ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

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Figure 2: The results of the fit experiment are plotted, showing the average score and the standard
deviation per guide design. The left graph shows the results with respect to translation, the right
graph with respect to rotation.
A statistical analysis was performed, using an analysis of variance (ANOVA) to look at
the difference in average score for all designs. Looking at the translational stability, an
F-value of 15.43 was found, which is fairly larger than the critical F-value of 3.35 for
this test. Given a p-value of 0.00003, which is much smaller than the significance level
α of 0.05, it can be concluded that the difference in group means is significant. A similar
analysis was conducted for the fit test with respect to rotational stability. Here an F-
value of 10.74 was found, where the critical F value was equal to 3.35. Given a p-value
of 0.0004 and a significance level α of 0.05, it can be concluded that the difference
between the group means is significant.
4. DISCUSSION
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This paper describes a methodology to assess the stability of a surgical guide design
based on methods developed in robotics and fixture lay-out research. We have applied
this method to the stability of a surgical guide. A quality score is then defined that
allows a comparison of different guide designs with respect to their stability.
Little research has been done so far about the stability of surgical guides and how this
can be predicted. Radermacher was the first to link the haptic feeling of a guide to the
stability of a robotic grasp [15]. Using the eigenvalues of the stiffness matrix
(equation 1), he defined a quality score based on the isotropy value, defined by Kim and
Khosla [16], to predict the subjective feeling of a guide. However, the method used was
not frame-invariant and no distinction was made between possible translation and
rotation of a surgical guide. We believe this decoupling has an additional advantage
when looking at the stability of a surgical guide, as it can be assumed that in certain
procedures a rotational error can have a more harmful effect than a translational error,
for example when correcting a rotational deformity of the lower limb.
The developed method was validated using a fit experiment, in order to compare the
predictive quality score to the haptic feeling of guide stability experienced by users. The
results of the analysis show a good match to the results of the fit experiment. The first
design was marked as the least stable in both tests, where the third design appears to be
remarkably more stable, found by both the computational model as the haptic feeling
scored by the users. This shows that the model developed here provides a reliable
method to compare different guide designs with respect to their stability.
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