An injectable bone graft substitute to enhance the primary stability of a novel miniscrew – The Sydney Mini Screw

179 © Australian Society of Orthodontists Inc. 2018 Introduction: Anchorage is crucial in controlling tooth movement during orthodontic treatment. Different designs have been introduced to increase the stability of miniscrews. A new miniscrew, The Sydney Mini Screw (SMS), with a hollow chamber and lateral port holes, has been developed to allow the diffusion of an injectable bone graft substitute (iBGS) into cancellous bone. The aim of this study was to analyse the optimum iBGS application with ideal chemo-mechanical properties to be used in conjunction with the novel SMS. Method: A composite calcium sulphate and calcium phosphate bone graft substitute was examined. The effects of powder particle size, and the powder-to-liquid ratio on the injectability of the iBGS through the SMS were investigated. The viscosity, injectability, and mechanical properties of the new composite mixtures were assessed using rheology and universal compression measurements. Results: The results showed that the optimised injectable formulation of the bone cement was acquired with the concentration of 2.5 g/ml. This concentration was readily injectable through the SMS, and its setting time was within 2–3 minutes, which is favourable for clinicians. In addition, the resulting structure fractured at 80 kPa compression stress. Conclusion: The result of this study identified the specific particle size and powder-to-liquid ratio of the iBGS that can be used in conjunction with the new SMS to enhance the primary stability of orthodontic miniscrew applications. (Aust Orthod J 2018; 34: 179-187)


Introduction
Temporary anchorage devices (TADs) were introduced almost three decades ago as an alternative to osseointegrated implants for skeletal anchorage. 1,2 TADs eliminate the need for patient compliance and provide absolute anchorage, aiding in maximising orthodontic treatment outcomes. Miniscrews are the most commonly used type of TAD and are of smaller dimension, lower cost, and can be placed chair-side. However, their failure rates remain unacceptably high and in the range of 0% to 41%. [3][4][5][6][7] While numerous factors such as bone quality, miniscrew design, soft tissue inflammation, root proximity and surgical technique have been documented to affect primary stability, cortical bone engagement is found to be the predominant factor. 4,8,9 Alterations in miniscrew design and placement techniques have been proposed to improve miniscrew success rates and acquire safer and more reproducible clinical outcomes. 10,11 The results of studies investigating different surface treatments and designs are conflicting as some show no statistical difference while others show increased stability. 12,13 Injectable calcium phosphate cements (iCPC) have been recently investigated for orthopaedic applications in dental and craniofacial procedures due to their exceptional biocompatibility, self-setting characteristic and osteo-conductive properties that allow successful repair of bone defects. 12,13 No significant anchorage increase has been shown between dental implants tested with or without the addition of an injectable form of calcium phosphate bone graft substitute (BGS) in implant beds. 14 However, when calcium phosphate BGS was applied in extraction sockets, it was shown to optimise angiogenesis and bone remodelling. 15 Like most ceramics, calcium phosphate BGS is brittle and cannot be used alone in load bearing applications. 16 A combination of calcium sulphate and calcium phosphate (CaSO 4 /CaPO 4; Pro-Dense ® ) was later introduced to address this problem. 17 In 2010, a novel orthodontic miniscrew, The Sydney Mini Screw (SMS) was designed to increase primary stability through the application of an injectable bone graft substitute through its hollow lumen into cancellous bone. 20 The aim of the present study was to assess the feasibility of using an injectable BGS (iBGS) with the SMS in an attempt to enhance the primary stability of miniscrews. The ease of injection, setting time and mechanical strength of the BGS were investigated.

The Sydney Mini Screw design
The specifications of the miniscrew tested in this study were 6 mm in length, and an outer thread diameter of 2 mm (Figure 1-A). The design of the SMS (Patent number: PCT2009014) was based upon the clinically proven Aarhus anchorage system (Medident, Hellerup, Denmark) with the addition of a hollow centre and exit holes. Titanium cannulated cylinder design miniscrews were manufactured by Russell Symes and Company Pty Ltd, Sydney, Australia. The central cannulated portion of the screw was 0.6 mm in diameter and extended from an open head to lateral portholes. The cannula at the head was widened to 0.92 mm for a depth of 3.8 mm to accommodate the thickness of a 20-gauge syringe tip. The two lateral portholes had a diameter of 0.40 mm and were positioned between the screw threads towards the bottom of the miniscrew body.

