Titanium-nickel shape memory clamps in small bone surgery

A Review on Design and Mechanical Properties of Additively Manufactured NiTi Implants for Orthopedic Applications

NiTi alloy has a wide range of applications as a biomaterial due to its high ductility, low corrosion rate, and favorable biocompatibility. Although Young’s modulus of NiTi is relatively low, it still needs to be reduced; one of the promising ways is by introducing porous structure. Traditional manufacturing processes, such as casting, can hardly produce complex porous structures. Additive manufacturing (AM) is one of the most advanced manufacturing technologies that can solve impurity issues, and selective laser melting (SLM) is one of the well-known methods. This paper reviews the developments of AM-NiTi with a particular focus on SLM-NiTi utilization in biomedical applications. Correspondingly, this paper aims to describe the three key factors, including powder preparation, processing parameters, and gas atmosphere during the overall process of porous NiTi. The porous structure design is of vital importance, so the unit cell and pore parameters are discussed. The mechanical properties of SLM-NiTi, such as hardness, compressive strength, tensile strength, fatigue behavior, and damping properties and their relationship with design parameters are summarized. In the end, it points out the current challenges. Considering the increasing application of NiTi implants, this review paper may open new frontiers for advanced and modern designs.

In silico analysis of Superelastic Nitinol staples for trans-sternal closure

BACKGROUND

Superelastic Nitinol staples, utilized routinely in foot surgeries, are proposed to be used for sternal closure application in this study. It is hypothesized that the shape memory induced superelasticity will allow multiple staples placed along the sternum to promote fast and safe recovery by maintaining constant clamping pressure at the sternotomy midline.

METHODS

Two different Nitinol staples of different alloying compositions, one representing the metal formed wire geometry and, the other, powder metallurgy manufactured rectangular geometry, are chosen from the literature. Austenite finish temperatures of both materials are confirmed to be appropriately below the body temperature for superelastic shape memory activation. The adopted finite element superelasticity model is first validated and, via design optimization of parametrized dimensions, the staple geometries for producing maximal clamping forces are identified. The performances of the optimized staples for full trans-sternal closure (seven staples for each) are then tested under lateral sternal loading in separate computational models.

RESULTS

The optimized metal formed staple exerts 70.2 N and the optimized powder metallurgy manufactured staple exerts 245 N clamping force, while keeping the maximum localized stresses under the yield threshold for 90° leg bending. Testing the staple-sternum constructs under lateral sternal loading revealed that the former staple can be utilized for small-chested patients with lower expected physiological loading, while the latter staple can be used for high-risk patients, for which high magnitude valsalva maneuver is expected.

CONCLUSION

Computational results prove that superelastic Nitinol staples are promising candidates as alternatives to routinely performed techniques for sternal closure.

Four-corner arthrodesis concentrator of nickel-titanium memory alloy for carpal collapse: a report on 18 cases

PURPOSE

To evaluate the treatment of carpal collapse using a four-corner arthrodesis concentrator of nickel-titanium memory alloy.

METHODS

From August 2006 to August 2010, 18 patients with carpal collapse had scaphoid excision and four-corner (capitate, lunate, triquetrum, and hamate) arthrodesis using a nickel-titanium memory alloy four-corner arthrodesis concentrator. The mean follow-up time was 30 months (range, 12-48). Various wrist parameters, including grip strength, wrist motion, and degree of pain were recorded and compared before and after surgery.

RESULTS

The average fusion time was 2.3 months (range, 2-4). Neither nonunion nor wound infection was found in any of the patients. At one year follow-up, the grip strength had reached 80% of that of the healthy side, whereas the range of motion was greater than 50% of the contralateral side. After the surgery, the mean pain scores were improved.

CONCLUSIONS

Four-corner arthrodesis using a nickel-titanium memory alloy four-corner arthrodesis concentrator effectively treated carpal collapse and preserved most wrist function.

Shape Memory Alloy Expandable Pedicle Screw to Enhance Fixation in Osteoporotic Bone: Primary Design and Finite Element Simulation

The properties of shape memory alloys, specifically the equiatomic intermetallic NiTi, are unique and significant in that they offer simple and effective solutions for some of the biomechanical issues encountered in orthopedics. Pedicle screws, used as an anchoring point for the implantation of spinal instrumentations in the spinal fracture and deformity treatments, entail the major drawback of loosening and backing out in osteoporotic bone. The strength of the screw contact with the surrounding bone diminishes as the bone degrades due to osteoporosis. The SMArtTM pedicle screw design is developed to address the existing issue in degraded bone. It is based on the interaction of bi-stable shape memory-superelastic elements. The bi-stable assembly acts antagonistically and consists of an external superelastic tube that expands the design protrusions when body temperature is attained; also an internal shape memory wire, inserted into the tube, retracts the assembly while locally heated to above the body temperature. This innovative bi-stable solution augments the pull-out resistance while still allowing for screw removal. The antagonistic wire-tube assembly was evaluated and parametrically analyzed as for the interaction of the superelastic tube and shape memory wire using a finite element model developed in COMSOL Multiphysics®. The outcomes of the simulation suggest that shape memory NiTi inserts on the SMArtTM pedicle screw can achieve the desired antagonistic functionality of expansion and retraction. Consequently, a parametric analysis was conducted over the effect of different sizes of wires and tubes. The dimensions for the first sample of this innovative pedicle screw were determined based on the results of this analysis.

