) during immediate loading. The results highlight that the porous structure increases the mechanical stimulation of the peri-implant callus tenfold, promoting cellular differentiation towards mature bone rather than fibrous tissue.

In practical terms, for the practitioner:

• Favour porous implants for immediate loading: their architecture drastically reduces the risk of stress shielding and better preserves the peri-implant cortical bone.
• Secure your protocols in low-density bone: the superior mechanical stimulation induced by porosity accelerates bone callus maturation, providing earlier biological stability.
• Optimise load transfer: the use of scaffold-based structures enables occlusal stresses to be transformed into beneficial osteogenic stimuli from the initial healing phase.

Technical glossary of the study

Stress shielding: Biomechanical phenomenon in which an overly rigid implant absorbs an excessive proportion of mechanical loads, depriving the peri-implant bone of the physiological stimulation necessary for its remodelling and long-term maintenance.

Immediate loading (IL): Prosthetic rehabilitation protocol where the restoration is placed at the time of implant placement, reducing treatment time but exposing the healing callus to a critical mechanical environment.

Prendergast-Huiskes stimulus: Mechanobiological model used to predict tissue differentiation (fibrous tissue, cartilage or bone) within the peri-implant callus based on physical stresses and strain rate.

TPMS (Triply Periodic Minimal Surface) structures: Porous lattice geometries (e.g. gyroid, diamond) produced via additive manufacturing, allowing the elastic modulus of the implant to be adjusted to closely match that of human trabecular bone.

Poroelastic materials: Mathematical model applied to bone tissues in this study to simulate their complex physical behaviour, including the interactions between the solid matrix and interstitial fluids.

Peri-implant callus: 0.2 mm thick tissue zone modelled around the implant to simulate the initial biological interface where mesenchymal stem cells differentiate during the healing phase.

Finite Element Method (FEM): Numerical simulation tool used here to analyse the distribution of stress and strain in the bone and prosthetic components under different loading conditions.

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