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Keywords:

  • shape memory alloy;
  • fracture healing;
  • rat model;
  • NiTi;
  • electromagnetic induction

Abstract

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES
  8. Supporting Information

Shape memory orthopaedic implants made from nickel–titanium (NiTi) might allow the modulation of fracture healing, changing their cross-sectional shape by employing the shape memory effect. We aimed to show the feasibility and safety of contact-free electromagnetic induction heating of NiTi implants in a rat model. A water-cooled generator–oscillator combination was used. Induction characteristics were determined by measuring the temperature increase of a test sample in correlation to generator power and time. In 53 rats, NiTi implants were introduced into the right hind leg. The animals were transferred to the inductor, and the implant was electromagnetically heated to temperatures between 40 and 60°C. Blood samples were drawn before and 4 h after the procedure. IL-1, IL-4, IL-10, TNF-α, and IFN-γ were measured. Animals were euthanized at 3 weeks. Histological specimens from the hind leg and liver were retrieved and examined for inflammatory changes, necrosis, and corrosion pits. Cytokine measurements and histological specimens showed no significant differences among the groups. We concluded that electromagnetic induction heating of orthopedic NiTi implants is feasible and safe in a rat model. This is the first step in the development of new orthopedic implants in which stiffness or rigidity can be modified after implantation to optimize bone-healing. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:1671–1676, 2010

Nickel–titanium (NiTi) shape memory alloys (SMAs) were described as early as 1963.1 NiTi medical devices have shown great mechanical strength and biocompatibility in vitro and in vivo, and exhibit pseudoelasticity and shape memory effects.2 Thus far, clinical applications have comprised stents in vascular surgery and gastroenterology,3 wires and brackets in orthodontic surgery,4, 5 porous implants in intervertebral body fusion,2 and staples in foot surgery.6

The shape memory effect is based on a reversibly martensitic phase transformation. Cooling the parent phase austenite to a critical temperature Ms (martensite start temperature) causes the monocrystalline structure to transform to twinned martensite. This transformation is finished by reaching the martensite finish temperature (Mf). In this state, the martensite can be mechanically deformed; the maximum reversible deformation (ε) is 8% for NiTi SMAs. If the temperature is raised above the austenite finish temperature (Af), the martensite converts back to austenite (Fig. 1),7 and the SMA returns to its initial predetermined state, exhibiting the so-called one-way memory effect.

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Figure 1. Illustration of the one-way memory effect. T, temperature; As(f), austenite start(finish) temperature; Ms(f), martensite start(finish) temperature. Adapted from Ref. 7.

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NiTi SMAs have been proposed as a means of applying bending forces for scoliosis correction in the growing spine.8, 9 Intramedullary (IM) application of curved NiTi nails in the rat femur results in a bowing of the bone, showing the potential to apply bending forces 10. Usually, body temperature is used to convert the implant from martensite to austenite. Our motivation to use NiTi implants to treat fractures is the dependence of bone healing on adequate biomechanical stimuli in addition to biological properties, such as blood supply, cellular immune response, and the availability of osteoinductive cytokines.11 In articular fractures, osteosynthesis aims for full reduction and bone healing via direct contact and without callus formation. In shaft fractures, the concept of absolute stability frequently leads to nonunions. Therefore, so-called biological osteosynthesis allowed for more dynamic fixation conditions with some degree of interfragmentary movement stimulating callus formation and bone healing.12

Fracture healing via callus formation can be divided into stages: inflammation, granulation tissue formation, and remodeling. The need for stability, in contrast to interfragmentary movement, is dependant on the stage of healing.13–15 Thus, in the repair of long bone fractures, lowering the rigidity of IM nail fixation by removing locking screws after a certain period of time, “dynamizing” the nail, has become a common procedure.16 Implants that have the property of inherently changing their rigidity during the course of fracture healing would allow for new approaches in bone repair modulation, avoiding additional invasive procedures. NiTi implants could provide this change in rigidity. If the austenite finish temperature is above the body temperature, implant conversion could be altered by heating at any desired time. Electromagnetic field induction is one method to heat the implant within the body without any contact with the implant. Therefore, we tested the hypothesis that in vivo noncontact electromagnetic induction heating of a NiTi SMA implant is feasible and safe in a rat model.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES
  8. Supporting Information

