The objective assessment of Gigli saws for hip arthroplasty

H L Stevenson, A C Fisher*, S J Scott, J S DavidsonDepts. of Musculo-Skeletal Science,University of Liverpool and

*Clinical Engineering, Royal Liverpool University Hospital

NHSRoyal Liverpool & Broadgreen University HospitalsNHS 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 B

C D

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 G G

R

cn

dn

cutn-1

cutn

cutn+1

Rgcgn

cutN-1

-y

-y

+x

Key:

R = radius of test piece

Rg = radius of cut

dn = depth of n th cut at apex (along y axis)

cn = chord length of test piece at n th cut apex (along x axis)

cgn = chord length of n th cut at R (along x axis)

?d = depth of a single cut ( ie. dn - dn+1)

Z = cut width (along z axis)

yz

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 cutn

The area of material removed at the nth cut comprises 2 parts:

A1n, the area included above the chord cn

and,

A2n, the area included above arc of cutgn and below chord cgn

Thus for a cut width of Z, Vn the volume removed,

R R dc

nn2 2

2

2 FHG

IKJb g

c R R dn n2 2 2b ge j

c R R dg g gn n 2 2 2b ge j

A c dn nn

N

1

0

A c c dn n nn

n

g2

0

b g

*

V Z c c c dn n n nn

n

n

N

g FHG

IKJ

b g00

*

V Z A An n n 1 2c h

viz n c cgn n , * for

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<0.01): differences between Depuy, New Splint and Judd were not significant (p>0.075 in all cases) by the Wilcoxon signed-ranks two-tailed test.

mean std Depuy New Splint Judd S&N

Depuy 6.39 1.7 S (p <0.01) NS NS

New Splint 14.24 2.8 NS S (p <0.01) NS

Judd 5.59 0.6 NS NS S (p <0.01)

S&N 6.29 1.0 NS NS NS

depth [mm] significant difference

NS: not significant