1. The objective assessment of Gigli saws for hip arthroplasty H L Stevenson, A C Fisher*, S J Scott, J S Davidson Depts. of Musculo-Skeletal Science,University of Liverpool and *Clinical Engineering, Royal Liverpool University Hospital NHS Royal Liverpool & Broadgreen University Hospitals NHS Trust Introduction A method for the objective quantitative assessment of Gigli saw cutting efficiency using mechanical and mathematical models is described. The Gigli saw, originally developed to open the symphysis pubis in obstructed labour, is now used routinely in orthopaedic surgery. The authors have experience of a number of makes of saw and are aware of, apparently, widely differing cutting performance. However, no reports are found in the literature where such observations are analysed objectively. Here we set out to address this shortcoming using a systematic laboratory-based approach. Methods Gigli saws were obtained from four manufacturers: Depuy; Judd; New Splint; and Smith & Nephew. Lengths were cut ( 2.5cm) and examined in the scanning electron microscope. See Photographs below A to D respectively. A BCD The test apparatus is shown in Photograph E to H and Figure 1. Ten test pieces (8cm length; 2.34cm diameter) were cut from a single length of hardwood dowel. Four cuts were made at approximately 0.3cm vertical spacing in 10 test specimens for each of the types of Gigli saw. A new saw was used each time. The cutting action was a 2Hz 16cm peak-to-peak sinusoid giving a cutting speed of: 80 (peak) and 64 (average) cm per second. The tension in the Gigli saw was 49N. Four cuts were made at approx 0.3cm vertical spacing in 10 test specimens for each of the types of Gigli saw. A new saw was used each time. E F GG R cncn dndn cut n-1 cut n cut n+1 RgRg cgncgn cut N-1 -y +x Key: R = radius of test piece R g = radius of cut d n = depth of n th cut at apex (along y axis) c n = chord length of test piece at n th cut apex (along x axis) c g n = chord length of n th cut at R (along x axis) ? d = depth of a single cut (ie.d n -d n+1 ) Z = cut width (along z axis) y z x As the experiment proceeds, we envisage a series of n cuts [ 0... N ] of thickness Z with constant arc radius of Rg equal to R, the radius of the test piece. By inspection, for any n and similarly, for any cut n The area of material removed at the n th cut comprises 2 parts: A 1 n, the area included above the chord c n and, A 2 n, the area included above arc of cut g n and below chord c g n Thus for a cut width of Z, V n the volume removed, Mathematical Model Results Mean and standard deviation (n=8) for depth of cuts at 60s in mm were 6.4 (1.7), 14.2 (2.8), 5.6 (0.6) and 6.3 (1.0) for Depuy, New Splint, Judd and S&N saws respectively. These corresponded to areas removed of 134.5 (34.5), 312.1 (50.2), 129.6 (14.2) and 146.4mm2, and volumes of 235.3, 358.9, 239.7 and 226.8mm3 respectively For both area and volume removed, New Splint was significantly different to Depuy, New Splint, Judd (p 0.075 in all cases) by the Wilcoxon signed-ranks two-tailed test. NS: not significant