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Table of Contents   
ORIGINAL RESEARCH  
Year : 2011  |  Volume : 22  |  Issue : 2  |  Page : 277-284
In vivo bone response and interfacial properties of titanium-alloy implant with different designs in rabbit model with time


1 Department of Periodontics and Oral Implantology, Guru Nanak Institute of Dental Science and Research, Kolkata, India
2 Department of Bioceramics and Coating Division, Central Glass and Ceramic Research Institute, Kolkata, India
3 Department of Veterinary Surgery and Radiology, West Bengal University of Animal and Fishery Sciences, Kolkata, India

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Date of Submission17-Dec-2009
Date of Decision09-Jul-2010
Date of Acceptance25-Sep-2010
Date of Web Publication27-Aug-2011
 

   Abstract 

Background: Using implants for dental applications are well-accepted procedures as one of the solutions for periodontal defect repair. Suitable design and materials, their reaction with the surrounding hard tissues and interfacial biomechanical properties are still considered to be the primary criteria which need to be addressed systematically. In the present study, a thorough and systemic approach was made to identify a suitable implant, considering the above criteria after both in vitro and in vivo animal trials.
Materials and Methods: Titanium alloy (Ti-6Al-4V) implants, with thread and without thread models, were implanted to the mid-metaphysial portion of the tibia of the right hind leg of three white Australian Chinchilla rabbit species and their effects and response to the surrounding bone were investigated. Parameters studied included hematological and biochemical features (serum alkaline phosphatase and calcium), both preoperatively and postoperatively, consecutively for 7 days and after 1-3 months. The interfacial integrity and compositional variation along the interface were studied using scanning electron microscope (SEM) with energy dispersive analysis of X-ray (EDAX) and histopathology from 1 to 3 months consecutively. Finally, biomechanical properties were studied with the help of push-out test.
Results: Bone remineralization started through the process of electro-physiological ionic exchanges, which helps in formation of osteoblastic cells in the area of bony injury. The SEM-EDAX results confirmed the initial stability for the Ti (with thread) implant, but the regeneration of new bone formation was faster in the case of Ti (Without thread) implant, and hence could be used for faster healing. These have also been substantiated through push-out and histopathlogical tests.
Conclusion: From the physico-chemical and biomechanical observations, it was found that that smooth type implants were well accepted in the physiological condition although chances of elemental leaching from the surface were also observed. Increase of the surface roughness can help into the formation of physico-chemical bondage with the surrounding hard tissues.

Keywords: Energy dispersive analysis of X-ray, histopathological evaluation, in vivo animal studies, push-out test, scanning electron microscope, Ti-alloy implants

How to cite this article:
Chakraborty A, Kundu B, Basu D, Pal TK, Nandi SK. In vivo bone response and interfacial properties of titanium-alloy implant with different designs in rabbit model with time. Indian J Dent Res 2011;22:277-84

How to cite this URL:
Chakraborty A, Kundu B, Basu D, Pal TK, Nandi SK. In vivo bone response and interfacial properties of titanium-alloy implant with different designs in rabbit model with time. Indian J Dent Res [serial online] 2011 [cited 2014 Apr 23];22:277-84. Available from: http://www.ijdr.in/text.asp?2011/22/2/277/84304
The improvement of the interface between bone and orthopedic or dental implants is still considered as a challenge because the formation and maintenance of viable bone, closely opposed to the surface of biomaterials, are essential for the stability and clinical success of non-cemented orthopedic/dental implants. It has been addressed to create a suitable environment where the natural biological potential for bone [1],[2],[3],[4] functional regeneration can be stimulated and maximized.There are many factors that could be demonstrated to influence implant osseointergation, viz., surface structure, biomechanical factors and biological response, [1],[5] as now-a-days, osseointergation is defined not only as the absence of a fibrous layer around the implant with an active response in terms of integration to host bone but also as a chemical (bonding osteogenesis) or physico-chemical (connective tissue osteogenesis) bond between implant and bone. [6],[7] Osseointegration, in turn, depends on the biomineralization into the surrounding tissue. Biomineralization normally happens when bony injury of normal bone tissue (cellular level) takes place. The process starts with the osteolysis through the osteoclastic cells from the neighborhood as well as from the systemic source. This is immediately followed by formation of a protein-rich matrix in the localized area (injury site) which is eventually mineralized with the inorganic ions, viz., calcium and phosphorous from the serum and the localized tissues. Once the nucleation of bone formation takes place at a very faster rate (approximately 10 days), then automatically further bone formation takes place with the incorporation of above-mentioned inorganic ions from the serum. [8] Now, titanium has become a widely used material for manufacturing osseointegrable implants, particularly after the studies of Branemark, [6] because of its high corrosion resistance [9],[10] and excellent biocompatibility. [11],[12],[13] The mechanical properties can be enhanced when titanium is alloyed. [14] According to Linder et al., titanium implants could achieve osseointegration without fibrous tissue at bone-implant interface. [15] in vitro, osteoblasts grow faster on these implants than any other metals like cobalt-chrome or SS316L alloy. [16] Further, wear particles from this alloy are less toxic than from cobalt-chrome. [17] Splendid biocompatibility of titanium is due to the formation of titanium-oxide (TiO 2 ) layer on the implant surface. [18],[19] The thickness of TiO 2 formed may be from a few nanometers to 200 nm, on the surface of which Ca and P bind, thus creating a very thin layer of apatite.

