Implant Surfaces: Treatments, Roughness and Impact on Osseointegration

The implant surface is the only part genuinely in contact with bone, and its characterisation has represented the main field of innovation in implantology over the past thirty years. Branemark's original implants had a machined, essentially smooth surface, with osseointegration times of six months in the maxilla and three in the mandible. Reducing those times while maintaining predictability has been the objective of all subsequent research.

The fundamental descriptive parameter is mean roughness, denoted Sa and measured in micrometres. Surfaces are conventionally classified into four categories: smooth with Sa below 0.5; minimally rough between 0.5 and 1; moderately rough between 1 and 2; rough above 2. This classification is not merely descriptive but predictive, as experimental literature has identified the moderately rough range as the biological optimum.

The reason for this optimum lies in cellular behaviour. An excessively smooth surface offers few anchorage points for matrix proteins and osteoblasts, slowing bone deposition. An excessively rough surface increases contact area but also favours bacterial adhesion and, if exposed, makes decontamination practically impossible. The one-to-two micrometre range represents the documented compromise.

Sandblasting with aluminium oxide or titanium particles was the first widely adopted subtractive treatment. It produces irregular roughness with values depending on particle size. The main limitation is residual abrasive particles embedded in the surface, which microanalytical studies have documented in variable percentages and which represent a foreign body at the interface.

Acid etching, generally with hydrochloric and sulphuric acid, produces more uniform microroughness free of abrasive residue. The combination of both treatments — sandblasting followed by etching, referred to in the literature as SLA — has become the reference standard and has the most extensive clinical documentation, with follow-up exceeding twenty years.

Anodic oxidation generates a thick, porous titanium oxide layer by applying current in an electrolytic solution. Layer thickness increases from the few nanometres of spontaneous passivation to several micrometres, with a porous morphology favouring mechanical anchorage. Clinical evidence indicates performance comparable to the SLA surface, with some reported advantage in low-density bone.

Surface hydrophilicity has emerged as a parameter independent of roughness. A hydrophilic surface is wetted immediately by blood at placement, favouring clot adhesion and cell migration. Surfaces treated and stored in isotonic solution, which maintain hydrophilicity by preventing atmospheric hydrocarbon contamination, have shown accelerated secondary stability in the first weeks.

The clinically relevant finding concerns the critical period. In implant healing there is a phase, between the second and fourth week, in which mechanical primary stability has already declined through remodelling while biological secondary stability is not yet sufficient. Hydrophilic surfaces reduce the depth of this dip, with direct implications for the safety of early loading protocols.

Hydroxyapatite-based coatings, widely used in the 1990s, followed an instructive trajectory. Chemical composition analogous to the mineral component of bone promised a direct biological bond, and early results were encouraging. Long-term follow-up, however, documented delamination and resorption of the coating, with loss of anchorage and late failures that curtailed its use.

Fluoride-incorporated surfaces represent a different approach: instead of adding a layer, the existing oxide layer is chemically modified. Experimental studies document increased osteoblastic activity and pull-out resistance, and available clinical evidence indicates performance in line with conventional moderately rough surfaces.

Behaviour in low-density bone constitutes the most meaningful test. In type D4 bone, typical of the posterior maxilla, available contact surface is reduced and primary stability limited. It is in this condition that differences between treatments become measurable: moderately rough, hydrophilic surfaces show documented advantages over minimally rough surfaces, whereas in D1-D2 bone the differences narrow until they become clinically irrelevant.

The relationship between roughness and peri-implantitis has generated prolonged debate. The hypothesis that rougher surfaces favour bacterial colonisation is biologically plausible and experimentally supported. Clinical evidence is, however, less clear-cut than theory would suggest: peri-implantitis prevalence correlates more robustly with periodontitis history, hygiene and smoking than with surface type.

A necessary clarification concerns the crestal portion surface. Some systems adopt a smooth or minimally rough collar in the transmucosal zone, aiming to reduce bacterial adhesion in the most exposed area. Evidence on this choice is conflicting: a smooth collar reduces plaque retention but also offers less bone anchorage, and the decision remains largely a matter of system philosophy.

Surface contamination represents an underestimated problem. Contact with gloves, saliva or non-dedicated instruments deposits hydrocarbons and proteins that alter surface energy and reduce hydrophilicity. The handling protocol — direct retrieval from the packaging with a dedicated instrument, no contact with non-sterile surfaces — is not formalism but the condition for the surface to behave as designed.

From a practical standpoint, the choice between available systems rarely turns on surface. All moderately rough surfaces from established manufacturers have adequate clinical documentation and performance differences that are difficult to perceive in daily practice. The criteria that genuinely matter are long-term component availability, connection quality and manufacturer support.

In conclusion, the implant surface has reached a plateau of maturity: the moderately rough range represents the documented optimum and recent innovations produce marginal improvements in secondary parameters. Research attention has rightly shifted towards the transmucosal portion surface and towards decontamination treatments for already exposed surfaces, where clinical problems remain unresolved.