Alloy Screws Research Articles

Magnesium alloys as a biomaterial for degradable craniofacial screws

Recently, magnesium (Mg) alloys have received significant attention as potential biomaterials for degradable implants, and this study was directed at evaluating the suitability of Mg for craniofacial bone screws. The objective was to implant screws fabricated from commercially available pure Mg and alloy AZ31 in vivo in a rabbit mandible. First, Mg and AZ31 screws were compared to stainless steel screws in an in vitro pull-out test and determined to have a similar holding strength (∼40N). A finite-element model of the screw was created using the pull-out test data, and this model can be used for future Mg alloy screw design. Then, Mg and AZ31 screws were implanted for 4, 8 and 12weeks, with two controls of an osteotomy site (hole) with no implant and a stainless steel screw implanted for 12weeks. Microcomputed tomography was used to assess bone remodeling and Mg/AZ31 degradation, both visually and qualitatively through volume fraction measurements for all time points. Histological analysis was also completed for the Mg and AZ31 at 12weeks. The results showed that craniofacial bone remodeling occurred around both Mg and AZ31 screws. Pure Mg had a different degradation profile than AZ31; however, bone growth occurred around both screw types. The degradation rate of both Mg and AZ31 screws in the bone marrow space and the muscle were faster than in the cortical bone space at 12weeks. Furthermore, it was shown that by alloying Mg, the degradation profile could be changed. These results indicate the promise of using Mg alloys for craniofacial applications.

Loss of mechanical properties in vivo and bone–implant interface strength of AZ31B magnesium alloy screws with Si-containing coating

In this study the loss of mechanical properties and the interface strength of coated AZ31B magnesium alloy (a magnesium–aluminum alloy) screws with surrounding host tissues were investigated and compared with non-coated AZ31B, degradable polymer and biostable titanium alloy screws in a rabbit animal model after 1, 4, 12 and 21weeks of implantation. The interface strength was evaluated in terms of the extraction torque required to back out the screws. The loss of mechanical properties over time was indicated by one-point bending load loss of the screws after these were extracted at different times. AZ31B samples with a silicon-containing coating had a decreased degradation rate and improved biological properties. The extraction torque of Ti6Al4V, poly-l-lactide (PLLA) and coated AZ31B increased significantly from 1week to 4weeks post-implantation, indicating a rapid osteosynthesis process over 3weeks. The extraction torque of coated AZ31B increased with implantation time, and was higher than that of PLLA after 4weeks of implantation, equalling that of Ti6Al4V at 12weeks and was higher at 21weeks. The bending loads of non-coated AZ31B and PLLA screws degraded sharply after implantation, and that of coated AZ31B degraded more slowly. The biodegradation mechanism, the coating to control the degradation rate and the bioactivity of magnesium alloys influencing the mechanical properties loss over time and bone–implant interface strength are discussed in this study and it is concluded that a suitable degradation rate will result in an improvement in the mechanical performance of magnesium alloys, making them more suitable for clinical application.

Insertion torques influenced by bone density and surface roughness of HA–TiO2 coatings

Bio-ceramic TiO2 coatings containing calcium (Ca) and phosphorous (P) were deposited onto Ti–6Al–4V alloy screws using plasma electrolytic oxidation (PEO) processes in an alkaline electrolyte with hydroxyapatite (HA) suspension. Coating on each screw had different surface roughness and morphology. Insertion torque (IT) of the coated screws in low (10pcf, pounds per cubic feet), medium–high (20pcf), and high (40pcf) density of artificial bones was measured in comparison with that of the uncoated and sandblasted screws having similar surface roughness. Higher insertion torques and final seating torques were obtained in the coated screws which may result in less micro-movement during the primary implantation stage and thus lower the risk of implant failure. Scanning electron microscopy (SEM) analysis indicated that all coatings still adhesively remained on the screw surfaces after inserted into the bones with different densities. The relationship between coefficient of friction and surface roughness was also addressed to better understand the results of insertion torque. It was found that a lower density bone (similar to aged bone) would need a surface-rougher coated screw to achieve a high torque while a high density bone can have a wide range of selections for surface roughness of the screw.

