Preparation and Characterization of Nitinol Bone Staples for Cranio-Maxillofacial Surgery
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The aim of this work was to form NiTi and TiNiCo body temperature activated and superelastic staples for clinical joining of mandible and face bone fractures. The alloys were obtained by VIM technique. Hot and cold processing was applied to obtain wires of required diameters. The martensitic transformation was studied by DSC, XRD, and TEM. The shape memory effects were measured by a bend and free recovery ASTM F2082-06 test. The superelasticity was recorded in the tension stress-strain and by the three-point bending cycles in an instrument equipped with a Hottinger force transducer and LVDT. Excellent superelastic behavior of TiNiCo wires was obtained after cold working and annealing at 400-500 °C. The body temperature activated shape memory staples were applied for fixation of mandibular condyle fractures. In experiments on the skull models, fixation of the facial fractures by using shape memory and superelastic staples were compared. The superelastic staples were used in osteosynthesis of zygomatico-maxillo-orbital fractures.
Keywordsmaxillofacial surgery nitinol bone staples osteosynthesis shape memory alloys
NiTi shape memory alloys with a chemical composition consistent with ASTM F2063-05 have been approved as metal biomaterials and are used in production of medical implants and devices (Ref 1-3). Products of NiTi alloys exhibiting phenomena of shape memory and superelasticity are widespread in such applications as orthodontic archwires, staples for osteosynthesis, stents, endodontic and surgical instruments (Ref 4-6). These implants show good mechanical properties, high corrosion resistance, and biocompatibility (Ref 7, 8). It is well known that improvement of shape memory and superelastic properties can be achieved by ternary additions and thermal or thermomechanical treatments (Ref 9-12). The NiTi staples applied to internal osteosynthesis in orthopedics and maxillofacial surgery operate usually as the shape memory staples activated by patient body heat (Ref 13, 14). For fixation of craniofacial bone fractures, the NiTi staples that are superelastic at the room temperature can also be used (Ref 15).
The studied NiTi-based implants were prepared at the Institute of Materials Science at the University of Silesia. They were applied to osteosynthesis of craniofacial bone fractures in the clinical experiments carried out by Department of Cranio-Maxillofacial Surgery of Medical University of Silesia in Katowice. Staples made of Ti50Ni48.7Co1.3 obtained agreement of the Bioethical Commission to be used for fixation of mandible bone fractures and were applied in 1990-1995 in clinical applications (Ref 16). Positive results of experiments carried out first for animals and then in human body applications showed that the NiTiCo implants as small staples may be used, for example, for fixation of mandibular condyle fractures or osteosynthesis of zygomatico-maxillo-orbital fractures.
Recently, it was confirmed that the NiTi alloys with Co additions in the range of 1-2 at.% should be considered for future medical applications (Ref 17).
The aim of these studies was to form NiTi and TiNiCo body temperature activated and superelastic staples for clinical joining of mandible and face bone fractures.
Materials and Experiments
In the experiments two types of alloys were used: commercial NiTi Euroflex wires (Ti-50.8 at.% Ni) with diameters of 1 to 1.4 mm and wires made of TiNiCo alloys of our own production by vacuum induction melting. The melted ingots were homogenized at 900 °C for 48 h under vacuum of about 10−5 Torr and followed pack hot rolled on the shape mill at 900-800 °C to rods with diameters of about 4 mm. After removing surface oxides, the rods were rotary forged to about 2 mm diameters and subsequently hot or cold drawn in sintered carbides wire drawing dies to wires with various diameters.
Samples of the wires 70 mm long and of different diameters were polished, washed in distillated water, scoured in acetone, and then subjected to proper heat treatment. They were solution treated at 700 °C for 15 min, cooled down in iced water, and then annealed at the temperature range of 300-500 °C for 15, 30, and 60 min. The cold worked wires with total deformation of 20-50% were annealed in the range of 300-600 °C in air atmosphere. Short annealing times from 5 to 15 min were used. Colored oxide layers were removed by chemical etching in water solution of HF and HNO3.
X-ray phase analysis was carried out using Philips X’Pert diffractometer with graphite monochromator on the diffracted beam. Cu Kα1 radiation was used. Phase transformation courses were studied by the DSC-7 Perkin Elmer calorimeter in the temperature range −100 to +80 °C. Cooling and heating speed was 10°/min. Shape recovery was studied for the wires deformed at low temperature (in martensite state) by bend-free recovery according to ASTM F 2082-06 test (Ref 18). The superelasticity and shape memory properties were measured in the elongation tests using Instron 4469 machine and in cyclic three-point bending tests using self-constructed machine. Compression forces of the staples were measured using digital FG-5000A gage.
Results and Discussion
In the specimen annealed at 400 °C one distinguished peak was observed at about 40 °C on cooling and two wide, hardly separated peaks could be seen on heating between 10-40 °C. This proves that the transformation course is B2 ⇔ R ⇔ B19′.
Similarly, two stage transformations were observed for specimens annealed at 500 °C. Increasing the annealing temperature causes that the transformation occurs directly from the B2 to B19′ martensite phases. It is worth to mention that for the specimen annealed at 400 °C the characteristic temperatures are higher than for the wires annealed at 500 and 600 °C. This can be explained by interaction of recovery and precipitation of the Ni4Ti3 phase processes.
This is typical for the alloys deformed and then annealed with specific dislocation structure (Ref 22). The recovery process causes the dislocation cellular structure, i.e., in the specimen two regions can be distinguished: with low dislocation density—inside the cells; and high dislocation density—in the cell boundaries. The transformation occurs in these two regions at different temperatures.
Recently in the Department of Skull and Maxillofacial Surgery of Silesian Medical University in Katowice carries out research on the development in implementation of a new method of subcondylar mandible fracture fixation by means of NiTi shape memory staples.
Prepared NiTi shape memory staples with desired dimensions and properties after their laboratory studies were selected and applied for fixation of low subcondylar fractures of mandible of patients with multiple condyle mandible fracture (Ref 24). The superelastic staples were used for joining of zygomatic bone fracture.
NiTi staples which recover the desired shape under the influence of the human body heat can be prepared from the studied wires after cold drawing and annealing in the temperature range from 400 to 500 °C for 15-30 min.
After cold drawing with 40-50% deformation and annealing at 400 °C for 30 min very good superelastic properties of TiNiCo wires were obtained.
Staples from the wire of a small diameter can be used for fixation of bone fractures as superelastic staples which may be mechanically opened at a room temperature with a pincer and inserted into the drilled bone holes. Those staples we proposed for fixation of face bone fractures, for example: for fixation of zygomatic bone fracture or the Le Fort I face bone fracture.
The use of shape memory and superelastic staples instead of titanium plates and screws is easier and shortens the operation time.
The work was financially supported by the Polish Ministry of Education and Science within the frame of the project Nr N N507 296339.
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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