Osseointegration. A Critical Appraisal

Research Paper (undergraduate), 2018
20 Pages, Grade: A


Table of Contents

1. Introduction

2. Biomaterials used in guided bone regeneration (GBR)

3. Surface modifications of Ti implants to improve osseointegration

4. Physicochemical methods

5. Biochemical methods

6. Animal models for the investigation of osteogenesis and osseointegration

7. Conclusion


1. Introduction

Osseointegration of dental implants Dental implantology, a special field of dentistry dealing with the rehabilitation of the damaged chewing apparatus due to loss of the natural teeth, is currently the most intensively developing field of dentistry. Missing teeth can be replaced by dental implants (artificial roots), which are inserted into the root-bearing parts of the mandible or maxilla. The success and long-term prognosis of implant prosthetic therapy depend primarily on the anchorage of the implant in the jawbone, i.e. on the osseointegration. Today, there are ever increasing demands from patients with missing teeth for masticatory function and aesthetic appearance of their replaced teeth to be restored and for shortening of the period of osseointegration of the implants, which takes a relatively long time (3-6 months). The successful insertion of a biocompatible material into living tissue with little to no evidence of rejection has revolutionized medicine and dentistry.

In the 1960s, Brånemark et al. stumbled upon this phenomenon when using titanium (Ti) in animal models, with little idea of the impact this discovery would have on the rehabilitation of future medical and dental patients. This phenomenon, described as “osseointegration”, was characterized by a number of clinical and ultra-structural observations. Osseointegration may broadly be defined as the dynamic interaction and direct contact of living bone with a biocompatible implant in the absence of an interposing soft tissue layer [1-3].

Although the clinical term osseointegration describes the anchorage of endosseous implants to withstand functional loading, it provides no insight into the mechanisms of bony healing around such implants. However, in the last decade it became clear that the long-term success of dental implants also depends on the complex bio integration of these alloplastic materials, which is determined by the responses of the different surrounding host tissues (the alveolar bone, the conjunctival part of the oral soft tissues and the gingival epithelium). Critical in developing biologic design criteria for implant surfaces is to understand the sequence of bone-healing events around endosseous implants is believed to be Bone growth on the implant surface can be subdivided into three distinct phases that can be addressed experimentally [4].

The first, osteoconduction, relies on the migration of differentiating osteogenic cells to the implant surface, through a temporary connective tissue scaffold. The second, de novo bone formation, results in a mineralized interfacial matrix, equivalent to that seen in cement lines in natural bone tissue, being laid down on the implant surface. The implant surface topography determines whether the interfacial bone formed is bonded to the implant. A third tissue response, the bone remodelling, creates a bone-implant interface comprising de novo bone formation. Treatment outcomes in dental implantology depend critically on the implant surface designs that optimize the biological response during each of these three distinct integration mechanisms.

Today, much effort is devoted to the design, synthesis and fabrication of Ti dental implants in order to obtain long term (lifelong) secure anchoring in the bone. Fundamentally, this means the ability of then implant to carry and sustain the dynamic and static loads that it is subjected to. The bulk structure of the material governs this ability. Evidently, it is important to achieve a proper function in the shortest possible healing time, with a very small failure rate and with minimal discomfort for the patient. These factors are also important for cost reasons. As regards osseointegration, i.e. the formation of a direct connection between the living bone and the surface of the load-carrying implants, the important question arises as to how to attain a better integration by modification of the implant surface morphology.

