SUCCESSFUL OSSEOINTEGRATION AND IMPLANT SURFACES

/Home/Dental Implants (Prosthodontics)/Dental Implants (Prosthodontics)Articles & Reports/Bone-Implant Interface Properties

SUCCESSFUL OSSEOINTEGRATION AND IMPLANT SURFACES
By: Melinda Mo, DDS
March 6, 2003
Compton Dental
Introduction:
Osseointegration is the basis of a successful endosseous implant. The process itself is quite complex and there are many factors that influence the formation and maintenance of bone at the implant surface. What characteristics does an implant surface need to possess in order for success? To more fully understand what influences osseointegration, it is important first to examine more closely the interface, the traits of a surface that allow for biocompatibility, and the common surfaces used and studied such as titanium oxide and hydroxyapatite.
The bone-implant interface: Osseointegration is a striking phenomenon in which bone directly opposes the implant surface without any interposing collagen or fibroblastic matrix. Numerous studies have all concluded that the strength of an osseointegrated implant is far greater than that of a fibrous encapsulated implant. Also, the strength of the interface between bone and implant increases soon after implant placement (0-12 weeks)1, 2. This strength may in fact be related to the amount of bone surrounding the implant surfaces. Other factors that may affect the strength of the interface is biophysical stimulation3 and time allowed for healing. Studies have shown that measurable increases in bone implant interactions take place for at least 3 years.4 Another factor that may affect the strength of the connection is an adhesive parameter that relates to the amount of surface attachment to bone.
The amount of surface attachment is affected by both time and chemistry. Gila et al. did a study which suggested that osteoblast adhesion and differentiation is enhanced when bioactive titanium is used in comparison to titanium that is untreated chemically.14 The interface between bone and implant is different from mineralizing bone matrix or osteoid. The histology reveals that the interface is rich in proteoglycans and glycoproteins.5, 6,7 In addition, osteoblastic matrix proteins such as osteopontin and alpha HS ǃÏglycoprotein help to form the interface.8,9 Kokubo et al showed that when chemically deposited bone like appetite was deposited on titanium and then soaked in simulated body fluid, better bonding to living bone was observed.15 Different implant surfaces have also been shown to have differing properties.
Characteristics of surfaces: There are many properties of implant surfaces that affect the biocompatibility and interaction with bone. For example, macrostructural threads, grooves and microstructural grit blasting, plasma spraying , acid etching and machining or polishing may all contribute to the behavior of the implant. In addition, surface characteristics such as topography, roughness, oxide thickness and composition can vary significantly. Surface roughness is a property that has been widely studied. Some researchers suggest that plasma spraying can offer a mechanical interlocking of bone that will enhance the strength of the bone-implant interface.10 In addition, pits that range from 1 to 10 ¨µm can augment bone formation and torsional strength. 11, 12,13 However, in a study done by Buser et al 37 after HA coated implants, grit blasting proved to have more appositional bone formation than plasma spraying, machining or electropolishing. Furthermore, surface topography as manufactured via grit blasting with titanium dioxide has been showed to improve bone adaptation and fixation when compared with turned titanium.16,17 In research done by Wennenberg et al it was found that there is an optimal range of surface roughness. An implant may in fact be too rough. The best surface may be one that has a balance of relative homogeneity of surface structure, augmented surface area and altered Ti ion release. These traits will all lead to superior mechanical interlocking of bone and load transfer.13,17,18
Titanium Oxide:
When Ti (Titanium) or Ti alloys are exposed to air or normal physiologic environments, there is a reaction with the oxygen that causes and oxide layer to be formed. Usually the oxide is in the form of TiO2. The oxide layer protects versus corrosion. Calcium and phosphate ions have been found in the oxide layers, which suggests that there is an active exchange of ions at the bone implant interface. 19 In studies done on animals, porous surfaces have been shown to boost ionic interactions, initiate a double physical and chemical anchor system and augment load bearing capacity. 20,21,22 Also, porous surfaces can increase the tensile strength via growth of bone three dimensionally as well as increased healing rates initially25,24 The majority of commercially available implants are covered via plasma spraying. Titanium plasma spraying involves molten droplets being sprayed in a powder form onto the implant surface at high temperatures. Thus, an increased surface area is obtained, increased bone contact is achieved and the ability to form a 3 dimensional interconnection is enhanced 23. The disadvantage of Titanium plasma spraying is the risk of scaling and cracking due to the high processing temperatures. Also, there is a risk of abraded material being implanted into the bone-implant interface.26,27 The amount of melting of the plasma sprayed titanium contributes to this abrasion. That is, the more the melting, the more abrasion resistant the surface.35 Hydroxyapatite:
Hydroxyapatite can also be applied via plasma spraying. The small particle, crystalline, ceramic hydroxyapatite powder also offers and increased surface area. This increased surface area improves the biomechanics as well as the load bearing capacity of the structure. Many studies have shown the advantages of a hydroxyapatite coating such as decreased corrosion rates, and an accelerated rate of bone formation and maturation in dogs.28,29 The histology of the interface when hydroxyapatite coatings have been used show a more organized bone pattern and a superior degree of mineralization than for other materials.30 In addition, the reliability of HA coated surfaces has been exceptional; even though the bone-HA interface has been shown to be stronger than the HA-implant interface. 29 Another very significant trait of HA is bone penetration, which improves fixation . This becomes especially important in regions where bone contact is limited initially. 31,32,33 The capability of bone bonding or bonding osteogenesis makes HA very unique.34 The continuity in mineral phases between forming bone and the implant surface may be the basis for the bonding. The initial rate of increasing strength of the bone-implant interface has been faster for HA than for Titanium.34 The rapidity of the mineralization at the HA surfaces may be due to protein adhesive characteristics that make HA osteoconductive. Also, when comparing with a Ti surface the HA surface appears to support a more rapid differentiation of osteoblasts (as defined by the expression of bone matrix proteins)36.
Conclusion:
The osseointegration of dental implants is a complex process that can be influenced by many factors relating to the surface of an implant. Time, chemistry, and an adhesive parameter all play an important role in osseointegration. In addition, characteristics of a surface such as how it is manufactured can also have a significant effect on the ingrowth of bone. Also, surface roughness, topography, oxide thickness and composition when varied ideally can significantly impact osseointegration. Titanium oxide surfaces have various advantages such as corrosion resistance, better bone contact due to an increased surface area, and an improved capacity to bear loads. However, there is a risk of cracking and scaling of the surface off of the implant, with abraded materials being left in the bone-implant interface. Hydroxyapatite coatings have also been much used and studied. HA coatings have the advantage of increasing surface area, decreasing corrosion rates , and accelerating bone formation via faster osteoblast differentiation. Also, due to the enhanced biomechanics HA coated implants are better able to withstand loads. Other advantages of HA include the more organized bone pattern and higher degree of mineralization at the interface, as well as increased bone penetration (which improves fixation). The bone ǃ?bondingǃ? capabilities of HA make it a very desirable surface and probably the most reliable surface up to date. However, controversies still exist. Some people argue that HA coatings may not provide any long term advantage for the prognosis of the implant system. Technology is constantly advancing, newer, better surfaces are being researched and tested. Modified titanium surfaces may show promising results in the future.