1.Introduction
2.Material and methods
3.Result
4.Discussion
5.Conclusion


  Abstract

    The purpose of the present study is to evaluate the significance of different surface textures by comparison of the removal forces for laser-treated and machined titanium screw 8 weeks after the installation in rabbit tibia.

    A total of 14 screw shaped, commercially pure titanium implants with a length of 5 mm, a diameter of 3.75mm were grouped as follows: Group A: seven implants left as-machined; Group B: seven implants treated with laser method (CSM implant, CSM company, Daegu, Korea) Topographic evaluation was performed with scanning electron microscope (Hitachi S-4200, Japan) to compare the surface structure of laser-treated and machined ones. Installation procedures were done according to Branemark protocol after pre-threading, machined implants were inserted in right tibia metaphysics and laser-treated surface implants were inserted in left ones.

    Eight weeks post surgically seven rabbits were sacrificed. The implant sites were exposed, and the bone and soft tissues that had formed on top of the implants were carefully removed. Subsequently, the force needed to unscrew the implants (n = 14) was measured using a digital torque gauge (Mark-10 corporation, USA).

    Scanning electron micrographs of the laser-treated and machined control groups demonstrated created a deep and regular honeycomb pattern with small pore, while machined treatment created the typical microscopically grooved and relatively smooth surface characteristic. Eight weeks after implant placement, the average removal torque was 23.58±3.71Ncm for the machined implants, 62.57±10.44Ncm for the laser-treated implants. The torque measurements yielded statistically significant differences between the machined group and the laser-etched group (p = 0.00055) (Wilcoxon's signed-rank test).

    The laser-treated group achieved higher removal torque values compared to the machined control group.

1. Introduction

    In an attempt to improve the quantity and quality of the bone-implant interface, numerous implant surface modi.cations have been used. Unlike other surface modifications, laser-etching technique has been introduced on material engineering originally [1]. This process can result in unique microstructures with greatly increased hardness, corrosion resistance, or other usefulsurface properties [1]. They also showed that laser processing is a new method of treating implant surfaces to produce a high degree of purity with enough roughness for good osseointegration [2]. The development and use of these surface modifications have been based on the theory that improved bone to implant contact can be achieved by increasing the topography or of the implant surface.

    As a positive correlation between the degree of one in contact with the implant and removal torque was reported the removal torque method were used to identify the bone to implant contact [3].

    The purpose of the present study is to evaluate the signi.cance of different surface textures by comparison of the withdrawal forces necessary for removal of otherwise identical laser-treated and machined implants of commercially pure titanium.

2. Material and methods

2.1. Animals and anesthesia

    Ten adult white rabbits weighing 3.1-4.0 kg were used in this study. The animals were anesthetized with a combination of ketamine (Ketara® ,Yuhan corporation, Seoul, Korea, 44 mg/kg of body weight) and xylazine (Rompun® , Bayer Korea, Seoul, Korea, 7 mg/kg of body weight) intramuscularly. Prior to surgery, 1.8 ml of lidocain 2% (Yuhan Corporation, Seoul, Korea) was injected locally into the surgical sites of tibia metaphysics. After the surgery, all the animals were injected IM with antibiotics (Baytril® Byer Korea, Seoul, Korea) at a dose of 0.3 mg per animal, analgesics (Nobin® Bayer Korea, Seoul, Korea) at 1ml per animal and metabolite (Catosal® Bayer Korea, Seoul, Korea) at 1ml per animal. They were allowed full weight bearing and movement post surgically. Eight weeks after surgery, the animals were sacri.ced using an overdose of anesthetics.

2.2. Surface treatment

    A total of 14 screw shaped, commercially pure titanium implants with a length of 5 mm, a diameter of 3.75mm were used in the present study. The implants were divided into two groups:
    Group A: Seven implants left as machined.
    Group B: Seven implants treated with laser method (CSM implant, CSM Company, Daegu, Korea).

