Filshie clip closure: determination of closure through the analysis of X-rays

The Filshie method is a tubal occlusion method commonly used to prevent pregnancy. In medical negligence cases where it is suspected that closure of a Filshie clip may be faulty, lawyers may call on expert surgeons to assess whether or not a clip is closed on the basis of visual examination of the X-rays. However, it is not uncommon for experts to disagree. The aim of this work was to reduce the uncertainty in determining whether or not Filshie clips had been correctly closed. An estimate of the error in the estimate of the clip height was made by propagating measurement errors through a mathematical model. The effects of angle of presentation of the clip, digitisation of the image and resolution of the measurements were studied and the method was applied to two cases. The analysis indicated that measurement errors were least when the digitisation of the image was at 600dpi, angle of presentation of the clip was less than 40 degrees and the measurements could be made to an accuracy of +/-1pixel. Under these conditions it was possible to determine clip closure height with an error of less than +/-0.2mm.