Injectable bone graft substitute
The iBGS used in the present study was a non-sterile commercially available synthetic bone graft composite (PRO-DENSE® Extremity Mixing Pack, Wright Medical Technology, Inc., TN, USA). The PRO-DENSE ® material was composed of 75% CaSO 4 and 25% CaPO 4 (brushite and granular TCP) powders and glycolic acid liquid. 17

Particle size
The particle size distribution of both CaSO 4 /CaPO 4 powders was determined with a laser granulometer (Mastersizer 2000, Malvern Instruments Ltd., UK). Three initial measurements were performed using sodium hexametaphosphate as a suspension medium for the original particle size of the commercially available product.
The commercially available iBGS was designed to pass through needles larger than 20-gauge. Therefore, the particle size had to be modified to be less than 63 μm. This was achieved by grinding the powder under dry conditions with an agate mortar and pestle. The ground powder was sieved using a stainless-steel frame and mesh of 63 μm aperture (Endecotts LTD, London, UK) to exclude particles above that mesh pore diameter.
Furthermore, the effect of different powder to liquid ratios (PLR) was examined by constant weight to varying liquid ratios to find out the appropriate combination for optimal injectability.
The iBGS was prepared under the same conditions and at room temperature prior to each experiment. The cement powders were prepared by proportioning and subsequent mixing of both CaSO 4 and CaPO 4 powders with the liquid using a flexible silicone cup and a plastic spatula, resulting in a viscous and cohesive mixture. For each formulation, iBGS pastes were prepared by mixing the cement powders and liquid by hand for 30 seconds. This resulted in a workable paste that was transferred to a disposable 1 cc syringe (BD Luer-Lok TM Tip, Singapore).

Injectability testing
Twenty gauge, blunt-type, 10-mm-long needles with inner and outer diameters of 0.6 mm and 0.9 mm, respectively (BD PrecisionGlide TM Needle, Singapore), were trialled. Cross-section apertures of needles and miniscrew lateral portholes were observed under a microscope (Olympus VE-3 Stereomicroscope, Tokyo, Japan) with 40× magnification to confirm the absence of interfering metal particles. The handling time was 1.5 minutes, which allowed mixing and loading of the iBGS into syringes.

Mechanical property analysis
Mechanical testing was used to determine the compressive strength of iBGS at different concentrations. The iBGS were transferred into a handmade translucent cylindrical mould with a height of 12 mm and a diameter of 6 mm, following the standard testing guideline of the American Society for Testing and Materials (ASTM) ASTM C-773. After two hours, the cylindrical specimen was removed from the mould and the dimensions were calculated. Measurement was performed using an electronic digital calliper (MAX-CAL; micrometer MFG, Co., Ltd, Japan) with an accuracy of 0.01 mm. The compressive strength was tested using an Instron Testing Machine 3366. A total of nine cylinders were tested using a crosshead speed of 1 mm/min until obvious specimen failure was observed. This was indicated by a substantial drop in the load curve.

Finite element analysis
FEA testing indicated that the 1.9 mm SMS with 0.4 mm diameter lateral ports under a torsional load (similar to bone insertion) exhibited maximum stress.
The macro-and micro-structure of the examined miniscrew is shown in Figure 1. The maximum tension stress achieved was 441 MPa as shown in Figure 2. The margin of safety for this design is approximately 150% above the design load, thus it performed well within all normal clinical parameters with an acceptable safety margin.

Particle size distribution analysis
Particle size distributions of calcium sulphate and calcium phosphate particles before and after grinding are shown in Figure 3. Upon grinding, the median size of the CaSO 4 and CaPO 4 powders decreased from 22 μ to 16 μ and 160 μ to 45 μ respectively.

iBGS mixing and injectability testing
Attempts using the commercially available iBGS in the pilot studies resulted in blockage of the small needles. Therefore, other than changing the particle size, the powder/liquid mass ratio of the iBGS was also gradually varied from 2.3 mg/ml to 3.1 mg/ ml. Decreasing the liquid to create mixtures with a PLR higher than 3.1 mg/ml resulted in granular and unworkable pastes that could not be injected with a force ≤ 100 N. 19 Therefore, for further evaluations and optimisation, iBGS concentration was limited to below 3.1 mg/ml.