Basic research on a cylindrical implant made of shape-memory alloy for the treatment of long bone fracture

The internal fixing materials made from shape-memory alloys (SMAs) have recently been reported for long bone fracture. We present a new internal fixation technique using a cylindrical SMAs implant in a rat femoral fracture healing. The implant was designed in a shape to circumferentially fix the fractured bone using resilient SMA claws. To evaluate the fixing ability of the implant, three-point bending and rotation tests were performed. Fifteen female Wister rats were treated surgically as an experimental model. All rats were killed at 16 weeks postoperatively, and the radiological and histological evaluations were performed. In biomechanical test, the good fixation ability of the implant was demonstrated. In animal model, no cases of postoperative infection or death were encountered and postoperative gait was stable in all cases. Radiological examination at 16 weeks postoperatively demonstrated the implant firmly fixed to the fractured part, endosteal healing, and no callus formation in all cases. In Histological evaluation, bone union in all cases was characterized by endochondral ossification from within the medullary cavity. In conclusion, our cylindrical SMA implant provided good fixation in biomechanical tests, and achieved bone union in all 15 rats. If a larger size is designed in the future, our implant will be a clinically applicable, useful fixing material for fracture of the human long bones.

Transverse fractures of the olecranon: a biomechanical comparison of three fixation techniques

INTRODUCTION

The gold standard for treating transverse olecranon fractures is tension band fixation. A problem with this technique is migration of the K-wires leading to premature hardware removal. The aim of this study is to compare stability provided by two new techniques designed to eliminate the problem with backing out of K-wires, with that of the recommended tension band technique, performed with a biomechanical in vitro investigation. Our hypothesis was that the two new techniques would provide at least equal stability as the traditional tension band fixation.

METHODS

Transverse olecranon osteotomies were created in human cadaveric elbows to simulate a type 21-B1.1 fracture. Three groups of 8 specimens were instrumented with: (1) recommended AO tension band technique; (2) modified K-wires with eyelets and tension band; (3) staples across the fracture with tension band. Each elbow was tested in a 90° flexed position. The triceps tendon was sinusoidally loaded by applying two load steps at 500 and 700 N for 4000 cycles each. Relative movements between the fragments were determined.

RESULTS

At the end of the first and second load step the displacement of the osteotomy at the posterior ulnar side was significantly less for the staples across the fracture with tension band as compared to both other groups. There were no significant differences between groups 1 and 2.

CONCLUSION

Since clinical results depend partly on stable fixation, it is concluded that using staples in the clinical situation might provide better results than the currently recommended tension band technique.

Biomechanical comparison of a new staple technique with tension band wiring for transverse patella fractures

BACKGROUND

The tension band wiring technique is the most common method of transverse patella fracture fixation. Since post-operative instabilities have been reported for this technique, alternative osteosynthesis solutions are of interest. We investigated the biomechanical behaviour of a new staple technique for treatment of transverse patella fractures in a cadaveric model.

METHODS

Eight human cadaveric knees with femur and tibia including soft tissue were used. A transverse osteotomy of the patella was created. Each specimen was fixed consecutively with tension band wiring and two Nitinol compression staples. Testing was performed by pull on the quadriceps tendon between a 90 degrees flexed position and full knee extension for up to 5,000 cycles.

FINDINGS

At 1,000 cycles, fracture site displacements in flexion and extension were significantly smaller for the staple group at the ventral aspect of the patella as compared to the tension band wiring group. With a failure criterion of 2mm fracture site displacement, cycles until failure were significantly smaller for the staple group.

INTERPRETATION

This study provides evidence based on a cadaveric model that compression staples have a promising potential to treat transverse patella fractures.

Four-corner arthrodesis using the Quad memory staple

Range of motion, pain, consolidation and complications were evaluated for nine patients who underwent four-corner arthrodesis using the Quad Memory Staple (QMS) at a mean follow-up of 44 months. The mean pre-operative range of motion was 50 degrees extension, 62 degrees flexion, 9 degrees radial deviation and 24 degrees ulnar deviation. The postoperative range of motion was similar to previous studies at 32 degrees extension, 31 degrees flexion, 15 degrees radial deviation and 20 degrees ulnar deviation. The grip strength was 28 kg pre-operatively and 26 kg postoperatively. The mean pain score improved from 41 to 23 and the Disabilities of Arm, Shoulder, and Hand (DASH) score from 24 to 20. Non-union, haematoma and wound infection were not seen and eventually all four-corner fusions were consolidated. The main advantages of the QMS are its compressive property and the simple fixation technique. It gives good stability, enables early rehabilitation and avoids the risks of pin fixation methods.

