Experimental Study of Bone Response to a New Surface Treatment of Endosseous Titanium Implants

Antonio Sanz R., DDS
Periodontist, Adjunct Professor in Oral Implantology, and Director of the Postgrade in Oral Implantology, Odontology Faculty, University of Chile, Santiago, Chile.

Alejandro Oyarzun, DDS
Biochemical and Oral Biology Unit, Odontology Faculty, University of Chile, Santiago, Chile.

Daniel Farias, DDS, Ivan Diaz, DDS
Specialist in oral implantology, Odontology Faculty, Postgraduate School, University of Chile, Santiago, Chile.

There is a need to find ways of achieving better and more efficient osseointegration in poor bone qualities found in different jaw regions and to attempt to reduce the preload cicatrization period. Success in this area will make possible the placement of implants in sites (such as the posterior areas of the maxilla and the mandible) where it would be difficult or impossible to currently achieve sound osseointegration.

Much of the current research is aimed at finding ways to modify the macrostructure as well as the microstructure of implants. These efforts may include the use of materials that could modify tissue response. The goal of obtaining an implant made with a biomaterial that will allow precise control of the superficial structure, absorption of protein, cellular adhesion, growth, and bone activation is noteworthy.

Different surface treatment of implants to improve their microstructure has been the goal of much experimental and clinical research during the last few years. Studies have demonstrated that the application of hydroxyapatite coatings increases the hardness of the bone-implant interface. Long-term outcomes for implants coated with hydroxyapatite are still under discussion.

Another type of implant surface treatment involves increasing the superficial roughness. Scientific evidence accumulated over the last 10 years suggests that titanium implants with roughened surfaces achieve significantly improved anchorage in the bone than do implants with machined surfaces. The first attempt at increasing surface roughness was made almost two decades ago18 with the application of titanium plasma sprayed onto the surfaces of the implants. Results obtained with this surface have been very satisfactory, as studies have shown.

Other treatments designed to alter the surface morphology of implants include grit blasting with different sized particles of sand, glass, or aluminum oxide to create varying degrees of roughness and acid etching, which produces a uniformly rough texture over the entire implant surface. These techniques have also been used in combination with promising results.

Recently, a new surface treatment called resorbable blast media (RBM) has been developed for application to implants. RBM involves blasting the implant with coarsely ground calcium phosphate (particle size, 180mm 3 425 mm), which gives the implant a coarse surface without leaving any residues. The calcium phosphate is a resorbable material that is not permanently imbedded into the surface of the implant primarily because of the passivation method used. The purpose of this study was to observe the biocompatibility of the RBM surface, analyze bone response, and make a topographical examination of the microstructure.

MATERIALS AND METHODS :

Two one-year-old, white New Zealand rabbits weighing approximately 4 kg each received four commercially manufactured titanium implants (diameter, 4 mm; length, 10 mm) (Restore, Lifecore Biomedical, Chaska, MN) with surfaces treated with RBM. These implants were placed in the mid-face of each tibia (proximal metaphysis). The housing care and experimental protocol were in accordance with guidelines set by the University of Chile Institutional Animal Care and Use Committee. After a 16-week cicatrization period, the rabbits were killed. The implants and all surrounding bone tissue were recovered from the tibial area and fixed in 4% paraformaldehyde in 0.1 mol/L phosphate buffer (pH, 7.4) for seven days. Subsequently, the samples were dehydrated in ascending ethanols, including LR White hard grade resin (London Resin Co., Hants, UK).

For microscopic observation, cuttings were made with diamond discs to a thickness of 100 mm. Their preparation was completed by abrasion to a thickness of 8 to 10 mm based on a method described by Donath.21 The cuttings were stained with methylene blue Azur II– basic fuchsin to observe their histology and take photographs with an Axioscop microscope (Carl Zeiss Inc., Thornwood, NY) on Kodak ASA 100 film (Eastman Kodak, Rochester, NY).

The electron microscopy used to observe the characteristics of the microstructure of the RBM implants and to compare it with the surface of the machined implants was performed on a scanning electron microscope (DSM 940, Carl Zeiss Inc.). Observation of the sample was performed directly and without a gold bath because the implants reflect the ions of the scanning beam.

