Soft tissue structural assessment using mechanical measurements

Abstract

The overall aim of the work presented in this thesis is the development of quantitative relationships between the structure (histological make-up and/or tissue architecture) and the mechanical properties of soft biological tissue. The purpose of the research is to contribute towards the assessment of “tissue quality” using mechanical probing (instrumented palpation). The work focuses particularly on two case studies; the eyeball, where tissue quality relates to the corneal stiffness and the intra-ocular pressure (IOP); and the periodontal ligament (PDL), where tissue quality relates to the load displacement-time behaviour of teeth to which external forces are applied (such as in orthodontic treatment). The experimental work involves static and dynamic testing of two porcine tissues (eyeballs and periodontal ligament) and also a mechanical system (mechanical eyeball) devised to investigate separately the components expected to influence mechanical behaviour; cornea stiffness, IOP, fluid inertia and leakage rate. Special test rigs were designed, calibrated and assessed for their measurement and process capabilities and the results were compared with quasi linear visco elastic (QLV) models to identify an appropriate mechanical way of characterising the tissue for comparison with its quality. The larger part of the work concentrated on the eye with the ultimate aim of identifying symptoms of glaucoma more accurately. Dynamic testing identified a suitable indentation frequency range of 20Hz to 24Hz, the amplitude ratio in this range being capable of measuring IOP within an error of ±7mmHg which is only slightly above the ±5mmHg target for the latest tonometers. The cornea tissue was found to have 20% viscous behaviour and 80% elastic behaviour. The data were analysed using dynamic visco elastic models with an additional term for the inertia of the fluid in the eyeball. The work on the mechanical eyeball showed that it is possible to separate the effects of IOP and the stiffness of the cornea, which is of great significance in determining the true IOP, as opposed to one derived from a tonometer reading which makes assumptions about cornea stiffness. The other main contribution is on the assessment of the periodontal ligament, which plays an important shock-absorbing role during mastication and is the initiator for orthodontic tooth movement (OTM), when loads are applied to teeth using orthodontic appliances. The force-relaxation behaviour of one lower premolar in pig mandibles was measured and the resulting force relaxation curves analysed using three different visco elastic spring damper models. The analysis showed that, when longer relaxation times are allowed three or even four parameter models are not adequate to describe the behaviour. It is suggested that a more appropriate model is a multi component Maxwell model which uses more or less Maxwell components depending on the allowed relaxation time. Overall, the work shows that instrumented palpation, supported by the development of suitable models can play a significant role in measuring tissue quality. Also, using simplified models of the stress-strain behaviour, it was possible to demonstrate that the measurements made here were in general accord with those reported in the literature for eyes, corneas and periodontal ligament.