Bibliography: pages 65-69. / The human lens has two functions to fulfil. It has to transmit light and it has to change its shape according to the requirements of the accommodative process. These functions are determined respectively by the optical and mechanical properties of the lens. It is well documented that cataracts contribute to the gradual changes of the lens matrix properties. The fact is that loss of transparency due to cataract is a universal phenomenon occurring in 70% of the population over 70 years of age and that the only effective treatment for cataracts is its operative removal. In cataract surgery, anterior capsulotomy is a highly defined and crucial procedure. It involves creating a circular opening in the lens by incising the anterior surface of the lens capsule. This gives access to the lens cortex which is then extracted and replaced with a permanent plastic lens. The most popular capsulotomy technique involves tearing the capsule in a circular fashion using forceps. However, there are many potential problems to this technique such as: it is difficult to master, it takes a long time to perform and above all, it runs the risk of creating tears on the periphery of the opening. Since the capsule is retained post-operatively and acts as a support and centralisation of the artificial lens, it is necessary that the integrity of the capsule remain intact. Since anterior capsulotomy is an unpredictable procedure in cataract surgery, there is a definite need for a surgical device that can perform a reliable incision on the capsule. Using ultrasound to perform a capsulotomy is an innovative technique and its application has to be thoroughly investigated. The investigation includes a numerical and experimental analysis of the lens capsule. The numerical analysis shows that the lens capsule reaches states of resonance at frequencies above 80 kHz. It is at resonance that the capsule oscillations are increased and the cellular bonds are strained and broken. Attempts were made to perforate the human lens capsule using experimental piezoelectric transducer systems operating at resonance frequencies of 81.6, 106 and 187 kHz. Although each ultrasonic system was able to denature the lens cortex, a perforation of the lens capsule was only achieved at a frequency of 81.6 kHz. However, the perforation was irregular and exhibited several tears. This result is not acceptable as one of the main design requirements is to produce a capsulotomy that has a smooth and continuous margin. The amplitudes for the higher frequencies were inadequate to strain and break the capsule, even when the crystals were driven at their maximum voltage of 400 Volts. The present investigation proves that it is highly unlikely that an ultrasonic tool can be designed within a safe margin of frequencies and voltages. As long as no other alternative method is devised, surgeons will have to contend with the occasional complication of radial tears that occur during anterior capsulotomy using current anterior capsulotomy techniques.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/17541 |
| Date | January 1998 |
| Creators | Giesecke, Tonia |
| Contributors | Vicatos, George |
| Publisher | University of Cape Town, Faculty of Engineering and the Built Environment, Department of Mechanical Engineering |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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