In the present study, cortical/cancellous bone and bone marrow response of an implanting material, viz., Ti-alloy, having two different design criteria, have been investigated. This paper outlines the clinical findings of the above-mentioned implanting materials into the mid-metaphysial portion of tibia of the right hind leg of three white Australian Chinchilla rabbit species, and their effects and response to the surrounding bone have been discussed with reference to the detailed hematological, biochemical, histomorphological and biomechanical results in terms of push-out test taken after consecutive 7 days, 1, 2 and 3 months postoperatively. The interface between the bone and implant has also been studied by means of scanning electron microscopy (SEM), energy-dispersive analysis of X-rays (EDAX) findings and their specific response has been correlated subsequently.


   Materials and Methods Top


Titanium alloy implant

Ti-6Al-4V metallic implants (cylindrical shape of tap drill size of 2.5 mm) were procured from M/s Mishra Dhatu Nigam (Hyderabad, India). Chemical composition and the mechanical properties of this implanting material are given in [Table 1]. Standard metric M3Χ0.5 thread was present in one set and the others were without any thread. In subsequent sections of this article, the first design would be designated as Ti (Thread) and the other design as Ti (Without thread) models.
Table 1: Properties of Ti-6Al-4V alloy used for this study


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In vivo trial on animal subjects

All the implanting materials were gamma ray sterilized (2.5 MRad) with Co 60 isotope (Gamma Chamber, GC5000, BRIT, Mumbai, India) prior to subsequent animal trial. In the in vivo study on animal subjects, nine male white Australian Chinchilla rabbit species, each of about 7 months of age, were chosen. They were divided into three groups (hereinafter groups would be designated as M1, M2 and M3). The naming of groups was based on the mean sacrificing of each set of three animals after 1, 2 and 3 months postoperatively. All of them were in the average weight range of about 1.2 kg. Basic scheme of the surgery was to make three holes in the mid-metaphysial portion of the tibia of the right hind leg of each of the species, using round burs (diameter ~2 mm), and placing implanting materials [Ti (Thread) and Ti (Without thread)] successively. The surgery was done under general anesthesia by injecting ketamine (35 mg/kg). The surgical area was anesthetized with local anesthetic agent (lignocaine with adrenaline ~2%) to achieve the bloodless field during surgical procedure. An incision was made through the skin and muscle layer with a surgical blade to expose the raw bone surface. Then, three holes were made over the bone surface for the placement of implants and one of these was kept as positive control. Then, the sutures were made with Catgut (3-0). Postoperatively, the rabbits were medicated with antibiotics such as taxim (IM), and for analgesic purpose, injection voveran (IM) was given for 3 consecutive days. Dosages were adjusted as per the advice of the Veterinary Surgeon. All the groups were subjected to routine body temperature measurement, hematological studies [including measurements of hemoglobin %, total count (TC), neutrophil %, lymphocyte %, monocyte %, eosinophil % and basophil %] and biochemical studies (the levels of calcium and alkaline phosphatase present in the serum) in both preoperative and postoperative situations, i.e, 7 days after surgery. Animal experiments were done by procedures conforming to the standards of the Institutions Animal Ethical Committee of the West Bengal University of Animal and Fishery Sciences, India.

