Implant element

Details Implant element Implant element

2000-02-14

2004-02-10

McDermott, Corrine (Department: 3738)

Prosthesis (i.e., artificial body members), parts thereof, or ai

Implantable prosthesis

Bone

C623S016110

Reexamination Certificate

active

06689170

ABSTRACT:

This application claims priority to PCT Application No. PCT/SE98/00891, filed May 14, 1998, and Swedish Application No. SE 9701872-5, filed May 16, 1997.
BACKGROUND OF THE INVENTION
Oral implants are made of syntetic materials and inserted in mucosal soft tissues and bone to serve as anchorage for prosthetic constructions. The choice of materials for bone anchorage has been discussed and considered over the years (reviewed in (Br{dot over (a)}nemark, 1996)) Osseointegrated titanium implants ad modum Br{dot over (a)}nemark have been successfully used for 30 years. There are several factors which are assumed to play important roles for the outcome of this treatment, for instance the choice of titanium with its adequate mechanical properties and corrosion resistance (Williams, 1981), surface topography and the relatively non-traumatic surgical procedures.
It is assumed that two-stage surgical procedure with an early post-operative period (3-6 months) without loading is important for the initial implant stability during the early healing phase. However, the two-stage surgical technique may be a disadvantage for the patient and requires more resources. In clinical practice, not only the materials and surgical procedures but also systemic and local host factors set the limits for treatment. It has been found that the failure rates are higher in the maxilla and the posterior mandible and that the success rates very much depend on the quality of bone (Esposito et al., 1997). It is therefore of importance to identify the beneficial and negative factors related both to the implant and the host in order to optimize the implant treatment. A reduction of the healing period and a maintenance of long-term stability during clinical loading conditions therefore appears essential.
A biomaterial is a material used in a medical device, intended to interact with biological systems (Black, 1992). The materials used in man-made structures may be divided into three classes: metals, ceramics and polymers. The classes are distinguished by the type of interatomic bonding (Cooke et al., 1996).
Metals consist of a large number of small crystallites. Each crystallite is an aggregate of atoms regularly arranged in a crystalline structure. When molten metals (which are amorphous) solidify small crystals (grains) start to grow. The irregularly arranged crystals eventually meet each other which gives rise to boundaries between the crystals, grain boundaries. The imperfect packing of atoms in the boundaries constitutes weak points in the material, which will be most strongly affected by a surface treatment such as etching or plasma cleaning and a groove will be created showing up as a darker line. The surface properties of a material is different from the bulk properties.
The term commercially pure (CP) titanium is applied to unalloyed titanium and includes several grades containing minor amounts of impurity elements, such as carbon, iron and oxygen. The amount of oxygen can be controlled at different levels to provide increased strength. There are four grades of titanium where grade 1 (used in the present thesis) contains the lowest amount of oxygen. The microstructure of CP titanium is essentially all &agr; titanium which has a HCP crystal structure.
Titanium dioxide, TiO
2
, is the most common and stable of the titanium oxides, while Ti
2
O
3
and TiO are more rare (Lausmaa, 1991).
TiO
2
can exist in three crystalline modifications; anatase (tetragonal structure), rutile (tetragonal), and brookite (orthotrombic). Rutile and anatase are the most usual forms whereas brookite is very rare (Keesman, 1966).
Techniques have been developed to alter and modify the surface properties of implants via mechanical and chemical procedures (Lausmaa, 1996; Smith et al., 1991a; Smith et al., 1991b). Plasma-spraying, sputter deposition, oxidation, vaporization, (grit, sand) blasting, grinding, etching, plasma cleaning and ion bombardment are examples of techniques available for this purpose.
Electropolishing is an electrochemical technique often used to obtain an improved surface finish by controlled dissolution of the surface layer of the metal. The amorphous surface layer produced by the machining of the implants is removed. After electropolishing a polycrystalline surface with a surface oxide consisting mainly of TiO
2
, typically 3-5 nm thick as measured by X-ray photoelectron spectroscopy (XPS), is found on the surface (Lausmaa, 1996).
Anodic oxidation is an electrochemical method used to increase the thickness of the oxide layer on metal implants. A current is applied in an electrolytic cell in which the sample is the anode. When a potential is applied on the sample, the current will transport oxygen containing anions through the electrolyte and a continuous oxide is formed on the metal sample. The stoichiometry of anodic oxides on titanium is mostly TiO
2
. The anodic oxides on titanium contain various structural features such as porosity (Lausmaa, 1996).
In order to characterize the surface properties after the modifications the following techniques were used; SEM and AFM for surface topography and roughness; ESCA and AES for surface composition and oxide thickness.
Interactions Between Titanium Surfaces and Proteins/Cells/Tissues
A review of the literature shows that surface modifications influence the biological response. The first events that take place when an implant is inserted in vivo is the exposure of the material surface to water and biomolecules, including plasma proteins. Both under in vitro and in vivo conditions serum proteins are known to adsorb to foreign material surfaces within seconds. The adsorption and desorption phenomena on different biomaterial surfaces have been studied intensely. A working hypothesis is that the biological response is directed by the initial protein adsorption which subsequently influence the cellular/tissue response and ultimately the performance of the implant (Horbett, 1996).
Three types of adsorption/desorption patterns have been described for metals and their oxides (Williams and Williams, 1988). For example, titanium was found to adsorb low levels of albumin, which remained low during a 48 h period. In addition, the albumin desorbed relatively easily. Other metal surfaces such as vanadium, showed an initially low amount of albumin, but the amount increased and desorption was slow. Gold was found to be characteristic for a surface with a high initial adsorption of albumin and the amount increased throughout the experiment.
A modification and variation on surface properties and the resulting effects on molecular adsorption to surfaces may provide important insights into the role of surface properties for biological reactions. Modified and characterized surfaces have been used to detect differences in the behaviour and adsorption patterns of proteins (McAlarney et al., 1991; McAlarney et al., 1996; Nygren, 1996; Shelton et al., 1988; Sunny and Sharma, 1990; Tengvall et al., 1992; Wälivaara et al., 1994; Wälivaara et al., 1992). Shelton et al (1988) found that a larger amount of proteins were adsorbed to negatively charged polymer beads than to positively charged beads but the roughness of the surface did not seem to influence protein adsorption or cellular behaviour. In general, rough surfaces are considered more wettable than smooth surfaces which may be an effect caused by an increase of the surface area as well as by an increased hydrophilicity of the surface (Curtis et al., 1983).
Nygren (1996) found two different reactions when hydrophilic and hydrophobic titanium surfaces were exposed to whole blood. On the hydrophobic surface, adherent platelets and fibrinogen were present while complement factor 1 (C1) and prothrombin/thrombin were present on the hydrophilic surface. Baier et al. (1982) has reviewed the principles of adhesive phenomena in diverse systems and he pointed out the wettability of a surface as the important parameter influencing the protein adsorption pattern.
The surface energy of a material is influenced by various cleaning procedures and the oxide t

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