A wide variety of materials have been used to produce endosseous implants [5, 6]. Currently, Ti and its alloys are the most commonly utilized dental and orthopaedic implant materials that meet the most important requirements [7, 8]. The properties of Ti and its surface, which is covered by a native oxide layer, are appropriate to allow its use as a biocompatible material [9]. At a cellular level, the relationship of an implant with the surrounding tissue is highly dependent on the interaction between the passive titanium oxide (TiO2) which is formed on the surface of a Ti implant, and biological elements such as collagen, osteoblasts, fibroblasts and blood constituents [2,10]. The TiO2 layer is very stable, corrosion-resistant and may be manipulated to have variable thickness. The clinician is often faced with the challenge of identifying the successful osseointegration of a dental implant. Clinical success is determined by a lack of mobility and by the ability of the implant to resist functional loading (chewing force) without mechanical deformation and to transfer the load onto the alveolar bone without deterioration of the bony interface [11].

Radio graphically, the bone should appear to be closely apposed to the implant surface. The resolution currently achievable in medical imaging, however, is several orders of magnitude less than what is required to observe a soft tissue cell. Accordingly, radiographic assessment alone is unsuitable to determine with certainty whether a soft tissue is present [12]. A number of studies have analysed this bone to Ti interface histologically and ultrastructurally, often with inconsistent findings. The difficulty arises primarily with the need to prepare and section the specimens without changing or damaging the interface. Recent studies have utilized CT scanning to obtain a 3-dimensional picture of the implant interface [13, 14].

2. Biomaterials used in guided bone regeneration (GBR)

The aesthetic and functional demands of the patients have recently increased enormously. In dental implantology, new biomaterials and available surgical techniques furnish excellent possibilities. However, there are certain fundamental weaknesses in the current technology. Patients must have suitable morphology and a sufficient amount of available jawbone for reconstruction to be a viable option. After extraction or the loss of teeth for any other reason, the edentulous alveolar ridge resorbs. Consequently, its dimensions and morphology, especially as concerns the labial plate, rapidly become inadequate for the appropriate accommodation of artificial roots. To preserve the height and width of the alveolar bone for future implantation therapy, guided tissue regeneration (GTR) procedures are used [15].

GBR has become a routinely applied method in dental implantology. Most of the dentoalveolar regenerative techniques require osteoconductive material in order to establish new bone formation in the necessary anatomical form. This surgical procedure makes use of barrier membranes to direct the growth of new bone at sites having insufficient volumes or dimensions for function or prosthesis placement. GBR is similar to any other GTR utilized in dental therapy, but is focused on the development of bony tissues instead of soft tissues of the periodontal attachment. Used in conjunction with a sound surgical technique, GBR is a reliable and validated procedure [16]. Xenograft bone substitutes originate from a species other than human, e.g. bovine. Xenografts are usually distributed only as a calcified matrix. Bio-Oss is a safe, effective xenograft: a deproteinized, sterilized bovine bone with 75-80% porosity. It is reported to be highly osteoconductive and biocompatible. It is known that Bio-Oss serves as a scaffold in GBR, but, due to its poor resorbability, it may exert a negative influence on the structure of the newly formed bone. The large-mesh interconnecting pore system facilitates angiogenesis and the migration of osteoblasts. It has been found clinically that its resorption is very similar to that of human bone [17-19]. Pure beta-tricalcium phosphates (TCP-β) such as Cerasorb are widely used osteoconductive materials. The chemical characteristics of Cerasorb allow it to resorb completely and quite rapidly during new bone formation. This may result in too early resorption in some cases without fulfillment of the clinical requirement of the space-maintaining function [20, 21].

These bone-substitute materials allow targeted bone regeneration as they facilitate construction of a base on which implants can be positioned and further stabilized. Cerasorb has good osteoconductive and resorption properties [21, 23]. Full resorption over a defined period of time, with simultaneous transformation into autologous bone, is of particular significance in this respect. Because of its rounded surface and chemical composition, Cerasorb is remarkably bio inert and is therefore particularly suitable for innovative procedures. The unique open porosity structure increases active cellular in-growth and improves nutrition, while the rough surface further increases osteoconductivity. The result is the rapid in-growth of local bone and a significantly shorter resorption time (6-12 months) compared with other ceramic products [22].