THE SYDNEY MINI SCREW WITH BONE CEMENT
The extruded volume of the paste at different concentrations was measured ( Figure 4). It was shown that 1 ml of the paste was extruded for the mixtures with a concentration of 2.3 mg/ml to 2.7 mg/ml. A further increase of powder concentration resulted in incomplete extrusion of the mixture (0.8 ml). For each test, maximum force for extrusion and at failure was also recorded (Figure 4).
The results of the injection test showed that iBGS samples with concentrations greater than 2.7 mg/ ml were not injectable through a 20-gauge needle. Therefore, concentrations above 2.7 mg/ml failed to exhibit a fundamental property and consequently were considered not suitable for use in conjunction with SMS.
Accordingly, for further evaluations and to optimise the gelling point and the mechanical properties of iBGS, only concentrations ranging between 2.3 mg/ ml to 2.7 mg/ml were used.

Gelling point
The gelling point of the paste with different powder concentrations was assessed using the rheology test. Increasing the powder concentration from 2.3 mg/ ml to 2.7 mg/ml resulted in a longer setting time, 62.5 seconds compared with 85 seconds ( Figure 5). Therefore, increasing the PLR led to slower gelation.
The results of the rheology test were also used to determine the fracture point of the iBGS ( Figure 6). Increasing the concentration of the powder from 2.3 mg/ml to 2.7 mg/ml resulted in a decrease of fracture point by approximately half. Therefore, increasing the concentration of the powder in the mixture resulted in formation of a more brittle material.

Mechanical property analysis
The mechanical properties of the paste also play an important role in obtaining a workable iBGS that is clinically applicable. Therefore, more concentrated pastes (2.3 mg/ml, 2.5 mg/ml, and 2.7 mg/ml) were selected for further analysis. Results from the rheology test were in agreement with the result gained from the compression test in which the ultimate failure load for the cement pastes decreased from 415 N to 300 N by increasing the concentration of the powder from 2.3 mg/ml to 2.5 mg/ml ( Figure 6).

