title: A surgical bone biopsy needle using ultrasonic-sonic frequency vibration
Creators: name: Li, Li
Publisher: name: University of Glasgow
Year: 2017

description: This thesis presents a surgical needle designed for bone biopsy, based on an ultrasonic-sonic drilling mechanism. Bone biopsy is an invasive diagnostic procedure where a bone sample is extracted for clinical analysis. For conventional bone biopsy methods, closed biopsy is normally adopted and uses a core needle. An intact and viable biopsy sample is required for clinical analysis. However, a particular limitation of closed biopsy is that the microarchitecture of the biopsy sample can be easily damaged due to the large force which is applied through the core needle to penetrate bone. In some cases, the bone biopsy samples are fractured or crushed during the biopsy process. Power ultrasonic surgical devices have improved many aspects of bone cutting procedures, such as lower cutting force, higher accuracy, and better preservation of the tissue around the cutting site. In this study, an ultrasonic-sonic needle (US needle) system is designed and used to extract an intact biopsy sample and the penetration performance is evaluated by the effective impulse delivered to the target. The ultrasonic-sonic drilling mechanism was originally invented for rock drilling in low environments. In the US needle system, a free mass oscillates between an ultrasonic transducer-horn and a surgical needle, converting the ultrasonic frequency vibration of the horn to sonic frequency vibration of the needle. Compared to other ultrasonic surgical devices that directly transfer the ultrasonic vibrations from the cutting tip to the tissue, the US needle allows sufficient time between impacts with the free mass for the tip vibration amplitude to be re-established in the horn. This can maintain penetration progress of the needle into bone, where the rate of progress has been shown to be proportional to the effective impulse delivered by the needle to the bone. To maximise the effective impulse, a numerical model is developed to simulate the dynamic behaviour of the needle system and optimise the US needle. 
To build the US needle system, the design and optimisation of the ultrasonic transducer-horn were investigated with the finite element method and experimental modal analysis, ensuring that the transducer-horn operates at the tuned frequency (50kHz) with a pure longitudinal mode. The configuration of the ultrasonic horn determines the momentum transferred to the free mass and hence also affects the effective impulse delivered to the target. The shape and dimensions of the ultrasonic horn were determined through the finite element model of the ultrasonic horn impacting the free mass, which focused on maximising the post-collision velocity of the free-mass. The dynamic components of the US needle were also investigated. 
A numerical model representing the dynamic behaviour of the needle system was developed, allowing the optimisation of each dynamic component, maximising the effective impulse delivered to the target. Each dynamic component of the US needle was modelled as a mass-spring-damper (MSD) system, which constituted the whole system dynamic model. The numerical model was validated by experiments using a prototype needle. The free-mass velocity, needle velocity and impact force predicted by the numerical model were compared with the results measured from experiments using 3D laser Doppler vibrometry, an ultra-high speed camera and a load cell, respectively. The numerical model results exhibit good agreement with the experimental results, indicating the numerical model can be used as a predictive tool to evaluate the performance of the US needle when different configurations are implemented. The configuration of the US needle is studied to maximise the effective impulse by the numerical model, through optimisation of the mass of the free mass, spring rate and spring pre-load. An optimised configuration of the US needle was determined by the numerical model and validated by experiments. The resulting prototype of the needle device was tested in ovine femur in vitro and was demonstrated to retrieve a cortical bone biopsy sample with a more cylindrical geometry, smoother surface and more intact sample than a cortical biopsy sample retrieved using a conventional trephine needle. Moreover, the penetration performance of the US needle was also compared with an ultrasonic resonant needle where the ultrasonic transducer and surgical needle resonate at the same frequency.