Study on the Typical Biological Soft Tissue Cutting for Needle Biopsy
|Course||Mechanical Design and Theory|
|Keywords||Tissue Cutting Needle Biopsy Cutting Performance Soft Tissue Needle|
Many medical conditions, including all cases of cancers, must be diagnosed by removing a sample of tissue from the patient and examined to determine if the tissue is benign or malignant. There are two methods to collect tissue specimens, i.e., needle biopsy and surgical operation. Needle biopsy is a medical procedure involving the removal of tissue samples, using medical needles, for examination by a pathologist under a microscope to identify and diagnose cancer and other diseases. To shorten recovery and to minimize trauma to the patient, biopsy, using fine gauge needles, is preferred. This procedure is performed with increasing frequency due to the emergence of early detection methods for cancer.The current biopsy procedures are painful and time consuming due to the lack of cutting efficiency of the existing biopsy needles. Each insertion only extracts a small piece of tissue. Sometimes, repetitve insertions at the same location should be peformed to obtain enough tissue specimens. Doctors in biopsy operations use a sharp edged needle to cut and remove the desired tissue specimen. It is a cutting process from mechanical perspective. The tissue corresponds to a work piece; the surgical instrument serves as a machine tool; and the tissue specimen resemble cutting chips, etc. Cutting soft tissue is difficult. Minimally invasive cutting of a small volume tissue sample, enough for pathology examination, is even more difficult. Based on the characteristics of needle biopsy, cutting performance of medical needles and biological soft tissue is studied.This dissertation involves the following major sections.(1) Needle Tip Cutting Edge Geometry and Its Cutting Performance. Based on the conception of traditional metal cutting, geometrical and mathematical models of the needle cutting edges are established. Cutting angles, i.e. inclination angle, normal rake angle, rake angle and wedge angle, along the needle cutting edge are calculated based on the space analytic geometry. The research shows that needle cutting angles only depend on the bevel angel and direction angle. Smaller bevel angle creates larger inclination angles and normal rake angles for a bias bevel needle, which means that it has better cutting performance. Other types of plane needles, such as symmetrical multi-plane needle, non-symmetrical multi-plane needle, and Cournand type needle, are systemically studied.(2) The Mechanical Properties of Typical Biological Soft Tissue. Biological tissue is very soft. It is hyperelastic and viscoelastic material. Therefore, tensile test of fresh porcine live is firstly performed and the parameters of the Ogden hyperelastic model are obtained. The tensile test shows that the cracking threshold is107.7±3.2KPa. However, there are many technical problems in tissue tensile test, such as sample making, size measurement and tissue fixture, etc. Consequently, indentation experiments are performed on the intact porcine liver. Neo-Hookean model and modified Kelvin model are selected to express the hyperelasticity and viscoelasticity. Hyperelastic parameters are C10=3.224±0.408KPa, D1=0.630±0.047MPa-1, and viscoelastic parameters are τ1=0.435±0.037s, τ2=8.769±1.630s, respectively. Using these material models, finite element analyses of indentation test are presented. The simulation results show that the stress declines fast during the initial period and the relaxation gradually becomes slower at a last stage.(3) Cutting Performance of Typical Biological Soft Tissue. Based on the orthogonal cutting theory, a theoretical cutting model of indentation type tissue cutting is established to analyse the cutting stress fields in the soft tissue cutting process. A test setup for tissue cutting is built, and cutting experiments are performed to quantitatively study the effects of cutting speed and cutting angles on the tissue cutting process. The results show that increasing inclination angle and normal rake angle lead to smaller cutting force and tissue deformation. Increasing cutting speed would results in larger cutting force, but it can reduce the tissue deformation before penetration. A new vibration assisted tissue cutting technology, transplanting ultrasonic scalpel technology to needle biopsy procedure, is proposed. A mathematical model of ultrasonic cutting is established to study the tissue separation. It is theoretically proved that ultrasonic vibration assisted cutting force is less than the critical cutting force. Vibration assisted needle insertion experiments are performed to quantitatively study the effect of vibration frequency. It is shown that this method could efficiently reduce the needle insertion force. (4) Needle Cutting Force Modelling Based on Tissue Oblique Cutting. Using the biopsy needle insertion setup, needle insertion force curve is obtained and analyzed. From the conception of cutting mechanics, a biopsy needle cutting force model was established. Using elementary cutting tools, porcine liver oblique cutting experiments are carried out on the soft tissue cutting setup. Experimental results are used to determine the specific cutting force model. The needle cutting force is the integration of the specific cutting force model along the needle cutting edge. This model quantitatively provides the relationship between cutting force and the needle tip geometry. This methodology can be used for design and evaluation of new needle, and preoperative planning of needle insertion trajectory. Needle biopsy test with different types of needles is conducted. It is proved that multiple plane needles can easily get longer and thicker tissue samples.The proposed activity extends the traditional cutting theory, and the transformative aspect of this research is to target a new class of work-material-biological tissue. The basic principles of cutting mechanics that have been developed in the past are transplanted to study medical issue-needle biopsy. This research will advance the state-of-the-art in needle-tip design and promote the development of the needle biopsy level.