Implant Surfaces: Present and Future
Paolo Trisi, D.D.S., Ph.D.
Implant surfaces are rapidly evolving in response to the increasing demand for faster healing and for success in more challenging cases. Osseointegrated implants seem to perform well in long-term studies, but short implants placed in posterior jaws with low-density bone showed lower percentages of success.
The improvement of the implant surface with a TPS- or HA-coating, and more recently, roughening of the CP-Ti by blasting or etching procedures improves the osseointegration rate.
While it has been widely demonstrated that an active surface may accelerate and enhance the rate of osseointegration, its effects on the early or immediate loading have not yet been documented in the literature. However, preliminary animal studies showed encouraging results1,2. Our clinical short-term studies showed rough surfaces have positive effects on the early loading after 2 months of healing3.
We analyzed the effects of rough surfaces on the osseointegration in human low-density bone and demonstrated a higher percentage of osseointegration4,5. Active or rough surfaces are able to enhance the percentage of osseointegration and HA-coated surfaces seem to have a better performance also in the regenerated bone 6,7,8,9.
Our studies4,5 in human low density bone showed that a modified surface may enhance the rate of bone-implant-contact (BIC), but it is not able to modify the bone density around the implant in the healing period, hence it is questionable if a modified surface is able to improve the long-term implant survival in the most critical areas.
Large porous-coated surfaces are widely used in orthopedic implants, where loading always starts immediately. The concept of bone penetration into the implant surface has been introduced in implant dentistry and currently under study are two implants with new surfaces: CSTi and Endopore. Some recent studies10 showed these surfaces might have some biomechanical advantages in a faster healing period and shorter implant length with good long-term prognoses. We have conducted an in vivo human study on CSTi™ that has shown a high percentage of ingrowth of bone inside the CSTi coating.
Experimental and clinical studies have reported that some biomechanical factors are the main reasons for loosening of the osseointegrated implants after loading: low-density bone, short implants in posterior areas and overloading in hard-biters.
Orthopedic studies have shown that bone reacts differently to different stress and strain conditions. The “mechanostat theory” of Frost11 hypothesizes four possible thresholds of load: low level of strain, i.e., the absence of earth’s gravity induces bone atrophy with resorption of long bone cortical walls; the standard level of strain is able to maintain constant the amount and density of the bone; a slightly higher level of strain is able to induce a positive modeling of the bone causing bone hypertrophy; the next step is towards the threshold of the overload that causes bone resorption, probably due to fatigue microcracks. According to this hypothesis implant loss could result from a range of load high enough to cause bone resorption due to microcracks.
The amount of load able to cause bone resorption seems to be in the range of occlusal forces and this could also be the reason for periimplant bone loss after initial loading. Continuous loading is also able to stimulate bone hypertrophy, thus enhancing the bone ability to bear load. When equilibrium is reached the implant will survive, but if the threshold of overloading is exceeded all around the implant surface, the implant will progressively loose the osseointegration.
According to this hypothesis, it is imperative to have a precise knowledge of the overloading threshold in each type of bone density and for each implant length and diameter, as well as for the different implant surfaces and shapes. Bone-implant interface conditions have a very strong effect on the bone loading patterns around the implant. Not only the level of the stress and strain, but also the stress and strain distribution in bone are highly affected by the interface conditions12.
The main focus of the future research should be directed towards a better understanding of these factors.
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