Sample preparation for SEM-EDAX

As the next part of the experimentation, M1, M2 and M3 were subjected to euthanasia after 1, 2 and 3 months, respectively, and before sacrificing the rabbits, biochemical analysis for serum calcium and alkaline phosphatase enzyme was done. The portion of the femur bone (where implantation had been made) was cut. Then, 5% glutaraldehyde phosphate solution was used for fixing the samples, washed twice for 30 minutes with phosphate buffered saline (pH 7.4) and distilled water and subsequently dehydrated in a series of graded alcohol solutions, followed by final drying with hexamethyldisilazane (HMDS). These were mounted on a resin block [10:1 of the Araldite AW 106 Standard Epoxy Resin (Huntsman, Delaware, USA) and hardener XY 95 (Vantico, Switzerland)] in such a way that the surface of each of the specimens of M1, M2 and M3 was exposed, and in cases where the surface was covered with the woven bone, the surfaces were fine ground with a diamond wheel (grit size of 600 mesh). The resin-mounted specimens were again vacuum-dried for a sufficiently long time to remove surface moisture of the specimens, if any. Prior to SEM (LEO 430 STEROSCAN, UK), the samples were cleaned ultrasonically and subsequently sputter coated with carbon having a coating thickness of 10-20 nm, for observation. Qualitative EDAX was then performed as spots on the different parts of the resin block. The detector used in EDAX was lithium drifted silicon detector operated at liquid nitrogen temperatures. The EDAX was performed first on the titanium alloy surface, then on the interface of the respective inserts and finally was done on the woven/matured bone.

Histopathology

Histological examinations were done from decalcified cross sections of the bone with the implants, 1 and 3 months after sacrificing the animals. The bone pieces were washed thoroughly with normal saline and fixed in 10% formalin for 7 days. Subsequently, they were decalcified in Goodling and Stewart's fluid containing formic acid 15 ml, formalin 5 ml and distilled water 80 ml solution and it was stirred daily and changed once in 3 days. The sections were checked regularly for the status of decalcification. They were considered as completely decalcified when sections became flexible and implants were pushed out easily from the bony holes. The decalcified tissues were processed in a routine manner and 4 ΅m sections were cut and stained with hematoxylin and eosin and observed under a light microscope.

Biomechanical studies: Push-out test

Push-out test was used for biomechanical studies which evaluated the interfacial bonding strength. To obtain the specimens for push-out testing, mid-metaphysial portions of tibia were retrieved and sectioned immediately following death of the rabbits, ensuring that the long axis of the specimen was parallel to the axis of the implant. A representation of the push-out test set-up including test and specimen geometries is shown in [Figure 1]. Each implant was pushed out of the bone with a plunger of circular cross section. The samples were placed on a support jig with a hole, 1 mm larger in diameter than the implant perimeter. For the push-out test, an Universal Testing Machine (UTM) (Instron 5500R, UK) was used at a constant crosshead speed of 0.5 mm/minute. After aligning the sample with the plunger axis, the load was applied until the failure load when the bone-implant interface ruptured or there was compressive collapse of the implant as observed from the peak on the load-displacement curve. Two samples each from the groups M1, M2 and M3 were tested within 1 day after the sacrifice. To calculate the contact area for each specimen, the thickness of the cortical bone around the implant was measured at four sites and their mean was approximated as the actual cortical thickness. The product of the height and circumference of the implant dissected from the segment was measured as the contact area between the implant and the surrounding bone. In each case, the failure load was divided by the contact area to calculate the bonding strength between the bone and the implant and the average value was recorded. Before each test, the machine was calibrated and special care was taken to ensure the linearity between the loading force and the long axis of each implant. Bone overgrowth at the outward, muscular site of the implant was removed by polishing, which also allowed an accurate alignment during the test.
Figure 1: Push-out test set-up

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   Results Top


Physical observation of the animals

All the animals implanted with different materials showed somewhat limping in the very first day of surgery and it resulted in inflammation in the region of surgery from the second day and continued for another 2 days. The pain was associated with the redness of the region and swelling. With the application of proper and suitable medication, the limping as well as the inflammation started decreasing from the 5 th day, up to the 7 th day of observation. In case of M2, in addition to the limping and inflammation, there was a nasal obstruction that may be due to some cold infection of that species. The body temperature values of all the animals both preoperatively and postoperatively were under control throughout the study period.