Screw loosening of different UCLA-type abutments after mechanical cycling

Aim: To evaluate the loss of applied torque (detorque) values in cast and pre-machined abutments for external hex abutment/implant interface of single implant-supported prostheses subjected to mechanical cycling. Methods: Ten metal crowns were fabricated using two types of UCLA abutments: cast and pre-machined with metal base in NiCrTi alloy and tightened to regular external hex implants with a titanium alloy screw, with an insertion torque of 32 N.cm, measured with a digital torque gauge. Samples were embedded with autopolymerizing acrylic resin in a stainless steel cylindrical matrix, and positioned in an electromechanical machine. Dynamic oblique loading of 120 N was applied during 5 x 10 5 cycles. Then, each sample was removed from the resin and detorque values were measured using the same digital torque gauge. The difference of the initial (torque) and final (detorque) measurement was registered and the results were expressed as percentage of initial torque. The results of torque loss were expressed as percentage of the initial torque and subjected to statistical analysis by the Student’s t-test (p<0.05) for comparisons between the test groups. Results: Statistical analysis demonstrated that mechanical cycling reduced the torque of abutments without significant difference between cast or pre-machined UCLA abutments (p=0.908). Conclusions: Within the limitations of this in vitro study, it may be concluded that the mechanical cycling, corresponding to one-year use, reduced the torque of the samples regardless if cast or pre-machined UCLA abutments were used.

Manufacture of magnesium alloy screws

This study reports the fabrication of LZ91 magnesium alloy screws using warm heading and thread-rolling processes. The proposed method consists of a two-stage warm heading process. This study presents an analysis of the material flow pattern of the billet inside the die by using finite element analyses. This study also presents the effects of upper die velocity, temperatures, and friction factors on the heading loads and product quality. This paper also investigates the effects of the friction factor on the effective stress, effective strain, and tooth height in a thread-rolling process. Finally, warm heading and thread-rolling experiments were conducted using a self-designed die set and a MoS2 lubricant. This study presents a comparison of the experimental values with the simulation results to verify the validity of the finite element models and the proposed warm heading procedures.

Torque Stability of Different Abutment Screws Submitted to Mechanical Cycling

To evaluate the torque stability of different UCLA retention screws of single implant-supported crowns submitted to mechanical cycling. Crowns fabricated from nickel-chromium-molybdenum alloy were attached to external-hexagon implants and grouped by the different retention screws used (n = 10): Ti, titanium screws (BRUNIHT, Biomet 3i); Au, gold-palladium screws with 24-carat gold coating (Gold-Tite, Biomet 3i); TiC, titanium alloy (Ti-6Al-4V) screw with diamondlike carbon coating (Neotorque, Neodent); and TiN, Ti-6Al-4V screw with aluminum-titanium-nitride coating (Ti-Tite, Conexão). Three initial removal torque (RT) values were obtained for each screw after torque insertion using an analog torque gauge. The final RT was measured after mechanical cycling (1 × 10⁶ cycles at 2 Hz under 130 N). Data were submitted to analysis of variance and the Fischer test. Statistically significant differences were observed between the initial RT in groups Ti and TiN, and between TiC and TiN. No statistically significant difference was seen between mean RT obtained before and after mechanical cycling, except for the Ti screws. All groups exhibited similar torque maintenance after mechanical cycling. Although no significant difference was observed among groups for the final percentage of torque maintenance, the final RT values of the coated screws were higher than those of the noncoated screws.

Study on Heading and Thread-Rolling Processes of Magnesium Alloy Screws

This study is to investigate the effects of the process parameters on the heading load and metal flow pattern during heading and thread-rolling processes of LZ91 magnesium alloy screws. A heading process composed of two stages is proposed. The material flow pattern of the billet inside the die is analyzed using the finite element analyses. The effects of the upper die velocity, temperatures and friction factors on the heading loads and product quality are discussed. On the other hand, in the thread-rolling process, the effects of the friction factor on the effective stress, effective strain, and tooth height are investigated. Finally, heading and thread-rolling experiments are conducted using a self-designed die set and a lubricant of MoS2. The experimental values are compared with the simulation results to verify the validity of the finite element models and the proposed heading procedures.