Smart intramedullary rod for correction of pediatric bone deformity: a preliminary study

We were interested in determining if a smart intramedullary rod made of nitinol shape-memory alloy is capable of correcting deformed immature long bones. Because of limitations in our study design the process was reversed in that we examined the smart rod's ability to create a deformity rather than to correct one. Smart rods of different lengths and diameters were heat-treated to resume a radius of curvature of 30 to 110 mm. The low and high temperature phases of the smart rods were set, respectively, at 0 degrees C to 4 degrees C and 36 degrees C to 38 degrees C. The preshaped smart intramedullary rods were implanted in the cooled martensite phase in the medullary canal of the tibia in eight rabbits, where they restored their austenite form, causing a continuous bending force. On a weekly basis anteroposterior and lateral radiographs of the surgically treated tibia and the contralateral tibia were obtained for comparison. Rabbits were euthanized 6 weeks after surgery and computed tomography scans of both tibias were used for image analysis. Smart rods with a larger radius of curvature showed only minimal signs of remodeling; however, rods with a radius of curvature of 50 and 70 mm generated enough force history to create bone remodeling and deformation. The amount of bone deformation was highly magnified when unicortical corticotomy on the tension side was done. Based on this preliminary study the technology of the smart intramedullary rod may provide a valuable alternative method to correct pediatric skeletal deformities.

Effect of porosity on the osteointegration and bone ingrowth of a weight-bearing nickel-titanium bone graft substitute

Porous nickel-titanium (NiTi) alloy is a promising new material for a bone graft substitute with good strength properties and an elastic modulus closer to that of bone than any other metallic material. The purpose of this study was to evaluate the effect of porosity on the osteointegration of NiTi implants in rat bone. The porosities (average void volume) and the mean pore size (MPS) were 66.1% and 259+/-30 microm (group 1, n=14), 59.2% and 272+/-17 microm (group 2, n=4) and 46.6% and 505+/-136 microm (group 3, n=15), respectively. The implants were implanted in the distal femoral metaphysis of the rats for 30 weeks. The proportional bone-implant contact was best in group 1 (51%) without a significant difference compared to group 3 (39%). Group 2 had lower contact values (29%) than group 1 (p=0.038). Fibrotic tissue within the porous implant was found more often in group 1 than in group 3 (p=0.021), in which 12 samples out of 15 showed no signs of fibrosis. In conclusion, porosity of 66.1% (MPS 259+/-30 microm) showed best bone contact (51%) of the porosities tested here. However, the porosity of 46.6% (MPS 505+/-136 microm) with bone contact of 39% was not significantly inferior in this respect and showed lower incidence of fibrosis within the porous implant.

Bone modeling controlled by a nickel-titanium shape memory alloy intramedullary nail

Nitinol (NiTi) shape memory metal alloy makes it possible to prepare functional implants that apply a continuous bending force to the bone. The purpose of this study was to find out if bone modeling can be controlled with a functional intramedullary NiTi nail. Pre-shaped intramedullary NiTi nails (length 26 mm, thickness 1.0-1.4 mm) with a curvature radius of 25-37 mm were implanted in the cooled martensite form in the medullary cavity of the right femur in eight rats, where they restored their austenite form, causing a bending force. After 12 weeks, the operated femurs were compared with their non-operated contralateral counterpairs. Anteroposterior radiographs demonstrated significant bowing, as indicated by the angle between the distal articular surface and the long axis of the femur (p = 0.003). Significant retardation of longitudinal growth and thickening of operated femurs were also seen. Quantitative densitometry showed a significant increase in the average cross-sectional cortical area (p = 0.001) and cortical thickness (p = 0.002), which were most obvious in the mid-diaphyseal area. Cortical bone mineral density increased in the proximal part of the bone and decreased in the distal part. Polarized light microscopy of the histological samples revealed that the new bone induced by the functional intramedullary nail was mainly woven bone. In conclusion, this study showed that bone modeling can be controlled with a functional intramedullary nail made of nickel-titanium shape memory alloy.

Structure and surface of TiNi human implants

The surface and the "bulk" structure of TiNi implants were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoemission spectroscopy (XPS), and scanning Auger microprobe analysis (AES). TiNi implants were compared with otherwise identically prepared non-implanted specimens, and sputter-cleaned and reoxidized samples. Non-implanted and implanted samples had essentially the same surface topography and microstructure. Ti, O, and C were the dominant elements detected on the surface. Trace amounts (approximately 1 at%) of Ni and Ca, N, Si, B, and S were also detected. Ti was present as TiO2 on the surface, while nickel was present in metallic form. A significant difference in Ni peak intensity was observed when retrieved or non-implanted control samples (a very low nickel content) were compared with sputter-cleaned and reoxidized samples (well-detected nickel). It is evident that the method of passivation is crucial for nickel loosening. No major changes occurred in the TiNi samples bulk structure or in the surface oxide during the implantation periods investigated.