Animal Model

All operations and procedures were approved by the district veterinary administration and complied with the Animal Protection Act of Germany. Male Lewis rats (n = 53, weight 180–220 g, Central Animal Laboratory, Hannover Medical School, Hannover, Germany) were randomly assigned to a control group without heat induction or one of five heat induction protocols with maximum temperatures of 40, 45, 50, 55, or 60°C. Animals were caged in pairs and fed ad libitum with the usual small animal food (Altromin 1310 and Altromin 1324, Altromin GmbH, Lage, Germany).

SMA Implants

To investigate the impact of the induction heating process on cell damage, NiTi SMA specimens with a nominal composition of 49.8–50.0% Ni and an intermediate transformation temperature (Af = 50–65°C) were used. The specimens (NiTi wires) had a diameter of 1.55 mm and a length of 10 mm (Memry GmbH, Weil am Rhein, Germany). The physical properties are presented in S-Table 1. The material was annealed and pickled by chemical etching, resulting in a blank metallic surface. The wire ends were mechanically ground to a flat surface. To connect several specimens with a temperature measuring fiber optic probe, a cylindrical bed with a depth of 0.6 mm and diameter of 0.65 mm was machined into the wires using electrical discharge machining (Fig. 2). Prior to implantation, the implants were washed, degreased, and autoclaved for 30 min at 120°C.

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Figure 2. Close-up view of the NiTi SMA wire. The left image shows the bed in which the fiber-optic probe was inserted. The right image shows the connection between the probe and the wire.

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Induction Device

The induction heating process was performed using a water-cooled generator–oscillator combination with a maximum power output of 10 kW (HFG 10, Eldec, GmbH, Dornstetten, Germany). The coil-shaped antenna (the induction coil) was made of three copper windings (inner diameter = 150 mm. The cross-section of the water-cooled coil was rectangular with a dimension of 20 mm × 10 mm (Fig. 3). Frequencies between 200 and 300 kHz have been used for heating stents.17, 18 Our induction device was set to a frequency of 250 kHz, which proved to be feasible for heating different specimens as NiTi wires and plates. To increase the temperature of the NiTi wires to a specified temperature, a control loop was integrated with the induction system. Because any conventional temperature measurement system, for example, thermocouples, are influenced by the electromagnetic field generated by the induction system, the temperature of the wire was measured using fiber-optic probes with a tip diameter of 0.55 mm (probe, TS 5; evaluation unit, FoTemp 1; OPTOcon, AG, Dresden, Germany). The temperature was determined by detecting a temperature-dependent shift in the optical band edge of a GaAs-crystal embedded in the tip of the probe. This method is unimpaired by electromagnetic interference. Furthermore, due to both the small heat dissipation and the small heat capacity, this method offers an accurate temperature measurement (minimal accuracy ± 1°C, resolution 0.1°C). Before applying the inductor in the in vivo experiments described below, a series of experiments on an isolated rat femur was carried out to determine the technical parameters needed to conduct the animal experiments.

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Figure 3. Control loop of the induction system.