Hematological and biochemical features

The routine hematological and biochemical reports showing the features of all animals are represented graphically in [Figure 2]a-c and [Figure 3], respectively, for both preoperative and postoperative conditions. In all the cases, hemoglobin and TC did not show any acute or chronic changes in hematological reports. Normal value of acute inflammatory cells like neutrophils shows that the animals did not show any remarkable acute immunological responses against the foreign body. The count of other white blood cells, especially lymphocytes, was within the normal range, which again confirmed absence of any chronic inflammatory reactions against the foreign body at the tissue level. The levels of eosinophils were also within the normal range and it shows that the animals did not show any allergic responses against the foreign body inserted within the tissue level discussed above. Alkaline phosphatases of serum, however, were increased from their respective preoperative values, while the calcium level had gradually decreased during the observation period.
Figure 2: Routine hematological report showing the variation in (a) hemoglobin % and TC; (b) neotrophil % and lymphocyte % and (c) monocyte %, basophil % and eosinophil % of the groups M1, M2 and M3 for 7 consecutive days postoperatively starting preoperatively

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Figure 3: Routine biochemical studies showing the variation in the serum alkaline phosphatase and serum calcium of the groups M1, M2 and M3 for 7 consecutive days and 1 month postoperatively starting preoperatively

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SEM-EDAX results

[Figure 4]a-e and [Figure 5]a-e show the SEM photomicrographs of the implants [Ti (Thread) and Ti (Without thread), respectively] with the surrounding tissues. EDAX performed on respective implants, their interfaces with surrounding tissues and on the woven/matured tissues is given as inset. It is very clearly seen in the SEM photomicrographs that Ti (Thread) and Ti (Without thread) forms of implants were placed within the mid-metaphysial portion of the tibia of the rabbit, with the holes made with drill-bit of the same diameter. The photographs as well as [Table 2] show that in due course of time, probably, the fibrous tissue along the marginal part of the respective implants grew. In the first month, the interface between the implant and the bone was measured and it was found that the interface was lesser in the case of Ti (Without thread) type of implant than that of the Ti (Thread) type of implant. In the second month, there were a sudden decrease of interfacial gap both between the Ti (Thread) and Ti (Without thread) type of implants [Table 2] and also it was found that the regeneration of bone along the smooth type of implants was faster than that of the Ti (Thread) type. Respective EDAX pictures in the interface show that the Ca, P ions gradually decreased from first to third months. The SEM pictures, however, show the opposite results, i.e., a gradual decrease in the width of interface between the bone and the inserted implant. It is seen from the respective EDAX that there was an accumulation of Ti ion at the interface in the first month, which gradually decreased to the lowest extent after the third month. Moreover, this observation was more pronounced in the case of the threaded model. Interestingly, aluminum ions could not be seen at the interface in the case of Ti (Thread) up to second month, although a small hump is noticed in the EDAX, showing the presence of aluminum at the same interface after third month. Aluminum ions, on the other hand, for Ti (Without thread) could be seen for the total observation period, during which there was almost a constant concentration of aluminum at the interface. This ion could be found in the matured bone too after 3 months of study. But there was no sign of vanadium at the interface for both the implants throughout the study period.
Table 2: Interfacial gap between the Ti (Without thread) and Ti (Thread) implants and the surrounding bone


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Figure 4: SEM and EDAX images of the Ti (Thread) implant with surrounding tissues taken after different intervals of time: (a) after 3 months (EDAX was carried out on implant surface); (b) after 1 month (EDAX at implant-bone interface); (c) after 2 months (EDAX at implant-bone interface); (d) after 3 months (EDAX at implant-bone interface); (e) after 3 months (EDAX was taken on the new bone surface)

