Miniature Ultrasonic Cutting Devices for High Precision Minimal Access Orthopaedic Surgical Procedures

Lead Research Organisation: Loughborough University
Department Name: Sch of Mechanical and Manufacturing Eng

Abstract

Currently, ultrasonic cutting devices consist of a Langevin piezoelectric transducer attached to a cutting blade both tuned to resonate in a longitudinal mode at a low ultrasonic frequency, usually in the 20-50 kHz range. Ultrasonic cutting devices are successfully used in several industries, especially the food industry, and have recently been designed for a variety of surgical procedures involving soft tissue, and even more recently for cutting of bone. Because the ultrasonic blade is a tuned component its length must be a half-wavelength or a multiple of the half-wavelength at the driving frequency. Also, because Langevin transducers can only deliver a few microns of vibration amplitude, the blade profile must be carefully designed to provide sufficient vibration amplitude gain to meet the requirements of the material to be cut. These two geometry requirements can be very restrictive in the design of devices; a half-wavelength at a low ultrasonic frequency leads to quite a large cutting device and profiling for high gain leads to very high stresses.This new research proposes to investigate adapting flextensional transducers for power ultrasonics applications. A flextensional transducer consists of piezoelectric rings bonded to two endcaps. When the ring contracts radially under an AC voltage, the endcaps flex providing an amplified longitudinal motion normal to the cap surfaces. For the proposed application a cutting blade will be attached to one of the vibrating endcaps with little effect on the operational frequency. Thus, the blade will behave nearly as a rigid body, without the need to be a tuned component of the device. The enormous benefit is that the cutting blade design can focus more closely on delivering the best interaction between the blade and bone to provide a highly accurate cut, and also the ultrasonic device can be miniaturised to allow the design of devices for delicate orthopaedic procedures involving minimal access surgery.Complementary to this work, it is required to investigate the interaction between the ultrasonic cutting blade and bone in order to understand how the ultrasonic vibrations enable accurate high quality cuts to be achieved. As bone is a complex hierarchical material with many layers of very different composition, a multi-scale modelling approach will allow both the micro and macro effects of bone penetration under vibro-impacts to be simulated. The simulations of ultrasonic cutting of bone will also allow parametric studies to be carried out to research the effects on cutting of various parameters, such as speed, vibration amplitude, frequency and also the geometry of the cutting edge of the blade. This will provide valuable input in to the design of the cutting blade. The cutting devices designed in this research project will be trialled both on human cadaver material and in animal studies. The results of these studies will provide valuable validations of the simulations as well as in depth assessments of the performance of the devices in bone for a wide range of orthopaedic surgical procedures.The research programme brings together the three academic institutions in the UK researching power ultrasonic penetration into bone. The three research groups involved have particular expertise and strong track records in power ultrasonic devices and ultrasonic bone cutting (Glasgow), multi-scale computational modelling of ultrasonic machining and bone drilling (Loughborough), and trialling of ultrasonic cutting devices and orthopaedic engineering (Edinburgh). To provide a commercial focus to the research and access to expertise in ultrasonic device design and manufacture, two industrial partners, Mectron Medical and Sonic Systems, are supporting the programme.

Publications

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Abdel-Wahab A (2011) Dynamic Properties of Cortical Bone Tissue: Impact Tests and Numerical Study in Applied Mechanics and Materials

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Abdel-Wahab AA (2011) Analysis of anisotropic viscoelastoplastic properties of cortical bone tissues. in Journal of the mechanical behavior of biomedical materials

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Vadim Silberschmidt (Author) (2012) Numerical modelling of impact fracture of cortical bone tissue using X-FEM in Journal of Theoretical and Applied Mechanics

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Alam K (2012) On-line analysis of cracking in cortical bone under wedge penetration. in Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine

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Abdel-Wahab A (2012) Experimental and numerical analysis of Izod impact test of cortical bone tissue in The European Physical Journal Special Topics

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Li S (2013) Analysis of fracture processes in cortical bone tissue in Engineering Fracture Mechanics

 
Description The random and heterogeneous microstructure of cortical bone contributes to a wide range of its mechanical properties. Using powerful statistical analysis tools, the correlations found between the variation of elastic modulus and histological sectors demonstrated a possible link between mechanical properties and the mechanically induced bone adaption. The theoretical calculations of the effective Young's modulus accurately reproduced our experimental results, which provide another evidence of the strong relationship between microstructure and elastic modulus. Beyond the everyday physiological conditions, corresponding to its linear-elastic mechanical behaviour, cortical bone demonstrates significant level of uncertainty in its overall stress-strain relationship as a result of various damage mechanisms, making prediction of their fracture rather challenging. The microstructure analysis confirmed this transition at microscopic level between anatomic quadrants.
Exploitation Route The spatial variability and anisotropy of cortical bone (of its various anatomical sectors) is important against a background of isotropic parameters, averaged for the entire bone or its cross-section, traditionally used in current models of bones and limbs. This should be properly incorporated in any quantitative tools assessing bone's structural integrity for different conditions (especially for surgical resections). More complex - but also more adequate - numerical models of bones, accounting for their microstructural variability and anisotropy of its mechanical behaviour, will contribute to finding personalised medical solutions for different patients.
Generally, this is also of high significance for the entire population, especially people with bone diseases (osteoporosis being a prominent example), since it could make more precise assessment of severity of different conditions, on the one hand, or provide targeted exercises, focused on specific regional bone parts (when combined with mechanostimulation models).
Sectors Healthcare

 
Description The acquired understanding of anisotropy and spatial variability in mechanical properties of cortical bones (its different sectors) led to development of new numerical approaches for assessment of fracture toughness of bones. Replacing the simplified schemes, more advanced numerical simulations demonstrated a significant effect of position of a bone part removed in operation on its structural integrity, thus potentially allowing medical practitioners to make more precise decisions for post-operation treatment.
First Year Of Impact 2015
Sector Healthcare
Impact Types Societal

 
Description Innovate UK
Amount £242,533 (GBP)
Organisation Innovate UK 
Sector Public
Country United Kingdom
Start 01/2015 
End 01/2017
 
Description Bone cutting 
Organisation University of Edinburgh
Country United Kingdom 
Sector Academic/University 
PI Contribution Development of numerical models of bones accounting for their microstructure
Collaborator Contribution Implementation of experimental tests on bone cutting
Impact New data on bone properties and cutting of bones
Start Year 2009