The effect of the screw pull-out rate on cortical screw purchase in unreamed and reamed synthetic long bones

Orthopaedic fracture fixation constructs are typically mounted on to human long bones using cortical screws. Biomechanical studies are increasingly employing commercially available synthetic bones. The aim of this investigation was to examine the effect of the screw pull-out rate and canal reaming on the cortical bone screw purchase strength in synthetic bone. Cylinders made of synthetic material were used to simulate unreamed (foam-filled) and reamed (hollow) human long bone with an outer diameter of 35 mm and a cortex wall thickness of 4 mm. The unreamed and reamed cylinders each had 56 sites along their lengths into which orthopaedic cortical bone screws (major diameter, 3.5 mm) were inserted to engage both cortices. The 16 test groups (n = 7 screw sites per group) had screws extracted at rates of 1 mm/ min, 5 mm/min, 10 mm/min, 20 mm/min, 30 mm/min, 40 mm/min, 50 mm/min, and 60 mm/ min. The failure force and failure stress increased and were highly linearly correlated with pull-out rate for reamed (R2 = 0.60 and 0.60), but not for unreamed (R2 = 0.00 and 0.00) specimens. The failure displacement and failure energy were relatively unchanged with pull-out rate, yielding low coefficients for unreamed (R2 = 0.25 and 0.00) and reamed (R2 = 0.27 and 0.00) groups. Unreamed versus reamed specimens were statistically different for failure force (p = 0.000) and stress (p = 0.000), but not for failure displacement (p = 0.297) and energy (0.054 < p < 1.000). This is the first study to perform an extensive investigation of the screw pull-out rate in unreamed and reamed synthetic long bone.