RESULTS:

Observation by Optic Microscopy

Optic microscopy reveals bone formation in close contact with the titanium without intervening fibers. There is direct bone apposition in hollow areas in the surface of the implant. Bone tissue appears in the cortical area, completely filling the threads of the implant. In the medullary area, there is progress of bone tissue in relation to the surface of the implant, producing a type of cortical bone of a different thickness based on the degree of progress of the osteogenesis. In the areas closest to the cortical bone, greater thickness is seen than in the more distal areas. In the middle, there is bone marrow in contact with the bone in proliferation. At greater magnification, mature bone of the lamellar type is observed in the cortical area with a configuration of osteons within a clearly identifiable haversian system. There is no evidence of inflammatory cells or fibrous tissue.

Scanning Electron Microscopy

The surface of the RBM implants as seen with a scanning electron microscope appears to be a coarse roughness that does not refract light except on the first three threads on which can be seen the machined titanium without surface treatment. In this area, the striae, characteristic of the implant turning process, can be distinguished. From the third thread onward, the surface of the implant is highly irregular and rough all over not only on the crest of the threads but also in the depressions between them. At higher magnification, the surface appears reticulated, with undermining and deformation of the metal remaining after impaction of the resorbable calcium phosphate material blasted under pressure on the surface of the implant.

DISCUSSION :

The surface remaining after calcium phosphate blast application is irregular, extremely rough, and comparable to that obtained with other types of treatments, such as the sand form, the application of acids, or a combination of these two. The roughness of the implant surfaces favors distribution of stress, retention of the implants in the bone, and cellular response. An increase in bone response and in the resistance of the bone-implant interface has been reported with bone trabeculae growing perpendicular to the surface of the roughened implant.

Studies have reported that adequate growth of bone in the interior of the pores or cavities left by the surface treatment requires that these must be approximately 100 mm in size. The growth of bone tissue into cavities of this size allows a mechanical interlocking of the implant with the bone. An increase in reverse torque values occurs in implants with pores between 10 mm and 40 mm. These pores do not allow maximum growth toward the inside of the bone. However, they do strengthen the mechanical union of the bone-implant interface. In vitro studies have also shown that the superficial roughness of the materials can influence cell function, matrix deposition, and mineralization. In terms of superficial roughness and pore size (2.5–4 mm), the RBM implants analyzed using scanning electron microscopy met the criteria, resulting in an improved boneimplant interface as described by various authors. Studies designed to measure the mechanical resistance of this type of surface appear to be absolutely necessary to support what has been described in relation to its microstructure. Using optic microscopy, there is evidence of direct apposition of bone tissue over the rough surface of this implant, just as was previously described by the authors in relation to other surfaces, such as commercially pure titanium, titanium plasma spray, and implants with a superficial coating of hydroxyapatite. In the cortical areas of the tibia, there is a complete filling of the implant threads, including anatomical repairs, with bone tissue appearing as mature bone of the lamellar type with haversian systems taking shape. In areas of spongy bone, there is bone apposition over the threads of the implant, surrounding them with a thin cortical bone of no more than 10–12 mm, bone tissue that grows at the expense of cortical areas and contact osteogenesis. This stretches toward the medullary area. This condition has also been described by Albrektsson and Johannsson25 in similar animal models and by Lederman, Schenk, and Buser in humans.

This bone property of growth over different surfaces is directly related to the mechanical rigidity of the substrate, its moistening ability, and the topography of the surface. The substrates with superficial tension greater than 30 dynes show greater bioadhesion; thus developing more points of insertion for cellular union. The property of greater moistening ability has been associated with rough implant surfaces. It is argued that it increases the points of initial fixation of the coagulum; thus avoiding its retraction and allowing greater bone contact with the implant.

Scientific evidence accumulated during the last 10 years conclusively indicates that the rough surfaces of titanium implants offer a significantly improved anchor to the bone than machined titanium surfaces. Future lines of research in this field are absolutely essential to consolidate the new knowledge and to prepare the way for the development of implants that achieve more effective and lasting osseointegration, even in clinical situations of poor bone quality.

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