Pullout of a lumbar plate with varying screw lengths

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Fig. 1
ALIF plate attached to foam block and screw lengths tested (inset).
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Fig. 2
Pullout testing setup.
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Fig. 3
Load-displacement curves for both foam densities.
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Fig. 4
Maximum pullout load plotted against screw length.
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Fig. 5
Stiffness plotted against screw length.

Table 1

Summary of maximum load to failure for each screw length and foam density

	Low density (0.08 g/cm³)		Medium density (0.16 g/cm³)
Screw length	Mean (N)	SD (N)	Mean (N)	SD (N)	Low density–medium density ratio
20 mm	302.8	11.0	1004	86.1	0.302
24 mm	388.2	35.6	1335	30.7	0.291
28 mm	479.4	43.5	1569	41.5	0.306
32 mm	585.9	12.5	1907	214.3	0.307
36 mm	707.8	33.9	2162	106.7	0.327

Table 2

Regression results for low-density (0.08-g/cm³) and medium-density (0.16-g/cm³) foam block specimens

Parameter	Slope (95% CI)	r Value	P value	Ratio
0.08 g/cm³
Maximum load	25.2 N (8.8%)	0.980	<.001
Displacement	0.03 mm (30.3%)	0.821	<.001
Stiffness	24.6 N/mm (24.2%)	0.872	<.001
0.16 g/cm³
Maximum load	72.2 N (11.2%)	0.968	<.001	2.87
Displacement	0.02 mm (60.9%)	0.584	.002	0.67
Stiffness	51.6 N/mm (30.5%)	0.816	<.001	2.10

NOTE. The 95% CI for the slope is given as the percentage of the slope. The slope is expressed as change per millimeter of screw length. The ratio is the medium-density slope divided by the low-density slope.