Intraoperative Neuromonitoring

OBJECTVIE: Intraoperative neuromonitoring of thoracic-level pedicle screw implantation for detecting breaches in the pedicle cortex has adopted methods originally developed in the early 1990s for stainless steel (SS) alloy screws used at lumbosacral levels. In our recent attempts to monitor thoracic-level pedicle screw placement, we were surprised to find that these widely used stimulation parameters were largely ineffectual when stimulating directly through titanium alloy (Ti-alloy) pedicle screws. The objectives of this study, then, were twofold: (1) to report the number of episodes in which intraoperative neuromonitoring of thoracic screw position failed to detect a medially directed breach (or malplacement) in a previously described and limited sample set; and (2) to compare the frequency-specific impedance of a sample of Ti-alloy pedicle screws to comparably sized screws made of SS alloys. We predicted that Ti-alloy screws would demonstrate impairment in conduction properties that could help explain the difficulties we, and others, have recently experienced with neuromonitoring of thoracic pedicle screw placement. Based on threshold values for train-of-four stimulation of spinal motor pathways, we quantified the incidence of medial breaches of thoracic-level pedicles in a small cohort of subjects. We also evaluated the conductive properties of Ti-alloy pedicle screws and compared these with SS screws. Eleven pedicle screws were examined using energy-dispersive x-ray spectroscopy to identify their alloys, after which DC resistance and AC impedance for each screw was measured. Furthermore, a subset of five screws was used to investigate the current delivery under dynamic testing conditions. Postoperative computed tomography of 6 subjects revealed 10 instances of significant medial screw malpositioning, out of a total of 88 screws placed. In each of these 10 instances, direct stimulation of thoracic pedicle screws at intensities considered in the literature to be clinically significant (i.e., ≤11 mA) failed to predict these medial pedicle breaches, yet each breach was reliably identified with low-intensity stimulation applied via a ball-tipped probe. For in vitro studies, most screws made of titanium alloys had higher resistance and impedance at tested frequencies compared with their SS counterparts. Moreover, there was widespread variability in conduction properties between Ti-alloy screws, whereas SS screws behaved in a more homogeneous manner. When compared with screws made of SS, most Ti-alloy pedicle screws behaved more like semiconductors, showing conduction properties that were highly frequency dependent. These properties likely contributed to the difficulties we encountered in interpreting thoracic screw placements based on stimulus-evoked electromyography from direct screw stimulation.

Carbon fiber reinforced PEEK Optima—A composite material biomechanical properties and wear/debris characteristics of CF-PEEK composites for orthopedic trauma implants

BackgroundThe advantageous properties of carbon fiber reinforced polyetheretherketone (CF-PEEK) composites for use as orthopedic implants include similar modulus to bone and ability to withstand prolonged fatigue strain. MethodsThe CF-PEEK tibial nail, dynamic compression plate, proximal humeral plate and distal radius volar plate were compared biomechanically (by four-point bending, static torsion of the nail, and bending fatigue) and for wear/debris (by amount of the debris generated at the connection between the CF-PEEK plate and titanium alloy screws) to commercially available devices. ResultsFour-point bending stress of the tibial nail and dynamic and distal radius plates yielded characteristics similar to other commercially available devices. The distal volar plate bending structural stiffness of the CF-PEEK distal volar plate was 0.542Nm2 versus 0.376Nm2 for the DePuy's DVR anatomic volar plate. The PHILOS proximal humeral internal locking system stainless steel plate was much stronger (6.48Nm2) than the CF-PEEK proximal humeral plate (1.1Nm2). Tibial nail static torsion testing showed similar properties to other tested nails (Fixion, Zimmer and Synthes). All tested CF-PEEK devices underwent one million fatigue cycles without failure. Wear test showed a lower volume of generated particles in comparison to the common implants in use today. InterpretationThus, these tested implants were similar to commercially used devices and can be recommended for use as implants in orthopedic surgery.