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Figure 5: SEM and EDAX images of the Ti (Without thread) implant with surrounding tissues, taken after different intervals of time: (a) after 3 months (EDAX was carried out on implant surface); (b) after 1 month (EDAX at implant-bone interface); (c) after 2 months (EDAX at implant-bone interface); (d) after 3 months (EDAX at implant-bone interface); (e) after 3 months (EDAX was taken on the new bone surface)

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Histopathological findings

Removal of uncoated smooth implant from the bony cavity after 1 month showed the presence of chronic inflammatory cells within the marrow spaces [Figure 6]a. This interprets the immunological reactions present due to the foreign body or the placement of the implant into the bone. The chronic inflammatory cells are lesser in number in the case of smooth type of implant than that of the threaded implant. This reaction helps in the formation of new bone in the drilled hole within the bony spaces. In [Figure 6]b, the matured bone formation with trabecular spaces and osteocyte cells embedded within the lamellated bony spaces is seen. The marrow spaces showing the reactionary part as chronic inflammatory cells are still present. The osteoblastic activity is still in process, as the inflammatory cells are found within the marrow spaces of the drilled hole bony cavity.
Figure 6: Histopathology of Ti (Without thread) implant after (a) 1 month (A: cortical part; B: marrow space showing inflammatory cells) and (b) 3 months (A: trabecular space; B: osteocytes; C: chronic inflammatory cells)

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Under a light microscope, the section in the case of Ti (Thread) extracted after 1 month [Figure 7]a showed chronic inflammatory cells with osteocyte cells within the marrow spaces. The osteoblastic activities are present although the immunological reactions are comparatively more than that of the smooth implant type. Subsequently, after 3 months, it is seen that matured lamellated bones with multiple osteocytes are in places and the marrow spaces are still infiltrated with chronic inflammatory cells [Figure 7]b. The presence of these inflammatory cells proves that the osteoblastic activities or the new bone formation are still present within the drilled hole socket. The broken portion of the endosteal bony section showing more bone formation and the removal of the implant was used to prepare the histopathological section.
Figure 7: Histopathology of Ti (Without thread) implant after (a) 1 month (A: cortical part; B: marrow space showing inflammatory cells) and (b) 3 months (A: trabecular space; B: osteocytes; C: chronic inflammatory cells)

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Push-out test results

The average interfacial strength of all the implants was calculated as per the methods described above and plotted [Figure 8]. It is found that the values of interfacial strengths were in the higher side in the case of Ti (Thread) and were found to be highest after 3 months. Initial stability of the Ti (Without thread) implant was very poor compared to that of the Ti (Thread), the reason for which is discussed subsequently. Interfacial gap between the matured bone and Ti (Without thread) implant was found to be the lowest, but the corresponding results of push-out test did not match with that of the Ti (Thread), in which the same gap was more after 3 months.
Figure 8: Interfacial strength as calculated from the push-out test for both implants [Ti (Thread) and Ti (Without thread)]

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   Discussion Top


The aim of the present study was to investigate the effect of different designs of Ti-alloy implants on the interfacial bone response and interfacial biomechanical strength with time. Titanium implant has been widely used in implantology, but its cation releasing property has also been reported. [20] After the placement of implants, for any bone injury, serum alkaline phosphatase level increases and it helps locally by depositing phosphate or it helps in the formation of collagen matrix in such a way that calcium can precipitate at that site. This means an elevated serum alkaline phosphatase can be due to rapid growth of bone since it is produced by bone-forming cells called osteoblasts. It makes sense that osteoblasts, by creating a local environment of alkalinity via alkaline phosphatase, help in regenerating the bone.

