Surface coatings are one of the most relevant areas of innovation in modern surgical implants. Biomaterials such as HA (hydroxyapatite), TiN, DLC or ceramic coatings improve osseointegration, reduce friction, minimize wear and extend implant life. But these benefits depend directly on the quality, adhesion and mechanical strength of the coating. In this article we explain what types of coatings exist, how they influence the durability of the implant and which tests are essential for their validation.
Types of coatings used in surgical implants
Coatings are applied to reinforce critical properties. Among the most commonly used are:
Bioactive coatings (hydroxyapatite, calcium phosphates).
Ceramic coatings (zirconia, alumina).
Metallic coatings (titanium by sputtering, plasma spray).
Antifriction coatings (titanium nitride – TiN, diamond-like carbons – DLC).
Antibacterial coatings (silver ions, nanostructured layers). Antibacterial coatings (silver ions, nanostructured layers).
Each requires specific testing methods to ensure that it will not degrade, detach or fracture during clinical use.
Factors determining the quality and long-term performance of a coating
The durability of an implant depends not only on the type of coating chosen, but also on how it is applied and its interaction with the biomechanical environment. The deposition technology, the layer thickness and the resulting microstructure are parameters that critically influence the performance of the coating over time.
Method of coating application
The main processes – such as plasma spray, PVD (Physical Vapor Deposition), CVD (Chemical Vapor Deposition), anodizing or electrochemical deposition – generate coatings with different mechanical and chemical properties.
For example:
- Plasma spray allows for thick, highly bioactive coatings, but more susceptible to delamination if not well anchored.
- PVD/DLC coatings have high surface hardness and low friction, but can be sensitive to impact or micro-cracking if the substrate is not properly prepared.
Coating thickness and uniformity
A coating that is too thick may fracture or generate residual stresses, while one that is too thin may lose effectiveness against wear or corrosion. Uniformity is equally crucial: micron variations can alter head-to-cup fit or increase friction in mechanical joints.
Adhesion to metal substrate
Adhesion is one of the most important indicators of durability. It depends on the:
- previous surface treatment (sandblasting, polishing, acid etching),
- hardness of the base material,
- chemical compatibility between coating and substrate.
Lack of adhesion can lead to progressive delamination, one of the most critical failures in coated implants.
Resistance to corrosion and biological environment
Body fluids can induce:
- galvanic corrosion,
- accelerated chemical degradation,
- release of ions or particles.
Coatings must be stable against physiological pH, temperature variations and cellular environment, especially in long-term implants.
Tribological behavior in joint prostheses
In joints such as the hip or knee, anti-friction coatings must maintain a low coefficient of friction to avoid:
- accelerated wear,
- particle generation,
- increase of temperature in articular surface,
- degradation of the synovial lubricant.
The tribological properties depend on both the coating and the final surface roughness of the component.
Biomechanical bone-implant interaction
Bioactive coatings (such as HA) promote osseointegration, but their early or excessive degradation can compromise primary and secondary implant stability. A proper balance between bioactivity and mechanical strength is critical.
Influences of the sterilization process
Methods such as plasma sterilization, gamma radiation or autoclaving can modify the microstructure of the coating or reduce its adhesion. Therefore, manufacturers should validate the stability of the coating after final sterilization of the product.
Why coatings affect implant durability
The longevity of an implant depends on its ability to withstand:
- Millions of load cycles (fatigue).
- Repeated stresses in critical areas.
- Friction between articular surfaces.
- Micro-movements at the bone-implant interface.
- Aggressive biological environments.
A poorly adhered coating can:
- De-laminate.
- Generate particles that cause osteolysis.
- Reduce the primary stability of the implant.
- Increase friction and wear.
Key tests to validate coatings
Essential mechanical and physicochemical tests include:
- Coating adhesion test (ASTM F1147)
Determines the force required to pull the coating off its metal substrate. - Shear test (ASTM F1044)
Evaluates the resistance of the coating to tangential stresses. - Wear and friction tests (ASTM F732, ISO 14242)
Specially applied to joint prostheses to determine their tribological behavior. - Fatigue test on coated implants
Verifies that the coating does not fail under repetitive loads. - Chemical stability analysis
Important for bioactive or degradable coatings.
See our accredited tests for implants with coatings at
Accredited femoral stem testing
