**2. Surface modification of orthodontic miniscrews**

The use of commercially pure titanium or titanium alloy (Ti-6Al-4 V) as an implant material has made it possible to predictably secure miniscrews into the maxilla and/or mandible by facilitating direct bone apposition to the implant surface and creating a unique bone-implant interface. This process is termed as "osseointegration" [6]. It is this intimate relationship between living bone and the titanium miniscrew surface that is responsible for its high degree of stability. Various surface treatments of titanium implants have been known to modify both the surface composition as well as its topography, thereby increasing the implant surface roughness and area, which might lead to enhanced bone-screw contact (BSC) [7–12]. Surface modification also enhances the interactions with biological fluids and cells, and thereby accelerates peri-implant bone healing as well as improves osseointegration at sites that lack sufficient quantity and/or quality of bone [7, 11–14]. Evaluation of BSC and removal torque (RT) can, therefore, be used as reliable measures of osseointegration of implants [4]. The improved osseointegration by surface modification is a characteristic exhibited by all titanium surfaces and hence, it applies equally to titanium orthodontic miniscrews [15].

*Surface Modification of Titanium Orthodontic Implants DOI: http://dx.doi.org/10.5772/intechopen.100038*

#### **Figure 2.**

 *Different types of surface modification techniques available for orthodontic miniscrews.* 

 Since the advent of titanium dental implants as prosthetic tooth replacements in the 1990s and titanium mini-plates and miniscrews as skeletal anchorage devices later in the same decade, a considerable amount of research has been done on surface treatments and modifications of these titanium devices. Broadly, these surface modifications can be categorized as either subtractive or additive methods ( **Figure 2** ). The subtractive methods are machining/turning, sandblasting, acid-etching, sandblasting (large-grit) combined with acid-etching (SLA), dual acid-etching and laser treatment. The additive methods are anodization (also known as anodic oxidization), fluoride surface treatment, plasma spraying (titanium or hydroxyapatite), sol–gel coating, sputter deposition, electrophoretic deposition, biomimetic precipitation (Ca-P) and most recently, nanoscale modifications with or without drug incorporation [ 16 , 17 ]. Many of these techniques have been used to augment the biomechanical potential of orthodontic miniscrews and have proven to be experimentally as well as clinically effective. Following is an account of all the surface modification techniques that have been used to enhance the biomechanical potential of orthodontic miniscrews.

#### **2.1 Sandblasting, large-grit, acid-etching**

 One of the earliest methods for surface treatment that was introduced, and one that has stood the test of time, is sandblasting with or without acid-etching. In this technique, alumina (Al 2O 3 ) particles at high pressures are blasted onto the implant surface, after which it may be treated with acidic solutions. The alumina particles are essentially large-grit particles with sizes ranging from approximately 250–500 μm, and the solutions used are highly concentrated acids like hydrochloric acid (HCl), nitric acid (HNO 3 ) and sulfuric acid (H 2SO 4 ). This process creates the desired roughness on the implant surface. The application of sandblasting using large-grit alumina particles followed by acid-etching is collective known as the SLA method ( **Figure 3** ). Wehrbein et al. was one of the first to study the effects of SLA surface treatment on orthodontic implants in humans. Histomorphometric findings revealed that the SLA technique was able to achieve up to 70–80% of BSC, which was remarkably high [ 18 ].

 Animal studies have routinely been carried out in this regard and have shown successful results. Various experimental studies conducted in rabbit tibiae and

#### **Figure 3.**

*Miniscrew surface modified with large-grit sand-blasting and acid-etching (SLA) (Taken from: Yadav et al. [20].)*

femurs that have compared smooth (machined or untreated) and SLA miniscrews have reported greater RT values and BSC in the surface treated miniscrews [19–22]. These results are suggestive of higher miniscrew stability especially in the early stages of healing thereby allowing immediate/early loading, and of an enhanced biological response due to increased osseointegration potential. Chang *et al.* compared conventional smooth miniscrews with SLA as well as alkaline-etched (SL/ NaOH) miniscrews in rabbit tibiae and found that both SLA and SL/NaOH groups had greater RT and BSC values than the conventional group [15]. However, as per a scanning electron microscope (SEM) analysis, the SLA surface showed roughness at two levels: (i) small micro-pits produced by the acid-etching procedure and (ii) microscopic pits superimposed on a sandblasted macro-rough texture, whereas the SL/NaOH surface showed only macroscopic surface properties. This indicates that alkaline-etching might not be as effective as acid-etching for surface treatment of miniscrews. Sirisa-Ard et al. reported that despite an increase in BSC values of SLA miniscrews over 8 weeks of healing in New Zealand rabbits, there was no significant increase of RT values as compared to machined miniscrews over a similar period, suggesting that SLA surface preparation did not have any added benefit in enhancing miniscrew stability [23].

Similar comparative studies between SLA and machined miniscrews have been carried out in other animals such as beagle, foxhound and mongrel dogs. Histomorphometric and micro-computed tomographic (micro-CT) analyses from those studies have revealed greater BSC values with SLA miniscrews indicating their increased osseointegration potential [24, 25]. Some studies have also reported variable torque values for SLA miniscrews at both insertion and removal, essentially indicating equal or improved stability when compared to machined miniscrews [25, 26]. Kim et al. used a digital device to measure the total energy at removal of miniscrews and found that the SLA group had greater values, thus indicating an enhanced biomechanical potential [26]. On the contrary, a similar torque analysis by Vilani et al.

concluded that since there was no significant difference between mobility and insertion torque (IT) or RT of the SLA and machined miniscrew groups, their stability was nearly comparable [27].

