Bone modeling controlled by a nickel–titanium shape memory alloy intramedullary nail
Introduction
The skeleton is a mechanically optimized biological system, whose composition and organization are greatly influenced by mechanical forces. The geometry of the cortical compartments and the trabecular structure are the result of functional adaptation to normal physiological loads [1], [2]. It has been shown that bone also adapts to externally applied forces [3], [4].
Nickel–titanium shape memory metal alloy, Nitinol (NiTi), is a functional material whose shape and stiffness can be controlled with temperature [5]. The metal undergoes a complex crystalline-to-solid phase change called martensite–austenite transformation. As the metal in the high-temperature (austenite) phase is cooled, the crystalline structure enters the low-temperature (martensite) phase, where it can be easily bent and shaped. As the metal is reheated, its original shape and stiffness are restored. NiTi has also been shown to have excellent springback and super-elastic properties [6], [7]. Biocompatibility studies have shown NiTi to be a safe implant material, which is at least equally good as stainless steel or titanium alloys [5], [8], [9], [10], [11], [12]. In orthopedic surgery, NiTi applications currently include NiTi compression bone stables used in osteotomy and fracture fixation [13], [14], [15], [16], [17], [18], NiTi rods for the correction of scoliosis [19], shape memory expansion clamps used in cervical surgery [20], clamps in small bone surgery [21], and fixator systems for suturing tissue in minimal access surgery [22].
NiTi can be used in functional intramedullary nails that are used to apply controlled force to bone. The nail can be fabricated to the desired shape for forced diaphyseal bone bending. Cooling down to the martensite phase enables insertion of the shaped nail into the medullary cavity. At body temperature, the nail begins to regain its original shape, causing a bending force. No such intramedullary bending device has been available before, and its effects on bone should therefore be studied.
The purpose of this work was to test the hypothesis that bone modeling can be controlled with a functional intramedullary nail made of nickel–titanium shape memory alloy. This is a preliminary experimental study with normal rats, and curved intramedullary nails were used to apply a continuous bending force to the femoral diaphysis.
Section snippets
Implants
We fabricated a set of intramedullary nails with different thickness and curvature characteristics to generate a variety of force ranges (Table 1). The material used was NiTi (55.7% Ni and 44.3% Ti by weight) melted in a vacuum high-frequency furnace. To fabricate a wire of diameters in the range of 1.1–1.5 mm, the ingot was hot-rolled followed by cold drawing accompanied by intermediate annealing. The initial round pieces for implants of curvature radii in the range of 25–37 mm were made by
Results
No rats died during the experimental period. No adverse effects were seen from the arthrotomies. Significant retardation of longitudinal growth in all the operated femurs compared to the contralateral normal femurs (p<0.001) was seen (Table 1). There was also a significant thickening of bones (p=0.001 and p=0.004 for DMAX and DMIN, respectively). These changes appeared to be more obvious when the thickest (1.4 mm) nail was used.
Discussion
This study showed that bone modeling can be controlled with a functional intramedullary nail made of nickel–titanium shape memory alloy. All bones were bent in the direction of the nail, as shown by the AP radiographs, but the degree of bending varied between animals. This is explained by the different thicknesses, curvatures and locations of the nails used. The thicker nails seemed to cause more bending. There was a trend towards more bending when the nail crossed the epiphyseal plate, while
Acknowledgements
The authors thank Mr. Pasi Ohtonen for assistance in statistical analysis. This work was supported in part by the National Technology Agency of Finland (40097/00, 40193/01).
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