    To ensure that the laser treatment did not severely change the shape of the screws, Nikon Measurescope 10 (Nikon, Tokyo, Japan) equipped with a digital counter was used to measure the outer diameter of live randomly chosen implants from the each groups.

2.3. Scanning electron microscope (SEM)

    Topographic evaluation was performed with SEM (Hitachi S-4200, Japan) to compare the surface structure of laser-treated Group B and machined Group A.

2.4. Energy dispersive spectroscopy

    Energy dispersive analysis of X-ray spectroscopy (EDAX: Horiba EX-300, Japan) was used in this experiment to evaluate the element analysis of the specimens.

2.5. Implant placement

    Prior to surgery, operation sites were shaved and decontaminated with mixture of iodine and 70% ethanol. The tibia metaphysics was exposed by incisions through the skin, fascia and periosteum. The bone of preparation sites were threaded using a 2mm in diameter round drill and 2 and 3.3mm in diameter twist drill under profuse irrigation and at low rotary speed. After pre-threading, machined implants (Group A) were inserted in right tibia metaphysics and laser etching surface implants (Group B) were inserted in left ones.

    All implants were allowed to penetrate the first cortical layer only. After the implant insertion, mucoperiosteum and muscle were sutured in separate layers using absorbable sutures.

2.6. Torque measurements

    Eight weeks post surgically seven rabbits were sacri.ced. The implant sites were exposed, and the bone and soft tissues that had formed on top of the implants were carefully removed. Subsequently, the force needed to unscrew the implants (n = 14) was measured using a digital torque gauge (Mark-10 corporation, USA). The result was recorded by measuring the maximum removal torque at which fracture occurred between implant and bone.

2.7. Statistical analysis

    Wilcoxon's signed-rank test was used to calculate probability value for torque measurement analysis between two groups.

3. Result

3.1. Topographic evaluation (SEM)

    By using the Nikon Measurescope 10, only small differences were found in the mean values of the outer diameter between the groups. The mean values of the outer diameters were 3.751mm for the machined control group and 3.737mm for the laser treatment group.

    Scanning electron micrographs of the laser-treated group and the machined control group demonstrated microscopic differences in the surface topography. SEM surfaces are seen in Fig. 1. Laser etching created a deep and regular honey-combpatte rn with small pore, while machined treatment created the typical microscopically grooved and relatively smooth surface characteristic. In the laser surface, the distance between the pore was 10-12 m.

3.2. Energy dispersive spectroscopy

    The results of EDAX (Horiba EX-300, Japan) showed no contamination during the laser-etching process (Fig. 2)

3.3. Removal torque measurements

    Eight weeks after implant placement, the average removal torque was 23.58 ± 3.71Ncm for the machined implants, 62.57 ± 10.44Ncm for the laser-treated implants.

Fig. 1		Fig. 2

    The removal torque results are summarized in Table 1 The torque measurements yielded statistically significant differences between the machined group and the laser-etched group (p<0.001). The higher removal torque corresponded to the laser-etched implants, while the lowest was demonstrated by the machined implant.

4. Discussion

    According to Roberts et al. [4] in rabbits it takes 6 weeks for the woven bone to be replaced by the lamellarbone with adequate strength for load bearing. So, we selected healing period in 8 weeks because a fast rate of bone is crucial for mechanical integrity of implant bone interface. But, longer follow-up periods (i.e. 12 weeks and 6 months) would be needed to reveal possible difference in removal torque value and bone-implant contact.

Table 1
Removal torque vaues (Ncm)
Animal no.	Machined	Laser etched
1	23.61	83.95
2	26.44	66.44
3	20.90	51.30
4	18.53	57.1
5	24.52	58.98
6	29.60	59.79
7	21.47	60.34
Mean	23.58	62.57
SD	3.71	10.44
*p value = 0.00055.

    Cordioli et al. [5] reported that average removal torque value was 25.28Ncm for the machined implants, 26.85Ncm for the grit-blasted implants, 29.57Ncm for the plasma-sprayed implants, and 40.85Ncm for the acid-etched implants under similar condition.