Discussion
The feasibility of using iBGS in conjunction with a hollow miniscrew was investigated in the current study. The CaSO 4 component in the composite CaSO 4 / CaPO 4 20 iBGS has been reported to exhibit fastdegradation allowing enhanced vascular infiltration and bone deposition. 21 Both of these properties are of great benefit to enhance the osseointegration and the primary stability of a miniscrew following its insertion.
The initial results of the injectability tests warranted modifications to the design of the SMS. A slight increase in the overall size of the miniscrew allowed the use of a larger gauge needle, 22 increasing the injectability of the BGS. A wider miniscrew hollow port with an internal diameter of 1.4 mm matched the outer diameter of a 20-gauge needle, which is known to be used for endodontic root canal fillings and to seal furcal perforations. 22,23 In the present study, the secure fit of the 20-gauge needle at the head of the miniscrew allowed optimum seal during injection of the BGS. These modifications minimised the tendency for the iBGS to extrude from the top of the miniscrew and allowed better delivery of the material through the miniscrew and into the cancellous bone.
The preliminary trials also suggested that the loss of pressure within the internal chamber of the miniscrew was the key to the injectability of iBGS. It was  hypothesised that by reducing the number of lateral portholes across the body of SMS, higher pressure will be distributed to each porthole and, therefore, overcome the frictional force according to Darcy's Law. 24 Furthermore, miniscrew fractures are more likely to occur during its insertion or removal if the patient has very dense cortical bone, and when smaller sized miniscrews are used, as the material is weaker. It was therefore plausible that the weakest point in this novel miniscrew design would be around the lateral extrusion portholes when the SMS was under torsion. 25 The initial FEA confirmed this, with the highest tensile stress expressed adjacent to the lateral portholes during torsional loading. For this reason, the design of the SMS was modified after initial testing to include fewer portholes for better extrusion of the iBGS, and to reduce the risk of fracture. The FEA analysis then confirmed that the improved design of the SMS could be considered for in-vivo testing.
The bioactivity 26 and osteo-conductivity of BGS in the osseous environment have shown promising results in orthopaedic and dental applications. 14,21,23,27,28 CaSO 4 and CaPO 4 are commonly used for fixation or repair purposes in orthopaedic surgery. 13,17,20 In the present study, the particle size of the iBGS was optimised to assure its complete and convenient extrusion through the 20-gauge needle. Phase separation caused filterpressing; 29 hence, in the present study, the cement was deemed injectable once the paste was extruded uniformly in a single phase composition. Two strategies to reduce filter-pressing of iBGS when using such small gauge needles include decreasing the mean particle size of the powder, and modifying the PLR. 22,[30][31][32] The inner diameter of the 20-gauge needle is 600 μm. Therefore, it was decided that the average particle size in the cement powder should be approximately below 100 μm to allow easier flow of the cement. The average particle sizes for CaSO 4 and CaPO 4 determined by the laser granulometer were below 100 μm and still within the acceptable range for convenient delivery of the biomaterial. Theoretical models were developed to investigate the effect of different parameters, such as particle size, on the injectability of the bone graft substitutes. Baroud et al. 32 stated that an increase of the PLR and a decrease of the particle size of the powder component contributed to improving the injectability of calcium phosphate pastes. The result of the present study confirmed that although smaller particle sizes were known to have higher reactivity, 30 it also helped reduce or even eliminate the filter-pressing phenomenon by improving the injectability of the paste 32 through very thin cannulae.
In addition to particle size distribution, the extrusion efficiency and the solidifying time were also of great importance to assure convenient thumb-driven extrusion of the iBGS. The extrusion efficiency and the injectability of the bone graft substitute, prepared with different concentrations, were also assessed. The results from the present in-vitro study in a simulated physiological condition showed that the extrusion efficiency significantly dropped upon increasing the concentration above 2.7 mg/ml (p < 0.001). Hence, for the other tests the PLR was kept within the range of 2.3 mg/ml to 2.7 mg/ml.
For clinical applications of iBGS, the post-mixing behaviour of the biomaterial should also be thoroughly assessed and evaluated, since fast bone graft solidification would block the needle and affect the delivery process adversely. To systematically investigate the gelling time of the iBGS, rheological behaviour was assessed using an oscillatory rheometer. The gelling point of the CaSO 4 /CaPO 4 was similar to the estimated initial setting time of the material, while the fracture point was closer to the start of the final setting of the material. The time between those two measurements indicates an estimate handling time for clinical applications. The present study sample demonstrated a rapid gelling, which needed to be taken into account when developing the setup for the dynamic test. 33 These results showed that elevating the concentration of iBGS from 2.3 mg/ml to 2.5 mg/ml significantly increased the gelling time by nearly two fold. This effect might be due to the presence of space hindrance in the iBGS in the higher concentration. Further increase of iBGS concentration from 2.5 mg/ml to 2.7 mg/ml had no significant effect on its gelation time. However, the iBGS prepared with PLR of 2.7 mg/ml was brittle and displayed the ultimate fracture load of 300 N. Those prepared with 2.5 mg/ml had a higher ultimate fracture load of approximately 350 N. Therefore, these results suggested that the iBGS prepared with 2.5 mg/ml of solid content is the optimum formulation as an injectable biomaterial to enhance the primary stability of SMS.
Hong et al. investigated the stability of miniscrew types that were defined by different shapes and thread count for comparison with a miniscrew of hollow design. The hollow miniscrew, similar to the SMS, was reported to show enhanced stability with better torque level and lateral displacement values. 34,35 The improved primary stability was provided by an increased cortical bone-to-implant contact surface and bone formation within its internal chamber. 34,35 The combined use of the hollow SMS with iBGS requires evaluation in future in-vivo studies.

Conclusion
The present study is the first to demonstrate the possible use of injectable bone graft substitutes with a novel hollow miniscrew for orthodontic applications. Using a finer powder and modifying the powder-toliquid ratio significantly improved the rheological properties and injectability of the chosen bone graft substitute. Further in-vivo studies will be required to confirm the applicability and stability of The Sydney Mini Screw and injectable bone graft substitutes.