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Fig. 10 Comparison between Fastening Strength, displaced bone volume and mean voxel
intensity, showing no statistical difference (p > 0.1) between plan and procedure,
therefore
confirming consistency between the planned and clinical standard Fastening Strength
displaced bone volume, and Fastening Strength), we used the pair Student t-statistic
to compare the paired (procedure vs. plan) results. While the average difference
between the plan-suggested and procedurally-implanted screws was on the order of
1 mm in diameter and 5 mm in length (i.e., typically one screw size, given the
dimensions of the available screws provided by the manufacturers), the paired
Student t-statistic revealed no difference between the parameters estimated from the
virtual plan and actual procedure (p > 0.1), therefore indicating that the results
suggested by planning platform were in agreement with those assessed based on the
post-procedure images, treated as clinical gold standard.
Figure
10
compares the Fastening Strength, displaced bone volume, and mean
voxel intensity of displaced bone volume between the pre-operative plan and post-
procedure outcome. While no statistical difference was noted between the plan and
procedure, higher Fastening Strength correlated with larger displaced bone volume,
and slightly larger implant dimensions.
The relationship used to estimate the Fastening Strength is available in
mechanical engineering and machine component design literature [
24
] and tradi-
tionally used to determine the holding power of screws, bolts and other fasteners.
Here we adapted to this concept the spine surgery application by relating the
material strength to a surrogate measure of bone mineral density derived from CT
image intensity. The Fastening Strength is computed based on the volume of bone
displaced by the screw, assuming a cylindrical model whose diameter is measured
across the thread, not just the screw shaft. To further emphasize the consistency and
utility of the Fastening Strength, we further analyzed the observed Fastening
Strength and displaced bone volume, in response to selected implant dimension.
Figure
11
illustrates the correlation between Fastening Strength and implant
dimension. As shown, as much as half of the holding power can be lost by un-
dersizing the implant diameter by up to 3.5 mm, and as much as 35 % of the
holding power can be lost by undersizing the implant length by up to 20 mm. These
measurements are consistent with the displaced bone volume measurements, which
are directly proportional to the implant dimension variability.
Moreover, as also revealed in Fig.
11
, given similar implant dimensions (no
difference in diameter or length), uncertainties on the order of 5 % were observed in
the displaced bone volume measurements, which, in turn, led to 8 % differences in