#### *Surface Modification of Titanium Orthodontic Implants DOI: http://dx.doi.org/10.5772/intechopen.100038*

The aforementioned positive effects of SLA surface treatment have been validated by a few *in vivo* studies on humans as well. Schätzle et al. compared the stability of standard SLA treated palatal implants with those modified by rinsing under nitride (N2) protection following SLA treatment to enhance their wettability [28]. Resonance frequency analysis (RFA) at various time points over a period of 12 weeks showed that the implant stability quotient (ISQ ) for both groups was similar at the beginning but gradually increased significantly for the experimental group by the end of the study period. This suggests that chemical modification of SLA miniscrews can positively influence their biologic potential and decrease healing time. While most of the research has been focused on evaluating BSC and individual implant stability, some authors have also reported the effect of SLA surface modification on the anchorage ability of miniscrews under orthodontic loads. Calderón et al. used a method of angular measurements on occlusal radiographs for evaluation of positional mini-implant stability and subsequently confirmed those readings on a cone-beam computed tomography (CBCT) occlusal view of just one patient from the study group [29]. As per their calculations, 65% mini-implants showed a ≤ 1degree shift, whereas 35% mini-implants showed a ≥ 2 degree shift. Kim et al. conducted a comprehensive 3-dimensional CBCT analysis of SLA treated mini-implants inserted in the posterior maxillary buccal alveolar region and found that there was no significant change in implant position over 9 months of en-masse retraction [30]. Both of these studies indicate that SLA modification of miniscrews may provide stable and stationary anchorage for orthodontic considerations. However, a couple of studies have reported that despite their relatively greater success rates and better IT values, SLA miniscrews do not have any significant advantage over conventional machined miniscrews in terms of initial stability or overall success [31, 32].

Results from clinical studies hold greater value if they are supplemented by similar proofs from experiments carried out at cellular and/or molecular levels, and viceversa. In an *in vitro* study, Proff et al. compared three groups: airflow treated, SLA treated and machined miniscrews, incubated in a fibroblast cell culture [33]. Using the AlamarBlue assay and fluorescence microscopy, they reported a slight reduction in metabolic cell activity after 24 hours in the airflow group but fibroblast survival and rate of cell proliferation were identical in all the three groups. In an *ex vivo* study of the peri-implant tissue surrounding SLA miniscrews obtained from beagle dogs after 1 and 4 weeks of healing, Nahm et al. carried out gene profiling analyses to reveal that genes encoding extracellular matrix (ECM) constituents were upregulated at the early stage of healing and that genes associated with bone mineralization, ossification, stem-cell fate regulation were upregulated at the later stage of healing [34]. Kim et al. attempted to study the chemical integration mechanism between human bone and titanium miniscrew surfaces at a nanoscale level [35]. A single SLA treated miniscrew was analyzed after 2 months of healing. High-resolution transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) showed evidence of crystalline hydroxyapatite and intermixing of bone with the oxide layer of the miniscrew surface. Scanning TEM (STEM) and electron energy loss spectroscopy (EELS) revealed that carbon existed in polysaccharides, calcium and phosphorus existed as tricalcium phosphate (TCP), and titanium existed in its oxidized form, all rather interesting results. Additionally, the oxygen energy loss near edge structures (ELNESs) showed a possibility of the presence of CaTiO3. The possible existence of the osseohybridization area and the form of the carbon suggests that osseointegration is not purely a mechanical bone-implant interaction and therefore, reconsideration of the standard definition of osseointegration is necessary. In a most recent study on this

topic, Kim et al. studied the molecular surface interaction of a titanium mini-implant (SLA treated) retrieved from a patient after 2 months of healing [36]. Layer profiling using atom probe tomography (APT) showed high concentrations of calcium (Ca) and phosphorus (P) in the bone, titanium oxide (TiO) in the interface, and titanium (Ti) in the implant. Such a nanoscale resolution showing atom-sharing zones at the implant-bone interface provides valuable insight into the process of osseointegration.

It is evident by now that SLA modification of the orthodontic miniscrew surface has some kind of positive biomechanical advantage over conventional machined miniscrews. One would think that this intimate bone-implant relationship comes at a cost of tissue damage to the surrounding bone while retrieval of miniscrews at the end of the treatment period. Studies have shown that despite SLA treated miniscrews having greater BSC and RT values on removal, there was no reported bone fracture or tissue destruction during unscrewing [30, 46]. Kim et al. recommended a nonloading period of fewer than 6 months before removal for optimal bone health and post-operative healing [37].

## **2.2 Microgrooving**

Machining/turning is one of the most basic and simplest forms of implant surface treatment. In actuality, it is an essential part of the manufacturing process that gives shape to the cutting surface and determines the pitch of the screw, which in turn affects the cutting capacity and biomechanical properties of the implant (**Figure 4**). Kim et al. extended this concept of surface turning to a micro-scale level and prepared miniscrews with microgrooves (50 μm pitch, 10 μm depth) on 300 μm of the upper cutting surface [38]. This experimental group (MG) was compared against conventional non-microgroove (NMG) miniscrews in beagle dogs after 16 weeks of orthodontic loading. Histomorphometry revealed higher BSC values on the pressure side of the MG group. Further histological analysis showed that gingival connective tissue

#### **Figure 4.**

*Microgrooving technique of surface modification. The microgroove shown here is 50* μ*m pitch and 10* μ*m depth in 300* μ*m on the surface. (Taken from: Kim et al. [38].)*

fibers (GCTF) in the MG group were oriented perpendicular to the miniscrew surface whereas in the NMG group they were parallel. Additionally, fluorescent microscopy showed more bone remodeling on the pressure sides in both groups as compared to the tension sides. This suggests that addition of microgrooves could exert some positive effects on the soft tissue adaptation and bone healing around orthodontic miniscrews.
