Cutting it fine

Surgeons could one day use miniature ultrasonic cutting devices to perform minimally invasive procedures on delicate areas of the body such as the spine.

The devices are being developed by researchers from three UK universities through a programme supported by Mectron Medical Technology and Sonic Systems. The goal of the three-year effort is to create a new kind of transducer that does not operate in resonance.

By accomplishing this the team will be able to shrink the size of current ultrasonic cutting instruments, said Margaret Lucas, a mechanical engineer from Glasgow University, one of the participants in the EPSRC-funded project.

Glasgow University was previously involved in the development of an ultrasonic scalpel for upper jaw surgery, which was commercialised by Mectron Medical Technology three years ago. The tool, while effective for jaw surgeries, is now being refined for use in delicate areas such as the spine.

Current ultrasonic cutting devices consist of a Langevin piezoelectric transducer attached to a cutting blade that is tuned to resonate in a longitudinal mode at a low ultrasonic frequency usually in the 20-50kHz range.

'The current systems have to be resonant, which means to maximise the vibrations you must create a device that is tuned to the frequency you're operating at,' said Lucas. 'Once it is tuned, it dictates what size the device has to be.'

The length of the tuned blade must be designed a half-wavelength or a multiple of the half-wavelength of the frequency driving the system. So the cutting device can only be designed relatively large.

Lucas will be leading efforts at Glasgow University to replace these transducers with flextensional transducers.

A flextensional transducer consists of piezoelectric rings bonded to two endcaps. When a voltage is applied, the ring will contract radially and the endcaps will flex, providing an amplified longitudinal motion.

Lucas and her colleagues at Glasgow have proposed attaching the cutting blade to one of the vibrating endcaps.

Lucas said this means the blade will behave like a rigid body without the need to be a tuned component of the device.

'Basically it would not rely on being resonant,' she added. 'It takes away that limitation in terms of the size and geometry of the device.'

It is claimed the cutting blade design can therefore be more tailored to providing better interaction between the blade and the bone and accurate cuts. It will also allow the overall ultrasonic device to be miniaturised.

The Glasgow design team will work with Loughborough University where a team of researchers will create computer models of ultrasonic machining and bone drilling. In three years, Lucas said the Glasgow researchers will likely have created prototypes of the ultrasonic cutting devices that can be trialled on both human cadaver material and animals at Edinburgh University.

Mectron Medical and Sonic Systems will support the team in their efforts to commercialise the technology. Ultrasonic cutting devices have already found commercial success in the food industry. The technology has also been designed for surgical procedures involving soft tissue and recently for cutting of bone.

Lucas said her team hopes the results of their project will allow ultrasonic cutting devices to be used in more surgical procedures than they are currently.

'The devices will be used in orthopaedics probably for spinal procedures and surgeries to the face,' she added. 'There are a lot of procedures that currently cannot by accessed by minimally invasive methods and we are hoping this will be possible with a small and very accurate ultrasonic device.'