Elsevier

Biomaterials

Volume 23, Issue 12, June 2002, Pages 2535-2543
Biomaterials

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

https://doi.org/10.1016/S0142-9612(01)00388-XGet rights and content

Abstract

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.

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).

References (53)

  • T.A Einhorn

    Bone strengththe bottom line

    Calcif Tissue Int

    (1992)
  • D.M Raab-Cullen et al.

    Periosteal bone formation stimulated by externally induced bending strains

    J Bone Miner Res

    (1994)
  • Y Hsieh et al.

    Effects of loading frequency on mechanically induced bone formation

    J Bone Miner Res

    (2001)
  • J Ryhänen

    Biocompatibility of Nitinol

    Minim Invasive Ther Allied Technol

    (2000)
  • Kapanen A, Ilvesaro J, Danilov A, Ryhänen J, Lehenkari P, Tuukkanen J. Behaviour of nitinol in osteoblast-like ROS-17...
  • Kapanen A, Ryhänen J, Danilov A, Tuukkanen J. Effect of nickel–titanium shape memory metal alloy on bone formation....
  • J Ryhänen et al.

    Bone healing and mineralization, implant corrosion, and trace metals after nickel–titanium shape memory metal intramedullary fixation

    J Biomed Mater Res

    (1999)
  • M Assad et al.

    Comparative in vitro biocompatibility of nickel–titanium, pure nickel, pure titanium, and stainless steelgenotoxicity and atomic absorption evaluation

    Biomed Mater Eng

    (1999)
  • R.G Tang et al.

    Application of a NiTi staple in the metatarsal osteotomy

    Biomed Mater Eng

    (1996)
  • K Dai et al.

    Studies and applications of NiTi shape memory alloys in the medical field in China

    Biomed Mater Eng

    (1996)
  • P.J Yang et al.

    Internal fixation with Ni–Ti shape memory alloy compressive staples in orthopedic surgery. A review of 51 cases

    Chin Med J (Engl)

    (1987)
  • V Brailovski et al.

    Review of shape memory alloys medical applications in Russia

    Biomed Mater Eng

    (1996)
  • P.P Kuo et al.

    The use of nickel–titanium alloy in orthopedic surgery in China

    Orthopedics

    (1989)
  • J.O Sanders et al.

    A preliminary investigation of shape memory alloys in the surgical correction of scoliosis

    Spine

    (1993)
  • F Mei et al.

    The biomechanical effect and clinical application of a Ni–Ti shape memory expansion clamp

    Spine

    (1997)
  • J Musialek et al.

    Titanium–nickel shape memory clamps in small bone surgery

    Arch Orthop Trauma Surg

    (1998)
  • Cited by (76)

    • Biomechanical design of a new percutaneous locked plate for comminuted proximal tibia fractures

      2022, Medical Engineering and Physics
      Citation Excerpt :

      Combining this study's optimization criteria to enhance callus formation (0.2 mm ≤ AIM ≤ 1 mm and SIM/AIM < 1.6), lessen stress shielding (σBINT for proposed plate > σBINT for titanium or steel plate), and reduce screw breakage (σSMAX < UTS of steel), the plate design recommendations are: 172.6 ≤ EP < 200 GPa (no KS screw); 79.8 ≤ EP < 100 GPa (1 KS screw); and 4.9 ≤ EP < 100 GPa (2 KS screws). Prototype plates could be made from materials already used or proposed for orthopaedics: polymers [23,66]; fiber-reinforced polymers [19,36]; metal foams [67,68]; shape memory alloys [69,70]; and porous 3D-printed metals [71,72]. Also, fiber metal laminates made of layers of metals and composites could be considered [73,74].

    • SMA biomedical applications

      2021, Shape Memory Alloy Engineering: For Aerospace, Structural, and Biomedical Applications
    View all citing articles on Scopus
    View full text