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Figures Fifty to Fifty three show that ESWL fi'agility varies with chemical type of stone. Increasing Augmentation was seen for the series struvite, uric acid, ammonium urate, apatite, calcium oxalate, and cystine stones. This is in agreement with clinical experience,

suggesting that this in vitro model provides a close representation of ESWL in vivo.

Struvite stones are known to respond well to ESWL and apatite and calcium oxalate stones are more difficult to ftagment. Cystine stones are clinically very difficult to fragment and this finding was confirmed by our results as the cystine stone remained in Augments larger than 5 mm after 1800 shock waves. The ESWL results also confirm that Augmentation is less in larger stones.

In vivo e^q>eriments aimed at fiiagmenting stones rely almost entirely on radiographic follow

up to assess the number and size of remaining Aagments. The clearance of firagments after ESWL relies on a number of factors including

• The size of the stone

• The chemical type of the stone

• The position of the stone (renal, ureteric etc)

• The type o f lithotripter used, and the number and energy o f the shock waves delivered

The variability associated with these factors means that comparisons between clinical trials

of different lithotripters are extremely difficult. The establishment of the ESWL Aagility score is a significant result as this is the first time (to my knowledge) that an attempt such as this at quantification of the results of ESWL has been attempted. So 6 r this numerical score has only been used for comparison and correlation with the other techniques to test for

predictive value. Further developments of this score are required including the effect of stone size and shape, as a smooth surface decreases the effect of cavitation and shock wave scatter [88], and this might be expected to reduce the amount of fragmentation.

As well as the ESWL score, the Augmentation pattern of different stones has been shown to

be different. Figure Fifty four shows that stones of different chemical types Augment after different numbers of shock waves, and that the resulting fragment sizes vary. Stones such as Struvite fragment early, and into fragments predominantly less than l-2mm. At the opposite end of the scale are Cystine calculi which require much laiger numbers of shock waves, at higher energies, to cause any amount of fragmentation, and even then break into fragments larger than 5mm, To this extent the ESWL score mentioned above does not really predict whether a stone will fragment or not, but says more about the energy required to break the stone, and the likely distribution of fragments resulting from this fragmentation.

This increases its usefulness because a similar approach is required in clinical studies - when treating a patient with a renal stone (for instance) it would be useful to have a table which predicted how much energy (shock wave number x kV per shock wave) would be required, and the likely size of fragments produced.

Figure Fifty six shows the correlation of ESWL fragility score with Vicker’s Hardness number. The correlation coefficient, r, was 0.50 (p=0.389) which suggests that ESWL fragility is poorly correlated to Vicker’s Hardness number. This is a further point of interest because it raises the question of what these two indices are really measuring. The fracture of solids depends on the mechanism by which force has been applied, and the intrinsic properties of the material [53]. Vickers Hardness testing relies on a constant compressive

force from the diamond indenter to produce a indent in the stone surfece. The mechanism by which ESWL fragments stones is currently not well understood, although current theories have been discussed. Microhardness as measured by the Vickers Hardness Number may therefore not be a useful way of predicting the fragility of stones in the clinical environment.

6.7 Conclusions and Future Research

A number of important findings have been made:

• The validity of the investigative techniques was assessed by determining whether

differences could be detected between stones of different chemical type. Clear trends existed for density (Figures Twenty eight and Twenty nine), and porosity (Figure Twenty seven), and to a certain extent for MRI (Figures Forty four to Forty six), but not for the LAXD results (Figure Thirty three).

• Each o f the four techniques under investigation was then correlated with the results of

Microhardness testing. None of the techniques, apart form density of Calcium Oxalate

Stones, showed a significant correlation with Vickers Hardness Number. Thus it appears that these tests are not useful for predicting stone hardness as assessed by microhardness testing.

• The ESWL experiment conducted for this thesis proved to be a good imitation of the

clinical situation and produced results which reproduced those found in clinical practice. The ESWL score, and the discovery of specific ESWL fragmentation patterns were

significant results which will be useful in future experiments of this kind. Porosity and density correlated with ESWL fiagility and thus might be of use for the fiiture. Neither MRI nor LAXD showed any significant correlations and thus at the moment show less promise.

The aims of this thesis were to determine whether a physical parameter of urinary stones might be determined which could be used to predict whether stones will Augment successfully with ESWL. At the present time the such a parameter remains to be determined although clearly the density and porosity results show some promise. The possibilities for fiiture research with density and porosity have been discussed (see above).

Research is continuing with LAXD in conjunction with the Medical Physics departments of University College London, and St Bartholomew’s Hospital, London. Three key areas are under investigation;

• A new x-ray source has been purchased which provides a continuous supply of low- intensity (up to 15 mA) x-rays. This will allow the experiments to run for longer periods which should produce data of a higher statistical quality, making identification of features much easier.

• Analysis of the spectra may be improved by the use o f neural nets. These are computer based programs which can be trained on high-quality spectra to identify key features. Unknown spectra could then be classified according to a range o f features across the whole spectrum.

• Work is under way with a tuneable x-ray detector. Such detectors could be programmed to register a “result” when a specific range of signals is provided - i.e. the detector could be taught to recognise specific patterns (possibly specific kinds of kidney stones).

This Thesis, although laboratory based, had its origins in a clinical problem. The

investigative research and results of the experiments lead me to speculate that further progress in the understanding of stone fiagmentation, and the ability to devise methods of predicting success o f fiagmentation techniques, lies in a better understanding of the physical nature of urinary stones, and how the microstructure of the stone affects the response to ESWL.