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Operative Procedure

All procedures were performed under general anesthesia. Rats were put into a Plexiglas chamber filled with oxygen and 4% isoflurane at a flow rate of 400 ml/min. When anesthetization was apparent, the animal was weighed and marked by perforating the ears with the number of the animal. The animal was randomized to a control or experimental group using numbered blocks that had been created prior to the procedure. A respiratory mask specially designed for rats was placed over the rat's mouth. Anesthesia was maintained with oxygen and 2.5% isoflurane at a flow rate of 400 ml/min. Normal saline (5 ml, 37°C) was injected subcutaneously on each side of the spine to prevent dehydration. The animal was placed in a supine position. The body temperature was measured rectally using a digital thermometer, and the hind leg was cleaned and shaved for surgery. Disinfection was performed using isopropanol. The knee was flexed, and a 5 mm skin incision made on the ventral aspect of the distal femur in a cranio-caudal direction. A deep incision was made between the lateral and intermedial part of the quadriceps muscle. A 1.6 mm Kirschner wire was used to locally dissect the quadriceps muscle from the femur shaft. The sterilized implant was introduced between the shaft and the quadriceps and positioned in the cranial part of the femur. A temperature probe consisting of an implant identical to the implant described above and a glass fiber connected to a measurement console was introduced in the same manner as the implant and placed in direct approximation of the implant. The skin was closed temporarily using 5/0 Dexon thread, and the leg was draped with a sterile pad. The body temperature was rectally measured again. The animal was then positioned in the inductor with the implant in the middle of the inductor. Electromagnetic induction heating was performed under the control of the implanted temperature probe. The target temperature was maintained for 20 s. The probe was then removed, and the skin closed using 5/0 Dexon thread. The animal was put into a cage with a warming bulb for waking under the supervision of the researcher. Postoperative analgesia was 400 mg Novaminsulfon per 500 ml drinking water. Postoperatively, animals were checked daily for signs of pain or infection.

Sample Preparation

Blood samples were drawn immediately after the onset of anesthesia and 4 h postoperatively. Animals were euthanized by putting them into a CO2 atmosphere for 2 min, followed by internal decapitation. The absence of vital signs was checked. The operated hind leg and the left lobe of the liver were harvested and stored at −80°C.

Histology

All specimens were fixed in 3.5% buffered formalin for 7 days. Hind leg specimens were prepared by removing the implants, skin, and subcutaneous tissue, leaving behind the femur with the adjacent muscle tissue. The hind leg specimens were embedded in a hard resin and dehydrated in a graded alcohol series of 70% ethanol for 6 days and xylol for 2 days, and embedded in methyl methacrylate (Technovit®, Kulzer GmbH, Wehrheim, Germany) using a standard technique.19 Liver specimens were dried in paraffin. Five micron sections were cut and stained with hematoxylin and eosin. The histological findings were estimated using a semi-quantitative score. The changes were scored using five grades from 0 to 4 for necrosis, mononuclear cell infiltration, polymorphonuclear leukocyte infiltration, and corrosion pits.

Cytokine Measurement

Plasma levels of interleukin (IL)-4, IL-6, IL-10, tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ) were measured using a rat cytokine cytometric bead array kit (Becton Dickinson, Heidelberg, Germany). The sample size for analysis was 50 µl. All assays were carried out according to the manufacturers' instructions.

Statistical Analysis

Plasma levels of cytokines at different time points were evaluated by analysis of variance (ANOVA) and subjected to Student's t-test. The level of significance was set at p < 0.05. Nonparametric data were subjected to the χ2-test or Fisher's exact test. The hypothesis was that no differences would be present among the groups.

RESULTS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES
  8. Supporting Information

Induction Characteristics

A NiTi SMA wire was connected to the temperature probe (Fig. 2) and implanted into the femur of a rat carcass. With respect to the following implantations, the femur of the carcass was arranged in the center of the induction coil. Figure 4 shows the measured temperature increase of a 15 mm NiTi SMA wire in relation to the power of the induction device over 40 s. The generator was switched on at 10 s, inducing a fast temperature increase followed by a decrease in the slope. As heat conduction increased with the temperature of the NiTi wire, higher output power was necessary to induce further increases.

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Figure 4. Influence of induction output power on the NiTi wire temperature.

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Depending on the output power, both the maximum slope of the temperature increase and the maximum temperature that was reached show almost linear correlations with generator power. The maximum temperature was defined as the temperature when further temperature increase was ≤0.1 K/s. A minimum output power (P) of 3 kW was determined to induce a temperature increase of 30 K for the following experiments.