    Several other studies have reported that rough implant surfaces of varying topography generally demonstrate increased bone apposition and higher removal torque when compared to machined surface [6-9]. Using the transcortical model, Thomas et al. [10] found that implants with a roughened surface had a great interface strength and higher surface coverage by bone than smooth implants. Buser et al. [11] found that increasing implant surface roughness generally correlated with increased surface coverage by bone. It is assumed that rough surface is one of the important factors in success of dental implants.

    Laser-etching technique is a kind of subtractive method. So, we measured the differences in implant outer diameters before and after laser treatment. This showed only small difference.

    The present study showed that the laser-treated group achieved higher removal torque values compared to the machined control group. The greater removal torque values may be related primarily to the higher bone to implant contact [12]. The higher torque values demonstrated in this investigation may be attributed to differences in implant surface.

    Wennerberg et al. [13] suggest that high surface roughness alone is not the only criteria to consider for optimal osseointegration. The pattern, size, and distribution of peaks and valleys that compose the surface roughness may signi.cantly in.uence the overall intimacy and mechanical interlocking of the bone-implant interface.

Fig. 3		Fig. 4

    Mechanical interlocking is based on basis of microporous surface structure, while biochemical bonding is based on calcium surface chemistry [14]. In author's experiment, the chemical composition of the two surface was similar (Fig. 4)), that means that the role of calcium could be deleted.

    It is evident that increasing the surface oxide thickness of titanium implants dose result in increase of the bone response; however, the reason for this reinforced bone response may not be the increase of the oxide thickness per se, but rather changes in other surface parameter, for example, the microporous surface structures, and so on [14].

    The reason why laser-treated surface showed more removal torque than that of machined surface would be the role of pore diameter could influenced the results, for this, it would be necessary to reveal the contact surface area of pore it self with bone and that of the area between the pores.

    Mustafa reported that the proliferation and differentiation of cells derived from human mandibular bone is enhanced by surface roughness of the titanium implant, but increasing the size of blasting particles to 300 µm does not further increase the initial attachment of the cells compared to turned surfaces and those blasted with 63-90 µm particles [15]. Itala et al. [16] studied that the optimal pore size for mineralized bone in growth is 100-400 µm. But, this pore size is notapplicable in clinical situations directly because oflimitation for non-load bearing conditions. For theinfluence of the depth of pore, it would be the future homework.

    Muller et al. [17] demonstrated that TEA-CO2 laser sirradiation resulted in enhanced adhesion of polymer on titanium surface and improved biological performance and improvement in bonding strength could be achieved with the help of creation of stable Ti oxides onto the surface [13]. Shigematsu et al. [18]reported that a surface processing using laser irradiation does not cause a deterioration of the mechanical properties of titanium, because laser beam heats only the surface of titanium to high temperature [18].

    Gaggl et al. [2] analyzed four different implant surfaces treated individually with special attention focused on laser surface treatment. Surfaces with machine roughness, titanium spray coating, treated by aluminum oxide and treated by laser were examined individually. Evaluation of the surface was carried out by SEM and degree of contamination was determined by energy disperse X-ray spectrophotometer analysis (EDS analysis).

    Gaggl et al. [2] reported that surfaces of laser-treated titanium implants showed a high purity with enough roughness for good osseointegration. Gaggle's surface had primary regular 30-50 µm troughs and secondary 10 µm trough structure of melting pearls. In laser treatment, EDS analysis revealed a high purity of the implant surface compared to the other treatment. Aluminum oxide-blasted implants caused surface contamination with alumina particles. Titanium plasma coating led to a high degree of contaminating particles without osseointegrative properties [2].

    The present study showed regular pattern ([Fig. 2]) of micropore of 10-12 µm interval and with diameter of 25 µm and depth of 20 µm ([Fig. 3]).

    The laser-treated surface, tested in this study, needs to compare the removal torque with other implant surfaces such as titanium plasma spray coated, blasted, acid etching and a combination of these.

