The effect of screw pullout rate on screw purchase in synthetic cancellous bone.

Clinically, orthopaedic fracture fixation constructs are mounted using screws inserted into cancellous bone, while biomechanical studies are increasingly using commercially available synthetic bones. The goal of this study was to examine the effect of screw pullout rate on cancellous bone screw purchase strength in synthetic cancellous bone. Sixty synthetic cancellous bone cubes (40x40x40 mm(3)) each had one orthopaedic cancellous bone screw (major diameter=6.5 mm) inserted to a depth of 30 mm. Screws were extracted to obtain outcome measures of failure force, failure shear stress, failure energy, failure displacement, resistance force, and removal energy. The ten test groups (n=6 cubes per group) had screws extracted at pullout rates of 1 mm/min, 2.5 mm/min, 5 mm/min, 7.5 mm/min, 10 mm/min, 20 mm/min, 30 mm/min, 40 mm/min, 50 mm/min, and 60 mm/min. The aggregate average results for failure force, failure stress, failure energy, failure displacement, resistance force, and postfailure removal energy for combined pullout rates were, respectively, 984.8 +/- 63.9 N, 3.5 +/- 0.2 MPa, 298.3 +/- 41.7 J, 0.53 +/- 0.08 mm, 453.8 +/- 19.6 N, and 5420.1 +/- 489.7 J. Most statistical differences (40 of 47) involved either the 5 mm/min or the 60 mm/min rates being compared to other rates. Failure force, failure stress, and resistance force increased and were highly linearly correlated with pullout rate (R-2=0.78, 0.76, and 0.74, respectively). Failure energy, failure displacement, and removal energy were relatively unchanged over the pullout range tested, yielding low correlation coefficients (R-2 < 0.05). Failure force, failure stress, and resistance force were affected by bone screw pullout rate in synthetic cancellous bone, while failure energy, failure displacement, and removal energy remained unchanged. This is the first study to perform an extensive investigation of cancellous bone screw pullout rate in synthetic cancellous bone.