To generate the preferably fast increase in temperature for a small induction period with precise temperature control, the influence of the loop control was evaluated. In particular, an exaggerated NiTi wire temperature during the induction heating process had to be avoided. Technically, the transfer coefficient (Kp) and the integral action coefficient (Ti) describe the setting parameters of the closed loop control that influence the transient time, that is, the time until the target temperature is reached, and the transient overshoot temperature.

Figure 5 shows the impact of Kp of the controlled system (Fig. 3) with Ti = 75 and output power (p = 3 kW) constant. A low Kp caused a fast temperature increase and only a marginal exaggeration of NiTi temperature while keeping Ti constant. However, the low transfer coefficient caused a slow increase to the target temperature in the in vivo experiments. To reach the target temperature quickly, a Ti of 100 and a Kp of 750 were used. Figure 6 shows the temperature profile during the induction heating process in one of our in vivo experiments with a target temperature of 55°C.

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Figure 5. Example of the influence of the transfer coefficient Kp on the vibration behavior of the control loop in the induction system. Ti = 75, p = 3 kW.

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Figure 6. Example of the temperature profile during the induction process. The target temperature of the NiTi SMA wire was 55°C. p = 4 kW, Ti = 100, Kp = 750.

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Procedures

Fifty-four animals successfully underwent the procedure. One animal had to be euthanized early because of a wound infection. Postoperative supervision showed no sign of undue limitations of the animals.

Histology

Six hundred seventy-five sections from the hind legs and 725 sections from the livers were available for evaluation. All liver specimens were scored 0 for necrosis or inflammation. Hind leg specimens revealed no to mild histological changes in terms of inflammation without any significance or trend of more inflammation or histological damage depending on the measured temperature of the NiTi wire following the induction heating process. No corrosion pits were found. S-Figures 6 and 7 show examples of hind leg and liver specimens from the 60°C induction group.

Cytokine Measurements

Only 26 blood samples were successfully drawn and only 13 blood samples provided cytokine level data. Therefore, statistical analysis was not feasible. No clear trend was found towards higher cytokine release after the procedure or towards a correlation with the temperature achieved during the electromagnetic induction of the NiTi implant (S-Fig. 8).

DISCUSSION

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES
  8. Supporting Information

To our knowledge, this is the first study that shows the feasibility of contact-free in vivo induction heating capable of inducing the shape memory effect in an orthopedic implant. Hitherto, shape memory effects in orthopedic implants were executed at the time of implantation. Kujala et al.10 used preshaped IM NiTi nails with a fully martensitic state at 0°C that restored their initial curved shape at roughly 25–30°C (fully austenitic state). Thus, implants were placed in the femurs of rats in a cooled state, with the shape memory effect exerted by the warming caused by the body's temperature. Wever et al.8 used a square-shaped memory metal rod with a transition temperature of ∼25°C in an experimental study of scoliosis correction. In its austenitic state, the rod featured a 40° bend; it was cooled and straightened for implantation, and the shape memory effect was activated by heating the rod to a temperature of 50°C with a high-frequency electric device.8 Su et al.20 conducted a clinical trial in which humeral shaft nonunions were treated with a “shape memory NiTi alloy swan-like bone connector.” The device was processed with one-way heat treatment and a reversion temperature of 33 ± 2°C. The device consisted of compression hooks and embracing arms connected to the main humerus plate. Before implantation, the device was held in ice-cold sterile water; after implantation, the device was heated with warm water to 40–50°C to recover the preset shape, thereby producing compression force to stabilize the fracture. Uniformly, these procedures implied the induction of the shape memory effect at the time of the surgical procedure, either by applying external heating or using the warming produced by the body.

Using an induction device, our method allows the transformation to the austenitic state to be performed independently from the surgical procedure. Thus, induction of a shape memory effect can be achieved at a point of time deemed the most suitable. In fracture repair, this point is likely some weeks after initial stabilization of the fracture, though the best time point is not yet known.12 Depending on the implant design and desired purpose, the SMA might lead to more or less stiffness or rigidity. Furthermore, the time point could be determined individually with respect to the kind of fracture, its location, and co-morbidities.