Influence of Abutment Screw Design and Surface Coating on the Bending Flexural Strength of the Implant Set

The purpose of this study was to analyze the influence of the setting and the presence of solid lubricant on the abutment screw surface on the flexural strength of the joint implant/abutment/screw. Forty abutments were connected to external hex implants, divided into 4 groups (n = 10): FE (titanium alloy screw threaded in the extremity), LE (titanium alloy screw with solid lubricant and thread in the extremity), FT (titanium alloy screw with threaded in all its length), and LT (titanium alloy screw with solid lubricant and thread in all its length). Through the mechanical flexural test, the implant/abutment resistance was evaluated with load applied perpendicular to the long axis in a mechanical testing machine (EMIC) under a speed of 0.5 mm/min. Data were submitted to a statistics test, and results showed statistically significant differences between the FE group and the other groups, and the FE group showed the lowest values. The LE group showed greater values than the LT group, and the values were statistically significant. According to the methodology used, it can be concluded that within noncoated titanium screws, a screw threaded along its entire length provided greater rigidity to the implant set, while with the screw containing solid lubricant, the screw threaded in all its length provided less rigidity of the implant set than screws with the thread only on the end. Among screws with the same geometry, those with the solid lubricant are statistically higher than those which do not have threads just at the end, but those with threads along their entire length do not show statistically significant differences.

Failure Analysis of Ti alloy Screws in Fixing Fractured Spines

Failure Analysis of Ti alloy Screws in Fixing Fractured Spines

Torque removal evaluation of prosthetic screws after tightening and loosening cycles: an in vitro study

The aim of this study was to evaluate the variation in removal torque of implant prosthetic abutment screws after successive tightening and loosening cycles, in addition to evaluating the influence of the hexagon at the abutment base on screw removal torque. Twenty hexagonal abutments were tightened to 20 regular external hex implants with a titanium alloy screw, with an insertion torque of 32 N cm, measured with a digital torque gauge. The implant/abutment/screw assemblies were divided into two groups: (1) abutments without hexagon at the base and (2) abutments with a hexagon at the base. Each assembly received a provisional restoration and was submitted to mechanical loading cycles. After this, the screws were removed and the removal torque was measured. This sequence was repeated 10 times, then the screw was replaced by a new one, and another cycle was performed. Linear regression analysis was performed. Removal torque values tended to decrease as the number of insertion/removal cycles increased, for both groups. Comparisons of the slopes and the intercepts between groups showed no statistical difference. There was no significant difference between the mean values of last five cycles and the 11th cycle. Within the limitations of this in vitro study, it was concluded that (1) repeated insertion/removal cycles promoted gradual reduction in removal torque of screws, (2) replacing the screw with a new one after 10 cycles did not increase resistance to loosening, and (3) removal of the hexagon from the abutment base had no effect on the removal torque of the screws.

Stability of beat tricalcium phosphate-coated mini-implants in rat tooth sockets

We investigated the potential of the osteoconductive material β-tricalcium phosphate to promote bone healing when coated on a titanium alloy screw implanted in the mandibular tooth socket of a healthy rat. Forty-eight 9-week-old male Wistar rats were divided into three groups: the Blast group, which received blast-treated screws; the βTCP group, which received β-tricalcium phosphate-coated screws; and the Control group, which received mechanically polished screws. Mandibles were removed at 3 or 9 weeks after screw implantation. Screw removal torques were measured using a handheld torque gauge and compared. At both 3 and 9 weeks after implantation, the screw removal torques were significantly greater in the βTCP group than in the Blast and Control groups (p<0.05). Removal torques did not differ significantly between the Blast and Control groups at either time point. These results suggest that a β-tricalcium phosphate coating on screws accelerates osseous healing around the screw and increases the retentive capacity of the screw. A β-tricalcium phosphate-coated implant fitted into the tooth socket immediately after extraction may therefore provide a favorable environment for early loading of implants.