5. Conclusion

    The laser-treated group achieved higher removal torque values compared to the machined control group.

    The torque measurement yielded statistically significant difference between the machined group (group A) and the laser etched group (group B).

References


    [1] Picraux ST, Pope LE. Tailored surface modi.cation by ion implantation and laser treatment. Science 1984;226:615-22.


    [2] Gaggl A, Schultes G, Muller WD, Karcher H. Scanning electron microscopical analysis of laser-treated titanium implants surfaces -a comparative study. Biomaterials 2000;21:1067-73.


    [3] Johansson CB, Albrektsson T. A removal torque and historphometric study of commercially pure niobium and titanium implants in rabbit bone. Clin Oral Implant Res 1991;2:24-9.


    [4] Roberts RW, Smith RK, Zibermann Y, Mozsary PG, Smith R. Osseous adaptation to continuous loading of rigid endosseous implants. Am J Orthod 1984;86:95-111.


    [5] Cordioli G, MajzoubZ, Piatelli A, Scarano A. Removal torque and histomorphometric investigation of 4 different titanium surfaces. Int J Oral Maxillofac Implants 2000;15:668-74.


    [6] Kieswetter K, Schwartz Z, Dean DD, Boyan BD. The role of implants surface characteristics in the healing of bone. Crit Rev Oral Biol Med 1996;7(4):329-45.


    [7] Boyan BD, Hummert TW, Dean DD. Role of material surfaces in regulating bone and cartilage cell response. Biomaterials 1996; 17:137-46.


    [8] Cochran DL, Simpson J, Weber HP, Buser D. Attachment and growth of periodontal cells on smooth and rough titanium. Int J Oral Maxillofac Implants 1994;9:289-97.


    [9] Brunette DM. The effects of implant surface topography on the behavior of cells. Int J Oral Maxillofac Implants 1998;3:231-46.


    [10] Thomas KA, Cook JKJF, Cook SD, Jarcho M. The effect of surface macrotexture and hydroxylapatite coating on the mechanical strengths and histologic pro.les of titanium implant materials. J Biomed Mater Res 1987;21:1395-414.


    [11] Buser D, Schenk RK, Steinemann S, Fiorellini JP, Fox CH, Stich H. In.uence of surface characteristics on bone integration of titanium imlants, a histomorphometric study in miniature pigs. J Biomech Mater Res 1991;25:889-902.


    [12] Johansson C, Albrektsson T. Integration of screw implants in the rabbit: a 1-year follow-up of removal torque of titanium implants. Int J Oral Maxillofac Implants 1997;2:69-75.


    [13] Wennerberg A, Albrektsson T, Lausmaa J. Torque and histomorphometric evaluation of c.p. titanium screws blasted wit 25- m and 75- m sized particles of A12O3. J Biomed Mater Res 1996;30:251-60.


    [14] Sul YT. On the bone response to oxidized titanium implants. Ph.D. thesis, Department of Biomaterials/Handicap Research, University of Gothenburg, Sweden; 2002.


    [15] Mustafa K, Wennerberg A, Wroblewski J, Hultenby K, Lope BS, Arvidson K. Determining optimal surface roughness of TiO (2) blasted titanium implant material for attachment, proliferation and differentiation of cells derived from human mandibular alveolar bone. Clin Oral Implant Res 2001;12:515-25.


    [16] Itala AI, Ylanen HO, Ekholm C, Karlsson KH, Aro HT. Pore diameter of more than 100 mm is not requisite for bone ingrowth in rabbits. J Biomed Mater Res 2000;58:679-83.


    [17] Muller WD, Seliger K, Meyer J. The improvement of adhesion of polymer of titanium surface after treatment with TEA-CO2 laser irradiation. J Mater Sci Lett 1994;5:692-4.


    [18] Shigematsu I, Nakamura M, Saitou N, Shimojima K. Surface hardening treatment of pure titanium by carbon dioxide laser. J Mater Sci Lett 2000;19:967-70.