In the present study, it was observed that after 1 month postoperatively, the interfacial gap formed after insertion of the threaded model into the bone was larger than that formed with the smoother one, which may explain that bone osteolysis was the predominant phenomenon that occurred in the interface initially, as the torque was applied during insertion of the threaded model, which was subsequently covered by the woven bone later and the interfacial gap was found to be almost same after 2 months for both with or without threaded model. Mechanical perturbation can provoke macrophages to release inflammatory agents that stimulate bone resorption. According to Malony et al, [21] the placement of Ti-alloy implants into the mid-metaphysial region of the rabbits causes bone lysis (osteolysis). The picture of the EDAX proves that the osteolysis process is gradually progressing, and thus, the X-ray peak intensity counts for Ca and P ions reduced in number [Figure 4] and [Figure 5]. At the same time, the SEM picture shows the opposite results, which is a gradual decrease in the width of interface in between the bone and the inserted implant. Surface roughness has a significant influence on cell response. Previous in vivo studies have shown that chemical etching of a titanium implant surface significantly enhances osseointegration [22] and increases the strength of osseointegration. [23] The extent of bone-implant interface positively correlates with an increasing roughness of the titanium implant surface. In addition, bone was induced on roughed titanium surface, whereas smooth surfaces were almost entirely covered by fibrous tissue. In this case, the implants were not surface treated with chemical; instead, the design of the surface had been modified macroscopically only by giving a thread pattern on the Ti-alloy surface. As the surfaces were not chemically treated, it was expected that the bone apposition into the close proximity of the implant should be the same and this was observed in our case. The titanium surface reacts with oxygen, forming a layer of titanium oxide film that significantly reduces, but is not able to completely avoid, ion release. The biocompatibility of titanium is for the most part attributed to its oxide film. In our EDAX results, it is clearly documented that titanium ion is released from both screw and smooth types of implant. In a recent study, [24] it has been shown that titanium produces small amount of ions which are rapidly drained through skeletal tissues. This Ti-alloy releases not only titanium but also aluminum and vanadium. We were not able to follow the fate of vanadium ions because in the EDAX method applied, vanadium K emission (4.95 keV) was covered by titanium K emission (4.93 keV). On the contrary, we could easily study the fate of aluminum, whose release from Ti-alloy screws can probably be ascribed to the activity of cells and biological fluids. Metal removal is so conspicuous that it may produce a large modification of the titanium surfaces. Titanium release is probably due to fretting mechanism caused by loads and interfacial displacement, which are inevitably associated with the application of cylinder in the mid-metaphysial region of the rabbit leg. It is indicated that titanium is released from device surface by soft tissues that then finds itsr way into the system. This event may be uneventful because titanium is believed to be free from inflammatory or allergic reactions. Another important observation could be made that aluminum ion is also released in due course of time. Despite being different biomaterials, skeletal tissues show the same behavior toward aluminum. In line with Shahgaldi et al., [25] our results indicate that aluminum is locally accumulated in soft tissue. A long period of aluminum draining follows, and aluminum accumulation in soft tissues eventually ceases if new release does not occur. This draining and diffusion of aluminum toward the surrounding soft tissues may explain the peculiar accumulation of aluminum in bone at sites distant from the biomaterials. In fact, newly formed lamellar bone had marked aluminum accumulation, and a large amount of aluminum was found inside the dense lamella. Aluminum is an element involved in severe neurological (e.g., Alzheimer's disease) and metabolic bone diseases (e.g., osteomalacia). Aluminum accumulation in bone is only a temporal soft tissue deprivation. Bone remodeling causes further aluminum release, even after the appliances no longer leach out aluminum, perpetrating the dangerous side effects of the element.

Different histopathological evidences show the presence of immunocompetent cells due to the surgical trauma at the site of insertion. The smooth implants showed less cellular reactivity, probably because of the lesser trauma caused during the insertion of the implant into the drilled hole. The formation of the new bone was seen in both the cases but the threaded type showed more bone formation along the implant surface in course of healing time. This helps to interprete the design of implant; threaded variety is more acceptable than the smooth type of implant as far as the healing part is concerned, although the nature of bone may r equire the smooth variety of implant, as during insertion it causes lesser trauma to the surrounding soft and hard tissues.

The interactions in the bone-implant interface are initiated from the time of implant insertion. The complex physiologic processes, comparable to those of fracture healing, are regulated by numerous different factors and involve participation of several cell types. [26] Osteoclastic resorption of the interface bone has been observed as one of the most dominating processes during the first week after insertion of an implant in cortical bone. The formation of callus is dependent on mechanical conditions and the distance and presence of gap between implant and bone. During stable mechanical conditions without gaps, intramembranous bone formation takes place directly after the inflammatory response. The presence of a gap over a certain size creates a different situation. It seems that small defects of less than 0.5 mm in diameter heal by direct intramembranous bone formation, whereas larger gaps heal through the cartilage stage and an initial scaffold of woven bone which subsequently turns into lamellar bone. In both the situations, the newly formed bone adapts to the new situation by orientation of the bone architecture. During unstable mechanical conditions, the inflammatory response is prolonged and a fibrous tissue membrane might develop. The magnitude of continuous micromotion in combination with the local environment determines whether the inflammatory response turns into formation of chondrocytes and endochondral ossification (secondary fracture healing). Factors other than mechanical stability and the presence of gaps might influence bone repair; such factors include access of joint fluid to the peri-implant gap, weight-bearing conditions resulting in various stresses at the bone-implant interface (loading versus unloaded implants), implantation site (cortical versus trabecular bone) and status of host bone.