Preparation and properties of C/SiC bolts via precursor infiltration and pyrolysis process

C/SiC composites with three kinds of preforms (carbon fiber stitched cloth, Type S; three-dimensional fine woven pierced felt, Type 3D; carbon fiber needled bulk felt, Type N) were fabricated by precursor infiltration and pyrolysis (PIP) process, and then machined into bolts and nuts with nominal diameter of M8 by lathe and alloy screw tap, respectively. The thread shape and mechanical properties were investigated. The results show that the external thread shapes from Type 3D and Type N are better than that from Type S. The shear strengths of the nuts and the tensile strengths of the bolts from the three preforms are 67.38, 32.47, 48.99, 143.70, 159.20, and 95.90 MPa, respectively. The fracture mode of Type S bolts is external thread spalling, and those of the other two are bolt rupture. Therefore, the composite of Type 3D is a good choice for the bolts, and that of Type S for the nuts.

Effect of miniscrew angulation on anchorage resistance

Even though the use of titanium miniscrews to provide orthodontic anchorage has become increasingly popular, there is no universally accepted screw-placement protocol. Variables include the presence or absence of a pilot hole, placement through attached or unattached soft tissue, and angle of placement. The purpose of this in-vitro study was to test the hypothesis that screw angulation affects screw-anchorage resistance. Three-dimensional finite element models were created to represent screw-placement orientations of 30°, 60°, and 90°, while the screw was displaced to 0.6 mm at a distance of 2.0 mm from the bone surface. In a parallel cadaver study, 96 titanium alloy screws were placed into 24 hemi-sected maxillary and 24 hemi-sected mandibular specimens between the first and second premolars. The specimens were randomly and evenly divided into 3 groups according to screw angulation (relative to the bone surface): 90° vs 30° screw pairs, 90° vs 60° screw pairs, and 30° vs 60° screw pairs. All screws were subjected to increasing forces parallel to the occlusal plane, pulling mesially until the miniscrews were displaced by 0.6 mm. A paired-samples t test was used to assess the significance of differences between 2 samples consisting of matched pairs of subjects, with matched pairs of subjects including 2 measurements taken on the same subject. One-way analysis of variance (ANOVA) with the post-hoc Tukey studentized range test was conducted to determine whether there were significant differences, and the order of those differences, in anchorage resistance values among the 3 screw angulations at maxillary and mandibular sites. The finite element analysis showed that 90° screw placement provided greater anchorage resistance than 60° and 30° placements. In the cadaver study, although the maximum anchorage resistance provided by screws placed at 90° to the cadaver bone surface exceeded, on average, the anchorage resistance of the screws placed at 60°, which likewise exceeded the anchorage resistance of screws placed at 30°, these differences were not statistically significant. Placing orthodontic miniscrews at angles less than 90° to the alveolar process bone surface does not offer force anchorage resistance advantages.

Analysis of Loss of Torque in Dental Implants Containing Gold Washers between Implant and Screw Head

This paper analyzes a novel solution of a dental prosthesis containing a gold washer between the implant and the screw head (made of a titanium alloy, Ti-6Al-4V), and compares its mechanical behavior under variable loading with those observed in two other solutions commonly used in dentistry, one of them having the implant, the abutment and the screw made of a titanium alloy, and the other one having a screw made of pure gold (the implant and the abutment being made of the same titanium alloy). The analysis has consisted in the measurement of the torque loss in the three types of prostheses after the application of a given number of variable loading cycles following ISO protocol 14 801. The investigation has been completed with the scanning electron microscopy (SEM) analysis of the different components after testing.The torque loss in this new solution is very similar to that obtained in the prosthesis containing a gold screw, and 30.7 % lower than that obtained in the prosthesis whose components are all made of a titanium alloy. This behavior can be accounted for by the lower friction between gold and titanium (when compared to titanium-titanium friction), which causes a higher preload in the screw. Also, the SEM analysis has revealed greater damage in Ti-Ti contacts than in Au-Ti contacts, and significant material transfer between the gold washer and the titanium alloy screw. The solution including a gold alloy washer combines low torque loss, as observed in the prosthesis with the gold screw, with the good mechanical (tensile) properties of the solution with the titanium alloy screw, which allows repairs in vivo to be performed more easily.

