The pedicle screw design, the number tested, and the augmentation techniques are shown in Table 1. Six screws were expandable and one was cannulated. The 20-mm and 40-mm expandable screw designs with either two (2F 20 mm and 2F 40 mm) or four (4F 20 mm and 4F 40 mm) fins were specially manufactured, and a standard pedicle screw was included as a control.

The expandable screws had a cannulated center into which an expansion pin was placed. The cannula was used to inject either PMMA or calcium phosphate cement before inserting the expansion pin (Fig. 1A). The cannulated screw had a central conduit with two 10-mm long and 2-mm wide holes at the distal end. Either PMMA or calcium phosphate cement was injected through the cannula (Fig. 1B). The standard pedicle screw was a conical, self-tapping screw. The design variables including thread pitch and width, proximal root radius, distal root radius, proximal half angle, and distal half angle were kept constant among the screws (Table 2). All screws were made of a titanium alloy according to American Standard of Testing Materials (ISO 5832-3, Ti6AI4V-ELI; AK Steel, Butler County, OH, USA).

Cellular polyurethane foam (Pacific Research Laboratories, Vashon, WA, USA) conforming to ASTM F1839-9720 [9] was used because it prevents the varying results that can occur with biomechanical tests using cadaver bone [1011]. Foam with a 0.16 gm/cm³ density, 23 Mpa compressive modulus, 2.3 MPa compressive strength, and 86% porosity was used to simulate cancellous bone with severe osteoporosis. Screws were inserted into the center of 4×6×6 cm foam block (Fig. 2) and were inserted with a length of 45 mm without pretapping. PMMA (CEMFIX cement;TeknimedSA,Bigorre,France) or calcium phosphate cement (Calsibon synthetic calcium phosphate bone substitute; Biomet Deutschland GmbH, Berlin, Germany) used as augmentation was injected with a 50 mL syringe and plastic adapter. Approximately 2–3 mL of cement was injected with hand pressure through the central bore of the screws (Fig. 3). The amount of cement delivered was limited by the amount of pressure developed by the syringe during injection. Bone cements are heat sensitive, and handling and temperature reduce the final setting time. Consequently, surgeons should rely on experience to judge when the cement has reached a viscosity appropriate for extrusion. This does not occur until after the cement has begun to thicken. A small amount of cement should be extruded from the syringe and visually assessed to ensure that the surface appears dull and excessive flow under gravity has ceased [12]. After injection of the cement, an expansion pin was inserted into the expandable screws for fin expansion. The foam block was held in a fixture frame prior to the application of a longitudinal extraction load to the screw head at a loading rate of 5 mm/min, conforming to ASTM F1691-96 [13]. An INSTRON Model 3382 device was used, and the peak pullout force was recorded (Fig. 4). As the screws were extracted, the load increased sharply at first and then rapidly dropped. When the screw stripped the polyurethane foam, the peak load recorded just before the rapid drop in the load deformation curve was defined as the peak pullout force (Fig. 5).

Analysis of variance was used to compare the differences of bending strength and pullout strength among the study groups and was conducted using SPSS software ver. 13.0 (SPSS Inc., Chicago, IL, USA). The level of significance was defined as p<0.05.

An increase from two to four fins, with the same fin length, significantly affected the peak pullout force. An increase in fin length, with the same fin number, also increased the peak pullout force, but the increase was not statistically significant. The 4F 40-mm screws had the greatest peak pullout force, and it was significantly greater than that of the 2F 20-mm and 2F 40-mm screws. The number of fins was thus the design variable most strongly associated with an increase in the pullout force (Table 3).

No significant difference was found in the performance of PMMA-augmented cannulated screws and the (best performing) 4F 40-mm nonaugmented expandable screws. Calcium phosphate-augmented cannulated screws had significantly lower peak pullout forces than nonaugmented 4F 40-mm screws (p<0.001).

Both calcium phosphate and PMMA-augmented 4F 40-mm pedicle screws had significantly greater peak pullout forces than the other expandable screws (p<0.001).

PMMA-augmented 4F 40-mm screws had significantly greater peak pullout forces than the other screws (p<0.001). All other test screws had significantly greater peak pullout forces than the standard pedicle screw (Table 4). Examination of the extracted augmented screws revealed that increases in fin length and fin number allowed the augmentation material to spread more widely within the polyurethane foam (Fig. 6), and minimum spread was seen with the cannulated screws (Fig. 7).

Finite element analysis of the pedicle screw results with a commercially available software (Algor Simulation Software ver. 23.1; Algor Inc., Pittsburgh, PA, USA) generated three-dimensional surface models of the screws by a helical sweep of a predetermined thread. The surface models were transformed to solid models by a Boolean operation. The screws were inserted into the center of polyethylene cylinders to simulate a bending test condition. The screw and cylinder materials were assumed to be linearly isotropic and were map-meshed with 8-node hexahedral elements, except for irregular contact surfaces, which were free-meshed with high-order 20-node hexahedral elements. The overall element size was 0.4 mm. Surface-to-surface contact elements with a frictional coefficient of zero were used to represent the interface between the pedicle screw and polyethylene, and axial rotation of the screws was not allowed. A simulated vertical force applied to the heads of the fully inserted screws heads showed that 40-mm long fins created a weak point on the screw neck (Fig. 8).