Calcium phosphate cements (CPCs, e.g. Vitalos) are an emerging class of bone-substitute materials that are capable of rapid setting to a hard mass, providing a scaffold for the bone remodelling process. The CPCs synthetic bone graft materials invented in the 1980s, consist basically of tricalcium phosphate and anhydrous dicalcium phosphate. Many different combinations of calcium and phosphate have been developed as commercial CPC materials [24]. Hydroxyl-apatite (HA) is the main component of VitalOs and the primary inorganic component of natural bone which makes the hardened cement biocompatible and osteoconductive. Over time, CPC is gradually resorbed and replaced by new bone. CPC has two significant advantages over pre-formed, sintered ceramics.

First, CPC paste can be sculpted during surgery to fit the contours of the wound. Second, the nanocrystalline HA structure of the CPC makes it osteoconductive, causing it to be gradually resorbed and replaced by new bone. Recent work with CPCs has focused on improving the mechanical properties, making premixed CPCs, giving the CPCs macro porous properties and seeding cells and growth factors into the cement [25]. CPCs are identified as alloplastic materials appropriate for osseous augmentation because of the unique combination of Osseo conductivity, biocompatibility, mouldability and malleability.

In contrast with conventional bone graft materials, CPCs can be directly moulded and shaped to fill intrabony defects. Moreover newly-developed CPCs are fully injectable, which ensures easy handling and appropriate application of these materials [26].

3. Surface modifications of Ti implants to improve osseointegration

The biological responses of the surrounding tissues to dental implants are controlled largely by their surface characteristics (chemistry and morphology). The biorecognition takes place at the interface of the implant and host tissue [27]. Biological tissues interact mainly with the outermost atomic layers of an implant, which measure about 0.1-1 nm. The molecular and cellular events at the bone-implant interface are not yet fully understood and there are still some uncertainties concerning the molecular structure of the bone-implant interface [28, 29]. The rationale for the surface modification of implants is straightforward: to retain the key physical properties of an implant, while modifying only the outermost surface layer to influence the bio-interaction. As a result, much research work is devoted to the elaboration of methods of modifying surfaces of existing implants (biomaterials) in order to achieve the desired biological responses. These responses can be several: in a healthy patient it may be a regular osseointegration process, but an older or even an ill patient a smaller bone quantity or a not ideal bone quality means a handicap in bio integration. These cases are often avoided by appropriate patient selection. As the length of the average human lifetime is increasing, more and more people are living with missing teeth and in widely differing status of health. There is a demand at present for the optimization of Osseo/bio integration processes (reducing the 3-6-month healing period) even for people in different status of health. For dental implants, as for other biomaterials, the bio- and osseointegration processes can be controlled at molecular and cellular levels by modification of the implant surface. There are various surface-modification possibilities, which are usually subdivided into physicochemical and biochemical methods [28].

4. Physicochemical methods

The most common physicochemical treatments are chemical surface reactions, e.g. oxidation, acid-etching, sand-blasting, ion implantation, laser ablation, surface coating with calcium phosphate, etc. These methods alter the energy, charge and composition of the existing surface, but can lead to surfaces with modified roughness and morphology. The surface energy plays an important role not only with regard to protein adsorption, but also as concerns cell attachment and spreading [30]. The surface charge influences both the molecular or cellular orientation and the cellular metabolic activity [31]. The roughness of the implant surface plays a significant role in anchoring cells and connecting together the surrounding tissues, thereby leading to a shorter healing period. These surfaces display advantages over smooth ones as the area of contact is enlarged by micro structuring the implant surface.


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Osseointegration. A Critical Appraisal
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The author of this text is not a native English speaker. Please excuse any grammatical errors and other inconsistencies.
Osseointegration, Osteointegration, Implantology, Research
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MDS, BDS Vinod Nair Sreekumar (Author), 2018, Osseointegration. A Critical Appraisal, Munich, GRIN Verlag, https://www.grin.com/document/418814


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