Miniature ultrasonic bone cutting device based on a cymbal transducer

Ultrasonic cutting devices have been successfully used in several industries, especially the food industry. This knowledge, developed for industrial procedures, has been exported to other areas where it is having great impact. In medicine, during the last 30 years, di↵erent ultrasonic devices have been designed for a wide variety of surgical procedures involving soft tissue, and even more recently for cutting of bone. The increasing numbers of surgeons adopting ultrasonic devices as the device of choice has in turn increased the demand for devices which are able to be used increasingly in new procedures with more dicult to access surgical sites. 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. The first commercial ultrasonic devices for bone cutting applications, designed by the Italian company Mectron and called Piezosurgeryr, were based on a Langevin piezoelectric transducer. Langevin transducers incorporate a piezoceramic stack capable of delivering a few microns of vibration amplitude, and therefore the transducer and the device as a whole must be resonant to achieve the required ultrasonic displacement amplitude at the cutting tip. 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 sucient vibration amplitude gain to meet the requirements of the material to be cut. Therefore the cutting blade itself incorporates high amplitude gain, which can lead to very high stresses, and the design of the blade geometry is somewhat restricted by the requirement for resonance. 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 thesis investigates adapting the class V flextensional ‘cymbal’ transducer for power ultrasonic applications. The cymbal transducer consists of piezoelectric rings bonded to two end-caps with truncated conical shape. When the ring contracts radially under an AC voltage, the end-caps flex providing an amplified motion normal to the cap surfaces. This thesis introduces a new prototype of an ultrasonic cutting device for bone surgery based on a cymbal transducer, optimised for use in power ultrasonics applications, which removes many of the geometrical restrictions on the cutting tip. For the proposed application, a cutting blade is attached to one of the vibrating end-caps with little e↵ect on the operational frequency. Thus, the blade behaves nearly as a rigid body, without the need to be a tuned component of the device. The enormous benefit of this technology 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. The results show how the cymbal transducer can excite suciently high vibration displacement amplitude at lower driving voltages, by adapting the configuration of the cymbal to remove the problem of epoxy layer debonding and by optimising the cymbal end-caps and geometry through finite element modelling supported with experimental vibration characterisation. Preliminary trials of the resulting prototype ultrasonic bone cutting device, which operates near to 25 kHz, are presented to illustrate the success of this novel device design