The strength of the interface between a biomaterial and bone is critical to the long-term performance of any load-bearing implant. Push-out testing is often utilized as a method of assessing the mechanical strength of a bone-biomaterial interface developed during in vivo experiments, generally concentrating on the bone-bonding performance of HAp-coated Ti-6Al-4V [27],[28] or the development of implant-bone interface strength with time. [29],[30],[31] The test measures the interfacial shear strength developed between a biomaterial and bone. Analogy could be taken for the present study of push-out test with single fiber push-and pull-out tests, increasingly used in the recent years for studying the interfacial shear behavior of the composites. [32],[33] When a compressive load is applied to the top face of the fiber, shear stresses are introduced at the interface with maximum value occurring at the region near the top face. When the applied load reaches a critical value, debonding is initiated. Once debonding occurs, the shear stress in the debonded zone drops and the region of maximum shear stress moves away from the top face when the applied load is increased. This results in debonding progression. When the load reaches the maximum value, the maximum shear stress reaches the critical value at the bottom face. As a result, the entire length of the fiber gets debonded and the fiber is pushed out of the matrix. This results in a sudden drop in the load because the resistance to further movement of the fiber is mainly due to friction and surface roughness. It has been shown that the following factors affect push-out test results: [34],[35] (i) induced residual stress; (ii) post-processed heat treatment (temperature and time); (iii) evolution of local interfacial chemistry (affects the initiation point); (iv) testing conditions including temperature and rate of loading and (v) test geometry (specimen thickness). All the factors mentioned have implications to our findings. It is evident that debonding forces resulting from the push-out test showed higher values in case of threaded model which is due the friction and surface roughness to the presence of pitch in the threaded model. As mentioned earlier, osteolysis at the interface could be another reason for higher interfacial gap at the interface of bone and the Ti (Thread) than in the case of Ti (Without thread). In the present study, the push-out test was performed at absolutely the same environmental conditions like temperature, humidity, etc., and it was carried out almost afresh.

In a nutshell, the present results allow us to speculate on the relationships and interactions between titanium and bone. In this case, we have to consider that bone apposition onto titanium surfaces is probably the only event that blocks the element release. There are many morphological findings showing a bone affinity for titanium. Titanium (or Ti-alloys) devices are bone metal implants with the best osseointegration; the titanium/bone interface shows a high resistance to mechanical stresses. Titanium has also been attributed a role in osteogenesis processes, as supported by Hong et al. [36] In this respect, a biotolerant coating layer over the dental implant surface prevents the metallic (titanium) ions leaching out in the surrounding hard tissues.


   Conclusions Top


Both types of dental implants [Ti (Thread) and Ti (Without thread)] were well accepted by the surrounding bone. Peak intensity counts of EDAX were found to be stronger in the case of both cortical bone and new bone; it was due to the greater relative percentage of calcium phosphate present in the bone. Primary stability was found in both types of dental implants. However, long-term or secondary stability is more important in terms of bearing the load of crowns. Implant (with thread) was found to be more stable (~27 MPa interfacial push-out strength compared to ~18 MPa for without thread model in the same study period) as far as long-term success is concerned (viz., 3 months in the present study). Different forms of threads are more acceptable and conclusively declared form of dental implants w.r.t. the results confirmed from histopathology and SEM-EDAX interfacial bone apposition study in the present investigation.

 
   References Top

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Correspondence Address:
Biswanath Kundu
Department of Bioceramics and Coating Division, Central Glass and Ceramic Research Institute, Kolkata
India
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DOI: 10.4103/0970-9290.84304

PMID: 21891900

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