A mini-implant (MI) is a device smaller than a conventional dental implant that provides skeletal anchorage for orthodontic tooth movement. As the MI is directly attached to the bony appendages, it provides an absolute anchorage with an ideal movement of the targeted teeth. The advantages of a MI over a dental implant include less invasiveness, relative comfort, and fewer limitations in the patient [1]. Hence, the use of MI application as an anchorage is favoured by the orthodontists and patients. However, the success rate of MI is lower than those of dental implants [2]. Therefore, it’s essential to find the affecting factors of the MI success rate [3].

A number of studies on surrounding bone volume and unique MI design suggested that primary stability was critical in ensuring the success of MI [4]. In addition, the drilling protocols for implant site preparation may be a factor in achieving robust primary stability [5], while the effect of implant site preparation technology on primary stability is not thoroughly studied [6].

At present, implant site preparation was typically achieved by mechanical drilling in clinical practices.

High intensity lasers have been widely used in machining applications in the past decade. Recently, ultrafast laser with picosecond to femtosecond pulses have been demonstrated with unique material processing capabilities that can produce novel structures in the micro to nanoscale [24, 25]. In the case of surgical applications, short (<nanosecond) pulse laser ablation, a non-contact technique usually associated with less thermal damage, is an alternative technology for MI site preparation. Lo et al. demonstrated accelerated bone healing after femtosecond laser ablation in a calvarial defect model [7]. Baek et al. compared the cut surface of the laser osteotomy with conventional mechanical osteotomy [8]. They found biologically open-cut surfaces in the laser osteotomy samples and a flattened smear layer in piezoelectric osteotomy samples. These results explained why laser osteotomy increases bone healing compared with conventional mechanical bone cutting. Femtosecond laser ablation technology was used for osteo-odonto-keratoprosthesis procedures by Berget et al. [9], who showed that the laser cut holes were as strong as burr drilled holes and with a smoother surface in SEM results. Consequently, laser osteotomy opens up the possibility to custom fit the hole precisely to the implant’s width, which represents a potential advantage of the laser over the conventional burr.

The goal of this study was to assess the effect of MI site preparation with a femtosecond laser ablation system on the MI primary stability. Bovine cortical bone samples at different thicknesses were used with MI site preparation performed with a mechanical drill or ultrafast laser ablation, respectively. After MI insertion, the lateral pull-out strength of MI was measured with a tensile testing machine, and the results were compared to a control group without site preparation.

The surface morphology of the bone sample was analyzed with a scanning electron microscope (SEM). Top-view SEM image of a hole prepared with a mechanical drill is shown in Figure 3. The surface characterization showed a cone-shape hole covered and closed by a coating resembling the smear layer. Top-view SEM image of a hole prepared with laser ablation is shown in Figure 4. The surface characterization showed a conical hole without a smear layer. No signs of carbonization or substantial cracking are visible in any of the bone samples. The quantitative lateral pull-out strength measurements are listed in Table 1.

Implant stability is the fundamental requirement for successful skeletal anchorage, and the implant must be maintained for the entire orthodontic period to avoid micro-movements. The stability is obtained from the cortical bones and the remodeling of adjacent bones during the healing process. Delgado et al. found that implant site preparation, cortical-bone thickness, and implant design strongly influenced MI’s primary stability for orthodontic anchorage [11].

To achieve better MI primary stability, less invasive insertion process and greater thickness cortical bone is recommended. However, cortical bone thickness of the implant site is limited in the middle and distal parts of the upper alveolar processes. Silvestrini et al. reported a mean thickness of the cortical bone in the maxilla of 1.10 mm buccally and 1.27 mm on the palatal side [12]. Hourfar et al. found the cortical bone thickness of the anterior palate with values ranging from 0.33 to 1.65 mm [13]. Mohlhenrich et al. measured bone density with cone-beam computed tomography [14]. They found cortical thickness ranged from 0.49 to 0.98 mm at 11 different positions in human cadaver maxillae. For testing the primary stability of MI, human cadaveric bone is a less suitable model as the thickness of cortical bone is variable. The variability of the cadaver bone may render it difficult to distinguish the effects between the thicknesses of cortical bone and site preparation technology. In this study, the standardized bovine bone samples were used to evaluate the effects of drilling technology on mini-screw implant’s primary stability. The bone samples were divided into respective groups according to the fabricated thickness (0.5, 1, 1.5 mm).