Concerns have been expressed regarding the biological safety of NiTi alloys. Wever et al. described a number of experiments on the safety of a 50% Ni/50% Ti alloy. Monolayers of skin fibroblasts exposed to NiTi extracts had no signs of cellular lysis, intracellular granulation, or morphological changes within 72 h, and there was no inhibitory effect on cell growth. Also, a sensitization test carried out on guinea pigs for 48 h showed no edema or erythema at the affected test sites. Finally, genotoxicity assays performed in Salmonella showed no evidence of significantly increased mutation.21 Ryhänen et al. introduced NiTi implants into the paraspinal muscle, toward the sciatic nerve, of rats. Also, rats received implants made from stainless steel or titanium alloy. Histological analysis revealed no necrosis, granuloma, or signs of dystrophic soft tissue calcification around any of the implanted materials. A mild inflammatory reaction was limited to the area close to the implant, regardless of the material.22 In our model, we looked for side effects due to the presence of NiTi SMA implants and their electromagnetic induction heating. Histological analysis of soft tissue surrounding the femur, the femur itself, or the liver as a likely site of a systemic inflammatory reaction revealed no differences among the groups. Local inflammatory reactions were mild and consistent with data from earlier reports.22

Corrosion is another concern. Ryhänen et al. investigated the concentrations of nickel, chrome, and iron in various organs 26 and 60 weeks after the implantation of either NiTi or stainless steel into adult rat femurs. Corrosion pits were found in the stainless steel, but not the NiTi group. No significant differences in Ni concentration were observed between the NiTi and stainless steel groups in any of the organs at any point in time.23 Consistently, we did not find any corrosion pits in our study.

Several cytokines have been described as sensitive markers of the systemic inflammatory response in patients.24, 25 Similarly, Ni ions from Ni-containing alloys cause the expression of inflammatory mediators in vitro, such as IL-1β and TNF-α.26 Prymak et al. investigated the release of IL-1ra, IL-6, IL-8, and TNF-α from peripheral blood mononuclear leukocytes (PBMC) and PMNs after co-incubation with porous NiTi samples, finding a significant increase in the secretion of IL-1ra, IL-6, and IL-8 from PBMC. No change was found in the secretion of IL-1ra and IL-8 from PMN compared to the spontaneous cytokine release in controls.2 Thus far, no data have been reported on the in vivo significance of these data. In our study, we concentrated on evaluating the cytokine response to NiTi implants and the following electromagnetic induction, which did not show any evidence of systemic inflammatory burden. The time period used to determine the change in cytokine level (4 h postoperatively) seems adequate. Gauglitz et al.,25 who worked on a rat burn model, showed a significant increase in IL-1β, IL-6, and IL-10 at 3 h after injury and only a mild increase at 6 h and later.

In conclusion, noncontact electromagnetic induction heating of an orthopedic implant made of NiTi is feasible in a small animal model. Cytokine measurements and histology of both the site of insertion and the liver did not raise concerns regarding adverse local or systemic side effects. The use of a shape memory effect independent from the time and approach of osteosynthesis allows for new strategies in the management of fracture repair.

Acknowledgements

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES
  8. Supporting Information

This work was funded by the “Deutsche Forschungsgemeinschaft” (DFG, SFB 599, TP D10). The authors would like to thank the DFG for their support.

REFERENCES

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES
  8. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES
  8. Supporting Information

Additional Supporting Information may be found in the online version of this article.

FilenameFormatSizeDescription
JOR_21171_sm_SupplTab1.doc35KSupplementry Table 1. Physical properties of NiTi
JOR_21171_sm_SupplFig6.tif4685KSupplementry Figure 6
JOR_21171_sm_SupplFig7.tif2684KSupplementry Figure 7
JOR_21171_sm_SupplFig8.tif1458KSupplementry Figure 8

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