Blake, Charlotte
(2021)
Additive manufacturing of mitral annuloplasty devices.
PhD thesis, University of Nottingham.
Abstract
Mitral valve annuloplasty is a common surgical procedure performed on thousands of patients each year across the world. A less invasive and more successful method of resolving mitral valve regurgitation, repair surgeries now outnumber replacement of the mitral valve in its entirety. As a result, a range of supportive annuloplasty ring devices for maintaining the surgical repair are now available for lifelong implantation. However, these devices underserve some populations leading to replacement surgeries, and rely on assumptions made on the natural, healthy anatomy of the mitral valve.
Additive manufacturing (AM) has, for the last few decades, become increasingly adopted into the medical industry. With applications ranging from educational aids to surgical instruments and long-term implantable devices, this field is rapidly expanding and encompassing a greater breadth of medical specialities. In particular, the manufacturing of patient-specific products with reasonable cost and high fidelity is a key area of development for medical applications of additive manufacturing methods. Significant research has already been undertaken in the fields of orthopaedics, regenerative medicine, and pharmaceuticals, producing long-term implantable metal devices, complex polymer scaffolds, and novel drug delivery methods.
Personalized annuloplasty rings could lead to greater surgery success rates enabling greater repair longevity, reduced reoperation rates, and reduced risk of future valve replacement. This project aimed to investigate the suitability of the AM technique, selective laser melting (SLM), to create annuloplasty rings tailored to each patient.
To achieve this goal, this research focussed first on comparing the existing design assumptions applied to commercial annuloplasty devices against human anatomy using cadaveric dissection and measurement. These studies concluded that whilst the assumed 3:4 ratio applied in annuloplasty design was a good average across a population, the ratio was inconsistent between subjects and could lead to difficulties in sizing devices appropriately for an individual patient.
Following this, methods of design and manufacturing were investigated, comparing various tools available in commercial medical-CAD software, Materialise Mimics®. The commonly applied “thresholding” method of isolating structures from patient scan data was found to be insufficient for isolation of soft tissue structures such as the mitral valve annulus from the surrounding cardiac tissue due to the similarity in densities reducing contrast on the scan. A method of single-point design using insertion points of the valve leaflets throughout the scan was shown to be sufficient to reproduce a mitral annular structure, which was then manufactured in the Ti6Al4V alloy, which has been shown to be biocompatible in some orthopaedic applications, using SLM.
Post-processing techniques appropriate for the specific application of this device into the cardiovascular system were also investigated. The novel electrolyte jet machining process was employed to moderate surface unevenness caused by inherent properties of the powder bed SLM process, such as stepping or loose powder particles. This process was tested with a range of parameter sets producing varying topographies and therefore applied to different needs of the annuloplasty device. Firstly, the process was applied for reduction of coagulation on the surface of Ti6Al4V alloy samples, and then for amplification of fibroblastic cell growth. The primary parameter sets were found to produce a small reduction in platelet adhesion when compared against as-built SLM surfaces, however failed to reduce the platelet activity to that found on conventionally manufactured Ti6Al4V samples. The secondary parameter sets did not produce any improvement in fibroblastic proliferation in short term studies, however SLM samples were found to be significantly more favourable to fibroblast growth than conventionally manufactured surfaces of the same material grade.
Finally, future avenues for work are discussed, including next steps for each of the three areas investigated in this thesis and a view to the future of novel annuloplasty devices as a whole. Recommendations for other applications of electrolyte jet machining are provided, including the potential for anti-biofouling surface processing given the lack of cell survivability found in these studies. Further design recommendations are considered, from computational modelling of the valve through to structured surgical prediction integrated with design of the annuloplasty device.
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