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Measurement of the stiffness of endoscopes—a plea for commonality
  1. G D BELL
  1. K BURN
  1. Sunderland University Medical Sciences Faculty
  2. University of Sunderland, Sunderland, UK
  3. School of Computing, Engineering and Technology
  4. University of Sunderland, Sunderland, UK
  1. Professor GD Bell, Endoscopy Unit, Sunderland Royal Hospital, Kayll Road, Sunderland SR4 7TP, UK.duncan_bell{at}

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Editor,—In a previous issue (OpenUrlPubMed), Brooker and colleagues described their experience with an exciting new variable stiffness colonoscope. They made the point that a stiffer colonoscope shaft reduces recurrent looping but makes passage through an angulated sigmoid more difficult and causes more stretching and hence pain when loops do occur. Conversely, the more flexible thinner paediatric instruments are better for negotiating a fixed or narrow sigmoid colon but then tend to allow recurrent loop formation later in the procedure. Their randomised trial using either a standard Olympus CF200HL (13.3 mm shaft diameter) or a prototype (Olympus XCF-SH230L—12.9 mm shaft diameter) variable stiffness colonoscope looked very promising although in one case a paediatric Olympus PCF230I (11.3 mm shaft diameter) was required to get past a fixed sigmoid secondary to diverticular disease.

In addition to Brooker et al, there are a number of research workers1-4 and endoscope manufacturers interested in colonoscope/flexible sigmoidoscope shaft stiffness and its relation to patient discomfort/procedure time, yet sadly there is no agreement as to the best way to express (and thus directly compare) results. The beam deflection technique adopted by Brookeret al appeared to us to be an entirely arbitrary one involving a strain gauge, 5 cm shaft deflection, and just three duplicate measurements every 10 cm along the three instruments.

We agree with Wehrmeyer and colleagues1 that flexural rigidity is a more precise, accurate, and reproducible engineering parameter to measure when trying to compare endoscope shaft stiffness. In beam bending theory, the flexural rigidity isEI, which is the product of the modulus of elasticity (or Young's modulus) E and the second moment of area I of the beam cross section about an axis through the centroid perpendicular to the plane of bending. EI is given by the following expression:

EI=WL 3/192δ

where W is the load applied at the centre of the beam, L is the length of the beam, and δ is the deflection at the centre. In our own studies, the value of W(typically either 0.5 to 1 Newtons) was selected such thatδ (mean of 10 readings) was less than 0.5% of the length of the 20 cm “beam”. An example of the results obtained is shown in fig 1 in which mean (SD) flexural rigidity values (in N cm2) are compared for (a) an Olympus PCF 240I (11.2 mm diameter) instrument, (b) a fibreoptic Olympus CF20HL (13.3 mm diameter) endoscope, and (c) an Olympus CF-240AL (12 mm diameter) variable stiffness colonoscope. These three instruments were taken as being the nearest we had available in our own unit to those employed in the study of Brooker et al. Although our results are expressed in different units, the shape of the curves are remarkably similar to those published by Brooker et al. We confirm that the now commercially available variable stiffness Olympus colonoscope can indeed significantly alter its shaft stiffness from being almost as floppy as a paediatric endoscope to as stiff as a standard Olympus 20HL near its most proximal end.

Figure 1

Mean (SD) flexural rigidity measurements (N cm2) of three different colonoscopes: (a) Olympus PCF 240I, (b) Olympus CF20HL, and (c) Olympus variable stiffness CF-240AL instrument in its “floppy” and “stiff” modes.

We agree with Brooker et al that modifications that may enhance the efficacy of a variable stiffness colonoscope might include “more floppiness in the paediatric setting and greater stiffness at the maximum stiffness setting” .

We welcome debate and discussion on how best to measure endoscope shaft stiffness. In the meantime, until a better way of expressing the results is suggested, it would seem to us that some form of simple beam displacement methodology to determine flexural rigidity has the advantage of at least being relatively easy, reproducible, and inexpensive to perform.