9H-14 低弾性医療用チタン合金スクリューの疲労特性評価(OS-13(5) 骨格系の医療デバイス)

9H-14 低弾性医療用チタン合金スクリューの疲労特性評価(OS-13(5) 骨格系の医療デバイス)

Biomechanical testing and degradation analysis of MgCa0.8 alloy screws: A comparative in vivo study in rabbits

The aim of this study was to compare the biomechanical properties of degradable magnesium calcium alloy (MgCa0.8) screws and commonly used stainless steel (S316L) screws and to assess the in vivo degradation behavior of MgCa0.8. MgCa0.8 screws (n=48) and S316L screws (n=32) were implanted into both tibiae of 40 adult rabbits for a follow-up of 2, 4, 6 and 8weeks. This resulted in a testing group of MgCa0.8 (n=12) and S316L (n=8) screws for each follow-up. Uniaxial pull-out tests were carried out in an MTS 858 Mini Bionix at a rate of 0.1mms−1. For degradation analysis of MgCa0.8 in vivo micro-computed tomography (μCT) was performed to determine the volume of metal alloy remaining. Retrieved MgCa0.8 screws were analysed for degradation by determination of weight changes, scanning electron microscopy and energy dispersive X-ray analyses. No significant differences could be noted between the pull-out forces of MgCa0.8 and S316L 2weeks after surgery (P=0.121). Six weeks after surgery the pull-out force of MgCa0.8 decreased slightly. In contrast, the S316L pull-out force increased with time. Thus, significantly higher pull-out values were detected for S316L from 4weeks on (P<0.001). The volume and weight of MgCa0.8 gradually reduced. A corrosion layer, mainly composed of oxygen, magnesium, calcium and phosphorus, formed on the implants. Since MgCa0.8 showed good biocompatibility and biomechanical properties, comparable with those of S316L in the first 2–3weeks of implantation, its application as a biodegradable implant is conceivable.

Dynamische dorsale Stabilisierung mithilfe des Pedikelschrauben-Systems DYNESYS®

The dynamic neutralization system for the spine (DYNESYS®) is a pedicle screw based system intending mobile restabilization substituting physiological tissue restraints and thus approximating the unstable motion pattern to a normal pattern. It consists of titanium alloy screws, connected by an elastic synthetic compound (PET band and PCU spacer) controlling motion in any plane. DYNESYS® can be used for mono- or multilevel stabilizations in the lumbar spine. DYNESYS® is indicated in the treatment of degenerative disease of the lumbar motion segment with instability and most often in combination with functional or structural spinal canal stenosis. Primary and secondary bone tumors of the spine. Spinal fractures. Infections. Lytic/isthmic spondylolisthesis. Degenerative spondylolisthesis > I°-II°. Facetectomy. Stabilization of thoracic and cervical spine. Allergies to components of DYNESYS®. Pregnancy. Standard midline approach. Decompression of the segment by laminotomy if indicated. Place the screws lateral to the facet joints. Measure the distance between the screw heads. Cut the spacers. Introduce spacer and cord on both sides. The length of the PCU spacer is determined by the distance of the pedicle screws and depends on the pathology of the lumbar motion segment. Tighten set screws under correct compression of the system. Mobilisation at the first day after surgery. No bracing necessary if there is no doubt about primary fixation of the screws. Physical strenuous work not earlier than 8 weeks postoperatively. DYNESYS® is a reliable dynamic stabilization system with good clinical results if used in correct indications.

Use of Ultrasound-Activated Resorbable Poly-D-L-Lactide Pins (SonicPins) and Foil Panels (Resorb-X) for Horizontal Bone Augmentation of the Maxillary and Mandibular Alveolar Ridges

Horizontal bone augmentation of the maxillary and mandibular alveolar ridges has been conventionally performed using mini titanium alloy screws. The titanium alloy screws are used to fixate corticocancellous block grafts to the recipient site or for tenting the mucoperiosteum to retain particulate bone grafts. Nonresorbable guided tissue regenerative membranes reinforced with titanium have also been developed to use with particulate bone grafts to augment alveolar ridge defects. This report demonstrates the use of resorbable ultrasound-activated pins and resorbable foil panels developed by KLS Martin for augmenting the alveolar ridges with particulate bone grafts.