The insertion torque, periotest, and resonance frequency analysis are used to evaluate the stability of mini-screw implants [15]. From a clinical perspective, the consensus is that the implant should bear the lateral pull-out strength (10 N), which is the strength used for tooth movement as an anchorage [16]. Therefore, the measurement of MI pull-out strength under loading perpendicular to the MI long-axis was to evaluate MI’s primary stability in this study. For simulating the process in actual clinical practice (the MI implant should be able to withstand tens to hundreds of cycles of pull-out), the 50 cycles of repeated lateral pull-out loads (2–10 N) were performed after initial lateral pull-out strength (10 N) was introduced. Then the lateral pull-out strength of the MI was measured.

In all three groups, the lateral pull-out strength increased with increasing cortical bone thickness (Table 1). The lateral pull-out strength was higher than 10 N in the group of cortical bone thicknesses of 1 or 1.5 mm, which means it can be used as a skeletal anchorage. However, in the group of cortical bone thicknesses of 0.5 mm, the lateral pull-out strength in the subgroups I and II was less than 10 N. On the other hand, the ultrafast laser prepared subgroup III shows lateral pull-out strength of 15.2 ± 2.6 N, which demonstrated that such improvement is clinically significant.

The decrease of pull-out strength in thinner samples is due to the lack of stabilizing torque (bending moment) to resist the lateral load applied to the screw head. In such thin samples, only the middle of the MI shaft is held in the bone, and the tip of the MI is unsupported. When the head of the MI is pulled, there is insufficient bone to resist the tipping of the MI. With thicker bone samples, more of the shaft of the MI is supported—providing a greater resistance (greater bending moment) to the lateral load at the head of the screw. The small improvement in lateral pull-out strength for even the thin bone samples demonstrates the importance of the stability of the bone bed. It is also reported that strain at the bone-implant interface rose with a decreasing MI diameter in finite element analysis [17]. Small diameter screws presented an increased risk of fracture [18] and more microcracks [19]. For the conventional MI site preparation with burr drilling, vibration and thermal damage might generate a side effect of microfracture. Hemanth et al. found the MI site preparation using drilling led to the occurrence of microdamage on the bone [20]. The lateral effects were seen around the implant surface and extend up to 1 mm [21]. These microcracks acted as areas of demineralization or voids and hampered the primary stability of the MI. Extensive microcracks with clear damage were also reported by other authors [22]. Although no signs of carbonization or strong cracking were visible in either of the bone samples, the electron microscopy images showed fewer smears and a smoother surface in laser ablated holes (Figure 4) than in electric drilled hole (Figure 3).

As for MI site preparation with laser ablation, vibration and thermal damage are related to the laser wavelength, pulse duration, and thermal relaxation constant. For a given type of laser-tissue interaction, if the duration of the laser pulse and subsequent ablation process is shorter than the thermal relaxation time of the irradiated volume, the ablation is thermally confined [23]. Our previous study showed that, using optical microscopy and Raman confocal microscopy, the ultrafast laser osteotomy showed no observable thermal damage, carbonization, or micro-cracks on bovine bone [23].

The result of the present investigation showed that primary stability was significantly influenced by MIs site preparation technology, especially for the site in which cortical bone thickness is less than 1 mm. As this study only evaluated the primary stability of the MIs, further animal studies are required to better understand the bone remodeling and osteointegration in response to the implant site preparation. Finally, clinical trials to evaluate the long-term stability of different implant site preparation technologies are essential for making clinical use possible.

In this study, we demonstrated that femtosecond laser ablation is a promising technology for implant site preparation in the thin cortical region (as in the posterior maxilla). Laser ablation prepared samples show significant improvements in MI primary stability, especially in thinner samples where mechanical preparation failed to meet clinical requirements.